Environmental Chemistry of Lakes and Reservoirs - ACS Publications

8

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on February 26, 2016 | http://pubs.acs.org Publication Date: May 5, 1994 | doi: 10.1021/ba-1994-0237.ch008

Long-Term Changes in Watershed Retention of Nitrogen Its Causes and Aquatic Consequences John L. Stoddard ManTech Environmental Technology, Inc., U . S . Environmental Protection Agency, Environmental Research Laboratory, Corvallis, O R 97333

Nitrogen saturation occurs when the supply of nitrogenous com pounds from the atmosphere exceeds the demandfor these compounds on the part of watershed plants and soil microbes. Several factors predispose forested watersheds to Ν saturation, including chronically high rates of Ν deposition, advanced stand age, and large pools of soil N. Many watersheds in the eastern United States exhibit symp toms of Ν saturation. A sequence of recognizable stages produces characteristic long-term and seasonal patterns of lake-water and stream-water NO concentrations that reflect the changes in rates and relative importance of Ν transformations as these watersheds become more Ν sufficient. The early stages of Ν saturation are marked by increases in the severity and frequency of NO episodes. The later stages of Ν saturation are marked by elevated baseflow con centrations of NO from groundwater. The most advanced symptoms of Ν saturation usually occur in regions with the most elevated rates of Ν deposition. Long-term increases in surface-water NO have important implications for surface-water acidification, but probably will not lead to freshwater eutrophication. 3-

3-

3-

3-

H I S T O R I C A L L Y , N I T R O G E N D E P O S I T I O N has not b e e n c o n s i d e r e d a serious threat to the i n t e g r i t y of aquatic systems. M o s t terrestrial systems have b e e n a s s u m e d to retain Ν strongly. I n s u c h cases t h e r e is a s m a l l p r o b a b i l i t y that d e p o s i t e d Ν w o u l d make its w a y to the surface waters that d r a i n these terrestrial systems. N i t r o g e n w i t h i n aquatic ecosystems c a n arise f r o m a 0065-2393/94/0237-0223$15.30/0 © 1994 American Chemical Society

In Environmental Chemistry of Lakes and Reservoirs; Baker, Lawrence A.; Advances in Chemistry; American Chemical Society: Washington, DC, 1994.


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ENVIRONMENTAL CHEMISTRY O F L A K E S A N D RESERVOIRS

variety of sources, i n c l u d i n g point-source a n d n o n - p o i n t - s o u r c e p o l l u t i o n , biological fixation of gaseous N , a n d d e p o s i t i o n o f n i t r o g e n oxides a n d a m m o n i u m . W h e n Ν was k n o w n to be affecting aquatic systems, some source o t h e r t h a n d e p o s i t i o n was assumed to b e responsible. T h e amounts of Ν s u p p l i e d to aquatic systems b y these other sources often o u t w e i g h b y a large m a r g i n the a m o u n t o f Ν p o t e n t i a l l y s u p p l i e d b y a t m o s p h e r i c d e p o s i t i o n . D u r i n g the past decade, h o w e v e r , o u r u n d e r s t a n d i n g of the transformations that Ν undergoes w i t h i n watersheds has increased greatly. I n areas of t h e U n i t e d States w h e r e n o n a t m o s p h e r i c sources of Ν are s m a l l , w e can b e g i n to infer cases i n w h i c h Ν d e p o s i t i o n is h a v i n g an i m p a c t o n aquatic systems. T h i s chapter w i l l establish h o w watersheds a n d surface waters are l i k e l y to change as the impacts of Ν d e p o s i t i o n b e c o m e m o r e severe. A b e r et a l . (I) d e s c r i b e d the changes that the terrestrial c o m p o n e n t s of u n d i s t u r b e d watersheds u n d e r g o as the effects o f Ν d e p o s i t i o n increase. I w i l l p r e s e n t the aquatic equivalents of the stages d e s c r i b e d b y A b e r et a l . (I) a n d o u t l i n e the k e y characteristics of these stages as t h e y i n f l u e n c e seasonal a n d l o n g t e r m aquatic Ν d y n a m i c s . T h e second p u r p o s e o f this c h a p t e r is to present a c c u m u l a t e d e v i d e n c e , p r i m a r i l y f r o m the eastern U n i t e d States, that o t h e r w i s e - u n d i s t u r b e d watersheds s h o w impacts f r o m e l e v a t e d Ν d e p o s i t i o n a n d that the severity o f these impacts corresponds to the h i s t o r i c a l levels o f Ν d e p o s i t i o n that the various sites have e x p e r i e n c e d . T h e focus o n w a tersheds that are u n d i s t u r b e d , o t h e r than b y e l e v a t e d rates of d e p o s i t i o n , is i m p o r t a n t . O t h e r disturbances (especially tree harvesting) can have sig nificant a n d l o n g - l i v e d effects o n Ν c y c l i n g . E l i m i n a t i o n of other p o t e n t i a l sources o f d i s r u p t i o n to the Ν cycle allows us to n a r r o w the f i e l d o f stresses that m i g h t be c o n t r i b u t i n g to the effects w e observe. E s t i m a t i o n of the effects of Ν d e p o s i t i o n o n aquatic systems is m a d e difficult b y the large variety o f forms of Ν f o u n d i n air, d e p o s i t i o n , w a tersheds, a n d surface waters, as w e l l as b y the m y r i a d pathways t h r o u g h w h i c h Ν can b e c y c l e d i n terrestrial a n d aquatic ecosystems. T h e s e c o m plexities separate Ν d e p o s i t i o n f r o m its effects a n d r e d u c e o u r a b i l i t y to attribute k n o w n aquatic effects to k n o w n rates of Ν d e p o s i t i o n . T h e orga n i z a t i o n of this chapter reflects this c o m p l e x i t y . Because an u n d e r s t a n d i n g of the ways that Ν is c y c l e d t h r o u g h watersheds is c r i t i c a l to o u r u n d e r standing of Ν effects, I b e g i n w i t h a b r i e f d e s c r i p t i o n of the Ν cycle a n d of the transformations of Ν that m a y o c c u r i n watersheds. I t h e n discuss the two most l i k e l y effects of Ν d e p o s i t i o n (acidification a n d eutrophication).

Nitrogen Inputs W a t e r s h e d s are generally several orders o f m a g n i t u d e larger t h a n the surface waters that d r a i n t h e m . T h u s most of the a t m o s p h e r i c d e p o s i t i o n that m a y potentially enter aquatic systems falls first o n some p o r t i o n of the w a t e r s h e d . N i t r o g e n m a y be d e p o s i t e d to the w a t e r s h e d or d i r e c t l y to water surfaces


8.

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225

Long-Term Changes in Watershed Retention of Nitrogen

i n a variety of forms, i n c l u d i n g N 0 ~ , N H , a n d organic Ν i n w e t a n d d r y d e p o s i t i o n . I n a d d i t i o n , plants may absorb gaseous Ν (as N O , N 0 , or n i t r i c acid vapor) (2, 3). Ν thus absorbed m a y s u b s e q u e n t l y e n t e r the w a t e r s h e d Ν b u d g e t as litterfall t h r o u g h the death o f plant biomass (4, 5). 3

4

+

2

Concentrations of N 0 " and N H i n precipitation vary widely through out N o r t h A m e r i c a ; t h e y d e p e n d largely o n the p r o x i m i t y of s a m p l i n g sites to sources of emissions. G a l l o w a y et a l . (6) r e p o r t m e a n concentrations of 2.4 μequiv/L for N 0 ~ a n d 2.8 μ e q u i v / L for N H for a site i n c e n t r a l A l a s k a s a m p l e d i n 1 9 8 0 - 1 9 8 1 . I n the S i e r r a N e v a d a M o u n t a i n s o f C a l i f o r n i a , m e a n concentrations of N 0 ~ a n d N H for the p e r i o d 1 9 8 5 - 1 9 8 7 w e r e 5.0 and 5.4 μequiv/L, respectively (7). I n a c o m p a r i s o n of Ν d e p o s i t i o n at lake and w a t e r s h e d m o n i t o r i n g sites i n n o r t h e r n U n i t e d States a n d s o u t h e r n C a n a d a , L i n s e y et a l . (8) f o u n d N 0 " concentrations r a n g i n g f r o m 15 to 40 μequiv/L a n d N H concentrations f r o m 10 to 50 μ e q u i v / L for 1 9 7 0 - 1 9 8 2 i n an area that was c o n s i d e r e d r e m o t e b u t m a y b e i n f l u e n c e d b y p r a i r i e dust and long-range acidic d e p o s i t i o n ; n e i t h e r i o n d o m i n a t e d o v e r the other. I n some areas closer to anthropogenic Ν sources (e.g., i n the northeastern U n i t e d States a n d southeastern Canada) v o l u m e - w e i g h t e d m e a n N 0 ~ c o n centrations range f r o m 30 μequiv/L (e.g., i n the A d i r o n d a c k a n d C a t s k i l l mountains of N e w York) to 50 μequiv/L (e.g., i n the eastern G r e a t L a k e s region). M e a n N H concentrations range f r o m 10 to 20 μ e q u i v / L i n the same areas (9). A m m o n i u m concentrations are highest (—40 μequiv/L) i n the a g r i c u l t u r a l areas of m i d w e s t e r n U n i t e d States. 3

4

+


3

4

3

4

+

+

3

+

4

3

4

+

S o m e u n c e r t a i n t y exists i n a l l estimates of N H deposition derived f r o m measurements made i n the N a t i o n a l A t m o s p h e r i c D e p o s i t i o n P r o g r a m / N a t i o n a l T r e n d s N e t w o r k ( N A D P / N T N ) because of the m e t h o d o f sample c o l l e c t i o n u s e d b y cooperators i n this p r o g r a m . Samples are c o l l e c t e d w e e k l y f r o m buckets that are c o v e r e d at a l l times except d u r i n g active p r e c i p i t a t i o n . P r e c i p i t a t i o n that falls early i n the w e e k m a y sit for several days before b e i n g c o l l e c t e d a n d f i l t e r e d for analysis. A h i g h p r o b a b i l i t y exists that some N H c o l l e c t e d i n N A D P collectors w i l l b e b i o l o g i c a l l y assimilated (transformed into organic forms o f N) before samples are f i l t e r e d . It is not c u r r e n t l y k n o w n w h a t the m a g n i t u d e of this p r o b l e m m a y b e . 4

+

4

+

D e p o s i t i o n of Ν d e p e n d s o n its concentration i n p r e c i p i t a t i o n , the v o l u m e of water falling as p r e c i p i t a t i o n , a n d the a m o u n t o f Ν i n d r y d e p o s i t i o n (2). T h e last o f these values (dry deposition) is difficult to measure a n d is often estimated as a fraction (e.g., 3 0 - 4 0 % ) of w e t d e p o s i t i o n (10). Sisterson et a l . (11) discussed d i r e c t measurements of d r y d e p o s i t i o n at r e g i o n a l l y representative sites. T h e y c o n c l u d e d that d e p o s i t i o n of Ν species ranges f r o m 4 0 - 5 0 % of w e t d e p o s i t i o n i n the northeast to —80% of w e t d e p o s i t i o n i n the southeastern U n i t e d States. G i v e n the range of concentrations p r e v i o u s l y m e n t i o n e d a n d the v o l u m e s of p r e c i p i t a t i o n falling i n different re gions of N o r t h A m e r i c a , estimates of Ν d e p o s i t i o n rates range f r o m less t h a n 12 e q u i v / h a p e r year i n A l a s k a to m o r e t h a n 800 e q u i v / h a p e r year i n northeastern U n i t e d States (Table I).


226


Table I. Rates of Nitrogen Deposition i n Several Areas of North A m e r i c a Area Alaska" (Poker Flat) Sierra Nevada, CA (Emerald Lake) Ontario, Canada (Experimental Lakes Area) British Columbia, Canada Upper M i d w e s t Southeastern U . S . ' (Walker Branch, TN) New Hampshire Catskills" Adirondacks c

c

d

1

c

r/

+

4

3

h


NH

N0 -

4.8

6.9

Total 11.7

Source 6

79

85

164

7

125

140

265

8

260

130 210 180

390

14

510 720

155 2

200 292

664 874 780

154 79

300 540 464 580 590

190

155

NOTE: All deposition values are in microequivalents per liter. "Dry deposition was estimated as 35% of total deposition. Dry deposition was sampled as part of snowpack; no correction was made for dry deposition. 'Bulk precipitation measurements; no correction was made for dry deposition. ''Values were corrected for dry deposition based on ratios of Hicks (201). 'Includes estimates for dry deposition and gaseous uptake of N. fo

Generally, N 0 dominates o v e r N H at sites close to e m i s s i o n sources (8,12). D i s s o l v e d organic n i t r o g e n ( D O N ) concentrations are h i g h l y variable i n p r e c i p i t a t i o n b u t often a m o u n t to 2 5 - 5 0 % o f i n o r g a n i c Ν d e p o s i t i o n (8, 12-14). I n some areas, D O N c a n occur i n greater concentrations than t h e inorganic species (15). 3

_

4

+

I n a d d i t i o n to w e t a n d d r y d e p o s i t i o n , m a n y h i g h - e l e v a t i o n sites m a y receive substantial inputs o f Ν f r o m clouds o r fog (15-17). F e w quantitative estimates o f c l o u d d e p o s i t i o n are available, b u t results f r o m o n e site o n W h i t e f a c e M o u n t a i n i n t h e A d i r o n d a c k s indicate that clouds a n d f o g c a n c o n t r i b u t e u p to 4 0 % of total d e p o s i t i o n (18). Rates of w e t a n d d r y d e p o s i t i o n at W h i t e f a c e M o u n t a i n w e r e comparable to t h e A d i r o n d a c k values g i v e n i n T a b l e I (—740 equiv/ha), b u t total d e p o s i t i o n rates ( i n c l u d i n g c l o u d a n d f o g deposition) averaged 1170 equiv/ha.

The Nitrogen Cycle A t m o s p h e r i c Ν can e n t e r aquatic systems either as d i r e c t d e p o s i t i o n to w a t e r surfaces o r as Ν d e p o s i t i o n to t h e terrestrial portions o f a w a t e r s h e d . N i t r o g e n d e p o s i t e d to the w a t e r s h e d is r o u t e d a n d transformed b y w a t e r s h e d p r o cesses. It m a y e v e n t u a l l y reach aquatic systems i n forms o n l y i n d i r e c t l y r e l a t e d to t h e o r i g i n a l d e p o s i t i o n . T h e transformations that Ν undergoes w i t h i n the w a t e r s h e d (e.g., i n soils, b y m i c r o b i a l action, a n d i n plants) play a major role i n d e t e r m i n i n g what forms a n d amounts o f Ν e v e n t u a l l y reach surface waters. M u c h o f the challenge o f d e t e r m i n i n g w h e n Ν d e p o s i t i o n is



8.

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227

affecting aquatic systems d e p e n d s o n o u r a b i l i t y to i d e n t i f y w h i c h Ν trans formations are o c c u r r i n g a n d w h i c h are not. A large part o f t h e f o l l o w i n g discussion is therefore focused o n terrestrial processes that alter t h e forms and rates o f Ν s u p p l y . M o s t o f these processes also o c c u r w i t h i n lakes a n d streams, a n d t h e i r strengths c a n d e t e r m i n e w h e t h e r t h e effects o f Ν d e p o sition w i l l b e felt i m m e d i a t e l y (e.g., t h r o u g h t h e e u t r o p h i c a t i o n of h e a d w a t e r lakes) o r d o w n s t r e a m (e.g., i n estuaries a n d coastal waters). I n a v e r y r e a l sense Ν c y c l i n g w i t h i n t h e terrestrial ecosystems controls w h e t h e r Ν d e position w i l l reach aquatic systems (and i n what concentrations), whereas Ν c y c l i n g w i t h i n lakes a n d streams controls w h e t h e r t h e Ν w i l l have a n y m e a surable effect. N i t r o g e n assimilation, m i n e r a l i z a t i o n , n i t r i f i c a t i o n , d e n i t r i fication, and n i t r o g e n fixation are i m p o r t a n t processes that affect t h e fate o f Ν from atmospheric deposition. Nitrogen Assimilation. N i t r o g e n assimilation is t h e uptake a n d m e t a b o l i c use o f Ν b y plants a n d soil m i c r o b e s ( F i g u r e 1). A s s i m i l a t i o n b y t h e terrestrial ecosystem controls t h e f o r m o f Ν e v e n t u a l l y released i n t o surface waters, as w e l l as affecting t h e a c i d - b a s e status o f soil a n d surface waters.

Figure 1. A simplified watershed nitrogen cycle, with major pathways (arrows) and their effects on the watershed hydrogen budget (numbers in circles) shown. Circled numbers represent the number of hydrogen ions transferred to the soil solution or surface water ( + 2) or from the soil solution or surface water (-1) for every molecule of Ν Of or NH that follows a given pathway. For example, nitrification follows the pathway for NH assimilation into microbial biomass ( + 1) and is leached out as N0 ~ ( + 1), for a total hydrogen ion production of +2 for every molecule ofN0 ~ produced. +

4

4

+

3

3


228


T e r r e s t r i a l assimilation is a major f o r m o f Ν r e m o v a l i n watersheds a n d m a y be sufficient to p r e v e n t a l l a t m o s p h e r i c a l l y d e r i v e d Ν f r o m r e a c h i n g surface waters (19). N i t r o g e n is the most c o m m o n l y l i m i t i n g n u t r i e n t i n N o r t h A m e r i c a n forest ecosystems (20, 21). T h e f o r m of Ν u s e d b y terrestrial ecosystems strongly affects the a c i d i f y i n g p o t e n t i a l o f Ν d e p o s i t i o n ( F i g u r e 1). A m m o n i u m uptake is an a c i d i f y i n g process (i.e., uptake of N H releases 1 m o l e of Η p e r m o l e of Ν assimilated). Downloaded by UNIV OF CALIFORNIA SAN DIEGO on February 26, 2016 | http://pubs.acs.org Publication Date: May 5, 1994 | doi: 10.1021/ba-1994-0237.ch008

4

NH

4

+

+ R O H

>R N H

2

+

+ H 0 + H

(1)

+

2

T h e b i o l o g i c a l uptake of N 0 " , o n the other h a n d , is an a l k a l i n i z i n g process (i.e., uptake o f N 0 ~ consumes 1 m o l e of Η p e r m o l e of Ν a s s i m i lated). 3

3

R O H

+ N0 - + H 3

+

> R*NH

+ 20

2

(2)

2

M o s t forested watersheds u n d e r g o a d o r m a n t p e r i o d , or at least a p e r i o d of m u c h r e d u c e d g r o w t h , d u r i n g the w i n t e r m o n t h s . Because assimilation o f Ν is also r e d u c e d d u r i n g this season, watersheds have a m u c h l o w e r a b i l i t y to retain Ν d u r i n g the w i n t e r a n d early s p r i n g . T h i s seasonality is r e s p o n s i b l e for the c o m m o n l y o b s e r v e d pattern of h i g h e r surface-water N 0 ~ c o n c e n trations i n w i n t e r a n d s p r i n g than i n s u m m e r a n d fall (discussed u n d e r Stage 0 Ν loss). If s p r i n g s n o w m e l t occurs before substantial forest g r o w t h begins i n the s p r i n g , s n o w m e l t N O " concentrations can be substantial, e v e n i n areas o f moderate Ν d e p o s i t i o n . C o n c e n t r a t i o n s o f N H i n surface waters, o n the o t h e r h a n d , are rarely elevated at any season because soil cation exchange; l o w m o b i l i t y ; a n d c o m p e t i t i o n a m o n g vegetation, m y c o r r h i z a l roots, a n d nitrifiers a l l c o n t r i b u t e to w a t e r s h e d N H retention. 3

i 3

4

4

+

+

M u l l e r a n d B o r m a n n (22) p r o p o s e d that s p r i n g e p h e m e r a l plants i n the understories o f forested watersheds m a y act as " v e r n a l d a m s " c o n t r o l l i n g the loss o f Ν from watersheds d u r i n g the s p r i n g . S p r i n g e p h e m e r a l plants m a y exploit the l i g h t , water, a n d nutrients available before forest g r o w t h and canopy closure make conditions less o p t i m a l i n later s p r i n g a n d s u m m e r . Zak et al. (23) u s e d radioactive Ν tracers to corroborate that s p r i n g e p h e m e r a l c o m m u n i t i e s are associated w i t h Ν r e t e n t i o n d u r i n g s p r i n g . H o w e v e r , t h e y f o u n d that most o f the r e t e n t i o n was attributable to assimilation b y soil m i c r o b e s , not plants. T h e r e has b e e n a t e n d e n c y to t h i n k of Ν assimilation as p r i m a r i l y a p l a n t - m e d i a t e d process, b u t assimilation b y m i c r o b e s m a y b e a v e r y i m p o r t a n t m e c h a n i s m m i n i m i z i n g Ν losses f r o m watersheds t h r o u g h out the g r o w i n g season. M o s t forest soils are c h a r a c t e r i z e d b y a v e r y large, relatively inert, p o o l o f organic Ν (24), w h i c h suggests that m i c r o b i a l uptake and i m m o b i l i z a t i o n of Ν are the d o m i n a n t processes i n most watersheds. Before w a t e r s h e d Ν d e m a n d (from b o t h forest a n d soils) can be m e t or


8.

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229

exceeded, uptake r e q u i r e m e n t s of b o t h vegetation a n d soil m i c r o b i a l c o m m u n i t i e s must be met. If one accepts the c o m m o n w i s d o m that forest v e g etation is a s u p e r i o r l o n g - t e r m c o m p e t i t o r for Ν (24, 25), t h e n the e v o l u t i o n of a N-sufficient w a t e r s h e d can b e seen to consist of two stages: f u l f i l l m e n t of vegetation Ν d e m a n d , f o l l o w e d b y f u l f i l l m e n t of soil m i c r o b i a l d e m a n d . A s s i m i l a t i o n b y aquatic plants, a k e y process i n the p o t e n t i a l e u t r o p h i cation of surface waters b y N , m a y also play a role i n t h e i r a c i d - b a s e status. U p t a k e of N 0 ~ i n lakes is an a l k a l i n i z i n g process ( F i g u r e 1) that may be stoichiometrically i m p o r t a n t i n some lakes (26). A q u a t i c plants generally favor the uptake of N H o v e r the uptake of N 0 ~ ; N H uptake is ener getically favorable because N 0 ~ must first be r e d u c e d before it is p h y s i o logically available to algae (27). M c C a r t h y (28) s u m m a r i z e d several studies that consistently show that potential (saturated) N H uptake rates are greatly e n h a n c e d i n N - d e f i c i e n t cells. T h i s relationship is n o w u s e d , a l o n g w i t h various other indices, as a basis for i d e n t i f y i n g the d e g r e e of Ν l i m i t a t i o n i n p h y t o p l a n k t o n (29, 30).


3

4

+

3

4

+

3

4

+

A c r u c i a l difference b e t w e e n aquatic a n d terrestrial ecosystems is that Ν additions d o not c o m m o n l y stimulate g r o w t h i n aquatic systems, as seems to be the case i n m a n y terrestrial systems. Ν l i m i t a t i o n m a y b e the e x c e p t i o n i n aquatic systems rather than the r u l e . T h e q u e s t i o n of w h e t h e r Ν l i m i t a t i o n is a c o m m o n occurrence i n surface waters w i l l p l a y a large role i n d e t e r m i n i n g w h e t h e r Ν d e p o s i t i o n affects the t r o p h i c state of aquatic ecosystems. T h e effects of Ν s u p p l y o n uptake a n d g r o w t h rates i n p h y t o p l a n k t o n a n d p e r i p h y t o n are the subject of v o l u m e s of literature, a s u m m a r y of w h i c h is b e y o n d the scope of this chapter. H o w e v e r , certain aspects of the l i m i t a t i o n of algal g r o w t h b y the s u p p l y of Ν a n d other nutrients w i l l be discussed later i n the section o n e u t r o p h i c a t i o n b y Ν d e p o s i t i o n . O t h e r details o n algal n u t r i t i o n can be f o u n d i n r e v i e w s b y G o l d m a n a n d G i l b e r t (31), B u t t o n (32), K i l h a m a n d H e c k y (33), a n d H e c k y a n d K i l h a m (34).

Mineralization. M i n e r a l i z a t i o n is the bacterial d e c o m p o s i t i o n of or ganic matter; it releases N H that can subsequently b e n i t r i f i e d to N 0 " . M i n e r a l i z a t i o n is an i m p o r t a n t process i n watersheds, as it recycles Ν that w o u l d otherwise be t i e d u p i n soil organic matter f o l l o w i n g the d e a t h of plants, or as leaflitter ( F i g u r e 1). I n a comparative study of m i n e r a l i z a t i o n i n soils, N a d e l h o f f e r et al. (35) f o u n d Ν m i n e r a l i z a t i o n rates r a n g i n g f r o m 6000 to 9600 e q u i v / h a p e r year u n d e r d e c i d u o u s tree species a n d from 2800 to 5800 e q u i v / h a p e r year u n d e r coniferous species. T h e s e rates s h o u l d b e c o m p a r e d to Ν d e p o s i t i o n rates of 6 0 0 - 9 0 0 e q u i v / h a p e r year for h i g h d e p o s i t i o n areas of the northeast (Table I). N a d e l h o f f e r et a l . (35) also re p o r t e d estimated rates of Ν uptake that w e r e 5 - 2 0 % h i g h e r than rates of m i n e r a l i z a t i o n . T h e s e rates suggest that i n t e r n a l c y c l i n g sources of Ν far o u t w e i g h external sources such as d e p o s i t i o n u n d e r most c o n d i t i o n s . 4

+


3

230


T h e effect of m i n e r a l i z a t i o n o n the a c i d - b a s e status of d r a i n i n g waters d e p e n d s o n the f o r m o f Ν p r o d u c e d . T h e c o n v e r s i o n of organic Ν (e.g., f r o m leaflitter) to N H consumes 1 m o l e of Η p e r m o l e of Ν p r o d u c e d ( F i g u r e 1); it can b e thought o f as the reverse of the reaction i n e q 1. O r g a n i c Ν that is m i n e r a l i z e d a n d s u b s e q u e n t l y o x i d i z e d (nitrified) to N 0 ~ (eq 3) p r o d u c e s a net of 1 m o l e of Η p e r m o l e of N 0 ~ p r o d u c e d . Because the p r o d u c t i o n of organic Ν (i.e., assimilation) can e i t h e r p r o d u c e or c o n s u m e hydrogen (depending on whether N 0 " or N H is assimilated), the net (ecosystem) effect of m i n e r a l i z a t i o n d e p e n d s o n b o t h the species e n t e r i n g the w a t e r s h e d a n d the species l e a v i n g the w a t e r s h e d ( F i g u r e 1). 4

+

3

3


3

4

+

M i n e r a l i z a t i o n often has the i n i t i a l effect (e.g., i m m e d i a t e l y after leaffall) of i m m o b i l i z i n g Ν (36). I n ecosystems w h e r e plant g r o w t h is l i m i t e d b y the availability o f N , m i n e r a l i z a t i o n is also l i m i t e d b y Ν i n the sense that a d d i t i o n of Ν to the leaflitter speeds decay a n d increases the rate at w h i c h Ν is i m m o b i l i z e d b y decomposers (37, 38). T h i s i n i t i a l i m m o b i l i z a t i o n p e r i o d is m a r k e d b y a net increase i n the Ν content o f leaflitter. N i t r o g e n l i m i t a t i o n of d e c o m p o s i t i o n follows i n part f r o m the l o w Ν content t y p i c a l o f l i t t e r , w h i c h arises f r o m the translocation of Ν out o f leaves d u r i n g senescence. T h e i m m o b i l i z a t i o n phase of m i n e r a l i z a t i o n is f o l l o w e d b y a p e r i o d of s l o w release o f inorganic Ν f r o m the soil m i c r o b i a l p o o l (36).

Nitrification. N i t r i f i c a t i o n , the oxidation o f N H to N 0 ~ , is m e d i a t e d b y bacteria a n d f u n g i i n b o t h the terrestrial a n d aquatic p o r t i o n s of watersheds. It is an i m p o r t a n t process i n c o n t r o l l i n g the f o r m of Ν released to surface waters b y watersheds, as w e l l as i n c o n t r o l l i n g the a c i d - b a s e status of surface waters ( F i g u r e 1). N i t r i f i c a t i o n is a strongly a c i d i f y i n g process, p r o d u c i n g 2 moles of Η for each m o l e of Ν ( N H ) n i t r i f i e d . 4

4

NH

4

+

+ 20

+

3

+

> N O , " + 2H+ + H 0

2

(3)

2

Because n i t r i f i c a t i o n i n forest soils transforms N H i n t o N 0 " , the a c i d i f y i n g p o t e n t i a l of d e p o s i t i o n (attributable to N ) is often d e f i n e d as the sum o f N H a n d N 0 ~ o n the a s s u m p t i o n that a l l Ν w i l l leave the w a t e r s h e d as N 0 " (39). U n l e s s N 0 " l e a v i n g the w a t e r s h e d is a c c o m p a n i e d b y base cations (e.g., C a ) , it w i l l acidify lakes a n d streams b y l o w e r i n g t h e i r a c i d n e u t r a l i z i n g capacity ( A N C ) . 4

4

+

+

3

3

3

3

2 +

N i t r i f i c a t i o n is l i m i t e d i n most soils b y the s u p p l y rate of N H (40, 41). C o m p e t i t i o n exists b e t w e e n nitrifiers a n d vegetation, w h i c h m a y b o t h be l i m i t e d b y the availability o f N H . T h i s m i c r o b i a l d e m a n d for N H , 4

4

+

+

4

+

c o u p l e d w i t h the h i g h cation-exchange capacity of most temperate forest soils, leads to surface-water N H concentrations that are usually u n d e tectable. N i t r i f i c a t i o n rates m a y also b e l i m i t e d b y inadequate m i c r o b i a l populations, lack of water, allelopathic effects (toxic effects p r o d u c e d b y i n h i b i t o r s m a n u f a c t u r e d b y vegetation), or b y l o w soil p H . 4

+


8.

STODDARD


231


A m o n g these potential l i m i t i n g factors, soil p H plays an o b v i o u s l y v i t a l role i n any discussion of the acidification of surface waters b y Ν d e p o s i t i o n . N i t r i f i c a t i o n has traditionally b e e n thought of as an acid-sensitive process (1, 42), b u t h i g h rates of nitrification have b e e n r e p o r t e d r e c e n t l y f r o m v e r y acid soils (i.e., p H < 4.0) i n the northeastern U n i t e d States (41, 43, 44) a n d i n E u r o p e (45, 46). I n the southeastern U n i t e d States, M o n t a g n i n i et a l . (47) w e r e unable to f i n d any ρ H effect o n nitrification or to stimulate n i t r i f i c a t i o n b y buffering a c i d soils. I n a survey of sites across the northeastern U n i t e d States, M c N u l t y et al. (48) f o u n d no correlation b e t w e e n n i t r i f i c a t i o n rates a n d soil p H , b u t a strong association ( r = 0.77) w i t h rates of Ν d e p o s i t i o n . T h e w e i g h t of e v i d e n c e suggests that nitrification w i l l o c c u r at l o w soil p H values as l o n g as the s u p p l y of N H is sufficient a n d that any i n h i b i t i o n of nitrification b y l o w soil p H can be o v e r c o m e b y excess Ν availability. 2

4

+

Rates of nitrification i n lakes a n d streams m a y also b e l i m i t e d b y l o w concentrations of N H . S u p p l y rates of N H f r o m watersheds are often l o w (except i n cases of point-source p o l l u t i o n ) , a n d n i t r i f y i n g organisms have little substrate w i t h w h i c h to w o r k . T w o exceptions to this generality are cases i n w h i c h N H d e p o s i t i o n is e x t r e m e l y h i g h (such as near a g r i c u l t u r a l areas) a n d i n w h i c h N H is p r o d u c e d w i t h i n the aquatic system. E x p e r i ments o n w h o l e lakes a n d i n mesocosms have c o n f i r m e d the a c i d i f y i n g p o tential of a m m o n i u m additions f r o m d e p o s i t i o n to surface waters (49, 50). A m m o n i u m d e p o s i t i o n is especially d e c e p t i v e because i n the atmosphere it can c o m b i n e as a n e u t r a l salt w i t h S 0 ~ to p r o d u c e p r e c i p i t a t i o n w i t h nearn e u t r a l p H values, as seen i n the N e t h e r l a n d s (51). O n c e d e p o s i t e d , h o w ever, the a m m o n i u m can b e assimilated (leaving an e q u i v a l e n t a m o u n t of hydrogen) or n i t r i f i e d (leaving twice the a m o u n t of hydrogen). C a n a d i a n w h o l e - l a k e experiments generated c o n f l i c t i n g e v i d e n c e that n i t r i f i c a t i o n i n lakes is an acid-sensitive process. R u d d et a l . (52) p r e s e n t e d data i n d i c a t i n g that nitrification was b l o c k e d at p H values less than 5.4 i n an e x p e r i m e n t a l l y a c i d i f i e d lake a n d that this situation l e d to a progressive a c c u m u l a t i o n of NH i n the water c o l u m n . S c h i n d l e r et a l . (49) r e p o r t e d n i t r i f i c a t i o n i n an e x p e r i m e n t a l l y f e r t i l i z e d lake at p H 4.6. R u d d et a l . (52) h y p o t h e s i z e d that nitrification w i l l occur i n l o w - p H lakes o n l y w h e n w i n t e r p H values are sufficiently h i g h ( p H > 5.4) to a l l o w nitrifiers to b e c o m e w e l l established before the l o w p H values r e s u l t i n g f r o m nitrification d e v e l o p . T h i s situation existed i n the e x p e r i m e n t a l lake d e s c r i b e d b y S c h i n d l e r et a l . (49). 4

4

+

4

+

4

+

4

4

+

2

+

High N H concentrations may also result t h r o u g h d e c o m p o s i t i o n i n lakes whose d e e p e r waters b e c o m e anoxic d u r i n g periods of stratification (usually late w i n t e r or late s u m m e r ) . N i t r i f i c a t i o n of this N H occurs w h e n lakes m i x d u r i n g s p r i n g or fall. T h e m i x i n g process supplies the o x y g e n necessary for n i t r i f y i n g organisms to s u r v i v e a n d m e t a b o l i z e (53). I n this case, the m a i n influence of nitrification is to recycle Ν w i t h i n the system a n d to s u p p l y N 0 " either to denitrifiers or to N - d e f i c i e n t algae. 4

+

4

+

3


232


Denitrification. D e n i t r i f i c a t i o n is the b i o l o g i c a l r e d u c t i o n of N 0 ~ to p r o d u c e gaseous forms of r e d u c e d n i t r o g e n ( N , N O , or N 0 ) (54). It is an anaerobic process (i.e., it occurs o n l y i n e n v i r o n m e n t s w h e r e oxygen is absent) whose e n d p r o d u c t is lost to the atmosphere ( F i g u r e 1). I n terrestrial ecosystems, d e n i t r i f i c a t i o n occurs i n anaerobic soils, especially boggy, p o o r l y d r a i n e d soils, or i n anaerobic microsites. It has t r a d i t i o n a l l y b e e n c o n s i d e r e d a r e l a t i v e l y u n i m p o r t a n t process outside o f wetlands (55). H o w e v e r , d e n i trification c o u l d be an episodic process o c c u r r i n g after s u c h events as s p r i n g s n o w m e l t a n d heavy rainstorms, w h e n soil oxygen tension is r e d u c e d (56). N o single equation can describe the d e n i t r i f i c a t i o n reaction because several e n d products are possible. H o w e v e r , d e n i t r i f i c a t i o n is always an a l k a l i n i z i n g process that consumes 1 m o l e o f H for e v e r y m o l e of Ν d e n i t r i f i e d ( F i g u r e 1). 3


2

2

D e n i t r i f i c a t i o n can be i n v o l v e d i n the p r o d u c t i o n or c o n s u m p t i o n o f N 0 , a p r o d u c t that may have considerable significance as a greenhouse gas (57, 58). I n a r e v i e w o f the effects of acidic d e p o s i t i o n o n d e n i t r i f i c a t i o n i n forest soils, K l e m e d t s s o n a n d Svensson (59) c o n c l u d e d that d e n i t r i f i c a t i o n rates are often l i m i t e d b y the availability of oxygen a n d m a y therefore b e r e l a t i v e l y insensitive to increases i n Ν d e p o s i t i o n . A c i d i f i c a t i o n of soils has an u n c e r t a i n effect o n o v e r a l l rates of d e n i t r i f i c a t i o n , b u t it strongly affects the e n d p r o d u c t of d e n i t r i f i c a t i o n a n d favors the p r o d u c t i o n o f N 0 o v e r N (56, 60). E v e n u n d e r e x t r e m e conditions, d e n i t r i f i c a t i o n is u n l i k e l y to be a significant sink for w a t e r s h e d N . N o n e t h e l e s s , it m a y b e significant i n the global atmospheric budget of N 0 (60, 61). 2

2

2

2

D e n i t r i f i c a t i o n plays a m u c h larger role i n Ν d y n a m i c s i n aquatic eco systems than it does i n terrestrial ones. I n streams, r i v e r s , a n d lakes, b o t t o m sediments are the m a i n sites for d e n i t r i f i c a t i o n (62), a l t h o u g h o p e n - w a t e r d e n i t r i f i c a t i o n has also b e e n r e p o r t e d (63). I n lake a n d stream sediments N 0 ~ is potentially available f r o m the water c o l u m n , b u t it is p r o d u c e d m a i n l y w h e n organic matter is b r o k e n d o w n w i t h i n the sediments a n d the resulting N H is subsequently o x i d i z e d (62). D e n i t r i f i c a t i o n is an especially i m p o r t a n t process i n large r i v e r s , for w h i c h S e i t z i n g e r (62) r e p o r t e d d e n i trification rates that w e r e 7 - 3 5 % of total Ν i n p u t s . D e n i t r i f i c a t i o n i n aquatic ecosystems is an a l k a l i n i z i n g process, c o n s u m i n g 1 m o l e of Η for e v e r y m o l e of N 0 ~ d e n i t r i f i e d . 3

4

+

3

Estimates of d e n i t r i f i c a t i o n rates range f r o m 54 to 345 μ ι η ο ΐ / m p e r h o u r i n streams w i t h h i g h rates of organic matter d e p o s i t i o n , 12 to 56 μΐΏθΙ/m p e r h o u r i n n u t r i e n t - p o o r o l i g o t r o p h i c lakes, a n d 42 to 171 μ η ι ο ΐ / m p e r h o u r i n e u t r o p h i c lakes (62). R u d d et a l . (64) r e p o r t e d an increase i n the rate of denitrification f r o m less than 0.1 to o v e r 20 μ η ι ο ΐ / m p e r h o u r i n an o l i g o t r o p h i c lake w h e n n i t r i c a c i d was a d d e d i n a w h o l e - l a k e e x p e r i m e n t a l acidification. T h i s result suggests that freshwater d e n i t r i f i c a t i o n m a y be l i m i t e d b y N 0 " availability. I n d e e p m u d s of slowflowing streams, the process can effectively r e d u c e N 0 " concentrations i n 2

2

2

2

3

3


8.

STODDARD


233

the water c o l u m n b y as m u c h as 200 μ β ς υ ΐ ν / ί . o v e r a 2 - k m l e n g t h of stream (65, 66). T h i s d e p l e t i o n amounts to 7 5 % of the d a i l y N 0 ~ i n p u t d u r i n g a g r o w i n g season. T h u s d e n i t r i f i c a t i o n can be c o n s i d e r e d as a m e t h o d for N 0 ~ r e m o v a l i n the management of some s l o w - m o v i n g streams w i t h a d e e p or ganic substrate (67). 3

3

Nitrogen Fixation. Gaseous a t m o s p h e r i c n i t r o g e n ( N ) can b e fixed to p r o d u c e N H b y a w i d e range of s i n g l e - c e l l e d organisms, i n c l u d i n g b l u e green algae (Cyanobacteria), a n d various aerobic a n d anaerobic bacteria. S y m b i o t i c n i t r o g e n - f i x i n g nodules are present o n the roots of some early successional forest species (68). I n headwater streams, n o d u l e s o n r o o t i n g structures of riparian vegetation (e.g., Alnus sp.) can also be i m p o r t a n t Ν fixers (69). O r d i n a r i l y , Ν fixation has no d i r e c t effect o n the a c i d - b a s e status of soil or surface waters. 2


4

+

N

2

+ H 0 + 2R-OH 2

>2RNH

2

+ § 0

(4)

2

N i t r o g e n fixation i n excess of b i o l o g i c a l d e m a n d , h o w e v e r , can l e a d to nitrification or m i n e r a l i z a t i o n of organic Ν a n d u l t i m a t e l y to acidification of soil or surface waters (70, 71). N i t r o g e n fixation counteracts d e n i t r i f i c a t i o n losses o f Ν f r o m surface waters a n d is f u n d a m e n t a l to r e p l e n i s h i n g f i x e d forms o f Ν i n a l l aquatic ecosystems. It is thought to be the m a i n process r e s p o n s i b l e for m a i n t a i n i n g surplus inorganic Ν i n lakes a n d streams. It is therefore basic to the c o n c e p t that p r i m a r y p r o d u c t i o n i n most lakes a n d streams is l i m i t e d b y p h o s p h o r u s (72). Rates of Ν fixation are generally r e l a t e d to t r o p h i c status i n fresh water. H o w a r t h et a l . (73) s h o w e d that fixation i n l o w - , m e d i u m - a n d h i g h - n u t r i e n t lakes is generally < 0 . 0 2 , 0 . 9 - 6 . 7 , a n d 1 4 . 3 - 6 5 6 . 9 m m o l / m o f Ν p e r year, respectively. F i x a t i o n is also closely c o r r e l a t e d w i t h the a b u n d a n c e of b l u e g r e e n algae (53). T h i s relationship suggests that the algae, rather t h a n bac teria, d o m i n a t e Ν fixation i n lakes. A l t h o u g h Ν fixation does o c c u r i n s e d i m e n t s , that source is of m i n o r i m p o r t a n c e c o m p a r e d to Ν fixation i n the water c o l u m n . O n l y i n v e r y n u t r i e n t - p o o r lakes, w h e r e Ν l o a d i n g f r o m a l l other sources is s m a l l , is Ν fixation i n sediments of o v e r a l l significance (e.g., 32 a n d 6 % of total inputs i n L a k e Tahoe a n d M i r r o r L a k e , respectively) (73). 2

Nitrogen Saturation M u c h of the debate o v e r w h e t h e r aquatic systems are b e i n g affected b y Ν d e p o s i t i o n centers o n the concept of n i t r o g e n saturation of forested w a tersheds. N i t r o g e n saturation can be d e f i n e d as a situation i n w h i c h the



234


s u p p l y of nitrogenous c o m p o u n d s from the atmosphere exceeds the d e m a n d for these c o m p o u n d s o n t h e part o f w a t e r s h e d plants a n d m i c r o b e s ( J , 74). U n d e r conditions o f Ν saturation, forested watersheds that p r e v i o u s l y r e t a i n e d nearly a l l o f Ν inputs d u e to a h i g h d e m a n d f o r Ν b y plants a n d m i c r o b e s b e g i n to have h i g h e r loss rates o f N . T h e s e losses m a y b e i n t h e f o r m o f l e a c h i n g to surface waters o r to t h e atmosphere t h r o u g h d e n i t r i f i cation. T h e s e t w o p o t e n t i a l loss pathways have p r o f o u n d l y different impacts o n t h e a c i d - b a s e status o f watersheds a n d surface waters (see f o l l o w i n g discussion). T h e i r relative i m p o r t a n c e i n a d v a n c e d stages o f Ν saturation w i l l b e a decisive characteristic d e t e r m i n i n g the severity of the i m p a c t o f Ν saturation. Progressive Stages. A b e r et a l . (I) p r o p o s e d a h y p o t h e t i c a l t i m e course for a w a t e r s h e d response to c h r o n i c Ν additions ( F i g u r e 2), d e s c r i b i n g b o t h t h e changes i n Ν c y c l i n g that are p r o p o s e d to o c c u r a n d t h e p l a n t responses to c h a n g i n g levels o f Ν availability. A b e r et a l . i n c l u d e i n t h e i r h y p o t h e t i c a l t i m e course a trajectory for the loss o f Ν to surface-water runoff, w h i c h suggests a s i m p l e response ( N leaching) i n t h e later stages o f Ν sat u r a t i o n . T h i s chapter seeks to establish w h e t h e r stages e q u i v a l e n t to those s h o w n i n F i g u r e 2 c a n b e d e s c r i b e d f o r surface waters a n d to d e t e r m i n e w h e t h e r t h e response o f surface waters to a d v a n c e d stages o f Ν saturation is as s i m p l e as is suggested i n F i g u r e 2. Stage 0 o f t h e A b e r et a l . (I) c o n c e p t u a l m o d e l is t h e p r e t r e a t m e n t c o n d i t i o n ; i n p u t s o f Ν f r o m d e p o s i t i o n are at b a c k g r o u n d levels a n d w a t e r s h e d losses of Ν are n e g l i g i b l e ( F i g u r e 2). Increased d e p o s i t i o n occurs i n Stage 1, b u t effects o n t h e terrestrial ecosystem are n o t e v i d e n t . F o r a l i m i t i n g n u t r i e n t s u c h as N , a f e r t i l i z a t i o n effect m i g h t result i n i n c r e a s e d ecosystem p r o d u c t i o n a n d tree v i g o r at Stage 1. R e t e n t i o n o f Ν is v e r y efficient a n d little or no Ν w o u l d b e lost a n n u a l l y to surface waters that d r a i n Stage 1 watersheds. M a n y forested watersheds i n t h e U n i t e d States w o u l d b e c o n s i d e r e d to exist at this stage. N e g a t i v e effects occur i n Stage 2 of the A b e r et al. (J) h y p o t h e t i c a l t i m e course. H o w e v e r , these effects are subtle, n o n v i s u a l , a n d r e q u i r e l o n g t i m e scales for d e t e c t i o n . O n l y i n Stage 3 d o t h e effects o n t h e forests b e c o m e v i s i b l e a n d result i n major e n v i r o n m e n t a l impacts. A b e r et a l . (J) e m p h a s i z e that different species a n d e n v i r o n m e n t a l c o n d i t i o n s c o u l d alter t h e t i m i n g of the effects i l l u s t r a t e d i n F i g u r e 2. A n u m b e r of factors m a y c o n t r i b u t e to a watershed's p r o g r e s s i o n t h r o u g h the stages o f Ν loss, i n c l u d i n g elevated Ν d e p o s i t i o n , stand age, a n d h i g h s o i l - N pools. H i g h rates o f Ν d e p o s i t i o n p l a y a clear r o l e , as t h e a b i l i t y o f forest biomass to accumulate Ν m u s t b e finite. A t v e r y h i g h l o n g - t e r m rates of Ν d e p o s i t i o n , t h e a b i l i t y o f forests a n d soils to a c c u m u l a t e Ν w i l l b e exceeded. T h e o n l y r e m a i n i n g pathway for loss o f Ν (other than runoff) is d e n i t r i f i c a t i o n . A l t h o u g h h i g h rates o f Ν d e p o s i t i o n m a y favor increased rates


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Ν mineralization ^ ^ ^ ^ ^ / CO


*c D Φ >

INU 3

,

/

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Ί

Ν inputs

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Additions Begins

Saturation

Decline

Saturation

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Stage: Net Γ Production CO

Foliar Biomass

"c D φ >

Foliar Ν Concentration

φ

Fine Root Mass Nitrate Assimilatioi 0 1

Addition Begins Stage:

1

Figure 2. Hypothetical time course of forest ecosystem response to chronic nitrogen additions. Top: relative changes in rates of nitrogen cycling and nitrogen loss. Bottom: relative changes in plant condition (e.g., foliar biomass and nitrogen content, fine root biomass) and function (e.g, net primary pro ductivity and nitrate assimilation) in response to changing levels of nitrogen availability. (Redrawn with permission from reference 1. Copynght 1989 American Institute of Biological Sciences.)



236


of d e n i t r i f i c a t i o n , m a n y watersheds lack t h e c o n d i t i o n s necessary f o r sub stantial d e n i t r i f i c a t i o n (e.g., l o w oxygen t e n s i o n , h i g h soil m o i s t u r e , a n d temperature). A n o t h e r i m p o r t a n t factor i n Ν loss f r o m watersheds is the age of the forest stands. A loss i n t h e ability to r e t a i n Ν is a n a t u r a l o u t c o m e o f forest m a t u r a t i o n , as d e m a n d for Ν o n the part o f m o r e s l o w l y g r o w i n g tree species m a y plateau i n later stages o f forest d e v e l o p m e n t o r d e c l i n e as forests achieve a "shifting-mosaic steady state" (75). U p t a k e rates o f Ν i n t o vege tation are generally m a x i m a l a r o u n d t h e t i m e o f canopy closure for conifers a n d somewhat later (at h i g h e r rates) i n d e c i d u o u s forests because o f t h e a n n u a l r e p l a c e m e n t o f canopy foliage i n these ecosystems (76). F i n a l l y , s o i l - N status m a y also affect Ν loss rates. W h e r e large s o i l - N pools exist, t h e y i m p l y that soil m i c r o b i a l processes that are o r d i n a r i l y l i m i t e d b y the availability o f Ν are instead l i m i t e d b y some o t h e r factor (e.g., a v a i l ability o f labile organic carbon) a n d c o n t r i b u t e to t h e l i k e l i h o o d that w a tersheds w i l l leach N 0 ~ (24, 77). Ν saturation occurs i n a sequence b e g i n n i n g w i t h the f u l f i l l m e n t o f vegetation Ν d e m a n d , w h i c h is f o l l o w e d b y t h e f u l f i l l m e n t o f soil m i c r o b i a l Ν d e m a n d . T h e existence o f large s o i l - N pools suggests that t h e second o f these r e q u i r e m e n t s m a y b e easily m e t . T h e possible i m p o r t a n c e o f a l l three factors (deposition, stand age, a n d soil N ) i n shifting watersheds f r o m one stage o f Ν loss to another w i l l b e discussed later i n t h e context o f surface water e v i d e n c e o f w a t e r s h e d Ν saturation. 3

Watershed Nitrogen Saturation. T h e loss o f Ν f r o m watersheds occurs i n stages that c o r r e s p o n d to t h e stages o f t e r r e s t r i a l Ν saturation d e s c r i b e d b y A b e r et a l . (I). T h e most obvious characteristics of these stages of Ν loss are changes i n the seasonal a n d l o n g - t e r m patterns of surface-water N 0 ~ concentrations, w h i c h reflect t h e changes i n Ν c y c l i n g that are oc c u r r i n g i n the w a t e r s h e d . T h e Ν cycle at Stage 0 is d o m i n a t e d b y forest a n d m i c r o b i a l uptake, a n d the d e m a n d for Ν has a strong i n f l u e n c e o n the seasonal N 0 ~ p a t t e r n o f r e c e i v i n g waters ( F i g u r e 3). T h e n o r m a l seasonal N 0 " p a t t e r n i n a stream d r a i n i n g a w a t e r s h e d at Stage 0 w o u l d i n c l u d e v e r y l o w , or i m m e a s u r a b l e , concentrations d u r i n g most o f the year a n d measurable concentrations o n l y d u r i n g s n o w m e l t (in areas w h e r e snowpacks a c c u m u l a t e o v e r the w i n t e r months) o r d u r i n g s p r i n g rainstorms. T h e s m a l l loss o f N 0 ~ d u r i n g t h e d o r m a n t season is a transient p h e n o m e n o n . It results because s n o w m e l t a n d s p r i n g rains c o m m o n l y o c c u r i n these e n v i r o n m e n t s before substantial forest a n d m i c r o b i a l g r o w t h b e g i n i n t h e s p r i n g . ( W i n t e r m i n e r alization of soil organic Ν m a y b e an e x c e p t i o n to this i n a c t i v i t y ; see ref. 78.) T h u s some o f t h e Ν stored i n soils a n d snowpack m a y pass t h r o u g h t h e w a t e r s h e d d u r i n g e x t r e m e h y d r o l o g i c a l events a n d generate a p u l s e o f e l evated N 0 " c o n c e n t r a t i o n ( F i g u r e 3b). T h e k e y surface-water characteristics of Stage 0 watersheds are v e r y l o w N 0 ~ concentrations d u r i n g most o f the year a n d m a x i m u m s p r i n g N 0 " concentrations that are s m a l l e r t h a n t h e concentrations t y p i c a l o f d e p o s i t i o n . 3

3

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Figure 3. Top panel: Schematic representation of nitrogen cycle in watershed at Stage 0 of watershed nitrogen loss. Sizes of arrows indicate the magnitude of process or transformation. Differences between winter-spring and sum mer-fall seasons are shown on opposite sides. At Stage 0, nitrogen transfor mations are dominated by plant and microbial assimilation (uptake), with little or no N0 ~ leakage from the watershed during the growing season. Bottom panel: Small amounts of N0 ~ may run off during snowmelt, producing the typical Stage 0 seasonal N0 ~ pattern. Data in lower panel are from Black Pond, Adirondack Mountains. (Data are taken from reference 157.) 3

3

3

A t Stage 1 t h e seasonal pattern t y p i c a l o f Stage 0 watersheds is a m p l i f i e d . T h i s amplification o f the seasonal N 0 " signal m a y b e t h e first sign that watersheds are p r o c e e d i n g t o w a r d the c h r o n i c stages (i.e., Stages 2 a n d 3 i n F i g u r e 2) o f Ν saturation (42, 79). T h i s suggestion is consistent w i t h t h e changes i n Ν c y c l i n g that are thought to o c c u r at Stage 1. A c o n c e p t u a l 3



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u n d e r s t a n d i n g o f these changes derives f r o m the most c o m m o n d e f i n i t i o n of n u t r i e n t l i m i t a t i o n . I m p l i c i t i n the d e f i n i t i o n of n u t r i e n t l i m i t a t i o n is the i d e a that "the c u r r e n t s u p p l y rate [of a n u t r i e n t ] p r e v e n t s the vegetation f r o m a c h i e v i n g m a x i m u m g r o w t h rates attainable within other environmental constraints" (ref. 80; emphasis added). D u r i n g the c o l d season these e n v i r o n m e n t a l constraints can be severe, a n d m a x i m u m attainable g r o w t h rates are clearly m u c h l o w e r than i n the w a r m m o n t h s . A l t h o u g h m u c h o f this discussion is c o u c h e d i n terms of forest trees, the same arguments a p p l y to soil m i c r o b i a l c o m m u n i t i e s (e.g., decomposers, nitrifiers) that m a y be as i m p o r t a n t as vegetation i n c o n t r o l l i n g Ν loss f r o m watersheds (80). O v e r a l l l i m i t a t i o n of forest g r o w t h i n the early stages of Ν saturation is characterized b y a seasonal cycle of l i m i t a t i o n b y p h y s i c a l factors (e.g., c o l d a n d d i m i n i s h e d light) d u r i n g late fall a n d w i n t e r a n d b y n u t r i e n t s ( p r i m a r i l y N ) d u r i n g the g r o w i n g season. T h e effect of i n c r e a s i n g the Ν s u p p l y (e.g., f r o m deposition) is to postpone the seasonal s w i t c h f r o m p h y s i c a l to n u t r i e n t l i m i t a t i o n d u r i n g the b r e a k i n g o f d o r m a n c y i n the s p r i n g ( F i g u r e 4) a n d to p r o l o n g the seasonal Ν saturation that is characteristic of watersheds at this stage. A t Stage 1, this s w i t c h is d e l a y e d e n o u g h that substantial N 0 ~ m a y leave the w a t e r s h e d d u r i n g extreme h y d r o l o g i c a l events i n the s p r i n g . W a tershed loss of Ν at Stage 1 is still a seasonal p h e n o m e n o n , a n d the a n n u a l Ν cycle is still d o m i n a t e d b y uptake ( F i g u r e 5), b u t N 0 ~ l e a c h i n g is less transient than at Stage 0. T h e k e y characteristics of Stage 1 watersheds are episodes of surface-water N 0 ~ that exceed concentrations t y p i c a l of d e p o sition ( F i g u r e 5b). E l e v a t e d N 0 ~ d u r i n g these episodes m a y result f r o m p r e f e r e n t i a l e l u t i o n of anions f r o m m e l t i n g snow (81, 82) or f r o m the c o n t r i b u t i o n of Ν m i n e r a l i z a t i o n to the soil p o o l of N 0 ~ that m a y be f l u s h e d d u r i n g h i g h - f l o w periods (43, 83). 3

3

3

3

3

I n Stage 2 of w a t e r s h e d Ν loss the seasonal onset of Ν l i m i t a t i o n is e v e n f u r t h e r d e l a y e d . A s a consequence, b i o l o g i c a l d e m a n d exerts no c o n t r o l o v e r w i n t e r a n d s p r i n g Ν concentrations, a n d the p e r i o d o f Ν l i m i t a t i o n d u r i n g the g r o w i n g season is m u c h r e d u c e d ( F i g u r e 6). T h e a n n u a l Ν c y c l e , w h i c h was d o m i n a t e d b y uptake at Stages 0 and 1, is instead d o m i n a t e d b y Ν loss t h r o u g h l e a c h i n g a n d d e n i t r i f i c a t i o n at Stage 2; sources o f Ν (deposition a n d mineralization) o u t w e i g h Ν sinks (uptake). T h e same m e c h a n i s m s that p r o d u c e episodes o f h i g h N 0 ~ d u r i n g extreme h y d r o l o g i c a l events at Stage 1 also operate at Stage 2. B u t m o r e i m p o r t a n t l y , N 0 " l e a c h i n g can also o c c u r at Stage 2 d u r i n g periods w h e n the h y d r o l o g i c a l cycle is characterized b y d e e p e r p e r c o l a t i o n ( F i g u r e 6). If biological d e m a n d is sufficiently d e p r e s s e d d u r i n g the g r o w i n g season, Ν begins to percolate b e l o w the r o o t i n g zone a n d elevated g r o u n d w a t e r N 0 ~ concentrations result. N i t r i f i c a t i o n b e c o m e s an i m p o r t a n t process at Stage 2 ( F i g u r e 2; also see réf. 1). L o w e r e d b i o l o g i c a l d e m a n d leads to a b u i l d u p of N H i n soils a n d n i t r i f i c a t i o n m a y b e s t i m u l a t e d . T h i s change is p i v o t a l i n the Ν cycle because n i t r i f i c a t i o n is such a 3

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»

]

J

F

M

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O

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D

Month Figure 4. Conceptual model of mechanism responsible for increased incidence and severity of N0 ~ episodes in watersheds in early stage of nitrogen satu ration (Stage 1 ). Solid line is potential growth rate of forest if all nutrients are sufficient and only environmental variables (e.g., temperature, light, and water) limit growth. Dashed line is potential growth rate if environmental variables are optimal, but Ν is in short supply. Points where dashed and solid lines cross are transitions from N-unlimited to N-limited growth and vice versa. Dotted line is potential growth rate if environmental variables are optimal and Ν supply is moderate. Shift in onset of Ν limitation in spring will result from increase in nitrogen supply and is likely to coincide with timing of snowmelt in northern forested watersheds. 3

strongly a c i d i f y i n g process ( F i g u r e 1). T h e k e y characteristics o f Stage 2 watersheds are elevated baseflow concentrations o f N 0 ~ , w h i c h result f r o m h i g h g r o u n d w a t e r concentrations ( F i g u r e 6b). E p i s o d i c N 0 ~ concentrations are as h i g h as Stage 1, b u t t h e seasonal pattern at Stage 2 is d a m p e d b y a n increase i n baseflow concentrations to levels as h i g h as those f o u n d i n d e position. I n Stage 3 t h e w a t e r s h e d becomes a n e t source o f Ν rather t h a n a sink ( F i g u r e 7). N i t r o g e n - r e t e n t i o n mechanisms (e.g., uptake b y vegetation a n d 3

3

microbes) are m u c h r e d u c e d . M i n e r a l i z a t i o n o f stored Ν m a y a d d substan tially to Ν l e a v i n g t h e w a t e r s h e d t h r o u g h l e a c h i n g o r i n gaseous forms. A s i n Stage 2, nitrification rates are substantial. T h e c o m b i n e d i n p u t s o f Ν f r o m d e p o s i t i o n , m i n e r a l i z a t i o n , a n d nitrification c a n p r o d u c e concentrations o f N 0 " i n surface waters that exceed i n p u t s f r o m d e p o s i t i o n alone. T h e k e y 3



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Figure 5. Top panel: Schematic representation of nitrogen cycle in watershed at Stage 1 of watershed nitrogen loss. Size of arrows indicates the magnitude of process or transformation. Differences between winter-spring and sum mer-fall seasons are shown on opposite sides. As in Stage 0, uptake dominates the nitrogen cycle during the growing season at Stage 1 and little or no N0 ~ leaks from the watershed during the summer and fall. The primary difference between Stage 0 and Stage 1 is the delay in the onset of Ν limitation during the spring season (see text and Figure 4). Bottom panel: Large runoff events (e.g., snowinelt or rainstorms) during the dormant season can produce episodic puhes of high N0 ~ concentrations, as shown in the typical Stage 1 seasonal N0 ~ cycle. Data in bottom panel are from Constable Pond in the Adirondack Mountains. (Data are taken from reference 157.) 3

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Figure 6. Top panel: Schematic representation of nitrogen cycle in watershed at Stage 2 of watershed nitrogen loss. Size of arrows indicates the magnitude of process or transformation. Differences between winter-spring and summer-fall seasons are shown on opposite sides. Uptake of nitrogen by forest plants and microbes is much reduced at Stage 2, resulting in loss of N0 ~ to streams during winter and spring and to groundwater during the growing season. Loss of gaseous forms of nitrogen through denitrification may also be elevated at Stage 2 if conditions necessary for denitrification are present (see text). Although episodes of higher N0 ~ concentrations continue to occur during high-flow events such as spring snowmelt, the primary difference between Stage 1 and Stage 2 is the presence of elevated N0 ~ concentrations in groundwater. Bottom panel: The typical seasonal N0 ~ pattern at Stage 2 includes both high episodic concentrations and high base-flow concentrations. Data in bottom panel are from Fernow Experimental Forest, Control Watershed No. 4, West Virginia. 3

3

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Figure 7. Top panel: Schematic representation of nitrogen cycle in watershed at Stage 3 of watershed nitrogen loss. Size of arrows indicates the magnitude of process or transformation. Differences between winter-spring and sum mer-fall seasons are shown on opposite sides. At Stage 3 no sinks for nitrogen exist in the watershed and all inputs, as well as mineralized nitrogen, are lost from the system either through denitrification or in runoff water. Because mineralization supplies nitrogen in excess of deposition, concentrations of Ν Of in runoff may exceed those in deposition. Bottom panel: Typical seasonal Ν Of pattern at Stage 3 includes concentrations at all seasons in excess of concen trations attributable to deposition and évapotranspiration, as at Dicke Bramke in Germany; data are from reference 190.


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characteristics o f Stage 3 watersheds are these e x t r e m e l y h i g h N 0 ~ c o n centrations a n d the lack o f any coherent seasonal p a t t e r n i n N 0 ~ c o n c e n trations ( F i g u r e 7b). 3


3

C o n c e p t u a l l y , the stages o f w a t e r s h e d Ν loss can b e thought o f as oc c u r r i n g sequentially, as a single w a t e r s h e d progresses f r o m b e i n g strongly Ν deficient to strongly Ν sufficient. T h i s sequence is consistent w i t h t h e c o n c e p t u a l m o d e l p r e s e n t e d b y A b e r et a l . (J; also F i g u r e 2) a n d c a n b e s u p p o r t e d b y t w o lines o f e v i d e n c e that are p r e s e n t e d i n the f o l l o w i n g sections o f this chapter. T h e first l i n e of e v i d e n c e comes f r o m space-for-time substitutions (84), w h e r e t h e occurrence o f various stages across a gradient of c u r r e n t Ν d e p o s i t i o n is u s e d as a surrogate f o r t h e t e m p o r a l s e q u e n c e that a single site m i g h t u n d e r g o i f i t w e r e exposed to c h r o n i c a l l y e l e v a t e d levels o f Ν d e p o s i t i o n . T h i s t e c h n i q u e is c o m m o n l y a p p l i e d to c u r r e n t e n v i r o n m e n t a l p r o b l e m s for w h i c h a good historical r e c o r d is not available (85). T h e second l i n e o f e v i d e n c e comes f r o m l o n g - t e r m t e m p o r a l trends at single sites, w h e r e increases i n Ν efflux f r o m watersheds (observable as i n c r e a s i n g trends i n N 0 ~ concentration) a n d changes i n t h e seasonal p a t t e r n o f N 0 ~ concentration c a n b e d i r e c t l y a t t r i b u t e d to t h e c o m b i n e d effects o f c h r o n i c Ν d e p o s i t i o n a n d o t h e r factors (e.g., forest maturation). T h e f e w cases i n w h i c h i n d i v i d u a l sites have progressed f r o m Stage 0 to Stage 1 o r Stage 2 of w a t e r s h e d Ν loss are especially useful i n establishing that Ν saturation occurs as a t e m p o r a l sequence i n areas o f h i g h Ν d e p o s i t i o n . T h e s e lines o f e v i d e n c e are discussed i n t h e f o l l o w i n g sections. 3

3

The Consequences of Nitrogen Loss from Watersheds T w o possible consequences o f w a t e r s h e d Ν loss are surface-water a c i d i f i cation a n d e u t r o p h i c a t i o n . T h e acidification processes i n lakes a n d streams are c o n v e n t i o n a l l y separated into c h r o n i c (long-term) a n d e p i s o d i c (eventbased) effects. Surface waters are generally c o n s i d e r e d acidic i f t h e i r a c i d n e u t r a l i z i n g capacity ( A N C ) is less than z e r o . T h e A N C o f a lake o r stream is a measure o f the water's capacity to buffer acidic i n p u t s . It results f r o m the presence o f carbonate o r bicarbonate (i.e., alkalinity), a l u m i n u m , a n d organic acids i n t h e water (86). I n the past decade a great d e a l o f emphasis was p l a c e d o n c h r o n i c acidification i n general, a n d o n c h r o n i c acidification b y S 0 ~ i n particular (87, 88). T h i s emphasis o n S 0 has r e s u l t e d largely because sulfur d e p o s i t i o n rates are often h i g h e r than those for N (S d e p o s i t i o n rates are a p p r o x i m a t e l y twice the Ν d e p o s i t i o n rates i n t h e N o r t h e a s t ; ref. 9) a n d because N 0 " appears to b e of n e g l i g i b l e i m p o r t a n c e i n surface waters s a m p l e d d u r i n g s u m m e r a n d fall i n d e x p e r i o d s (89). A s m e n t i o n e d p r e v i ously, s u m m e r a n d fall are seasons w h e n w a t e r s h e d d e m a n d for Ν is n o r m a l l y v e r y h i g h . T h i s t i m i n g creates a l o w p r o b a b i l i t y that N , i n any f o r m , w i l l b e l e a c h e d into soil a n d surface waters unless t h e watersheds are i n stages 2 a n d 3 o f Ν loss. U n d e r t h e usual conditions (i.e., i n Stages 0 a n d 1), Ν 4

2

4

2 _

3



244


l e a k i n g f r o m terrestrial ecosystems is m o r e l i k e l y to b e a transient (or sea sonal) p h e n o m e n o n than a c h r o n i c one. E u t r o p h y generally refers to a state of n u t r i e n t e n r i c h m e n t (53), b u t the t e r m is c o m m o n l y u s e d to refer to conditions of increased algal biomass a n d p r o d u c t i v i t y , the p r e s e n c e of nuisance algal populations, a n d a decrease i n oxygen availability for h e t e r o t r o p h i c organisms. E u t r o p h i c a t i o n is the process w h e r e b y lakes, estuaries, a n d m a r i n e systems progress t o w a r d a state of e u t r o p h y . I n lakes, e u t r o p h i c a t i o n is often c o n s i d e r e d a natural process, progressing gradually over t h e i r l o n g - t e r m e v o l u t i o n . T h e process can b e significantly accelerated b y the a d d i t i o n a l i n p u t of n u t r i e n t s f r o m a n t h r o p o g e n i c sources. T h e subject of e u t r o p h i c a t i o n was extensively r e v i e w e d b y H u t c h i n s o n (90), the N a t i o n a l A c a d e m y of Sciences (91), a n d L i k e n s (92). Chronic Acidification. T h e most c o m p r e h e n s i v e assessment of c h r o n i c acidification o f lakes a n d streams i n the U n i t e d States comes f r o m the N a t i o n a l Surface W a t e r S u r v e y ( N S W S ) . It was c o n d u c t e d as part of the N a t i o n a l A c i d P r e c i p i t a t i o n Assessment P r o g r a m ( N A P A P ) . T h e N S W S sur v e y e d the a c i d - b a s e c h e m i s t r y o f b o t h lakes a n d streams b y u s i n g an i n d e x p e r i o d concept. T h e goal o f the i n d e x p e r i o d concept was to i d e n t i f y a single season o f the year that e x h i b i t e d l o w t e m p o r a l a n d spatial v a r i a b i l i t y a n d that, w h e n s a m p l e d , w o u l d a l l o w the general c o n d i t i o n o f surface waters to be assessed (89). F o r lakes, the i n d e x p e r i o d selected was a u t u m n o v e r t u r n , the p e r i o d w h e n most lakes are m i x e d u n i f o r m l y f r o m top to b o t t o m . F o r streams the chosen i n d e x p e r i o d was s p r i n g baseflow, the p e r i o d after s p r i n g s n o w m e l t a n d before leafout (93). Because of the strong seasonality of the Ν cycle i n forested watersheds (described earlier), the choice of i n d e x p e r i o d plays a v e r y large role i n the assessment of w h e t h e r Ν is an i m p o r t a n t c o m p o n e n t of c h r o n i c acidification. T h e E a s t e r n L a k e S u r v e y (89) was based o n a p r o b a b i l i t y s a m p l i n g of lakes d u r i n g fall o v e r t u r n , a n d the N a t i o n a l S t r e a m S u r v e y ( N S S ; ref. 94) was based o n a p r o b a b i l i t y s a m p l i n g of streams d u r i n g a s p r i n g baseflow p e r i o d . T h e results suggest that Ν c o m p o u n d s make o n l y a s m a l l c o n t r i b u t i o n to c h r o n i c acidification i n N o r t h A m e r i c a . H e n r i k s e n (95) p r o p o s e d that the ratio o f N 0 ~ : ( N 0 ~ + S 0 ~ ) i n surface waters b e u s e d as an i n d e x of the i n f l u e n c e o f N 0 " o n c h r o n i c acidification status. T h i s i n d e x assesses the i m p o r t a n c e o f N 0 ~ relative to the i m p o r t a n c e of S 0 ~ , w h i c h is u s u a l l y c o n s i d e r e d m o r e i m p o r t a n t i n c h r o n i c acidification. A value greater than 0.5 indicates that N 0 ~ has a greater i n f l u e n c e o n the c h r o n i c a c i d - b a s e status of surface waters than does S 0 ~ 3

3

4

2

3

3

4

2

3

4

2

M e d i a n values of N 0 " : ( N 0 ~ + S 0 ~ ) ratios for acid-sensitive regions of the U n i t e d States s a m p l e d i n the N S W S are s h o w n i n T a b l e II. D a t a are taken m a i n l y f r o m refs. 89 a n d 96, w i t h data f r o m a d d i t i o n a l r e g i o n a l surveys i n c l u d e d i n Table II for c o m p a r i s o n (97-99). I n a l l regions the m e d i a n values of N 0 ~ : ( N < V + S 0 ) are a l l less than 0.5, a n d most are less than 0.2. 3

3

4

3

4

2

2 _



c

/

7

6.98 6.56 7.09 6.91 6.81 6.77 6.71 7.02 6.96 6.60 7.05 5.98 6.99 6.80 7.33 6.62 5.48 6.60 5.58 6.70

1986 1986 1986 1986 1986 1986 1986 1986 1986 1984-86 1984-86 1987

pH

1985 1985 1985 1985 1985 1985 1985 1985

Year

3

8 2 16 1 5 29 61.1 0

—

6 30 10

3.1 1.0 0.7 0.2 0.8 0.3 0.6 0.7

b

3

N0

3

2

—

17 48 58 59 22 138 76.1 72

169 171 154

31.8 93.7 57.1 74.6 141.1 101.2 118.7 159.3

b

SO/ 3

3

N0 :(N0 -

—

0.28 0.03 0.32 0.02 0.19 0.17 0.44 0.00

0.03 0.14 0.09

0.09 0.01 0.01 0.00 0.01 0.00 0.01 0.00

+ SO/')

3

"Values are weighted medians for each region. ''Values are given in microequivalents per liter. 'Data are from reference 89. ''Values for p H are for entire region (94); medians for NO3-, S O / and the N0 -:(N0 - + SO/") ratio exclude sites with potential agricultural or other land-use impacts (96). 'The influence of agricultural and land-use practices could not be ruled out for any of the sites in the mid-Atlantic coastal plain (96). 'Median value for repeated measurements at 51 streams (data are from reference 79). "Mean value for repeated measurements at five streams (data are from reference 99). ''Median value of 341 streams (data are from reference 98).

Eastern Lake Survey Southern Blue Ridge Florida Upper Midwest Maine Southern New England Central New England Adirondack Mountains Catskill-Poconos National Stream Survey'' Poconos-Catskills Northern Appalachians Valley and Ridge Mid-Atlantic Coastal Plain" Southern Blue Ridge Piedmont Southern Appalachians Ozarks-Ouachitas Florida Catskill Regional Survey Great Smoky Mountains Stream Survey* Virginia Trout Stream Survey '

0

Region

3

Table II. Concentrations and Ratio of N0 " to (N0~ + SO4") in Lakes and Streams of Acid-Sensitive Regions of the United States


I

m

Ο

«τΐ-

50

1 w 1-

I a 5S aCO rcs

ana

?

C/3 H Ο D Ό > so Ό

00

246


T h e s e values suggest that N 0 ~ does not c o n t r i b u t e significantly to c h r o n i c acidification i n most regions of the U n i t e d States. S e v e r a l southeastern re gions exhibit ratios i n the 0 . 2 - 0 . 4 range, p r i m a r i l y because t h e i r c u r r e n t S O / " concentrations are r e l a t i v e l y l o w . T h e S o u t h e r n B l u e R i d g e , i n par ticular, has the lowest S 0 ~ concentrations f o u n d i n the N S S , a n d the r e l atively h i g h N 0 ~ : ( N 0 ~ + S0 ~) ratios i n this r e g i o n c o u l d be c o n s i d e r e d m i s l e a d i n g . T a k e n i n total, r e g i o n a l survey data suggest that the C a t s k i l l s , Northern Appalachians, Valley and Ridge Province, and Southern Appala chians a l l show some p o t e n t i a l for c h r o n i c acidification attributable to N 0 " . H o w e v e r , i n a l l of the regions s h o w n i n T a b l e II c h r o n i c acidification is m o r e closely t i e d to S O / " than to N 0 ~ . 3

4

3

2

3

4

2


3

3

T h e highest N 0 " : ( N 0 ~ + S 0 ) ratios a n d the highest m e a n N 0 ~ concentrations w e r e r e c o r d e d i n streams of the G r e a t S m o k y M o u n t a i n s , w h e r e baseflow N 0 ~ concentrations w e r e as h i g h as, or i n some cases h i g h e r than, baseflow S 0 ~ concentrations (99). C o o k et al. (99) stressed that this study i n c l u d e d a s m a l l n u m b e r of streams, that the r e g i o n is k n o w n for geological sources o f S 0 ~ (the A n a k e e s t a geological formation), a n d that h i g h S 0 ~ a n d N 0 " concentrations t e n d to o c c u r s i m u l t a n e o u s l y . T h e results are e n t i r e l y consistent, h o w e v e r , w i t h soil l y s i m e t e r studies c a r r i e d out i n the G r e a t S m o k y M o u n t a i n s . A c c o r d i n g to these studies, N 0 " c o n c e n t r a tions i n d e e p soil lysimeters are h i g h e r than i n p u t concentrations f r o m d e p osition a n d t h r o u g h f a l l (172). Because d e e p soil-water concentrations can b e assumed to approximate g r o u n d w a t e r a n d baseflow stream-water c o n c e n trations, these streams a n d watersheds i n the G r e a t S m o k y M o u n t a i n s are the o n l y k n o w n examples i n the U n i t e d States of watersheds at Stage 3 o f w a t e r s h e d Ν loss, as d e s c r i b e d earlier. 3

3

4

2

3

3

4

2

4

4

2

2

3

3

C h r o n i c acidification r e s u l t i n g f r o m Ν d e p o s i t i o n is m u c h m o r e c o m m o n i n E u r o p e than i n N o r t h A m e r i c a (39). M a n y E u r o p e a n sites s h o w c h r o n i c increases i n Ν export f r o m t h e i r watersheds (101, 102), a n d at sites w i t h the highest stream-water N 0 ~ concentrations (i.e., L a n g e B r a m k e a n d D i c k e B r a m k e i n W e s t G e r m a n y ) N 0 ~ concentrations no l o n g e r show the sea sonality that is expected f r o m n o r m a l w a t e r s h e d processes (39). H e n r i k s e n a n d B r a k k e (101) r e p o r t e d r e g i o n a l c h r o n i c increases i n surface-water N 0 " i n S c a n d i n a v i a i n the past decade. T h e s e increases i n N 0 " c o n c e n t r a t i o n are associated w i t h increasing concentrations of a l u m i n u m , w h i c h is toxic to m a n y fish species (103, 104). S o m e e v i d e n c e (105) suggests that N 0 " has a greater a b i l i t y to m o b i l i z e toxic a l u m i n u m f r o m soils t h a n does S 0 " . C h r o n i c acidification attributable to a m m o n i u m d e p o s i t i o n has also b e e n d e m o n strated i n the N e t h e r l a n d s (51, 106). A s d e s c r i b e d earlier, a m m o n i u m i n d e p o s i t i o n can be n i t r i f i e d to p r o d u c e b o t h N 0 " a n d h y d r o g e n ions, w h i c h are s u b s e q u e n t l y l e a k e d into surface waters. Rates of N 0 ~ a n d N H dep osition are m u c h h i g h e r i n E u r o p e (in some places d e p o s i t i o n is > 2 0 0 0 e q u i v / h a p e r year; ref. 107) than i n the U n i t e d States (Table I). C h r o n i c Ν acidification m a y be m o r e e v i d e n t i n E u r o p e t h a n i n N o r t h A m e r i c a because Ν saturation is f u r t h e r d e v e l o p e d i n E u r o p e . 3

3

3

3

3

4

2

3

3


4

+

8.

STODDARD

247


Episodic Acidification. E x c e p t i n extreme cases, Ν loss f r o m w a tersheds is m o r e l i k e l y to be an e p i s o d i c or seasonal process than a c h r o n i c one. T h e r e f o r e , data u s e d to assess the c o n t r i b u t i o n of N 0 ~ to acidification must be c o l l e c t e d o n an intensive schedule so s h o r t - t e r m effects of N 0 ~ increases can be characterized. T h i s situation places severe l i m i t a t i o n s o n the type of data that can b e used i n this assessment. D a t a f r o m r e g i o n a l surveys, although they p r o v i d e excellent spatial coverage, are h a m p e r e d b y the n e e d to collect samples d u r i n g a stable i n d e x p e r i o d . I n most areas the s p r i n g p e r i o d w h e n N 0 ~ effects are most l i k e l y to be observable w o u l d not meet the r e q u i r e m e n t s d e s c r i b e d earlier for a stable i n d e x p e r i o d . A s a result the r e g i o n a l i m p o r t a n c e a n d severity of e p i s o d i c acidification have not b e e n q u a n t i f i e d ; that is, the regional i n f o r m a t i o n o n c h r o n i c acidification that was gained f r o m the N S W S has no p a r a l l e l i n e p i s o d i c acidification, a n d a l l conclusions must be based o n site-specific data. E v e n w i t h i n a g i v e n area, such as the A d i r o n d a c k M o u n t a i n s , major differences can b e e v i d e n t i n the o c c u r r e n c e , nature, location (lakes or streams), a n d t i m i n g of episodes at different sites. 3


3

3

It has b e e n estimated that 1 . 4 - 7 . 4 times as m a n y streams i n the eastern U n i t e d States u n d e r g o e p i s o d i c acidification than are c h r o n i c a l l y acidic (108). S i m i l a r l y , the n u m b e r of e p i s o d i c a l l y acidic A d i r o n d a c k lakes is e s t i m a t e d to be 3 times h i g h e r than the n u m b e r of c h r o n i c a l l y a c i d i c lakes (J 08). W i g i n g t o n et al. (J09) r e p o r t e d that acidic episodes o c c u r i n a w i d e range of geographic locations i n the northeastern, southeastern, a n d w e s t e r n U n i t e d States, as w e l l as i n Scandinavia, E u r o p e , a n d C a n a d a . A n u m b e r of processes c o n t r i b u t e to the t i m i n g a n d severity of a c i d i c episodes (42). D u r i n g high-discharge p e r i o d s the most i m p o r t a n t of these processes are • d i l u t i o n of base cations (110); « increases i n organic a c i d concentrations (111); • increases i n S 0

4

2 _

concentrations (112); a n d

• increases i n N 0 " concentrations (110, 3

113,

114).

I n a d d i t i o n to these factors, w h i c h p r o d u c e the c h e m i c a l c o n d i t i o n s char acteristic of e p i s o d i c events, the l i k e l i h o o d of an acidic episode is also i n f l u e n c e d b y the c h e m i c a l conditions p r e v a i l i n g before the episode begins. E p i s o d e s are m o r e l i k e l y to be acidic, for e x a m p l e , i f the baseflow A N C of the stream or lake is l o w . I n this way, acidic anions, especially S 0 , can c o n t r i b u t e to the severity of an acidic episode b y l o w e r i n g the baseflow A N C of the stream or lake, e v e n t h o u g h they do not increase d u r i n g the event (97). 4

2

I n m a n y cases a l l of these processes w i l l c o n t r i b u t e to episodes i n a single aquatic system. D i l u t i o n , for e x a m p l e , p r o b a b l y plays a role i n a l l e p i s o d i c decreases i n A N C a n d p H i n a l l regions of the U n i t e d States (109).

American Chemical Sociefif Library

In Environmental Chemistry of Lakes and Reservoirs; Baker, Lawrence A.; 1155 16th St, N.W.Society: Washington, DC, 1994. Advances in Chemistry; American Chemical

248


D i l u t i o n results f r o m the increased rate of r u n o f f a n d the c h a n n e l i n g o f r u n o f f t h r o u g h shallower soil layers that occurs d u r i n g storms or s n o w m e l t . T h e shorter contact t i m e p r o d u c e s runoff w i t h a c h e m i c a l c o m p o s i t i o n closer to that of a t m o s p h e r i c d e p o s i t i o n than is t y p i c a l of baseflow c o n d i t i o n s (115-117). Because p r e c i p i t a t i o n is usually m o r e d i l u t e than stream or lake water, s t o r m r u n o f f p r o d u c e s surface waters that are m o r e d i l u t e t h a n d u r i n g n o n r u n o f f p e r i o d s . I n a sense, d i l u t i o n sets the baseline c o n d i t i o n to w h i c h is a d d e d the effects of organic acids a n d a t m o s p h e r i c a l l y d e r i v e d S 0 " a n d 4

2


NO3-. L i t t l e i n f o r m a t i o n exists about the effects of changes i n organic acids d u r i n g episodes. D r i s c o l l et a l . (118) a n d E s h l e m a n a n d H e m o n d (119) c o n c l u d e d that organic acids d i d not c o n t r i b u t e to s n o w m e l t episodes i n the A d i r o n d a c k s o r i n Massachusetts, r e s p e c t i v e l y . A t H a r p L a k e i n C a n a d a , organic a c i d i t y is b e l i e v e d to r e m a i n constant (120) or decrease (121) d u r i n g s n o w m e l t episodes. H a i n e s (122) a n d M c A v o y (123) d o c u m e n t e d increases i n organic acidity d u r i n g rain-caused episodes i n coastal M a i n e a n d i n M a s sachusetts. Storage of S 0 ~ i n watersheds a n d its subsequent release ~ d u r i n g episodic events are w e l l d o c u m e n t e d i n m a n y parts of E u r o p e (109), b u t the process has not b e e n f o u n d c o m m o n l y i n the U n i t e d States. Sulfate episodes have b e e n d e s c r i b e d for the L e a d i n g R i d g e area of P e n n s y l v a n i a (124) a n d at F i l s e n C r e e k i n M i n n e s o t a (125), b u t they are not w i d e s p r e a d . Sulfate does c o n t r i b u t e to e p i s o d i c a c i d i t y , h o w e v e r , i n the sense that concentrations m a y r e m a i n h i g h d u r i n g events a n d c o n t r i b u t e to a l o w e r baseline A N C . T h e effects of other factors, s u c h as increased N 0 ~ , are s u p p l e m e n t a l to any constant effect of S 0 i n l o w e r i n g the baseline A N C (97). 4

2

4

2

3

4

2 _

I n the discussion of the c o n t r i b u t i o n of N 0 " to c h r o n i c acidification, the i m p o r t a n c e of N 0 " was r e l a t e d to the i m p o r t a n c e o f S 0 ~ , the p r i m a r y acidic a n i o n i n anthropogenically a c i d i f i e d lakes a n d streams. F o r e p i s o d i c acidification, the i m p o r t a n c e of s h o r t - t e r m increases i n N 0 ~ c o n c e n t r a t i o n m u s t be assessed i n relation to the other processes that c o n t r i b u t e to acidic episodes. I n the A d i r o n d a c k s , for example, strong N 0 " pulses i n b o t h lakes (110, 113) a n d streams (118) are a p p a r e n t l y the p r i m a r y factor c o n t r i b u t i n g to d e p r e s s e d A N C a n d p H d u r i n g s n o w m e l t . Schaefer et a l . (126) e x a m i n e d i n t e n s i v e m o n i t o r i n g data f r o m 11 A d i r o n d a c k lakes a n d c o n c l u d e d that the m a g n i t u d e of the episodes e x p e r i e n c e d b y lakes d e p e n d s strongly o n t h e i r base-cation concentration. T h e y c o n c l u d e d that lakes w i t h h i g h base-cation concentrations (and therefore h i g h A N C values) u n d e r g o episodes that are largely the result of d i l u t i o n b y s n o w m e l t . L o w - A N C lakes, o n the o t h e r h a n d , u n d e r g o episodes that result largely f r o m increases i n N 0 " c o n c e n trations. A t i n t e r m e d i a t e A N C levels, lakes are affected b y b o t h base-cation d i l u t i o n a n d N 0 ~ increases. T h e r e f o r e these lakes m a y u n d e r g o the greatest increases i n a c i d i t y d u r i n g s n o w m e l t episodes ( F i g u r e 8). M u r d o c h a n d S t o d d a r d (127) r e p o r t e d s i m i l a r results for streams i n the C a t s k i l l M o u n t a i n s , 3

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e l u d e d f r o m this analysis because m a n y of t h e m m a y have e x p e r i e n c e d anthropogenic inputs of Ρ (53, 144). T h i s test is therefore conservative for Ν l i m i t a t i o n , b o t h because the D I N : T P value chosen (20%) of p o t e n t i a l l y N - l i m i t e d lakes. T h e smallest p r o p o r t i o n s o f N - l i m i t e d lakes w e r e f o u n d i n the N o r t h e a s t a n d Southeast (9 a n d 10%, r e s p e c t i v e l y , of the lakes i n these regions e x h i b i t e d l o w D I N : T P ratios). O n e surprise i n this analysis is the n u m b e r of lakes i n the u p p e r M i d w e s t that appear to b e N - l i m i t e d . T a k e n as a w h o l e , 2 8 % of this region's lakes h a d D I N : T P ratios less t h a n 2. A m o r e d i r e c t i n d i c a t i o n of n u t r i e n t l i m i t a t i o n t h a n is available f r o m n u t r i e n t ratios can b e gained f r o m bioassay e x p e r i m e n t s . I n this p r o c e d u r e a s m a l l v o l u m e of natural lake water is e n c l o s e d a n d various k n o w n c o n c e n trations of p o t e n t i a l l y l i m i t i n g nutrients are a d d e d (145-147). A g r o w t h response (usually m e a s u r e d as an increase i n biomass) i n treatments c o n t a i n i n g an a d d e d n u t r i e n t constitutes e v i d e n c e o f l i m i t a t i o n b y that n u t r i e n t . T h e results o f s u c h e x p e r i m e n t s are available for o n l y a f e w selected n u t r i e n t p o o r lakes, h o w e v e r . T h e y indicate a v a r i e t y of responses, i n c l u d i n g strong Ρ l i m i t a t i o n (148), l i m i t a t i o n b y Ρ a n d i r o n (247), simultaneous Ν a n d Ρ l i m i t a t i o n i n w h i c h the t w o nutrients are so closely b a l a n c e d that a d d i t i o n of one alone s i m p l y leads to l i m i t a t i o n b y the other (247), a n d l i m i t a t i o n p r i m a r i l y b y Ν (142, 149). N o clear p a t t e r n of Ν or Ρ l i m i t a t i o n d e v e l o p s f r o m an examination of these f e w studies. T h e p o t e n t i a l for Ν d e p o s i t i o n to c o n t r i b u t e to the e u t r o p h i c a t i o n of freshwater lakes is p r o b a b l y q u i t e l i m i t e d . E u t r o p h i c a t i o n b y a t m o s p h e r i c inputs o f Ν is a c o n c e r n o n l y i n lakes that are c h r o n i c a l l y N - l i m i t e d . T h i s c o n d i t i o n occurs i n some lakes that receive substantial i n p u t s of a n t h r o p o genic Ρ a n d i n m a n y lakes w h e r e b o t h Ρ a n d Ν are f o u n d i n l o w concentrations (e.g., T a b l e III). I n the f o r m e r case the p r i m a r y d y s f u n c t i o n of the lakes is an excess s u p p l y of P, a n d c o n t r o l l i n g Ν d e p o s i t i o n w o u l d be an ineffective m e t h o d of w a t e r - q u a l i t y i m p r o v e m e n t . I n the latter case the p o t e n t i a l for e u t r o p h i c a t i o n b y Ν a d d i t i o n (e.g., f r o m deposition) is l i m i t e d b y l o w Ρ concentrations; additions of Ν to these systems w o u l d soon l e a d to N-sufflcient, a n d p h o s p h o r u s - d e f i c i e n t , c o n d i t i o n s . T h e results of the N S W S s h o w n i n Table III, for e x a m p l e , can b e u s e d to calculate the increase i n Ν concentration that w o u l d be r e q u i r e d to p u s h N - l i m i t e d lakes i n t o Ρ l i m i tation (assuming total Ρ concentrations do not change). A n increase o f o n l y


8.

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0.5 μπιοί/L i n Ν concentration w o u l d be sufficient to i n d u c e Ρ l i m i t a t i o n i n h a l f of the N - l i m i t e d lakes i n the N S W S target p o p u l a t i o n . I n the eastern U n i t e d States the largest Ν increases (0.7 μπιοί/L) w o u l d b e r e q u i r e d i n F l o r i d a a n d S o u t h e r n N e w E n g l a n d , w h e r e a n t h r o p o g e n i c i n p u t s of Ρ are most l i k e l y . T h e largest increases of all (1 μπιοί/L) w o u l d be r e q u i r e d i n the S o u t h e r n R o c k y M o u n t a i n s , w h i c h has the highest Ρ concentrations of any of the w e s t e r n subregions (median = 0.26 μπιοΙ/L). T h i s Ρ c o n c e n t r a t i o n p r o b a b l y results f r o m natural sources of Ρ (e.g., v o l c a n i c bedrock). Increases i n Ν d e p o s i t i o n to some of the regions i n T a b l e III w o u l d p r o b a b l y l e a d to measurable increases i n algal biomass i n those lakes w i t h l o w D I N : T P ratios and substantial total Ρ concentrations, b u t the n u m b e r of lakes that m e e t these c r i t e r i a is l i k e l y to be quite s m a l l .

Regional Analysis of the Stages of Watershed Nitrogen Loss If elevated rates of Ν d e p o s i t i o n c o n t r i b u t e to w a t e r s h e d Ν loss, t h e n patterns of Ν loss m i g h t be expected to m i m i c patterns of Ν d e p o s i t i o n . I n the f o l l o w i n g discussion, the d i s t r i b u t i o n of m o n i t o r i n g sites i n the eastern U n i t e d States w i t h N 0 ~ patterns suggestive of the v a r y i n g stages of w a t e r s h e d Ν loss is p r e s e n t e d as o c c u r r i n g across a gradient i n Ν d e p o s i t i o n f r o m r e l a t i v e l y l o w to v e r y h i g h . T h i s space-for-time s u b s t i t u t i o n is a u s e f u l tool i n assessing w h e t h e r Ν saturation is p r o g r e s s i n g i n h i g h - d e p o s i t i o n areas. A l t h o u g h i m p o r t a n t exceptions do occur (e.g., see discussion of P e n n s y l v a n i a streams i n the next section), lakes a n d streams w i t h Stage 0 patterns t e n d to occur i n l o w - to m o d e r a t e - d e p o s i t i o n areas; sites w i t h N 0 ~ patterns t y p i c a l of later stages of Ν loss t e n d to occur i n h i g h - d e p o s i t i o n areas. Results are p r e s e n t e d as a series of maps, a n d the discussion progresses f r o m rela t i v e l y l o w - d e p o s i t i o n areas i n the northeastern c o r n e r of the c o u n t r y ( M a i n e ) to h i g h e r d e p o s i t i o n areas i n w e s t e r n N e w E n g l a n d a n d N e w Y o r k , a n d moves south i n t o the v e r y - h i g h - d e p o s i t i o n areas of s o u t h e r n P e n n s y l v a n i a a n d W e s t V i r g i n i a . A d d i t i o n a l l y , results f r o m the f e w m o n i t o r i n g sites i n the w e s t e r n U n i t e d States, w h e r e d e p o s i t i o n rates are u n i f o r m l y l o w , are p r e sented separately. 3

3

I n general, sites are i n c l u d e d i n this analysis i f p u b l i s h e d data of m o r e than 2 years' d u r a t i o n i n the 1980s are available a n d i f at least a seasonal s a m p l i n g schedule was u s e d to collect the data. E a c h site has b e e n assigned to one of six classes. I n a d d i t i o n to four classes r e p r e s e n t i n g the four stages of w a t e r s h e d Ν loss, m a n y o f the Stage 1 a n d Stage 2 sites have data of l o n g e n o u g h d u r a t i o n to d e t e r m i n e w h e t h e r they are e x p e r i e n c i n g u p w a r d trends i n N 0 ~ concentrations; Stage 1 a n d Stage 2 sites w i t h u p w a r d N 0 " trends are i n c l u d e d as separate classes o n the maps for the f o l l o w i n g sections. A s m e n t i o n e d earlier, t e m p o r a l trends i n N 0 ~ p r o v i d e i m p o r t a n t v e r i f i c a t i o n that the stages o f Ν loss o c c u r as a sequence i n sites e x p e r i e n c i n g c h r o n i c a l l y elevated rates of Ν d e p o s i t i o n . 3

3

3



256

ENVIRONMENTAL CHEMISTRY O F LAKES A N DRESERVOIRS

The Northeastern United States. F o r a p p r o x i m a t e l y 100 sites i n the Northeast, the data are intensive e n o u g h to d e t e r m i n e t h e i r status w i t h respect to w a t e r s h e d Ν loss ( F i g u r e 9). F r o m the standpoint o f N , the N o r t h e a s t is t h e most data-intensive r e g i o n o f N o r t h A m e r i c a . Seasonal data are available f r o m a spatially extensive n e t w o r k of sites, so that geographic patterns i n w a t e r s h e d Ν loss can b e i n f e r r e d . I m p o r t a n t l y , these geographic patterns i n w a t e r s h e d Ν loss f o l l o w t h e geographic p a t t e r n i n Ν d e p o s i t i o n , w i t h t h e most severe effects observable i n t h e A d i r o n d a c k a n d C a t s k i l l m o u n tains, w h e r e d e p o s i t i o n rates are h i g h , a n d little Ν loss i n M a i n e , w h e r e rates o f Ν d e p o s i t i o n are r o u g h l y 5 0 % l o w e r (42).

Figure 9. Location of acid-sensitive lakes and streams United States where the importance of N0 ~ to seasonal be determined. Only data from undisturbed watersheds are from references 109, 114, 118, 119, 151b, 154, 191-194.) 3

in the northeastern water chemistry can are included. (Data 156-159, 162, 164,


8.

STODDARD


257

D a t a f r o m sites i n M a i n e , V e r m o n t , a n d the A d i r o n d a c k a n d C a t s k i l l mountains are available f r o m the U . S . E n v i r o n m e n t a l P r o t e c t i o n A g e n c y ' s ( E P A ) L o n g - T e r m M o n i t o r i n g ( L T M ) Project (150). I n most cases the records f r o m these sites are sufficiently l o n g (7-10 years) to detect trends w h e r e they are present. M o s t of the other sites i n F i g u r e 9 have data o f shorter d u r a t i o n , a n d it is not k n o w n w h e t h e r they e x h i b i t l o n g - t e r m t r e n d s . A l l of the sites i n the state of M a i n e s h o w some seasonality i n N 0 ~ concentrations, w i t h peak concentrations of less than 20 μ ε ς υ ί ν / ί d u r i n g s p r i n g s n o w m e l t a n d n e g l i g i b l e concentrations d u r i n g a l l o t h e r seasons; the concentrations suggest t y p i c a l Stage 0 N 0 patterns. D a t a f r o m five L T M lakes i n M a i n e e x h i b i t no l o n g - t e r m trends i n N 0 " concentrations (151a).


3

3

3

Seasonal data are available f r o m two sites i n N e w H a m p s h i r e , w h e r e d e p o s i t i o n rates are slightly h i g h e r than i n M a i n e . B a i r d et a l . (151b) s t u d i e d e p i s o d i c acidification d u r i n g s n o w m e l t at C o n e P o n d , N e w H a m p s h i r e , a n d w e r e u n a b l e to detect any N 0 i n i n l e t water. Researchers at the H u b b a r d B r o o k E x p e r i m e n t a l F o r e s t i n N e w H a m p s h i r e have b e e n s t u d y i n g the w a t e r s h e d processes a n d t h e i r effects o n stream-water c h e m i s t r y since 1963 (J52). I n reference W a t e r s h e d N o . 6, stream-water N 0 " concentrations u n d e r g o strong seasonal cycles w i t h peak concentrations as h i g h as 85 μ e q u i v / L d u r i n g s n o w m e l t , s i m i l a r to Stage 1 w a t e r s h e d Ν loss. B o t h N 0 " a n d h y d r o g e n i o n concentrations increase d u r i n g s n o w m e l t at H u b b a r d B r o o k , although S 0 ~ concentrations decrease slightly (153, 154). N o l o n g t e r m t r e n d i n stream-water N O ~ concentration is detectable for the 23-year p e r i o d of r e c o r d at H u b b a r d B r o o k (155). 3

_

3

3

4

2

i 3

I n the state of V e r m o n t , 6 of 24 L T M lakes e x h i b i t strong seasonal N 0 " cycles, a n d i n these lakes seasonal N 0 " increases are the most i m p o r t a n t m e c h a n i s m c o n t r i b u t i n g to s p r i n g A N C m i n i m a (256). T h e s e six watersheds exhibit the characteristics of Stage 1 of Ν loss, a n d the r e m a i n i n g 18 lakes e x h i b i t Stage 0 patterns. N o n e of the sites e x h i b i t trends i n N 0 " c o n c e n trations for the p e r i o d 1981-1989. 3

3

3

D r i s c o l l a n d V a n D r e a s o n (257) r e p o r t e d data f r o m 16 A d i r o n d a c k L T M lakes c o l l e c t e d b e t w e e n 1982 a n d 1990. T h e patterns i n a c i d - b a s e c h e m i s t r y at C o n s t a b l e P o n d ( F i g u r e 10), one of the A d i r o n d a c k L T M lakes, can b e c o n s i d e r e d typical of surface waters i n the N o r t h e a s t w i t h e p i s o d i c a l l y ele vated concentrations of N 0 ~ (e.g., at Stages 1 a n d 2). C o n s t a b l e P o n d becomes acidic o n l y d u r i n g seasons of h i g h runoff, as a result of base-cation d i l u t i o n a n d e p i s o d i c N 0 " increases. Sulfate concentrations are r e l a t i v e l y invariant. I n C o n s t a b l e P o n d , as i n other A d i r o n d a c k lakes a n d streams, 3

3

s h o r t - t e r m changes i n N 0 " w e r e h i g h l y c o r r e l a t e d , a n d c h e m i c a l l y c o n sistent, w i t h changes i n the concentrations of a c i d i c cations ( h y d r o g e n a n d a l u m i n u m ) (255). A s m e n t i o n e d earlier, a l t h o u g h d i l u t i o n o f base cations a n d increases i n N 0 ~ appear to be the p r i m a r y causes of e p i s o d i c acidification i n C o n s t a b l e P o n d , these episodes are excursions f r o m an already l o w base l i n e A N C , w h i c h can be a t t r i b u t e d largely to h i g h S 0 ~ concentrations. 3

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E N V I R O N M E N T A L CHEMISTRY O F L A K E S A N D RESERVOIRS

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Figure 10. Temporal patterns in lake-water N0 ~, acid-neutralizing capacity (ANC), base cations (Ca^ + M g + Na + K ) , S O / " , and inorganic monomeric aluminum (Ah) at Constable Pond, a long-term monitoring site in the Adirondack Mountains. Trend lines are shown for variables with significant trends (p < 0.10 in seasonal Kendall tau test). Seasonal pattern is typical of Adirondack lakes, with seasonal minima in ANC coincident with seasonal maxima in N0 ~ and Al Many Adirondack lakes exhibited upward trends in N0 ~ in the 1980s; the primary increase was in episodic NOf concentrations. (Data are from reference 157.) 3

+

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3


8.

STODDARD


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T h e p e r i o d of r e c o r d is l o n g e n o u g h for the A d i r o n d a c k L T M lakes that l o n g - t e r m trends i n N 0 " concentrations can b e e s t i m a t e d (Table I V ; ref. 157). N i n e out o f sixteen A d i r o n d a c k lakes e x h i b i t e d significant increases i n N 0 ~ concentrations, w i t h t y p i c a l slopes of trends b e i n g + 1 μ β ς υ ί ν ^ p e r year. D a t a from the seven r e m a i n i n g lakes a l l suggested u p w a r d trends i n N 0 ~ that w e r e not significant (Table I V ) . A d d i t i o n a l l y , plots of t e m p o r a l 3

3

3

N 0 " patterns indicate that the p r i m a r y change i n these lakes is i n t h e i r s p r i n g values ( F i g u r e 10) a n d that baseflow N 0 ~ concentrations are r e l a t i v e l y u n c h a n g e d . T h i s analysis suggests that these lakes are i n Stage 1 o f w a t e r s h e d Ν loss a n d that t h e i r c o n d i t i o n is w o r s e n i n g . 3


3

In l o w - A N C streams of the C a t s k i l l s increases i n N 0 " , base-cation d i l u t i o n , a n d high-baseline S 0 " concentrations a l l c o n t r i b u t e to a c i d i c e p isodes (97). I n B i s c u i t B r o o k , an i n t e n s i v e l y s t u d i e d headwater stream i n the C a t s k i l l s , concentrations of N 0 " approach or exceed those of S 0 ~ d u r i n g episodes ( F i g u r e 11; ref. 127). Values for the ratio of N 0 " : ( N 0 " + S 0 ~ ) , as p r e s e n t e d i n Table II, illustrate b o t h the general i m p o r t a n c e of N 0 ~ to the a c i d - b a s e d y n a m i c s of this stream a n d the increase i n i m p o r t a n c e of N 0 " d u r i n g h i g h - f l o w events ( F i g u r e 11). N i t r a t e c o n c e n 3

4

2

3

4

2

3

3

4

2

3

3

Table IV. Trends i n N 0 " Concentrations for Adirondack Long-Term Monitoring Lakes 3

Change in N0 ~ (pjequwlL per year) 3

Lake Name

n

Arbutus Lake Big Moose Lake Black Lake Bubb Lake Cascade Lake Clear Pond Constable Pond Dart Lake Heart Lake Lake Rondaxe Little Echo Pond Moss Lake Otter Pond Squash Pond West Pond Windfall Lake

90 98 99 99 98 97 99 99 98 99 97 99 94 93 99 99

fl

+ + + + + + + + + + + + + + + +

1.06 0.92 0.00 0.32 0.15 0.48 1.65 0.90 0.83 0.72 0.00 0.42 1.33 1.80 0.41 0.46

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Ρ

Environmental Chemistry of Lakes and Reservoirs - ACS Publications

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