Sources and Fates of Aquatic Pollutants - ACS Publications

15 Cycles of Nutrient Elements, Hydrophobic Organic Compounds, and Metals in Crystal Lake Downloaded by UNIV OF LEEDS on June 18, 2016 | http://pubs.acs.org Publication Date: June 15, 1987 | doi: 10.1021/ba-1987-0216.ch015

Role of Particle-Mediated Processes in Regulation David E. Armstrong, James P. Hurley, Deborah L. Swackhamer,1 and Martin M. Shafer Water Chemistry Program, University of Wisconsin-Madison, Madison, WI

53706

Measurements of the chemical composition and fluxes of particulate matter were used to assess the particle-mediated cycling of selected nutrient elements, hydrophobic organic compounds, and metals in Crystal Lake, located in north central Wisconsin. The absence of surface water input simplified the analysis of in-lake cycles. Sediment incorporation and accumulation fluxes were cal culated on the basis of an assumption of negligible sediment fo cusing. Removal of 210Pb was rapid with negligible recycling, and 210 Ρο was partly recycled in the water column. The nutrient ele ments (C, Ν, P, and Si) contained in deposited particles were partly recycled (~50%) into the water column, but most of the Ρ recycled was subsequently redeposited through interaction with Fe(III) formed near the sediment-water interface. Hydrophobic organic compounds such as polychlorinated biphenyls (PCBs) were also removed rapidly to the sediment-water interface by particle deposition but apparently returned partly to the water column during particle incorporation into surface sediments. Although PCBs were partly returned to the sediments by redeposition, recycling from bottom sediments increased the residence time of PCBs in the water column. Differences among chemical constituents in particle-mediated fluxes were regulated by differ ences in biogeochemical processes. 1 Current address: School of Public Health, Environmental and Occupational Health, University of Minnesota, Minneapolis, ΜΝ 55455

0065-2393/87/0216-0491$08.00/0 © 1987 American Chemical Society

Hites and Eisenreich; Sources and Fates of Aquatic Pollutants Advances in Chemistry; American Chemical Society: Washington, DC, 1987.

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SOURCES AND FATES OF AQUATIC POLLUTANTS

Downloaded by UNIV OF LEEDS on June 18, 2016 | http://pubs.acs.org Publication Date: June 15, 1987 | doi: 10.1021/ba-1987-0216.ch015

CHEMICAL LIMNOLOGY IS FUNDAMENTALLY CONCERNED

with the pro cesses regulating the chemical composition of lakes. Advances in the field involve improved qualitative and quantitative understanding of the controlling processes. Regulation occurs through both external and internal processes. External processes include transport of materials by air and water, input of light and heat, and water circulation by wind energy. Internal processes involve changes in chemical forms by biologi cal and chemical reactions and transport of materials by advection and diffusion. Models based on quantitative input-output relationships, which treat most internal processes empirically, have been very useful tools in lake management (J). However, element-specific internal processes often play a major role in controlling chemical composition (2-4), and prediction of lake composition and response to changes in external fac tors requires an understanding of internal processes and cycles. Particle-mediated processes play a major role in the internal regula tion of the chemical composition of lakes. Essential elements are incor porated and released by particles through photosynthesis and respira tion. Similarly, the uptake and release of other chemical substances by particles occurs through adsorption-desorption and precipitation-disso lution reactions. Combined with particle transport by settling, these particle-mediated reactions have a major influence on chemical com position. Although laboratory experiments have provided major contributions toward understanding the mechanisms of particle-mediated reactions in lakes, simulation of the complex interactions among physical, chemical, and biological processes is difficult. Thus, whole-lake measurements and experiments are useful in resolving the role of particle-mediated pro cesses in controlling lake composition. Our analysis focuses on Crystal Lake, located in the Northern Highlands area of north central Wisconsin (Vilas County). Crystal Lake is an oligotrophic lake that has no surface water inlets or outlets. Direct precipitation accounts for about 90$ of the water input (Table I). The remainder occurs through groundwater inflow. Consequently, external inputs are low, and Crystal Lake is well suited for investigation of the regulation of chemical composition by internal processes. Three groups of chemical constituents are examined in this chapter: the nutrient elements (C, Ν , P, and Si), hydrophobic organic compounds [represented by the polychlorinated biphenyls (PCBs)], and metals ( 2 1 0 Pb and 2 1 0 Po). The role of particle-mediated processes in regulating biogeochemical cycles is emphasized.

Nutrient Elements The biogeochemical cycles of nutrient elements in lakes are complex. Some processes are common to all nutrient elements, including incorpoHites and Eisenreich; Sources and Fates of Aquatic Pollutants Advances in Chemistry; American Chemical Society: Washington, DC, 1987.


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ration of inorganic forms from lake water into cells (particles) through primary production. Other processes are unique to specific elements or chemical forms, such as adsorption or precipitation reactions and the biological transformations among the inorganic forms of nitrogen (nitrification and denitrification). Biogenic particles link the cycles of nutrient elements together, and similarities in behavior result. Primary production incorporates the elements into particles in characteristic proportions, and particle settling leads to similar vertical transport rates. However, element-specific reactions can uncouple the nutrient elements as particles flow through the lake system. This uncoupling leads to enrichment or depletion of the relative concentration of the nutrient element in the lake water. Thus, particle-mediated cycling influences both the absolute and relative supply of nutrients to the photic zone of lakes. Biological Regulation. For the nutrient elements, active uptake from lake water by phytoplankton has a major influence. Although bacteria and higher plants also assimilate inorganic forms of nutrients, phytoplankton play a dominant role in the pelagic zones of lakes. The influence of phytoplankton on the nutrient elements is related to their requirements for growth and organic matter production. As ultimately determined by the chemical composition of cell components, phytoplankton remove nutrients from lake water in specific proportions. The resulting average stoichiometry of the phytoplankton is the basis of the Redfield model (equation 1) of the influence of nutrient assimilation and mineralization on nutrient elements dissolved in lake water (5). 106CO 2 + 16NH 3 + 15H 4 Si0 4 + H2PO4- + H + + IO6H2O ^ ± R

[ (CH2O)10e(NH3)ie(H4SiO4)l5H3PO4] + IO6O2

(1)

phytoplankton The nutrient elements are incorporated into biogenic particles during photosynthesis (P) and regenerated during cell decomposition, represented as respiration (R). The stoichiometry in equation 1 (CioeNi6Sii5Pi) is representative of diatoms. For nonsiliceous algae, the C:N:P stoichiometry is similar (CioeNi6P). The Redfield model is an approximation because the nutrient stoichiometry of the phytoplankton varies with species and nutrient concentrations in lake water (6). Nevertheless, stoichiometry is extremely useful in modeling the biological cycling of the nutrient elements. Chemical Regulation. Chemical processes also control nutrient element concentrations in particulate phases. These chemical reactions may result in deviations from the stoichiometry predicted by the Redfield model. In general, the important processes include (1) the precipiHites and Eisenreich; Sources and Fates of Aquatic Pollutants Advances in Chemistry; American Chemical Society: Washington, DC, 1987.

SOURCES AND FATES OF AQUATIC POLLUTANTS

494

tation of inorganic carbon as calcium carbonate, (2) adsorption of the ammonium ion by cation-exchange reactions, (3) the solubility and disso lution kinetics of amorphous silica contained in diatom frustules and the formation of silicate minerals, and (4) adsorption and precipitation reac tions involving phosphate. The alkalinity, calcium concentration, and p H of Crystal Lake preclude calcium carbonate formation (Table I). How ever, chemical reactions may play a role in retaining ammonium ion, silica, and phosphate in particles, particularly near the sediment-water interface.


Table I. Limnological Characteristics of Crystal Lake Characteristic Surface area Volume Terrestrial drainage area Direct precipitation Groundwater inflow Groundwater outflow Phosphorus Total dissolved Ρ Total particulate Ρ Nitrogen N 0 3 (N) N H 4 (N) Particulate Ν Silicon Dissolved reactive Si Particulate biogenic Si Calcium Chlorophyll Conductivity v^uiiuuunviiy Alkalinity pH Mass sedimentation rate Sediment mixed-layer depth

Value 36 ha 3.8 Χ 106 m 3 negligible 346 Χ 103 m 3 year" 1 30 Χ 103 m 3 year - 1 127 Χ 103 m 3 year - 1 2.7 μg L " 1 2.5 μg L 1 18 μg L~ 28 μg L - 1 34 μ% L " 1 l

16 μ g L -"ι 23 μg L -"ι 1.1 mg L " 1 1.5 μg L " 1 -1 j .12 ^ μ$ c m "

-1

15 μequiv L 5.85 8 mg c m - 2 year 4 cm

1

SOURCE: Data are from references 19, 20, 24, and 27. NOTE: Concentrations represent volume-weighted mean annual values.

Sediments of lakes in the Northern Highlands area, including Crys tal Lake, are generally rich in iron (7). Thus, phosphate tends to be retained in sediments through interactions with iron. The removal of phosphate from lake waters by iron is linked to the iron reduction-oxi dation cycle, which mobilizes Fe(II) in anoxic zones and forms Fe(III) in oxic zones. This "ferrous wheel" (8) scavenges phosphate from lake waters above anoxic-oxic boundaries. If Fe(III) is formed in the pres ence of phosphate, a basic iron phosphate [Fe2(OH) 3 P0 4 ] with a Fe:P stoichiometry of 2:1 is apparently formed (9). This reaction can be


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represented as a combined Fe(II) oxidation-basic iron phosphate pre cipitation reaction:


2Fe 2 + + | 0 2 + H 2 P 0 4 + 2 H 2 0

>Fe 2 (OH) 3 P0 4 (s) + 3 H +

(2)

Alternatively, if Fe(III) is formed and hydrolyzed before interaction with phosphate, adsorption of phosphate by Fe(OH) 3 (s) results in Fe:P ratios > 5:1 (10). However, either reaction is efficient in removing phos phate from solution if the Fe:P ratio is sufficiently high. Below the oxicanoxic boundary (e.g., after incorporation into surficial sediments), Fe(III) reduction in association with organic matter oxidation by sedi ment bacteria tends to solubilize the iron-bound phosphate. For the basic iron phosphate, the reaction can be depicted as Fe 2 (OH) 3 P0 4 (s) + | C H 2 0 + 3 H + — • » 2Fe 2 + + H 2 P 0 4 + | C 0 2 + | H 2 0

(3)

Similarly, reduction of Fe(OH)3(s) results in solubilization of adsorbed phosphate. The removal of phosphate from lake waters by iron is also related to reactions that control F e 2 + concentrations and transport from the anoxic zone into the lake water, especially reactions that result in FeC0 3 (s) and FeS(s) formation (11). In lakes of the Northern High lands, the Fe:S and Fe:C (inorganic) ratios are high and lead to a high mobility of F e 2 + in anoxic waters (12). Although relatively selective for phosphate, Fe(OH) 3 (s) also adsorbs smaller proportions of other nu trient elements including silica, organic N , and organic C (13). Amorphous silica is typically undersaturated in lake waters, and this undersaturation results in a tendency for dissolution of biogenic (diatom) silica. Because of the relative rates of silica dissolution and diatom sink ing, dissolution in lakes occurs mostly after diatoms sink to the sedi ment-water interface (14, 15). Consequently, some of the amorphous silica is buried and retained in the lake sediments (14, 16). In addition, silica subsequently released by dissolution in sediments may be incorpo rated into aluminosilicates (17). Although sediment particles usually possess a net negative charge and the capacity to adsorb N H 4 and other cations, the exchange capac ity is typically small relative to the organic Ν content (18). This capac ity indicates that most of the N H 4 formed by mineralization of biogenic particles will not be retained by adsorption to the particulate phase. In summary, chemical regulation of nutrient elements in particulate matter in Crystal Lake may be important for inorganic phosphorus and silica. Chemical reactions, especially involving Fe(OH) 3 , may also retain organic forms of carbon and nitrogen.


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SOURCES A N D FATES O F AQUATIC

Particle-Mediated

Nutrient

Cycling

in Crystal

POLLUTANTS

Lake


In this section, data from Crystal Lake (19 ) is used to examine links in nutrient element behavior through particle-mediated processes. Specifi cally, the processes investigated include the removal of nutrient elements by particle production and settling, the regeneration of nutrient elements through mineralization of particulate organic matter, and the uncoupling of nutrient elements through element-specific interactions occurring in association with regeneration.

Particulate Matter Compartments. The biogeochemical cycling of nutrient elements in association with particulate matter is evaluated by dividing the lake into four vertically segregated particulate-matter com partments; suspended particulate matter, recently deposited material collected by sediment traps, the surficial sediment layer mixed by bioturbation, and the underlying buried sediment layer. This approach views the particulate matter as originating as suspended particulate matter in the water column. Settling of the suspended particulate matter forms a thin layer of recently deposited material at the sediment-water interface. This material is subsequently incorporated into the surficial sediment by advection and mixing (bioturbation in Crystal Lake) and eventually leaves the surface layer by burial as additional sediment is deposited at the surface. Nutrient regeneration (removal from the particulate phase) as the particulate matter moves through these four compartments can be eval uated by measuring the composition of the particulate matter in each compartment and the rate of gain or loss from each compartment. For the nutrient elements (19), suspended particulate matter was obtained by filtration (0.4 μτη), and recently deposited particulate matter was col lected by sediment traps. The rate of deposition of an element was cal culated as the product of the particulate matter mass flux (g cm 2 year"1) and the element concentration in the deposited particulate mat ter (mmol g"1). The sedimentation rate and mixed-layer depth were cal culated from 2 1 0 Pb profiles (20). Temporal Changes in Composition and Fluxes. The composi tion and flux of suspended particulate matter in Crystal Lake reflect the annual primary production cycle (Figures 1 and 2). Details of the chemi cal and biological composition of the particulate matter are given else where (19). Phytoplankton are an important component of the sus pended particulate matter. Diatoms (e.g., Asterionella) are prevalent in early spring and late fall, but nondiatom algae, some containing silica (e.g., Dinobryon), are also relatively abundant throughout the ice-free season. Nonalgal components of the suspended particulate matter and


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ARMSTRONG E T AL.

Spring-Summer

Winter

Summer-Fall

ice covered

stratified

1.5

497

Processes

Suspended Particulate Matter

i.O

0.5 0

Silica Concentration DRSI PBSI

40


30 20h 10

100

300

200

400

500

Day, 1982-1983

A

M

J

J

A

S

O

N

O

J

F

M

A

M

Figure 1. Particulate matter and silica cycling in Crystal Lake. Suspended particulate matter concentration versus day of the year (top), and concen trations of dissolved reactive silicon (DRSI) and particulate biogenic silicon (PBSI) versus day of the year (bottom). The scale represents the months of the year.

trap particulate matter include detritus, zooplankton (mainly copepods) and their fecal matter, and pollen. The particulate matter concentration and flux rise from low values in the early spring to a maximum in the midsummer and decline to low levels during the winter ice-covered period (Figures 1 and 2). This fluc tuation reflects the importance of in-lake primary production as a source of particulate matter in Crystal Lake. Erosional transport to the lake of soil from the surrounding landscape is unimportant. Atmospheric input and groundwater are the main external sources of chemical components. However, atmospheric deposition of particulate matter (21 ) corresponds to less than 2% of the in-lake depositional flux of autochthonous particu late matter. The deposition rates of the nutrient elements (C, N , Si, and P) generally reflect the pattern for total particulate matter abundance, and maximum values occur in the midsummer (Figure 2). However, fluxes of Ρ are higher in fall than in spring. In a related manner, Fe was not


SOURCES A N D FATES O F AQUATIC POLLUTANTS

Particulate Matter

20 10 0 7.5

Carbon


5.0 25 0 1.0

Nitrogen

0.50

-,

0 0.50|

,

ι

Γ

1

Silicon

025 »

g

o. §

0 0.15 Phosphorus

0.10 0.05 0 0.75

Iron

0.50 0.25 0

Τ

Q075

!

1

Manganese

0.05 0.025| 0

\

A

»

M

I

4

J

A

S

O

N

D

J

F

M

A

Month, 1982-83 igure 2. Deposition rates of particulate matter, nutrient elements (C, N, Si, and P), iron, and manganese in Crystal Lake.



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detected in sediment traps in the spring, but high deposition rates of Fe were observed in midsummer and fall. Mn was also deposited during this period, but in small amounts in comparison with Fe. These results suggest a primary link of N , Si, and Ρ deposition to biogenic organic matter deposition and a secondary link between Ρ and Fe deposition. Even though the particulate material is derived mostly from autochthonous primary production, chemical composition varies con siderably with time (19). This variability reflects, in part, differences in phytoplankton and zooplankton assemblages, alteration by secondary biological processes (phytoplankton autolysis and grazing by zooplank ton, and bacterial decomposition), varying proportions of biogenic detritus, and chemical processes (biogenic silica dissolution and pre cipitation of Fe and Mn hydrous oxides). Variations in concentration of C, N , Si, and Ρ are illustrated in Figure 3. The data are grouped according to the three seasonal periods depicted in Figure 1. In general, variations within a season (as shown by the 95% confidence limits) are of the same magnitude as variations between seasons. Seasonal concentra tions of nutrient elements in the particulate matter were usually within 80X-120X of the mean annual concentration. However, some seasonal patterns are apparent. Higher Si concentrations in the suspended particulate matter during spring-summer are associated with diatom production. High Ρ concentrations in the deposited particulate matter during summer-fall and winter reflect the increased rate of Ρ deposition during these seasons. Changes in composition and deposition rates of the nutrient ele ments over the annual cycle (Figures 2 and 3) provide some insight into the importance of particle production and removal events in controlling these nutrient elements. The major event appears to be primary produc tion, which leads to large relative differences in deposition rates with season, smaller average changes in composition with season, and rela tively large within-season variations in composition. Superimposed on this pattern are the production of specific types of phytoplankton. The production of diatoms and other Si-containing algae is reflected in both the seasonal and within-season variations in amorphous Si concentration. Changes in phytoplankton have a smaller influence on C and N . Varia tions in Ρ within season are relatively high and are consistent within the well-established range in Ρ concentration of algal cells as influenced by species and external supply (6). Deposition of Ρ in association with Fe appears as a secondary pattern in addition to the influence of primary production. Biological alteration of organic matter during deposition no doubt causes variation in both concentrations and deposition rates but does not emerge as "temporal" patterns in the nutrient concentration data sets. In the following sections, the nutrient composition of the particulate



Summer-Fall [ν

Figure 3. Ratios of mean seasonal to mean annual nutrient element concentrations

Spring-Summer

in particulate

Winter


matter.

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matter compartments and intercompartmental fluxes are used to quan tify nutrient cycling among compartments in Crystal Lake. To enable comparison of the bottom sediments with other compartments, annual weighted average composition and fluxes are used. This approach is necessary because short-term (seasonal) changes in composition of the surface sediments cannot be resolved. The validity of this approach is supported by the similarity among seasons in major element composition of the suspended and trap particulate matter. Althoughfluxesare highly seasonal, composition varies as much within season as between seasons (Figure 3 ) . Thus, using an annual time scale should not bias the analysis.

Changes in Stoichiometry during Deposition and Burial. Changes in stoichiometry as particulate matter moves through the lake from suspended particulate matter to sediments are shown in Figure 4. The average stoichiometry reflects a general similarity to the composition of phytoplankton (CioeNieSii5P for diatoms). The suspended particulate matter is somewhat depleted in Ρ and is consistent with the expected lower Ρ concentration in P-limited algal cells (6). Also, Si is lower than

Suspended PM C

I 0 6 I 4 . 3 4 . I 0.32 N

S,

P

Deposition (10.4)

Sediment

Deposited PM

^0-0.14 cm C

I06 l l . 7 N

S i

W a t e r

2 . 8 0 . 6 € ^2.4 P

Incorporation (8.0) Surface Sediment

0.14-4 cm C

I06 7 . l N

S ,

ll.8 0.S9 i.8 P

F e

Burial (8.0) Buried Sediment

4-8 cm C

I06 N 6.6 S i l0.8 P0.5I Fe0.7l|

Figure 4. Nutrient element stoichiometry of particulate matter in Crystal Lake. Compartments (boxes) show composition relative to carbon on a molar basis. Values in parentheses denote the mass fluxes of particulate matter (mg cm 2 year1).



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SOURCES A N D F A T E S O F AQUATIC P O L L U T A N T S

expected for diatoms and reflects a mixed population of diatoms and nonsiliceous algae. Comparisons of the particulate matter compartments show that the major changes in stoichiometry are depletion of C and Ν during incor poration of recently deposited particulate matter into surficial sedi ments, apparent enrichment of Si during incorporation, and enrichment of Ρ and Fe in the recently deposited particulate matter over the other compartments. Although the stoichiometry suggests a partial depletion of Ν and Si during deposition, the concentrations (mmol/g) show this results in part from an increase in the carbon content of the trap particu late matter (Figure 5). The concentrations of Ν and Si in suspended particulate matter and deposited particulate matter are similar. The lower C and Ν levels in the active sediments indicate that organic matter mineralization occurs mainly after deposition and that Ν is mineralized to a greater extent than C . Presumably, Ρ is mineralized similarly to Ν during incorporation. However, the inorganic phosphate formed by mineralization is partly retained as Fe-bound P. Although Ρ contained in plankton is organic, about 45% of the Ρ in Crystal Lake sediment is inor ganic (7). Some of the inorganic Ρ is released from the anoxic sediments but converted to particulate Ρ near the sediment-water interface by association with Fe(HI) oxides formed from soluble Fe(II) also released from the sediments. This ferrous wheel accounts for the enrichment of Ρ in the recently deposited particulate matter. In contrast to the other elements, Si shows a marked enrichment in the surface sediments in comparison to suspended and deposited partic ulate matter (Figure 4). The source of this Si is unclear (see discussion later in the chapter). The increase in Si concentration in the sediments could arise in part from a corresponding loss of organic matter. How ever, comparing the deposited particulate matter and surface sediment compartments shows that a loss of 9 mmol/g of organic C from the trap particulate matter would increase the Si content to only about 1 mmol/g, as compared to the observed concentration of 2 mmol/g (Figure 4). Also, this "concentrating" effect of organic matter loss would be partly offset by the increased Al content of the sediments (21 ). Changes in nutrient element composition during burial were minor. This fact no doubt reflects the relatively high mixed-zone thickness (4 cm or 0.35 g cm" 2 ), low sedimentation rate (0.008 g cm" 2 year' 1 ), and corresponding long residence time of particles (—40 years) in the mixed zone of Crystal Lake sediments. Most alterations in composition involv ing mineralization of biogenic particles likely occur during this time period. Although the stoichiometry (Figure 4) shows some decreases in N, Si, and Ρ relative to C during burial, the concentration data (Figure 5) show these changes are due mainly to a higher C content in the buried sediment zone. One notable exception to this trend is shown by Fe. The


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Processes Air Water

Suspended P M

Thermocline

Sources and Fates of Aquatic Pollutants - ACS Publications

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