The Dynamics of Nitrogen and Phosphorus Cycling in the Open

40 The Dynamics of Nitrogen and Phosphorus Cycling in the Open Waters of the Chesapeake Bay J A M E S J. M C C A R T H Y

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Department of Biology, Harvard University, Cambridge, Mass. 02138 W. ROWLAND TAYLOR and JAY L. TAFT Chesapeake Bay Institute, The Johns Hopkins University, Baltimore, Md. 21218 The use of radioisotopes and rare stable isotopes in field investigations of plankton nutrition both in lakes and the ocean is now widely practiced. The introduction of the C technique to measure phytoplankton photosynthesis (1) revolutionized the study of aquatic primary productivity. The use of P in plankton studies has received less attention (2,3,4), and more recently N has proven to be a useful tool (5,6). The literature of the last two decades which includes various applications of these isotopes in studies of aquatic biology is heavily burdened by discussions of problems in experimental design and data interpretation (7,8,9,10). in general the application of such techniques to aquatic biology has progressed more slowly than in other fields of study, and ecologists have been criticized (11) for their naive and often erroneous use of isotopes in studies of transfer processes within aquatic ecosystems. Recently, innovative applications of isotope tracer studies to plankton nutrition have included tritiated substrates in the study of heterotrophy (12) and Si (13) and Ge (14)in the study of the silicious nutrition of diatoms. D i r e c t and i n d i r e c t evidence supports t h e c o n t e n t i o n t h a t n i t r o g e n i s o f t e n the element most l i k e l y t o l i m i t p l a n t prod u c t i v i t y i n the sea (15,16,17). I n a d d i t i o n , s t u d i e s o f t h e nitrogenous n u t r i t i o n o f plankton a r e a t t r a c t i v e i n t h a t n i t r o gen provides perhaps the s i m p l e s t p e r s p e c t i v e from which t o view a m a t e r i a l balance i n a p l a n k t o n i c ecosystem. Whereas c e l l u l a r n i t r o g e n i s p r i m a r i l y bound i n s t r u c t u r a l m a t e r i a l , b o t h carbon and phosphorus a r e l a r g e l y i n v o l v e d i n c e l l u l a r metabolic a c t i v i ties i naddition to their structural roles. Blue-green algae capable o f f i x i n g gaseous n i t r o g e n a r e c o n s i d e r a b l y more common i n fresh-water than i n t h e sea and p a r t i a l l y because of t h i s , the a v a i l a b i l i t y o f n i t r o g e n i s considerably less l i k e l y to regulate plant productivity i n freshwater. Phosphorous i s f r e q u e n t l y looked t o as the most important n u t r i e n t i n both the r e g u l a t i o n o f p r o d u c t i v i t y and l i m i t a t i o n o f biomass i n fresh-water and the open waters o f e s t u a r i e s 14

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(18) . In most natural waters dissolved combined nitrogen i s present i n more than a single form. Concentrations of nitrate, n i t r i t e , and ammonium i n the sea vary temporally and spatially (19) and recently, urea has been reported from a l l areas which have been investigated (20,21,22^,23). Amino acids are usually detected (24,25), and although the concentrations are always low, they may at times contribute to phytoplankton nitrogenous nutrition (26,27). Although we know what forms of combined inorganic nitrogen do and do not commonly exist i n the sea, we are largely ignorant of the identity, origin, and fate of a sizeable pool of dissolved organic nitrogen. This material i s temporally and spat i a l l y ubiquitous (27,28,29) and the implication i s that the bulk of i t i s highly refractory and has l i t t l e potential i n a l gal nitrogenous nutrition (27,28). The a v a i l a b i l i t y of labeled forms of the plant nutrients thought to be of greatest importance, enables one to assess the relative significance of several forms of nitrogen i n the n u t r i tion of natural plankton assemblages. A few océanographie laboratories i n this country now have the capacity to rapidly process plankton samples for 1 % enrichment, and the recent a v a i l a b i l i t y of optical emission spectrometers may greatly extend the use of N i n plankton studies. Shipboard studies with large volume cultures established by enriching natural seawater with nutrients have demonstrated that estimates of phytoplankton assimilation derived from short interval measurements of 15

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uptake agree closely with measurements of both the decrease i n dissolved nitrogenous nutrient and increase i n particulate nitrogen (30). Field studies of nitrogen fixation have also employed " Ν (5), but the acetylene reduction assay for nitrogenase a c t i v i t y (31) i s far more convenient and i t has become widely accepted as the method of choice. Because of the low natural concentrations of some of the phosphorous and nitrogenous forms of interest, experiments must be initiated quickly. In most cases phytoplankton activity i s measured after removal of the zooplankton; such removal certainly reduces the natural supply of the rapidly recycled forms of both phosphorus and nitrogen to the captured phytoplankton. With zooplankton inclusion, phytoplankton are consumed during the experiment, animal metabolites are released, and the inter pretation of data i s immensely complicated. Since ambient sub strate levels should be determined before the i n i t i a t i o n of an experiment, there has been much interest i n improving the capa city to quantitate phosphorous and nitrogenous nutrients with specific, accurate, precise, and convenient methodology. Most of the techniques of choice are described in both manual and automated forms by Strickland and Parsons (1972) (32). We have streamlined these manual methods to permit us to rapidly process 5ml samples with both precision and sensitivity which exceeds



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those o f the automated methods. A small number o f samples can be r o u t i n e l y analyzed by a s i n g l e a n a l y s t f o r n i t r a t e , n i t r i t e , ammonium, urea, and s o l u b l e r e a c t i v e phosphate w i t h i n one h a l f hour a f t e r sample c o l l e c t i o n . In the study o f plankton n u t r i t i o n one has to s e l e c t an i n c u b a t i o n technique which best s u i t s the p a r t i c u l a r problem under i n v e s t i g a t i o n . An i n s i t u i n c u b a t i o n i n v o l v e s the c o l l e c t i o n o f a water sample, enclosure i n a s u i t a b l e b o t t l e , a d d i t i o n of isotope t r a c e r m a t e r i a l , and resuspension o f the b o t t l e to the depth from which the sample was c o l l e c t e d . The o b j e c t i v e i s t o execute the i n c u b a t i o n a t near n a t u r a l l i g h t and temperat u r e . C a r e f u l and r a p i d handling a r e r e q u i r e d to minimize physiol o g i c a l shock which may r e s u l t from exposure o f deep water samples to both f u l l i n c i d e n t s o l a r i r r a d i a t i o n and great changes i n temperature. Any p o t e n t i a l problems r e l a t e d to pressure d i f f e r e n t i a l s a r e u s u a l l y ignored. The i n s i t u i n c u b a t i o n has been perf e c t e d with a s i n g l e u n i t which captures, i n o c u l a t e s with materi a l o f choice (e.g. isotope s t o c k s ) , and serves as an i n c u b a t i o n chamber (33). A f t e r the completion o f the i n c u b a t i o n the chamber i s brought to the s u r f a c e and the sample i s processed. Advantages of such a system a r e obvious, but the disadvantages a r e p r i m a r i l y that f o r n u t r i e n t a s s i m i l a t i o n s t u d i e s one must make isotope t r a cer a d d i t i o n s without s p e c i f i c knowledge o f the ambient n u t r i e n t levels. A l s o , zooplankton which consume phytoplankton and r e l e a s e p l a n t a s s i m i l a b l e forms o f n i t r o g e n and phosphorus cannot be excluded from the i n c u b a t i o n chamber. Because major sea-going v e s s e l s u s u a l l y cannot remain on s t a t i o n s f o r extended p e r i o d s , and there may not be an opportunity to r e t u r n t o a buoy, b i o l o g i c a l oceanographers f r e q u e n t l y use simulated i n s i t u i n c u b a t i o n s . A deckboard incubator i s f i t t e d with o p t i c a l f i l t e r s to simulate l i g h t a t depth o f sampling, and flowing seawater c o n t r o l s temperature. Some o f the problems a s s o c i a t e d with measuring l i g h t and s e l e c t i n g appropriate f i l t e r s have been i n v e s t i g a t e d and discussed (34). The Chesapeake Bay i s a h i g h l y productive c o a s t a l p l a i n estuary with a w e l l defined two-layer c i r c u l a t i o n (35). I t i s approximately 300 km i n l e n g t h and i t i s p r i m a r i l y an estuary o f the Susquehana R i v e r . The n u t r i e n t l o a d i n g r e s u l t i n g from both the Susquehana R i v e r drainage from western Pennsylvania and c e n t r a l New York and the m e t r o p o l i t a n discharges o f the D i s t r i c t of Columbia and Baltimore, Maryland, a r e p o t e n t i a l l y l a r g e . I t i s indeed i n t e r e s t i n g to note i n the data presented by Carpenter, P r i t c h a r d , and Whaley (1969) (36), that the e f f e c t o f n u t r i e n t l o a d i n g from the adjacent m e t r o p o l i t a n areas i s v i r t u a l l y undet e c t a b l e as d i s s o l v e d i n o r g a n i c n u t r i e n t i n the Bay proper below both Baltimore and the j u n c t i o n o f the Potomac River w i t h the Bay. I t has been demonstrated that the heavy w i n t e r - s p r i n g r u n o f f from the Susquehana River i s the primary source o f n i t r a t e i n the Bay, but no p o i n t sources f o r e i t h e r ammonium o r phosphate can be i d e n tified.



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Data from some coastal regions, such as for Buzzards Bay, Massachusetts, suggest that transport of nitrogen remineralized in the sediment i n t e r s t i t i a l water to the overlying water may contribute significantly to the phytoplankton nitrogen requirement (37). In another study, v e r t i c a l transport from the sand bottom off La J o l l a , California, was shown to provide an insignificant portion of the nitrogen u t i l i z e d by the phytoplankton in the overlying water (38). Carpenter, Pritchard, and Whaley (1969) (36) concluded from calculated v e r t i c a l exchange rates for spring and summer that the transport of dissolved nitrogen and phosphorus from the sediments into the euphotic zone was insufficient to account for more than a few per cent of the approximated rates of phytoplankton nutrient utilization.More recently, Bray, Bricker, and Troup (1973) (39) calculated that upward diffusive flux across the sediment-water interface i n the Chesapeake Bay would amount to a weekly addition of 5% to the total orthophosphate in the water column. This i s considerably less than the rate at which phytoplankton remove phosphorus from the euphotic zone of the Bay (40). It can be seen, therefore, that the observations relating to nutrient a v a i l a b i l i t y and plankton biomass i n the Chesapeake Bay are apparently paradoxical: High phytoplankton standing stocks and high phytoplankton productivity persist i n the presence of low ambient levels of dissolved inorganic nitrogen and phosphorus throughout much of the year. The short-term temporal s t a b i l i t y in these patterns suggests an equilibrium i n the biological and chemical processes, and Carpenter, Pritchard, and Whaley (1969) (36) hypothesized that the productivity of the open waters of the Chesapeake Bay i s regulated largely by a dynamic nutrientphytoplankton-zooplankton-nutrient cycle. Our i n i t i a l efforts to stimulate primary productivity by enriching natural water samples from various locations within the Chesapeake Bay were unsuccessful. The results clearly demonstrated the unsuitability of the nutrient enrichmentincubation technique i n the investigation of a highly dynamic planktonic ecosystem, and supported the hypothesis that only through short-term isotope tracer experiments could one come to an understanding of the plankton nutrition i n this estuary. The phytoplankton may grow at rates sufficient to double their biomass i n one or two days and yet such increases i n biomass are rarely observed i n the Bay. In fact, with the exception of localized blooms, the phytoplankton standing stock varies only by approximately a factor of 5 in the entire main body of the Bay throughout an annual cycle. Consider further a summer condition of nearly constant low levels of plant nutrients i n waters containing a population of small herbivorous zooplankters capable of ingesting 2 to 3 times their body mass daily, and one can begin to appreciate both the dynamic nature of estuarine planktonic communities and the great effort required to investigate their nutrition.



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What f o l l o w s i s a d e s c r i p t i o n of our recent e f f o r t to study the plankton n u t r i t i o n i n the Chesapeake Bay w i t h i s o t o p e t r a c e r techniques. This program remains i n progress, and most of our r e s u l t s to date are e i t h e r i n press or i n a s t a t e of p r e p a r a t i o n . Our i n t e n t i o n here i s to demonstrate through a preview of some of our data the u t i l i t y of i s o t o p e t r a c e r s i n developing a dynamic image of plankton n u t r i t i o n which permits i n s i g h t i n t o the mech anisms by which p l a n k t o n i c p r o d u c t i v i t y i s r e g u l a t e d . We conducted a s e r i e s of 7 c r u i s e s of 2 weeks d u r a t i o n and a t 6 week i n t e r v a l s to sample and study 8 g e o g r a p h i c a l l y f i x e d s t a t i o n s i n the open waters of the Chesapeake Bay and one s t a t i o n on the adjacent c o n t i n e n t a l s h e l f (Figure 1 ) . N u t r i e n t measure ments were made and experiments were i n i t i a t e d to q u a n t i t a t e pbytoplankton i n c o r p o r a t i o n r a t e s f o r carbon, n i t r o g e n , and phos phorus. Numerous other p h y s i c a l , chemical, and b i o l o g i c a l measurements were made, and the c o l l e c t e d b i o t a were a l s o used f o r a d d i t i o n a l experiments. Each s t a t i o n r e q u i r e d a f u l l day of e f f o r t . With the exception of the p r e v i o u s l y mentioned modi f i c a t i o n s , a l l shipboard a n a l y t i c a l methods f o l l o w e d the proce dures o u t l i n e d by S t r i c k l a n d and Parsons (1972) (32). A l l n u t r i e n t , biomass, and n u t r i e n t uptake r a t e measurements which w i l l be reported are averages of two samples from d i f f e r e n t depths i n the euphotic zone, and they are considered to be r e p r e s e n t a t i v e of the upper l a y e r of the estuary. The near constancy of c h o r o p h y l l a_ c o n c e n t r a t i o n s ( h e r e a f t e r r e f e r r e d t o as c h l o r o p h y l l ) w i t h depth i n the p r o f i l e s throughout the upper l a y e r supports the n o t i o n t h a t t h i s l a y e r , and l i k e w i s e the euphotic zone, i s w e l l mixed. In order to minimize the e f f e c t o f any s m a l l s c a l e inhomogeneities, a l l i n d i v i d u a l measurements were made w i t h m a t e r i a l c o l l e c t e d i n a composite sample (the contents of 6 Van Dorn b o t t l e s which were c a s t to the same depth were mixed and a l i q u o t s were withdrawn). C l a b e l e d carbonate, 32p l a b e l e d phosphoric a c i d , and l^N l a b e l e d n i t r a t e , n i t r i t e , ammonium, and urea were added to separ ate b o t t l e s and each was incubated on the deck of the s h i p under simulated i n s i t u c o n d i t i o n s . At the t e r m i n a t i o n of the e x p e r i ments |jje p a r t i c u l a t e m a t e r i a l was analyzed f o r i s o t o p i c e n r i c h ment. C and Ρ were determined by l i q u i d s c i n t i l l a t i o n spectro metry and Ν by mass spectrometry. The d i s s o l v e d organic phos phorus and d i s s o l v e d polyphosphate were a l s o examined f o r e n r i c h ment, and s i m i l a r measurements f o r d i s s o l v e d organic carbon and n i t r o g e n are p a r t of our c o n t i n u i n g program. Because of the frequent c o n d i t i o n of low n u t r i e n t s , h i g h biomass, and hence r a p i d turnover of a v a i l a b l e n u t r i e n t , i n c u b a t i o n s were of o n l y a few hr d u r a t i o n a t midday and e x t r a p o l a t i o n s to d a i l y r a t e s were made using o c c a s i o n a l 24 h r sequences of short i n c u b a t i o n s . The d i s t r i b u t i o n of p l a n t biomass, as c h l o r o p h y l l , i s r a t h e r s u r p r i s i n g l y uniform i n the main body of the Chesapeake Bay (Figure 2 ) . In A p r i l and June higher biomass was observed i n the v i c i n i t y of the Potomac R i v e r discharge, and i n June and



Figure 1.

Location of the Chesapeake Bay and station positions


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August higher biomass was observed below Baltimore. But i n general, there i s l i t t l e temporal or spatial heterogenity: > 50% of the chlorophyll values were between 10 and 20 /ig*liter , and 90% were between 5 and 25 ug«liter*" . The cause of the increased biomass below Baltimore may be the result of greater nutrient a v a i l a b i l i t y , but there i s not evidence for this i n either the nitrogen or phosphorus data throughout most of the year. There i s sufficient nitrogen i n this region to support a greater plant biomass than i s observed and, as demonstrated by Taft, Taylor, and McCarthy (1975) (40), there i s no apparent relationship between plant biomass and soluble reactive phosphorus. The same conclusions with respect to nitrogen and phosphorus apply to the area immediately below the Potomac River i n August when compared to the stations further north. There are numerous possible explanations for the occasional association of higher plant biomass with regions downstream from the discharges of Baltimore and D i s t r i c t of Columbia, and some of the more obvious include: greater a v a i l a b i l i t y of minor nutrients; unidentified stimulating material associated with the discharge of the metropolitan wastes; less herbivorous grazing potential which may or may not be related to the discharges; and reduced thickness of the near surface mixed layer. We have no data to permit evaluation of hypotheses concerning minor nutrients and other stimulating materials. It would be exceedingly d i f f i c u l t to detect the small reduction in herbivorous grazing potential which could within a few days result i n plant biomass increases comparable to those observed. The pycnocline was quite shallow below Baltimore i n August, and we are further evaluating the relationship of both phytoplankton biomass and phytoplankton productivity on a volume basis to thickness of both the upper mixed layer and the euphotic zone. McCarthy, Taylor and Loftus (1974) (41) demonstrated that throughout the Chesapeake Bay and over an annual cycle,approximately 90% of both the biomass and the productivity of the phytoplankton community i s found in the unicellular forms which pass 35 pm mesh. There are well documented occurences of algal blooms of moderate proportions i n the Chesapeake Bay. During our study a major v i s i b l e bloom was never identified on our regular stations. Loftus, Subba Rao, and Seliger (1972) (42) followed the development and dissipation of a bloom which appeared i n the open Bay near the Severn River following an intensive rain storm. Their data demonstrate that for the portion of the bloom i n the Bay proper there was never adequate nitrogenous nutrient available to permit more than a fractional doubling of plant biomass (1-13%). Therefore,with nutrient limited growth, the bloom was either doomed to dissipate through physical forces or sufficient nutrient had to be delivered through rapid recycling processes within the bloom. Oxytoxum sp. was the dominant phytoplankter and the dominant herbivore, a r o t i f e r (Euchlanis sp.), reached densities of 10^ individuals- l i t e r . They - 1

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demonstrated that this density of Euchlanis sp. could almost totally^consume maximum observed bloom phytoplankton densities (4 χ 10 /ig c h l o r o p h y l l ' l i t e r " ) in 12 hr. A r o t i f e r of similar size (Brachionus sp.) ingests 3 times i t s mass per day (43), and i t can be concluded that the high metabolic rate per unit biomass for such small zooplankters would result i n considerable return of ingested nitrogen and phosphorus to the water i n dissolved forms. Time permits the consideration of only a single cruise i n any detail, and we have chosen to discuss PROCON 10 of June 1973. It represents an extreme in chlorophyll variations along the major axis of the Bay, and i t demonstrates the preferential algal u t i l i z a t i o n of the more reduced and rapidly recycled forms of nitrogen i n the presence of high concentrations of NO^. Time course measurements with multiple samplings during an incubation of a few hours repeatedly demonstrated that Ρ uptake occurred i n i t i a l l y at a rapid rate which could not, from compari sons with photosynthesis as measured by fixation, be equated with net uptake associated with growth (40). The subsequent phase of reduced uptake can however, with a few exceptions be shown to approximate phytoplankton synthesis of new cellular material. Table I and Figure 3 demonstrate this phenomenon.


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TABLE I.

ATOMIC RATIO OF CARBON TO PHOSPHORUS UPTAKE (40) Initial Subsequent Station Phase Phase 834G 818P 744 724R 7070 0707V

0.16 0.001 0.001 0.002 0.015 0.45

44 28 60 88 410 6

From the laboratory culture data of Parsons, Stephens, and Strickland (1961) (44), one can calculate atomic ratios for carbon to phosphorus composition of marine phytoplankton which were grown in a phosphorus sufficient medium. For 8 phytoplankters the values ranged from 28 to 102 and had a median of 81. A phosphorus deficient natural phytoplankton population existing in a medium which contains a low constant level of orthophosphate (and no alternate source of phosphorus) would be expected to have less intracellular phosphorus than phytoplankton i n a phosph orus sufficient medium (45). The well documented phytoplankton "luxury uptake" of orthophosphate (46) complicates efforts to make meaningful extrapolation from short-term rate measurements. That i s to say, that even after taking into consideration the above mentioned dual phase uptake, rate measurements for the second phase may s t i l l suggest greater net uptake per unit time than can be reasonably anticipated from independent estimates of



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Chlorophyll g /zg-literH

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Figure 2. Chlorophyll concentration in the Chesa peake Bay from December 1972-December 1973

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Figure 3. Uptake of P orthophosphate into the particulate fraction with time, after Ref. 40.

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phytoplankton growth. If the algae are phosphorus stressed as a result of lengthy exposure to medium with lower than usual levels of orthophosphate, and sufficient phosphorus suddenly be comes available, an extremely rapid uptake would be expected i n the second phase. Such a stress would result i n a low carbon to phosphorus uptake value such as that observed at station 0707V (Table 1). Conversely, ample intracellular stores of phosphorus could support high rates of algal carbon fixation concurrent with very slow phosphorus uptake resulting i n a high carbon to phosph orus uptake ratio such as that observed at station 7070 . Stations 7070 and 0707Vwere anomalous in other respects as well. The high specific activity of 32p preparations permits one to make estimates of uptake without measurably increasing the orthophosphate i n the medium (radioactive isotope additions con tributed 1-1.5 /ig atom N - l i t e r " , 95% of the n i t r o genous ration o£ the phytoplankton w i l l be met with NH4 and urea. This NH4 value i s similar to that found sufficient to suppress ΝΟβ" u t i l i z a t i o n i n the culture studies mentioned above. If one were to determine the effect of ambient NH4 con centration on NO3 uptake with a unialgal culture, a hyperbolic relationship might be predicted. The scatter i n Figure 9 i s probably the result of combining data from 120 natural phytoplank ton assemblages sampled in waters with s a l i n i t i e s ranging from 2 to 30 0 / 0 0 and temperatures ranging from 4 to 29°C. This plot does not necessarily suggest that NH4 concentra tions < 1-1.5 /ig atom Ν·liter"" w i l l induce NO3" uptake, but rather i t demonstrates that i n excess of this concentration, l i t t l e i f any NO3 i s u t i l i z e d when available, and hence the phytoplankton are probably not growth rate limited by_nitrogenous nutrient. The kinetics of phytoplankton uptake of NO3" and NH4 for unialgal cultures are well documented (57), and i f one views the uptake of NO3" and NH4 , somewhat paradoxically, as analagous to Holling's "vertebrate" feeding response (58), then the value of 1.5 ug atom Ν-liter" may represent a concentration of total nitrogenous nutrient above which there w i l l be no nitrogen l i m i tation of primary productivity. +

+

1

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+

1

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gj§ Insufficient jj Jjj Nitrogenous Nutrient t§| Β for α Single Doubling | | Μ of the Particulate If |j§ Nitrogen

Sttfg IBIS

Figure 8. Portion of the Chesapeake Bay and portion of the 13-month study in which available nitrogenous nutrient was insufficient to permit a single doubling of the particulate nitrogen

904 Ν

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Figure 9. Effect of ambient ammonium concentration on the algal selection of nitrate. Data are for the entire Chesapeake Bay from December 1972 through December 1973.

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At the present time our greatest uncertainty i n the nitrogen and phosphorus budgets of the main body of the Chesapeake Bay rests with the question of local nutrient supply. We have con cluded that in general neither v e r t i c a l transport from the sedi ments nor r a i n f a l l (unpublished data) can be considered as major sources. A large body of data provide both direct and indirect evidence which suggests that herbiverous zooplankters are capable of consuming the phytoplankton productivity. The smaller the zooplankton, the greater the fractional return of ingested n i t r o gen and phosphorus to the water via excretion. We are i n the process of evaluating the importance of this pathway to local nutrient replenishment i n the Bay. Bacteria in the water column, whether free-living or assoc iated with larger particles, may in part be responsible for both supply and loss of the plant nutrients discussed above. Unfor tunately, i t i s extremely d i f f i c u l t to quantitate with even f a i r accuracy the role of the bacteria, and the significance of bacter i a l activity in these processes has not been evaluated for any large area of the open Bay. Indirect evidence suggests, however, that the role of bacteria is minor when compared to those of both phytoplankton and zooplankton. The general impression which we would l i k e you to obtain from this presentation is that plankton nutrition must be viewed as a dynamic process. One can be totally deceived i n an effort to understand plankton nutrition solely from measurements of bio mass and nutrient concentrations, and, therefore, unless one partitions the nutrient pool and actually measures rates of u t i l ization, l i t t l e useful information can be obtained from f i e l d programs designed to investigate various links in the nutrientphytoplankton-zooplankton-nutrient cycle.

Acknowledgment This work was supported by grant GA-33445 and grant DES75-02846 from the National Science Foundation and contract AT (11-1) 3279 with the U.S. Atomic Energy Commission. LITERATURE CITED 1. 2. 3. 4. 5.

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