The response of planktonic and microbenthic algal assemblages to nutrient enrichment in shallow coastal waters, southwest Sweden

The response of planktonic and microbenthic algal assemblages to nutrient enrichment in shallow coastal waters, southwest Sweden

J. Exp. Mar. Biol. Ecol., 1985, Vol. 85, pp. 253-268 253 Elsevier JEM 414 THE RESPONSE ASSEMBLAGES OF PLANKTONIC TO NUTRIENT AND MICROBENTHIC ...

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J. Exp. Mar. Biol. Ecol., 1985, Vol. 85, pp. 253-268

253

Elsevier

JEM 414

THE RESPONSE ASSEMBLAGES

OF PLANKTONIC

TO NUTRIENT

AND MICROBENTHIC

ENRICHMENT

WATERS, SOUTHWEST

EDNA GRANBLI

and KRISTINA

ALGAL

IN SHALLOW COASTAL

SWEDEN

SUNDBACK

Department of Marine Botany, University of Lund, Box 124, S-221 00 Lund, Sweden

(Received 23 January 1984; revision received 24 September 1984; accepted 17 October 1984) Abstract: Field and laboratory nutrient (nitrogen and phosphorus) enrichment experiments were performed using natural phytoplankton and microphytobenthic assemblages from the brackish water tiresund, SW. Sweden. The response of algae from a low-nutrient area (Falsterbo Canal) was compared to that of algae from a polluted, nutrient-rich area (Lomma Bay). The biomass (measured as chlorophyll a) of both phytoplankton and microphytobenthos from the Falsterbo Canal increased after the addition of nitrogen. Phytoplankton growth was stimulated by the addition of phosphorus to the nitrogen-rich water of the polluted Lomma Bay. Sediment chlorophyll a showed no significant increase after the addition of nutrients in the Lomma Bay. In containers without sediment, phytoplankton uptake was calculated to account for z 90% of the disappearance of inorganic fixed nitrogen from the water. In the sediment containers the microphytobenthos was estimated to account for x 20% of the nitrogen uptake. The rest was presumably lost mainly through denitrification. When containers with microphytobenthos from Lomma Bay were kept in the dark, phosphorus was released at a rate of up to z 180 PM. m - a. day - ‘. We suggest that by producing oxygen microbenthic algae keep the sediment surface oxygenated thereby decreasing phosphorus transport from the sediment to the overlying water. Key words: nutrient enrichment; microbenthic algae; phytoplankton;

nitrogen; phosphorus

INTRODUCTION

Nitrogen is often the main nutrient-limiting phytoplankton biomass (in the sense of Liebig) in coastal waters (Goldman, 1976). However, little work has been done on nutrient limitation in microphytobenthos, as it has been assumed that these algae have access to an inexhaustible nutrient supply from the interstitial water of the sediment (Van den Hoek et al., 1979). Nutrient enrichment experiments in salt marshes have given varying results; changes in microflora diversity (Van Raalte et al., 1976a), stimulation of growth (Darley et al., 1981), no effect at all (Sullivan, 1981). Nitrogen additions have been shown to stimulate phytoplankton growth in surface waters from the &esund and Kattegat, S.W. Sweden (Gram%, 1978, 1981; Nyman & Graneli, 1983). Although work on the microphytobenthos has been done in the &esund (The Sound) (Gargas, 1970; Sundback & Persson, 1981; Gargas & Gargas, 1982; 0022-0981/85/$03.30 0 1985 Elsevier Science Publishers B.V. (Biomedical Division)

254

EDNAG~N~LIANDKRISTINASUNDB~CK

Sundback, 1983), nu~ent~~~roph~o~~os relations have not been investigated. The coasts of aresund are mainly sandy with shallow water and the microphytobenthos contribute significantly to the total primary production (Sundback, 1983). Our experiments were designed to answer the following questions. (1) Are the phytoplankton and microphytobenthos limited by the same nutrient (nutrient limited at all)? (2) Is the percentage biomass increase the same for the phytoplankton and the microphy-tobenthos when a limiting nutrient is supplied? (3) Do the ph~opl~kton and the ~croph~obenthos Iiving ina eutrophicated area and pop~ations in an area not affected by po~ution react in the same way to extra nut~e~t supply? (4) How much of the added nutrients (phosphorus and nitrogen) becomes incorporated into the microalgae and how much is converted by bacterial and chemical processes, e.g. denitritication?

MATERIALS AND METHODS

Two field experiments (A and B) and two laboratory experiments (C and D) were conducted. In each of the four experiments, two series of containers were used; one series with uteri sea water, and a second with sediment and filtered sea water ( = sediment containers). Water and sediment were collected from the basin at the southern entrance to the Falsterbo Canal (Expts. A, B and C), and from the Lomma Bay (Expt. D), both on the S.W. coast of Sweden (Fig. l), in shallow water (OS-2 m) with sandy bottoms. The Falsterbo Canal is not directly influenced by sewage whereas the Lomma Bay receives a heavy load of municipal sewage from a tertiary treatment plant (phosphorus removal). (See Graneli, 198 1, for a detailed account of the phytoplankton field experiments Expts. A, B.) Briefly, a series of 200-I polythene bags hung from a wooden frame were filled with 150 1 of unfiltered sea water. For the laboratory experiments (Expts. C, D) 4-l spherical flasks filled with 150 pm filtered sea water were used. For the microph~obenthos field expe~ments we used PVC frames pressed into the sediment covered on top and sides with pol~ene (Expt. A, Fig. 2). As the flux of nutrients in the cylinders was affected by fluctuations in the outside water level, the second experiment (Expt. B) was performed in aquaria (0.8 1) outdoors. In the laboratory 8-l aquaria were used in Expt. C and 0.8-l in Expt. D. For the aquaria experiments (B, C, D), we scraped off 1 cm from the sediment surface (0.5 m water depth). The sediment was sieved through a 0.5-mm mesh size sieve. The bottoms of the aquaria were covered with a 4-cm thick layer of sediment. The aquaria were then filled with Whatman GF/F-filtered sea water. Additions of phosphorus (as K,HPO,) and nitrogen (as NaNO,) were added to the ph~opl~kton and microph~obenthos alone (P and N containers) or in combination (PN).

N AND P EFFECT ON PHYTOPLANKTON

AND MICROPHYTOBENTHOS

255

For the microphytobenthos assays C and D, two additional containers with added PN were used: one with sieved sediment kept in the dark (Dark), to estimate the nutrient uptake in the dark, and one with unsieved sediment (Unsieved), to obtain a gross estimate of the importance of bioturbation in the flux of nutrients between sediment and water. (For more information on experimental design see Table I.) Expts. C and D were run in a 16 : 8 light : dark cycle. The temperature in Expt. C was 20 k 1 “C, and 13 + 1 “C in Expt. D. In all four experiments involving microphytobenthos the top 5 mm of the sediment was sampled with a Plexiglas corer (20 mm ID, Expt. A; Gargas, 1970) or a cut-off syringe (9 mm ID, Expts. B, C, D). Quadruplicate sediment samples were taken for all experiments for analyses of chlorophyll and primary production. For all experiments 300-500 ml water samples were taken for nutrient analyses. When 0.8-l aquaria were used (Expts. B, D), the sampled water was replaced together with new nutrient additions.

DENMARK\

Q

\SWEDEN

a Baltic Fig. 1. besund,

showing experimental and sampling sites (a),

256

EDNA GRANl?LI AND KRISTINA SUNDBACK

PVC -4Ocm

frame 1:

l+Rubber

8.

band

:+Polythene sheet : (0.1 mm thick)

Fig. 2. PVC frame used for investigating the microphytobenthos

in situ (Expt. A).

ANALYSES {ALL FOUR EXPERIME~S)

Nutrient uptake rates were obtained by measuring changes in phosphorus and nitrogen concentrations in the water. In addition, the nutrient concentrations of the interstitial water were measured at the beginning and end of Expt. C. The inorganic nutrients (PO,, NH,, NO,, N03) were analysed immediately after sampling following the methods in Carlberg (1972). The response of the microaigae to the addition of nutrients was obtained by measuring changes in chlorophyll a content and primary production rates in the water and in the sediment. For phytoplankton chlorophyll a measurements, the water was filtered through ~a~~ GFjF filters and the chIorop~yi1 ff content was dete~in~ according

N AND P EFFECT ON PHYTOPLANKTON

AND MICROPHYTOBENTHOS

251

to Jeffrey & Humphrey (1975). Phytoplankton primary production was estimated as 14C uptake. (For further details see GranCli, 1981.) The chlorophyll u of the microphytobenthos was extracted from the sediment samples with 15 ml 90 y0 acetone for 1 h in darkness at 8 ‘C. The chlorophyll flasks were shaken every 10 min. Chlorophyll a was calculated according to Lorenzen (1967) modified for phytobenthos by Wetzel8z Westlake (1969). Primary production of the sediment was measured by 14Cuptake according to Gargas (1970). (For further details see Sundback, 1983.)

RESULTS

For the phytoplankton from the Falsterbo Canal nitrogen addition stimulated chlorophyll a production compared to the control values (Figs. 3, 4,5). In the two field experiments the microphytobenthos did not show any clear response to any nutrient enrichment (Figs. 3,4), although a slight chlorophyll a stimulation by nitrogen addition was noted in the second experiment (Fig. 4). However, in the

2

3

4

5

6

Sediment

Days Fig. 3. Response of phytoplankton and microphytobenthos (sediment) expressed as chlorophyll a, to different nutrient additions in the field Expt. A (Falsterbo Canal): C, control; additions: P, phosphorus, N, nitrogen.

258

EDNA GRANBLI AND KRISTINA SUNDB;iCK

Falsterbo laboratory experiment (Fig. 5), nitrogen addition doubled the chlorophyll a amount in relation to the control. Phosphorus enrichment had no effect on the phytoplankton or on the microphytobenthos in the Falsterbo experiments (Figs. 3,4,5). In the Lomma Bay experiments, nitrogen had no effect on chlorophyll a production while phosphorus increased the chlorophyll a production for the phytoplankton, but not for the microphytobenthos (Fig. 6). Phosphorus and nitrogen added together (PN) had the strongest effect (compared to N and P alone) on chlorophyll a increase, primary production and assimilation numbers for both phytoplankton and microphytobenthos from the Falsterbo Canal (Table II). The microphytobenthos primary production responded more strongly than chlorophyll a to phosphorus + nitrogen addition to Falsterbo water. For Lomma water, however, no marked effect was obtained (Table II). In Lomma Bay, initial chlorophyll a values in the sediment were about three times higher than in the Falsterbo sediment (see e.g. Fig. 7). Nutrient uptake was higher at the beginning of the experiment for the Falsterbo phytoplankton and microphytobenthos (Fig. 5). An opposite pattern was found in the

Phytoplankton

+ C AP

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‘1234567

Sediment

1234567 Days Fig. 4. Response of phytoplankton and microphytobenthos (sediment) expressed as chlorophyll a, to different nutrient additions in the field Expt. B (Falsterbo Canal): additions as in Fig. 3.

N AND P EFFECT ON PHYTOPLA~TON

AND MIC~OPHYTOBENTHOS

259

Lomma experiment, i.e. lower nutrient uptake at the beginning of the experiment increasing towards the end. This difference is seen clearly when comparing sediment containers from Falsterbo and Lomma with PN additions and under light exposure (Fig. 7). The nutrient disappearance rate was higher in the sediment than in the phytoplankton containers for the Lomma experiment (Fig. 6). Maximum P and N uptake by phytoplankton were 1.4 and 46 PM - day- r, respectively, in the containers with phosphorus and nitrogen added together (Expt. C, Falsterbo). Also in the PN containers, maximum values of P and N uptake were obtained for the microph~obenthos, viz: 2.2 PM P-day- ’ (or 0.2 mm01 P*m-2.day-‘, = Falsterbo Canal) and 80,uM N.day-’ (or 5.1 mm01 N~m-2.day-’ = Lomma Bay; Fig. 7).

Phytoplankton

Sediment

C

2 i

c?

P

E

1 4

8 11 1619 23

Days

Fig. 5. Response of phytoplanlcton and microphytobenthos (sediment) expressed as chlorophyll a to different nutrient additions and the rate of disappearance of inorganic nitrogen (NH, + NO, + NO,) and phosphorus (PO,) in the different containers in the laboratory Expt. C (Falsterbo Canal): additions as in Fig. 3.

23 days

(19 Oct.-IO Nov. 1981) 10 days (28 Sept.-l Nov.,

1979)

C

(laboratory, Falsterbo) D (laboratory,

Lomma)

2.1

0.8 179

3.5

0.8

1.3

Sed.

Plankt.

Plankt. Sed.

Sed. Plankt.

Sed.

Plankt.

1.8*

1.8

11.1 17.7

11.3 1.13

0.77

0.77

Day I

PO,

0

0

0 0

0.35 0.9

0.07

0.07

Daily

* From the third day onwards 1.8 PM of P and 304 PM of N were added to the sediment containers after every sampling.

(1-7 1979)July,

0.25

0.26

6 days (23-28 Aug.,

1978) 7 days

PO,

Inorg. N (NH, + NO, + N03)

Initial cont. in the water (PM)

60.7*

60.7

607 607

250 25

28.6

28.6

Day 1

0

0

0

0

28.6 11

1.1

1.1

Daily

Inorg. N (NH, + NO, + NO,)

Nutrient additions (,uM)

and additions of inorganic phosphorus and nitrogen to the water in four experiments.

Duration

;eld Falstkrbo)

Falsterbo)

$eld,

Experiment

Initial concentrations

TABLE I

R

3 g:

2

$

G ;;

3

F

$ p

2

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N AND P EFFECT ON PHYTOPLANKTON

AND MICROPHYTOBENTHOS

261

Sediment

Phytoplankton 180

C 120 100

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Fig. 6. Laboratory Expt. D (Lomma Bay): for explanation see Fig. 5.

Sediment from the unpolluted Falsterbo Canal held phosphorus within the sediment for a much longer period than did the nutrient-rich Lomma Bay sediment. In the former there was a net release of P from the sediment only after 20 days under dark incubation, while in the latter P was released during the first 3 days (Fig. 7). Phosphorus was kept in the Lomma Bay sediment when the containers (sieved or unsieved) were incubated in light (Fig. 7).

262

EDNA GRANfiLI AND KRISTINA SUNDBACK

Lomma

Falsterbo Sieved

( dark

) 15or

Sieved

(

light

light

)

B

) 180 150

a. -9

-z

Days

Fig. 7. Chlorophyll a content in the sediment and the rate of disappearance of inorganic nitrogen (NH4 + NO, + NO,) and phosphorus (PO.,) from the water in the sediment containers with addition of both P and N under varying conditions in Expts. C (Falsterbo Canal) and D (Lomma Bay): additions as in Fig. 3.

DISCUSSION

The results show that both phytoplankton and microphytobenthos are nitrogenlimited in the Falsterbo area. The phytoplankton, however, responded with a much higher increase in chlorophyll (w 80 times the initial value) than the microphytobenthos (z 2 times the initial value) (Table II). The slower increase in microphytobenthic biomass can be explained in two ways: (1) the biomass of microphytobenthos is initially high because of adequate nutrient conditions, and (2) the microphytobenthos receive

Initial C P N PN

Lemma (Expt. D)

Initial C P N PN

Falsterbo (Expt. C)

4.6 113 208 160 200

1.6 2.8 2.3 9.1 127.8

Plankton (mg.me3)

143 188 173 146 170

38.3 47.1 47.1 14.3 75.6

Sediment (mg.m-‘)

Chlorophyll a

6.6 320 645 428 645

4.4 8.9 14.1 33 368

Plankton (mg C.m-3.h-‘)

121 150 120 105 101

50.7 54.4 53.5 98.2 242

Sediment (mg C.m-*.h-‘)

Primary production

1.42 2.84 3.22 2.68 3.3

2.84 4.38 2.85 5.41 10.02

0.84 1.21 I .05 1.12 0.73

1.33 1.46 1.65 1.72 3.21

Sediment Plankton (mg C.mg Chl a-‘.h-‘)

Assimilation number

Maximum values for chlorophyll a content, primary production and assimilation number in the different containers in the laboratory experiments C (Falsterbo Canal) and D (Lomma Bay).

TABLE II

264

EDNA GRANkLI AND KRISTINA SUNDBikK

light only through the sediment so that light becomes the factor that is most icing, not nutrient availability, Diatoms were the main constituents of both floras, but while planktonic diatoms may divide up to 4 times a day, benthic diatoms may only divide 0.3 times a day (Admiraal et al., 1982). Phytoplankton was nitrogen-limited in all experiments using water from Falsterbo, which is in agreement with previous experiments in this area (Gram%, 1978, 1981). Microphytobenthos also may be limited by N, as was the case in Expt. C. Additions of nutrients have been found to increase chIorophyII LIcontent and primary production of edaphic algae in sah marshes (Sullivan & Daiber, 1975; Van RaaIte et al., 1976b; Darley et al., 1981). No direct effect was noticed by Estrada et ai. (1974). On tidal flats even deleterious effects on benthic diatoms have been observed after large additions of ammonium in combination with high light intensity (Admiraal, 1977). Water from Lomma Bay, which contained high concentrations of inorganic nitrogen (mostly as nitrate), stimulated phytoplankton growth markedly, and, when P was added as well, an even higher biomass was obtained. Phosphorus limitation in coastal waters is usually found in places where freshwater inflow is high as in the Trondheim Fjord (Sakshaug et al., 1983), Both&n Bay (Alasaarela, 19791, and in the inner part of the Bay of Marseilles (Berland et al., 1980). Phosphorus limitation in Lomma Bay is caused by discharge of tertiary-treated sewage with a high N/P ratio. The microph~obenthos from Lomma Bay showed only a slight increase during the lo-day experiment (D). Why were these algae not stimulated by the high amounts of nutrients availabie? Sullivan & Daiber (1975) found chlorophyll a values of up to 280 mg - m - ’ in the top 1 cm of sediment after the addition of nutrients to a Delaware salt marsh. Our values reached a maximum of 188 mg’rne2 in the top 0.5 cm of sediment. This may be the maximum value that the benthic microalgae can reach in Lomma Bay. Assimilation numbers were much higher for the phytoplankton than for the microphytobenthos. Low values were also found for the microphytobenthos by Darley et al. (1981). The ~sappe~~ce of nu~ents from the water column in the euphotic zone is usually considered to be the result of ~co~oration into ph~op~~kton cells. At the sediment surface other factors such as bacterial activity may be more important. In shallow waters microphytobenthos may also play an important part in the uptake of nutrients. The processes of denitrification, nitrif?cation and ammonification make it difficult to evaluate the N uptake of algae. The interstitial water is generally considered to be the most important source of nutrients for microbenthic algae (see e.g. Van den Hoek et al., 1979). Phytoplankton on the other hand usually depend on rapid remineralization in the euphotic zone to maintain a high level of production in coastal waters during the summer (Harrison et al., 1977). The uptake rates of P and N in the ph~op~~kton containers reached values of 1.4 PM - day- 1 for P and 46 ,uM - day- I for N. N~ajima et al. (1981), working in an

N AND P EFFECT ON PHYTOPLANKTON

AND MICROPHYTOBENTHOS

265

experimental pond, simulated a plankton bloom by adding P and N and found uptake rates of up to 143 PM - day- ’ for N and 9pM * day- ’ for P. These values were similar to ours, while bearing in mind the fact that their chlorophyll a values were 3 times higher than in our experiments. In Expt. C, x 90% of the initial pool of N disappeared during the 23 days in the phytoplankton containers with N additions. The total fixation of carbon by phytoplankton during this experiment was about 45 g C . m - 3. This production corresponds to an N uptake of 574 PM using the Redfield C : N ratio 6.6 (by atoms) (Fig. 8) which agrees well with the net disappearance of N (559 PM). The net decrease in N from the water column in the sediment containers was higher than in the phytoplankton containers; 99.8% of the initial concentration of N in the water disappeared during Expt. C. If we assume that the uptake of N from the water column by autotrophic organisms in the sediment is equal to the difference between the uptake in light and dark (see Fig. 7), the microphytobenthos would account for x 20% of the total decrease of N in the water. This value does not, however, tell us anything about the possible uptake from the interstitial water. Since we measured the nutrients in the interstitial water in Expt. C, we can try to roughly calculate the uptake of N by the microphytobenthos from the total initial pool of inorganic N (overlying water + interstitial water) during the experiment. The initial pool of inorganic N in the container with P + N additions was 765 PM or 55.6 mmol*m-2 (53.1 + 2.5; Fig. 8). At the end of the experiment ~0.4 mm~l*rn-~ remained i.e. 55.2 mm01 . rnp2 ( = 99%) had been lost. In the dark less N was lost, i.e. 34 mmol. m-’ ( = 61%). If we assume that the difference between the two amounts represents uptake by microbenthic algae in light, then this uptake is equal to 38% of the total net decrease in N or 2 1 mm01 * m - 2. The total microbenthic primary production during Expt. C was z 8.5 g C. me2 (a factor of 0.2 was used to correct for overestimation in the incubation method; see Sundback (1983). If we use the C : N ratio 22 : 1 (by atoms) for marine berrthic plants (Atkinson & Smith, 1983) this production would correspond to 32 mmol. me2 or 58% of the total net loss of N in the P + N container kept in light (55.2 mmol . m- ‘). According to the above calculations w 40-60 % of the inorganic N was assimilated by the microphytobenthos. The rest (40-60x) disappeared through other processes, mean daily loss being l-l.5 mm01 - rnm2. day- ‘. Denitrification may account for much of this loss, but we do not know the rate of this process in our experiments. Kaplan et al. (1979) found denitritication rates of l-5 rng.rnW2.h-’ (~1.7-8.5 mmol. rnm2. day- ‘) in salt marshes, while Nakajima et al. (1981) found a rate of 30 mmol . m _ ’ . day - ’ in a eutrophicated pond. They estimated that only a third of the inorganic N was taken up by algae, the rest being denitrified. The activity of macrofauna did not affect the uptake rate, probably because of the low animal density (see sieved/unsieved, Fig. 7). The N concentration decreased more rapidly in the containers with material from the

266

EDNA GRANBLI AND KRISTINA SUNDBACK

eu~ophicat~ Lomma Bay than in the Falsterbo Canal containers. Since we found no distinct increase in microbenthic growth in the Lomma sediment, we can fairly safely assume that denitrification was proportionately higher in Lomma Bay than in the Falsterbo Canal. Denitrification rate in the sediment has been shown to be proportional to the concentration of nitrate in the overlying water (see e.g. Nedwell, 1982). Anoxic conditions develop fast in the sediment of eutrophicated areas because of the

Phytoplankton Addition 607

Sediment Addition 52.8 (607)

p j

I

__ _______ j g-Jpf+q--------------;J----j _--.--_:T-

f

Bacteria

, _ _ _ _ _ _ . _ _ _ . -,

1 I

I

~__-_J___-------~

i I

,

I

Fig. 8. Budget for inorganic nitrogen (NH, -t NOa + NO,) for the phytoplankton and sediment (sieved) containers with additions of P + N calculated from carbon fixation during Expt. C (23 days): for the phytoplankton container N content is expressed as PM and for the sediment container as mmol. m - a, concentrations expressed as PM are shown in brackets; numerals marked with * show the uptake and loss of N respectively calculated from the difference between uptake in the container kept in light and in the dark.

N AND P EFFECT ON PHYTOPLANKTON

AND MICROPH~OBENTHOS

267

high amounts of organic matter present. This leads to phosphors release which can be counteracted by production of oxygen by microbe&Cc algae, as we have found in the Lomma Bay experiment (Fig. 7).

ACKNOWLEDGEMENTS

We are grateful to Dr. T. von Wachenfeldt for making available working facilities at Uimpinge Research Station (National Swedish Environmental Protection Board) and to Dr. W, Gran&i for valuable criticism of the m~usc~pt. Financial support was received from the Royal Physio~aphi~ Society of Lund. REFERENCES ADMIRAAL,W., 1977. Tolerance of estuarine benthic diatoms to high concentrations of ammonia, nitrite ion, nitrate ion and orthophosphate. Mar. Biol., Vol. 43, pp. 307-3 15. ADMIRAAL,W., H. PELETIER & H. ZOMER, 1982. Observations and experiments on the population dynamics of epiphelic diatoms from an estuarine mudflat. Estuarine Cousrul shelf Sci., Vol. 14, pp. 471-487. ALASAARELA,E., 1979. Phytoplankton and environmental conditions in central and coastal areas of the Bothnian Bay. Ann. Bot. Fenn., Vol. 16, pp. 241-274. ATKINSON,M. J. & S.V. SMITH, 1983. C : N : P ratios of benthic marine plants. LimnoZ. Oceunogr., Vol. 28, pp. 568-574. BERLAND, B.R., D.J. BQNIN & S.Y. MAESTRINI, 1980. Azote ou phosphore? Considerations snr le “paradoxe nut~tionneln de la mer Mtditerranee. Oceunol. Actu., Vol. 3, pp. 135-141. CARLBERG,S. (ed.), 1972. New Baltic manual with methods for sampling and analyses of physical, chemical and biological parameters. ICES Coop. Res. Rept. Ser. A, Vol. 29, 145 pp. DARLEY,W. M., C. L. MONTAGUE,F. G. PLUMLEY,W. W. SAGE & A.T. PSALIDAS,1981. Factors limiting edaphic algal biomass and productivity in a Georgia salt marsh. J. Phycol., Vol. 17, pp. 122-128. ESTRADA,M., I. VALIELA& J. M. TEAL, 1974. Concentration and distribution of chlorophyll in fertilized plots in a Massachusetts salt marsh. J. Exp. Mar. Biol. Ecol., Vol. 14, pp. 47-56. GARGAS, E., 1970. Measurements of primary production, dark fixation and vertical distribution of the microbenthic algae in the &esund. Ophelia, Vol. 8. pp. 231-253. GARGAS, M. & E. GARGAS, 1982. Growth physiological conditions of marine microalgae in the topmost 10 cm of the sediment, Y&ten, Vol. 38, pp. 189-198. GOLDMAN,J.C., 1976. Identification of nitrogen as a growth-limiting nutrient in wastewaters and coastal marine waters through continuous culture algal assays. B’rrter Res., Vol. 10, pp. 97-104. GRANBLI,E., 1978. Algal assay of limiting nutrients for ph~opl~~on production in the &esund. Vutren, Vol. 34, pp. 1 l7-128. GRANI?LI, E., 1981. Bioassay experiments in the Falsterbo Channel - nutrients added daily. Kiefer Meeresforsch., Sonderh. 5, pp. 82-90. HARRISON,W.G., F. AZAM, E.H. RENGER & R.W. EPP~EY, 1977. Some experiments on phosphate assimilation by coastal marine plankton. Mar. Biol., Vol. 40, pp. 9-18. HOEK, VANDEN C., W. ADMIRAAL,F. COLIJN & V.N. DE JONGE, 1979. The role of algae and seagrasses in the ecosystem of the Wadden Sea: a review. In, Flora and vegetation ofthe Wadden Seu, Part 3, edited by W.J. Wolff, A.A. Balkema, Rotterdam, pp. I-206. JEFFREY,S. W. & G. F. HUMPHREY,1975. New spectrophotometric equations for determining chlorophylls a, b, c, and ca in higher plants, algae and natural phytoplankton. Ph_ysiol.Pj7unzen (BBP), Vol. 167, pp. 191-194. KAPLAN,W., I. VALIELA& J.M. TEAL, 1979. Denitrification in a salt marsh ecosystem. Limnol. Oceanogr., Vol. 24, pp. 726-734. LORENZEN,C. J., 1967. Determination of chlorophyll and pheopi~ents: spectrophotometric equations. Limnol. Oceanogr., Vol. 12, pp. 343-346.

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AND KRISTINA

SUNDB;lCK

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