Interactions of light, temperature and inorganic nitrogen in controlling planktonic nitrogen utilisation in the Tagus estuary

The effect of light, temperature and ammonium on inorganic nitrogen uptake by phytoplankton was investigated from June 1994 through December 1995 at three sites in the Tagus estuary (Portugal), during high tide of neap tides. Ammonium concentrations higher than 10 M reduced nitrate uptake down to 24% but never prevented it. Below this threshold concentration, nitrate uptake was neither inhibited nor changed. Uptake of both nitrate and ammonium as a function of light intensity exhibited a saturation response. Uptake reduction occurred in the near bottom phytoplankton populations, particularly for nitrate. The ammonium uptake system was less limited by light than the nitrate uptake system, indicating the importance of ammonium as a nitrogen source for the phytoplankton which is likely to experience high changes in light in the well-mixed water column of this estuarine environment. Ammonium uptake was exponentially related to temperature in the upper estuary whereas in the mid and lower estuary this relationship was linear. The effect of temperature on nitrate uptake was linear but far less marked than for ammonium uptake.     

Restricted use of nitrate and a strong preference for ammonium reflects the nitrogen ecophysiology of a light‐limited red alga

Ammonium and nitrate are important sources of inorganic nitrogen for coastal primary producers. Nitrate has higher energy requirement for uptake and assimilation, compared with ammonium, suggesting that it might be a more efficient nitrogen source for slow‐growing, light‐limited macroalgae. To address this hypothesis, we examined the nitrogen ecophysiology of Anotrichium crinitum, a rhodophyte macroalgae common in low‐light habitats in New Zealand. We measured seasonal changes in seawater nitrate and ammonium concentrations and the concentration of nitrate and ammonium stored internally by A. crinitum. We determined the maximal uptake rates of nitrate and ammonium seasonally and grew A. crinitum in the laboratory with these nitrogen sources under two ecologically relevant saturating light levels. Our results show that field‐harvested A. crinitum has a high affinity for ammonium and although it will grow when supplied exclusively with nitrate, internal nitrate pools are low and it is unable to take up nitrate without several days of acclimation to saturating light. Our data predict that A. crinitum would be able to sustain growth with ammonium as the sole source of nitrogen, a strategy that would help it survive under low‐light conditions that prevail in the field.     

Phytoplankton uptake of ammonium, nitrate and urea in the Neuse River Estuary, NC, USA

Uptake rates of ammonium, nitrate and urea were measured during the spring, summer and autumn (2001) in the eutrophic, nitrogen (N) limited Neuse River Estuary (NRE), North Carolina, USA. Ammonium was the dominant form of N taken up during the study, contributing approximately half of the total measured N uptake throughout the estuary. Nitrate uptake declined significantly with distance downstream comprising 33% of the total uptake in the upper estuary but only 11 and 16% in the middle and lower estuary, respectively. Urea uptake contributed least to the total pool in the upper estuary (16%), but increased in importance in the middle and lower estuary, comprising 45 and 37% of the total N taken up, respectively. The importance of regenerated N for fuelling phytoplankton productivity in the mesohaline sections of the NRE is demonstrated. The contribution of urea to the regenerated N pool suggests that internal regeneration of dissolved organic N may support a large proportion of the phytoplankton primary production and biomass accumulation in the middle and lower NRE. These results suggest that N-budgets based on dissolved inorganic N uptake rates alone will seriously under estimate phytoplankton N uptake.     

Response of coastal phytoplankton to ammonium and nitrate pulses: seasonal variations of nitrogen uptake and regeneration

Seasonal variation in uptake and regeneration of ammonium and nitrate in a coastal lagoon was studied using ¹⁵N incorporation in particulate matter and by measuring changes in particulate nitrogen. Uptake and regeneration rates were two orders of magnitude lower in winter than in summer. Summer uptake values were 2.8 and 2.2 mol N.l^–1.d^–1 for ammonium and nitrate, respectively. Regeneration rates were 2.9 and 2.1 mol N.l^–1.d^–1 for ammonium and nitrate respectively. Regeneration/uptake ratios were often below one, indicating that water column processes were not sufficient to satisfy the phytoplankton nitrogen demand. This implies a role of other sources of nitrogen, such as macrofauna (oysters and epibionts) and sediment. Phytoplankton was well adapted to the seasonal variations in resources, with mixotrophic dinoflagellates dominant in winter, and fast growing diatoms in summer. In winter and spring, ammonium was clearly preferred to nitrate as a nitrogen source, but nitrate was an important nitrogen source in summer because of high nitrification rates. Despite low nutrient levels, the high rates of nitrogen regeneration in summer as well as the simultaneous uptake of nitrate and ammonium allow high phytoplankton growth rates which in turn enable high oyster production.     

PULSES OF PHOSPHATE PROMOTE DOMINANCE OF THE TOXIC CYANOPHYTE CYLINDROSPERMOPSIS RACIBORSKII IN A SUBTROPICAL WATER RESERVOIR1

The role of dissolved inorganic phosphorus (DIP) in promoting dominance of the toxic nitrogen (N)‐fixing cyanobacterium Cylindrospermopsis raciborskii (Wo?osz.) Seenayya et Subba Raju was examined in a subtropical water reservoir, Lake Samsonvale (=North Pine reservoir). A novel in situ bioassay approach, using dialysis tubing rather than bottles or bags, was used to determine the change in C. raciborskii dominance with daily additions of DIP. A statistically significant increase in dominance of C. raciborskii was observed when DIP was added at two concentrations (0.32 μM and 16 μM) in a daily pulse over a 4 d period in three separate experiments in the summer of 2006/2007. There was an increase in both C. raciborskii cell concentrations and biovolume in two DIP treatments, but not in the ammoniacal N + DIP treatment. In addition, overall phytoplankton cell concentrations increased with DIP addition, indicating that Lake Samsonvale was DIP limited at the time of experiments. Given the bioassay response, it is likely that dominance of C. raciborskii could increase in Lake Samsonvale with periodic injections of DIP such as inflow events.     

Uptake and regeneration of inorganic nitrogen in coastal waters influenced by the Mississippi River spatial and seasonal variations

The Mississippi and Atchafalaya Rivers introduce large amountsof nutrients to surface waters of the northern Gulf of Mexico.This paper reports the most complete data to date on inorganicnitrogen uptake and regeneration in a broad range of coastalenvironments influenced by the river water, along with informationon nutrient concentrations and including pico-, nano-, and microplanktonspecies composition. Nitrate in surface waters is greatly reducednear the river plume, at salinities between 5 and 25 PSU, wherethe largest variance in uptake rates was observed, and was coincidentwith peaks in surface chlorophyll. Despite the depletion ofnitrate, nitrogen limitation was a rare event during the study,because of relatively high ammonium concentrations (>1 µmolNH₄⁺ I^–1 and regeneration rates. Two contrasting situationscharacterize the seasonal nitrogen dynamics in surface shelfwaters. High nitrate input during the spring caused a largebloom in which the cells were well adapted to use nitrate.Thedominant phytoplankton species were chain forming diatoms, alsoreported in sediment-trap studies in the area. Ammonium regenerationonly accounted for a small fraction of the nitrogen requirementsduring the bloom. In contrast, the low flow of river water duringsummer resulted in low nitrate concentrations in surface water.In this case phytoplankton productivity was highly reduced andmay depend greatly on ‘in sita’ ammonium regeneration.     

Nitrogen transformations in a small mountain stream

Ammonium, urea, and nitrate were added to Bear Brook, a second and third order stream in the Hubbard Brook Experimental Forest, New Hampshire. Removal of ammonium and urea during downstream transport coincided with the release of nitrate. Nitrate removal did not occur when it was added alone or with dissolved organic carbon. Laboratory experiments showed that coarse particulate organic material (detritus) and bryophytes taken from the streambed were active in the removal of ammonium from enriched stream water, and in the release of nitrate upon the addition of ammonium.The patterns of removal and release observed in these experiments suggest a biologically mediated, oxidation process. Budgetary calculations show that the in-stream transformation of nitrogen inputs during summer and autumn could represent 12 to 25 percent of the nitrogen exported as nitrate during winter and spring from heterotrophic streams like Bear Brook. This type of internal cycling affects the timing and form of nitrogen export from small streams draining forested watersheds in the northeastern United States.     

Contribution of several nitrogen sources to growth of Alexandrium catenella during blooms in Thau lagoon, southern France

A monitoring program with a weekly sampling frequency over a 15-month period indicates that urea concentrations above a certain threshold level may trigger the blooms of Alexandrium catenella in Thau lagoon. However, urea concentrations were also sometimes related to ammonium and dissolved organic nitrogen concentrations, indicating that the role of urea may not be a direct one. An original approach is used to assess the relative contribution of several nitrogen sources (nitrate, nitrite, ammonium, urea) to growth of A. catenella by comparing nitrogen uptake rates to nitrogen-based growth rates estimated from dilution experiments during four blooms over a 4-year period (2001–2004) in Thau lagoon. Nitrate and nitrite contributed 0.1–14% and 0.1–5% respectively of growth requirements. Ammonium and urea were the main N sources fueling growth of A. catenella (30–100% and 2–59%, respectively). Indirect estimates indicated that an unidentified N source could also contribute significantly to growth at specific times. Concerning ammonium and urea uptake kinetics, half-saturation constants varied between 0.2 and 20 μgat N L⁻¹ for ammonium and between 0.1 and 44 μgat N L⁻¹ over the 4-year period, indicating that A. catenella can have a competitive advantage over other members of the phytoplankton even under low concentrations of ammonium and urea. However, the observed large changes in ammonium and urea uptake kinetics on a short time scale (days) during blooms preclude more precise estimates of those contributions to growth and require further investigation.     

Uptake of Nitrate by Mutants of Arabidopsis thaliana, Disturbed in Uptake or Reduction of Nitrate

A study of nitrate and chlorate uptake by Arabidopsis thaliana was made with a wildtype and two mutant types, both mutants having been selected by resistance to high chlorate concentrations. All plants were grown on a nutrient solution with nitrate and/or ammonium as the nitrogen source. Uptake was determined from depletion in the ambient solution. Nitrate and chlorate were able to induce their own uptake mechanisms. Plants grown on ammonium nitrate showed a higher subsequent uptake rate of nitrate and chlorate than plants grown on ammonium alone. Mutant B25, which has no nitrate reductase activity, showed higher rates of nitrate and chlorate uptake than the wildtype, when both types were grown on ammonium nitrate. Therefore, the uptake of nitrate is not dependent on the presence of nitrate reductase. Nitrate has a stimulating effect on nitrate and chlorate uptake, whereas some product of nitrate and ammonium assimilation inhibits uptake of both ions by negative feedback. Mutant B 1, which was supposed to have a low chlorate uptake rate, also has disturbed uptake characteristics for nitrate.     

Uptake of 13C and 15N (ammonium, nitrate and urea) by Microcystis in Lake Kasumigaura

The uptake rate of carbon and nitrogen (ammonium, nitrate andurea) by the Microcystis predominating among phytoplankton wasinvestigated in the summer of 1984 in Takahamaira Bay of LakeKasumigaura. The V_max values of Microcystis for nitrate (0.025–0.046h^–1) and ammonium (0.15–0.17 h^–1) were considerablyhigher than other natural phytoplankton. The ammonium, nitrateand urea uptake by Microcystis was light dependent and was notinhibited with nigh light intensity. The K₁ values were farlower than the I_k values. The carbon uptake was not influencedby nitrogen enrichment. Microcystis accelerated the uptake rateby changing V_max/K _s value when nitrogen versus carbon contentin cells declined. Nitrate was scarcely existent in TakahamairiBay during the summer, when Microcystis usually used ammoniumas the nitrogen source. However, the standing stock of ammoniumin the water was far lower than the daily ammonium uptake rates.Therefore, the ammonium in this water had to be supplied becauseof its rapid turn-over time (–0.7–2.6 h).     

Nutrient biogeochemistry in the water column (N,P, Si) and porewater (N) of sandy sediment of the Scheldt estuary (SW-Netherlands)

The Scheldt river drains a densely populated and industrialized area in northern France, western Belgium and the south-west Netherlands. Mineralization of the high organic load carried by the river leads to oxygen depletion in the water column and high concentrations of dissolved nitrogen and phosphorus compounds. Upon estuarine mixing, dissolved oxygen concentrations are gradually restored due to reaeration and dilution with sea water. The longitudinal redox gradient present in the Scheldt estuary strongly affects the geochemistry of nutrients. Dissolved nutrients in the water column and dissolved nitrogen species in sediment porewaters were determined for a typical summer and winter situation. Water column concentration-salinity plots showed conservative behaviour of dissolved Si during winter. During summer (and spring) dissolved Si may be completely removed from solution due to uptake by diatoms. The geochemistry of phosphorus was governed by inorganic and biological processes. The behaviour of nitrogen was controlled by denitrification in the anoxic fluvial estuary, followed by nitrification in the upper estuary (prior to oxygen regeneration). In addition, nitrogen was taken up during phytoplankton blooms in the lower estuary. Dissolved inorganic nitrogen species in porewaters from the upper 20 cm of sediments were obtained from a subtidal site in the middle of the lower estuary. Dissolved nutrient concentrations were low in the upper 10–15 cm of the sandy and organic poor (<1% POC) sediments mainly as a result of strong sediment mixing. The porewater profiles of ammonium and nitrate were evaluated quantitatively, using a one-dimensional steady-state diagenetic model. This coupled ammonium-nitrate model showed ammonification of organic matter to be restricted to the upper 4 to 7 cm of the sediments. Total nitrification ranged from 3.7–18.1 mmol m^?2 d^?1, converting all ammonium produced by ammonification. The net balance between nitrification and denitrification depended on the season. Nitrate was released from the sediments during winter but is taken up from the water column during summer. These results are in good agreement with data obtained from the independently calibrated water column model for the Scheldt Estuary (VAN GILSet al., 1993).     

Nitrogen dynamics in lower Narragansett Bay, Rhode Island. I. Uptake by size-fractionated phytoplankton populations

The contribution of nanoplankton (< 10 µm fraction)to winter – spring (1977 – 78) and summer (1978,1979) phytoplankton nitrogen dynamics in lower NarragansettBay was estimated from ammonium, nitrate and urea uptake ratesmeasured by ¹⁵N tracer methods. During the winter – spring,an average of 80% of chlorophyll a and nitrogen uptake was associatedwith phytoplankton retained by a 10 µm screen. In contrast,means of 51 – 58% of the summer chlorophyll a standingcrops and 64 – 70% of nitrogen uptake were associatedwith cells passing a 10 µm screen. Specific uptake ratesof winter – spring nanoplankton populations were consistentlylower than those of the total population. Specific uptake ratesof fractionated and unfractionated summer populations were notsignificantly different. Ammonium uptake averaged between 50and 67% of the total nitrogen uptake for both the total populationand the < 10µm fraction. The total population and the10 µm fraction displayed similar preferences for individualnitrogen species. Though composed of smaller cells, flagellatedominated nanoplankton assemblages may not necessarily takeup nitrogen at faster rates than diatom dominated assemblagesof larger phytoplankters in natural populations. ¹Present address: Australian Institute of Marine Science, P.M.B.No. 3, Townsville M.S.O., Qld. 4810, Australia     

Phytoplankton nitrogen demand and the significance of internal and external nitrogen sources in a large shallow lake (Lake Balaton,Hungary)

Since the middle of 1990s the trend of Lake Balaton towards an increasingly trophic status has been reversed, but N₂-fixing cyanobacteria are occasionally dominant, endangering water quality in summer. The sources of nitrogen and its uptake by growing phytoplankton were therefore studied. Experiments were carried out on samples collected from the middle of the Eastern (Siófok) and Western (Keszthely) basins between February and October 2001. Ammonium, urea and nitrate uptake and ammonium regeneration were measured in the upper 5-cm layer of sediment using the ¹⁵N-technique. Ammonium was determined by an improved microdiffusion assay. N₂ fixation rates were measured by the acetylene-reduction method. Ammonium regeneration rates in the sediment were similar in the two basins. They were relatively low in winter (0.13 and 0.16 μg N cm^?3 day^?1 in the Eastern and Western basin, respectively), increased slowly in the spring (0.38 and 0.45 μg N cm^?3 day^?1) and peaked in late summer (0.82 and 1.29 μg N cm^?3 day^?1, respectively). Ammonium uptake was predominant in spring in the Eastern basin and in summer in the Western basin, coincident with the cyanobacterial bloom. The amount of N₂ fixed was less than one third of the internal load during summer when external N loading was insignificant. Potentially, the phytoplankton N demand could be supported entirely by the internal N load via ammonium regeneration in the water column and sediment. However, the quantity of N from ammonium regeneration in the upper layer of sediment combined with that from the water column would limit the standing phytoplankton crop in spring in both basins and in late summer in the Western basin, especially when the algal biomass increases suddenly.     

Seasonal variation in marine phytoplankton and ice algae at a shallow antarctic coastal site

The phytoplankton population near Davis, Vestfold Hills, Antarctica was monitored throughout 1982. Chlorophyll-a determinations and counts of living cells in both the water column and sea ice demonstrated a marked seasonality in phytoplankton abundane and species composition. From April to October nanoplanktonic organisms contributed most of the chlorophyll-a in both the sea ice and water column. Blooms of diatoms occurred in May, November and December in the bottom of the sea-ice and in January and February in the water column. Phaeocystis pouchetii was dominant during December in the water column. Large numbers of dead diatoms were found in winter. The concentrations of nitrate, dissolved inorganic phosphate and dissolved silicate increased throughout the year until December, when the concentrations of nitrate and silicate fell sharply, followed a month later by a reduction in phosphate concentration. The diversity of phytoplankton was greatest during the summer months.     

FACTORS RELATED TO THE DOMINANCE OF CYLINDROSPERMOPSIS RACIBORSKII (CYANOBACTERIA) IN A SHALLOW POND IN NORTHERN TAIWAN1

Seasonal succession of phytoplankton was investigated in a shallow pond in northern Taiwan from August 2009 to January 2011, with particular reference to the dynamics of Cylindrospermopsis raciborskii (Woloszynska) Seenayya et Subba Raju. The abundances of the representative species in the pond increased during high‐temperature seasons, whereas only C. raciborskii became dominant in the pond from summer to autumn in both 2009 and 2010. The high shade tolerance of C. raciborskii was likely one of the factors that enabled the cyanobacterium to grow during the summer when the transparency was low. Moreover, the heterocyst production of C. raciborskii was enhanced during summer when the concentration of dissolved inorganic nitrogen was low, implying that nitrogen fixation also played an important role in supporting the growth of C. raciborskii. Autumnal rainfall was a critical factor in the collapse of C. raciborskii blooms. C. raciborskii formed blooms with relatively small trichomes, whereas larger trichomes dominated during winter. The dependence of the trade‐off between growth rate and trichome size on temperature was assumed to be an adaptation strategy of C. raciborskii.     

Seasonal differences in the effects of oscillatory and uni‐directional flow on the growth and nitrate‐uptake rates of juvenile Laminaria digitata (Phaeophyceae)

The influence of oscillatory versus unidirectional flow on the growth and nitrate‐uptake rates of juvenile kelp, Laminaria digitata, was determined seasonally in experimental treatments that simulated as closely as possible natural environmental conditions. In winter, regardless of flow condition (oscillatory and unidirectional) or water velocity, no influence of water motion was observed on the growth rate of L. digitata. In summer, when ambient nitrate concentrations were low, increased water motion enhanced macroalgal growth, which is assumed to be related to an increase in the rate of supply of nutrients to the blade surface. Nitrate‐uptake rates were significantly influenced by water motion and season. Lowest nitrate‐uptake rates were observed for velocities <5 cm · s^?1 and nitrate‐uptake rates increased by 20%–50% under oscillatory motion compared to unidirectional flow at the same average speed. These data further suggested that the diffusion boundary layer played a significant role in influencing nitrate‐uptake rates. However, while increased nitrate‐uptake in oscillatory flow was clear, this was not reflected in growth rates and further work is required to understand the disconnection of nitrate‐uptake and growth by L. digitata in oscillatory flow. The data obtained support those from related field‐based studies, which suggest that in summer, when insufficient nitrogen is available in the water to saturate metabolic demand, the growth rate of kelps will be influenced by water motion restricting mass transfer of nitrogen.     

Contribution of several nitrogen sources to growth of Alexandrium catenella during blooms in Thau lagoon,southern France

A monitoring program with a weekly sampling frequency over a 15-month period indicates that urea concentrations above a certain threshold level may trigger the blooms of Alexandrium catenella in Thau lagoon. However, urea concentrations were also sometimes related to ammonium and dissolved organic nitrogen concentrations, indicating that the role of urea may not be a direct one. An original approach is used to assess the relative contribution of several nitrogen sources (nitrate, nitrite, ammonium, urea) to growth of A. catenella by comparing nitrogen uptake rates to nitrogen-based growth rates estimated from dilution experiments during four blooms over a 4-year period (2001–2004) in Thau lagoon. Nitrate and nitrite contributed 0.1–14% and 0.1–5% respectively of growth requirements. Ammonium and urea were the main N sources fueling growth of A. catenella (30–100% and 2–59%, respectively). Indirect estimates indicated that an unidentified N source could also contribute significantly to growth at specific times. Concerning ammonium and urea uptake kinetics, half-saturation constants varied between 0.2 and 20 μgat N L⁻¹ for ammonium and between 0.1 and 44 μgat N L⁻¹ over the 4-year period, indicating that A. catenella can have a competitive advantage over other members of the phytoplankton even under low concentrations of ammonium and urea. However, the observed large changes in ammonium and urea uptake kinetics on a short time scale (days) during blooms preclude more precise estimates of those contributions to growth and require further investigation.     

Microplanktonic regeneration of ammonium and dissolved organic nitrogen in the upwelling area of the NW of Spain: relationships with dissolved organic carbon production and phytoplankton size-structure

Ammonium regeneration and dissolved organic nitrogen (DON) releasewere studied experimentally in the euphotic zone of shelf andoceanic waters of NW Spain in relation to coastal upwellingdynamics and the size-structure of phytoplankton communities.Incubations of plankton labelled with [¹⁵N]ammonium were madeduring four cruises, two of which also included size-fractionateddeterminations of chlorophyll a and primary production, andexperimental determinations of production rates of dissolvedorganic carbon (DOC) using ¹⁴C. Inorganic nitrogen concentrationswere mainly related to nitrate enrichment by upwelling pulses,while ammonium concentrations were generally low in all situations.Ammonium did not accumulate in the study area, suggesting adaily time scale coupling between regeneration and uptake. Incontrast, DON largely exceeded inorganic nitrogen in all situationsand generally increased from spring to autumn. Ammonium regenerationwas positively correlated with DON release and both rates showedthe largest variation in summer, with minimum values duringactive upwelling and maximum values when upwelling relaxed.Comparison of DON stocks and rates in different shelf areassuggests that DON release near the coast during summer was morepersistent in the water than DON release in off-shelf and oceanicareas. The carbon:nitrogen ratio of DOC and DON release rateswas highly variable, revealing a large excess of DOC comparedwith DON. This excess can be attributed to either an underestimateof total DON release (as only release from ammonium was measured)or to an enhanced production of carbon-rich organic substancesby diatoms in coastal areas. By considering a broad range oftrophic situations, this study reveals a fundamental differencebetween short term release of DOC and DON by plankton. Physiologicalprocesses (such as carbohydrate exudation by diatoms) seem tobe the cause of large DOC excess, whereas trophic processes(such as grazing) are more likely the cause of enhanced DONrelease.     

Do cyanobacteria dominate in eutrophic lakes because they fix atmospheric nitrogen?

1. The sources of nitrogen for phytoplankton were determined for a bloom‐prone lake as a means of assessing the hypothesis that cyanobacteria dominate in eutrophic lakes because of their ability to fix nitrogen when the nitrogen : phosphorous (N : P) supply ratio is low and nitrogen a limiting resource. 2. Nitrogen fixation rates, estimated through acetylene reduction with ¹⁵N calibration, were compared with ¹⁵N‐tracer estimates of ammonium and nitrate uptake monthly during the ice‐free season of 1999. In addition, the natural N stable isotope composition of phytoplankton, nitrate and ammonium were measured biweekly and the contribution of N₂ to the phytoplankton signature estimated with a mixing model. 3. Although cyanobacteria made up 81–98% of phytoplankton biomass during summer and autumn, both assays suggested minimal N acquisition through fixation (<9% for the in‐situ incubations; <2% for stable isotope analysis). Phytoplankton acquired N primarily as ammonium (82–98%), and secondarily as nitrate (15–18% in spring and autumn, but <5% in summer). Heterocyst densities of <3 per 100 fixer cells confirmed low reliance on fixation. 4. The lake showed symptoms of both light and nitrogen limitation. Cyanobacteria may have dominated by monopolizing benthic sources of ammonium, or by forming surface scums that shaded other algae.     

Does N2 fixation meet the nitrogen requirements of heterocystous blue-green algae in shallow eutrophic lakes?

Summary Phytoplankton net carbon uptake and nitrogen fixation were studied in two shallow, eutrophic lakes in South Sweden. Ranges of diurnal net carbon uptake were estimated by subtracting 24-h respiration rates corresponding to 5–20% of P _max, respectively, from daytime carbon uptake values. total nitrogen requirement of the phytoplankton assemblage was determined from the diurnal net carbon uptake, assuming a phytoplankton C:N ratio of 9.5:1. Nitrogen supplied by nitrogen fixation only occasionally corresponded to the demands of the total phytoplankton assemblage. When heterocystous algae made up a substantial proportion (10%) of the total phytoplankton biomass, nitrogen fixation could meet the requirements of heterocystous blue-green algae on c. 50% of the sampling occasions. Nitrogen deficiencies in heterocystous algae were most probably balanced by the simultaneous or sequential assimilation of dissolved inorganic nitrogen. It was concluded that uptake of ammonium or nitrate, regenerated from lake seston and sediment, is the main process by which growth of phytoplankton is maintained during summer in the lake ecosystems studied.