Nitrogen uptake by size-fractionated plankton in permanently well-mixed temperate coastal waters

Nitrogen uptake by net- (15–200 µm), nano- (1–15µm) and picoplankton (<1 µm) was measured overseasonal cycles at two stations with different patterns of biologicaland chemical cycles in the Morlaix Bay (western English Channel).Though assimilable dissolved N nutrient pool at both stationswas nitrate-dominated, characteristics of biomass and N uptakeby netplankton differed from conventional patterns in two respects.In the first, biomass (26–30%) and N uptake (36–43%)were less important than those of nanoplankton. In the second,the netplankton did not show any marked preference for nitrateover ammonium (nitrate to ammonium uptake ratios of 0.98 and1.08). In contrast, nanoplankton had a preference for ammoniumover nitrate (ammonium to nitrate uptake ratios of 2 and 1.2).N uptake by picoplankton was only 8% of total N uptake at bothstations and was supported mainly by regenerated N (66% ammoniumand 17% urea), with nitrate uptake detectable in only one instanceand nitrite uptake in none. Substrate-dependent uptake of ammoniumin all fractions and a higher ammonium uptake in the nanoplanktonfraction in summer at both stations when ambient ammonium concentrationswere high indicated that while nitrate may satisfy a part ofN requirements, availability of ammonium and its flux throughnanoplankton determine the magnitude of total N uptake in thesewaters. Most of the N uptake in picoplankton appears to be autotrophic,suggesting that a substantial part of heterotrophic uptake,if any, could be localized in the fractions >1 µm,and mediated by free-living and particle-bound bacteria.     

Nitrogenous nutrient uptake by phytoplankton and ammonium regeneration by microbial assemblage in Lake Biwa

In situ rates of nitrate, ammoniwn and urea uptake by the phytoplanktonassemblage, and the regeneration rate of ammonium by the microbialassemblage, in Lake Biwa were measured using the nitrogen 15tracer method from 1985 to 1987. The rate of total nitrogen(sum of ammonium, nitrate and urea) uptake was in the rangeof 62–594 ng N^–1 r^–1 h^–1. The percentagecontribution of ammonium uptake was 41–92%, that of urea4–58% and that of nitrate <1–28% of total uptake.The annual mean new production which was supported by nitrateuptake was 18% of the total production in 1986. The phytoplanktonassemblage in Lake Biwa preferentially utilized regeneratednitrogen, such as ammonium and urea, whose concentration wasmuch lower than that of nitrate throughout the observation penodwithout in summer. The in situ nitrogen uptake rate was almostsufficient to meet the nitrogen requirement of the phytoplanktonassemblage, except in midsummer when the nitrate concentrationwas below the detection limit of 0.3 µg N r^–1. Inthe trophogemc layer, the rate of ammonium regeneration was66–272 ng N 1^–1 h^–1 Although the ambient ammoniumconcentration in the trophogenic layer was maintained at aroundthe half-saturation constant for ammonium uptake kinetics, theammomum uptake rates were always highly correlated with ammoniumregeneration rates. From the size fractionation experimentsand estimates from the literature, it was suggested that themicrobial assemblage <1 µm may have been the most importantagent responsible for the ammonium regeneration processes inthe trophogenic layer.     

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     

Uptake of dissolved nitrogen by phytoplankton in a eutrophic subtropical lake

Uptake rates for ananonium, nitrate, urea and dinitrogen byphytoplankton in Lake Okeechobee ranged from 0.58 to 1.52 µmol1^–1 h^–1 among four representative stations duringa short-term study period. Ammonium accounted for 53% of theuptake rates, followed by nitrate (19%), urea (16%) and dinitrogen(12%). Half-saturation constants for nitrogen (N) uptake rangedfrom 8.70 µmol 1^–1 for ammonium, 2.07 iimol 1^–1for urea and 2.21 µmol 1^–1 for nitrate at Southstation. This study reveals spatially varying N uptake rates,particularly N fixation, within a large eutrophic lake.     

Carbon Fixation, Ammonium Uptake and Regeneration in an Equatorial Lake: Biological Versus Physical Control

Ammonium uptake and regeneration were measured in the euphoticzone of Petit Saut Lake, French Guyana, to examine nitrogencycling in this recently flooded equatorial forest environment.Am-monium regeneration rates were extremely high (mostly inthe range 1–6 µmol N l^–1 h^–1), and aredue to the very high grazing rates of the microzooplankton,which consumed between 56 and 95% of the phytoplankton productionin any given incubation. These regeneration rates were aboutan order of magnitude higher than the net ammonium uptake rates.This imbalance is probably due to dissolved organic nitrogenrelease during grazing. At the bottom of the euphotic zone (4–5m), photosynthetic bacteria are responsible for ammonium uptake.Diffusion-driven ammonium fluxes are an order of magnitude lowerthan biologically driven fluxes. Therefore, ammonium fluxesare dominated by biology rather than by physics in this lake.     

Planktonic nitrogen transformations during a declining cyanobacteria bloom in the Baltic Sea

The uptake of ¹⁵N-labelled nitrogen nutrients (ammonium, urea,nitrate) was studied during the decline of a bloom of nitrogen-fixingcyanobacteria in the Baltic Sea. This was done by sampling anorth-south transect of stations, representing different stagesof the bloom. Comparison with nitrogen fixation data showedthat this process was of minor importance, and that the nitrogenuptake was dominated by regenerated nitrogen, mainly ammonium.From time series incubations for studying nutrient uptake, itappears that the regeneration of ammonium was substantial, butthat the production of urea or nitrate was slow. The integrateddaily uptake was calculated for the 0–15 m interval atfour stations and values ranged between 6 and 21 mmol N m^–2day^–1, of which the regenerated nutrients, ammonium andurea, constituted 71–93%. Nitrate was of minor importanceand the highest nitrate uptake rates were found close to thethermocline (at 15 m) and in the southern part of the Baltic.Comparison with carbon fixation data reported from simultaneousmeasurements at two stations gave C/N uptake ratios of 4.9 and2.1 for integrated daily uptake. Contrary to earlier findings,the concentration of DON increased with increasing salinity(from 15 to 17 µmol l^–1). This was correlated withthe declination of the bloom and is suggested to be a resultof a gradual release of less easily utilized DON from the degradationof cyanobacteria. The C/N ratio of DOM was high, 21–23.     

Summer nutrient dynamics of the Middle Atlantic Bight: nitrogen uptake and regeneration

Nitrate and ammonium uptake and ammonium regeneration rates(by zooplankton, microplankton and benthos) were measured onthe Atlantic continental shelf (Middle Atlantic Bight) duringsummer, 1980. Euphotic zone profiles of NO₃^– and NH₄⁺uptake rates were similar in magnitude and vertical structureover a large geographical area. Microplankton NH₄⁺ regenerationrates, although measured less frequently, also showed a relativelyconsistent vertical structure; rates were positively correlatedwith uptake rates. Nitrate assimilation (‘new’ production)was used to estimate vertical eddy diffusivity and paniculatesinking rates. Eddy diffusion estimates ranged from <0.1to >2.0 cm² s^–1 and were positively related to arealprimary production. Estimated particulate sinking rates averaged5 mg at Nm^–2d^–1 and compared favorably with sedimentationrates measured from free-floating and moored sediment traps.Benthic nitrogen regeneration rates represented <10% of thispaniculate nitrogen flux. Within the mixed layer, NH₄⁺ assimilation(‘regenerated’ production) represented 50–80%of the total (NO₃^– + NH₄⁺ ) nitrogen productivity and33% for the euphotic zone. Of this, 30% was attributed to zooplankton,63% to microplankton (<100 µm) and 7% to benthos. Onthe average, 74% of the microplankton NH₄⁺ regeneration wasassociated with organisms passing 1 µm filters.     

Nitrogen uptake by heterotrophic bacteria and phytoplankton in Arctic surface waters

We estimated rates of heterotrophic bacterial and phytoplanktonuptake of nitrate, ammonium, and urea using ¹⁵N-labelled nitrogenand specific metabolic inhibitors of prokaryote and eukaryotenitrogen metabolism in the surface waters of the North Water(northern Baffin Bay) during autumn that were characterizedby the absence of cyanobacteria (comprising prochlorophytes).The percentage of nitrate + ammonium uptake by heterotrophicbacteria ranged between 44 and 78% of the measured total uptakeand was the highest when the phytoplankton biomass was relativelylow (<2 µg Chlorophyll a L^–1). Phytoplanktonaccounted for a larger fraction (e.g., 58–95%) of ureauptake than heterotrophic bacteria. When our results are combinedwith those from previous studies carried out in diverse temperateand polar areas, it appears that heterotrophic bacteria accountfor 25% (14–40%; median and interquartile range) of thetotal nitrate uptake in surface waters with chlorophyll biomass<2 µg L^–1. Estimates of new production computedfrom phytoplankton carbon uptake and f-ratios may be stronglyoverestimated in regions where nitrate uptake by heterotrophicbacteria is high and the biomass of phytoplankton is low.     

Contribution of heterotrophic plankton to nitrogen regeneration in the upwelling ecosystem of A Coruna (NW Spain)

The contribution of heterotrophic plankton to nitrogen (N) regenerationin the water column, and its significance for the requirementsof phytoplankton, were studied at the seasonal scale in thecoastal upwelling ecosystem of A Coruña (Galicia, NWSpain). During 1995–1997, monthly measurements were takenof hydrographic conditions, dissolved nutrients, and abundanceand biomass of microplanktonic heterotrophs (bacteria, flagellatesand ciliates), phytoplankton and mesozooplankton (>200 µm).Additionally, series of experiments were conducted to quantifyN fluxes, including primary production (¹⁴C method), phytoplanktonuptake of nitrate, ammonium and urea (¹⁵N-labelling techniques),microheterotrophic regeneration of ammonium, mesozooplanktongrazing (chlorophyll gut-content method) and excretion of ammoniumby mesozooplankton. Two N budgets were built for the averagesituations of high (>100 mg C m^-2 h^-1) and low (<100 mgC m^-2 h^-1) primary production. The results revealed that phytoplanktonrelied strongly on regenerated ammonium all year round (33 and43% of total N uptake in high and low production situations,respectively). This demand for ammonium was closely matchedby regeneration rates of microplankton (0.14–0.25 mmolN m^-2 h^-1), whereas zooplankton contributed on average <10%to N regeneration. Likewise, zooplankton grazing had littledirect control on phytoplanktonic biomass. The results obtainedindicate that in the A Coruña upwelling system, N biomassof heterotrophic plankton is generally higher than phytoplanktonN biomass. The high rates of N regeneration measured also suggestthat a large proportion of the organic matter produced afteran upwelling pulse is recycled in the water column through themicrobial food web.     

Microbial ammonium cycling in the Mississippi River plume during the drought spring of 2000

Microbial potential uptake and regeneration rates of ammonium(NH₄⁺) were studied along a salinity gradient (salinities 0.2–34.4)in the Mississippi River plume during an extreme drought inspring 2000. Chlorophyll concentrations up to 30 µg L^–1were highest in the low- and mid-salinity regions (salinities8.5–28.2) and comparable to records of other years butextended over smaller areas than during periods of normal riverflow. Bacterial biomass (5.1–28.3 µg C L^–1)was at the low end of the range observed in normal flow years,decreased with distance from the river mouth and did not peakwith chlorophyll. Heterotrophic nanoflagellate abundance (1.4–4.0µg C L^–1) did not reflect phytoplankton and bacterialspatial distribution but peaked at 9.2 µg C L^–1at salinity 8.5. Microbial NH₄⁺ regeneration rates were estimatedby ¹⁵NH₄⁺ isotope dilution experiments for the whole microbialcommunity, under light and dark conditions, and for the <2µm bacterium-dominated size fraction. Microbial NH₄⁺ regenerationrates (0.018–0.124 µmol N L^–1 h^–1) werelow relative to previous reports and peaked at salinity 28.Total NH₄⁺ regeneration rates were higher than those in the<2 µm size fraction at only four stations, suggestingthat bacterial mineralization was a significant component ofNH₄⁺ recycling in some parts of the river plume. Higher NH₄⁺regeneration in whole-water samples versus <2 µm fractionsprovided evidence for microbial grazing in regions where chlorophylland regeneration rates peaked and at two full-salinity stations.     

Seasonal nitrogen assimilation and carbon fixation in a fjordic sea loch

Carbon (C) fixation and nitrogen (N) assimilation rates havebeen estimated from ¹⁴C and ¹⁵N techniques for a 12 month periodin a Scottish sea loch. The maximum rate of nitrogen assimilated(29.92 mmol N m^–2 day^–1) was in April at the mostseaward station; similar high rates were experienced duringMay at the other stations. Carbon fixation rates were maximal(488–4047 mg C m^–2day^–1) at the time of highphytoplankton biomass (maximum 8.3 mg m^–3 chlorophylla) during May, whilst nitrate concentrations remained >0.7µ.mol l^–1. C:N assimilation ratios suggest nitrogenlimitation only during the peak of the spring bloom, althoughat times nitrogen (nitrate and ammonium) concentration fellto 0.2 µmol l^–1 in the following months. The verticalstability of the water column, influenced by tidal and riverineflushing, varied along the axis of the loch, resulting in markeddifferences between sampling stations. Although ammonium waspreferentially assimilated by phytoplankton, >50% of productionwas supported by nitrate uptake and only during the summer monthswas the assimilation of ammonium quantitatively important.     

Seasonal patterns of urea regeneration by size-fractionated microheterotrophs in well-mixed temperate coastal waters

Urea regeneration by size-fractionated plankton was measuredover an annual cycle at a coastal station in the permanentlywell-mixed waters of the western English Channel. Rates of urearegeneration in the <200 µm fraction varied from 0.6to 20.6 nmol N L^–1 h^–1. Regeneration rates werelowest in winter and highest in summer. The ratio of the ratesof regeneration to uptake of urea was close to 1 on all time(seasonal and nycthemeral), and space (vertical) scales indicatingthat regeneration by microheterotrophs supplied the totalityof urea used by phytoplankton. On an annual basis, urea regeneratedby the microheterotrophs (0.98 mol N m^–2 year^–1)was equivalent to 33% of the total regenerated N (urea + ammonium).The major part of urea regeneration was due to the nanoplankton(51%) and microplankton fractions (36%). Regeneration of ureain the picoplankton was detectable only from April to Octoberand represented, on an average, 25% of the total urea regeneratedduring this period. Urea regeneration in micro- and nanoplanktonfractions was mainly associated with ciliates and in the picoplanctonfraction with bacteria.     

Regeneration and uptake of ammonium by plankton in an Amazon floodplain lake

Planktonic regeneration and uptake of ammonium was measuredin Lake Calado on the floodplain of the Amazon River (Brazil).In the epilimnion of the lake, regeneration and uptake averaged0.86 ± 0.15 and 1.9 ± 0.4 µmol NH₄1^–1h^–1 respectively. Uptake exceeded regeneration under theammonium-enriched conditions of our experimental incubations,but uptake and regeneration are in near-balance at in situ ammoniumconcentrations of <1 µM. In experiments that simulatedlake overturn, samples from the meta- and hypolimnion had higherrates of regeneration (1.4 ± 0.6) and lower rates ofuptake (0.89 ± 0.53) than epilimnetic samples. Duringinflow of water from the Amazon River and a local stream, ammoniumregeneration rates were similar to those measured in the epilimnion(0.84 ± 0.22), but the uptake rates were lower (0.49± 0.13). Although regeneration always exceeded uptakein the overturn and inflow experiments, ammonium did not accumulate.Rapid isotope dilution of the ammonium pool occurred in theincubations, and the effect of not accounting for isotope dilutioncould result in underestimating uptake rates by a factor of2–5 in the epilimnion of this tropical lake.     

Nitrate and ammonium uptake by plankton in an Amazon River floodplain lake

Uptake of ammonium and nitrate by plankton was measured in tropicalLake Calado, Brazil. Nitrate uptake was strongly influencedby light and was light saturated at {small tilde}340 µEm^–2 s^–1. In contrast, uptake of ammonium was lessinfluenced by light, and saturated at {small tilde}250 µEm^–2 s^–1. Uptake rates of both forms of nitrogenwere inhibited by up to 80% at light intensities higher thanthose required for saturation. Concentrations of ammonium andnitrate also had a strong influence on uptake rates. Half-saturationconstants (0.3–5 µM) were usually greater than ambientconcentrations (0.1–0.6 µM), indicating that uptakerates at ambient concentrations were less than one-half of thesaturated rates. Ammonium is the more important type of inorganicnitrogen for plankton of Lake Calado because nitrate concentrationsremain low to undetectable except during periodic inputs ofnitrate-rich water from the Amazon River. Using the observeddependence of uptake on concentration and light, maximum uptakerates per unit chlorophyll were computed to be in reasonableagreement with rates derived from P^B_m values for carbon uptake. ¹ Present address: Florida Department of Natural Resources,Marine Research Laboratory, St Petersburg, FL 33701, USA     

Inorganic nitrogen uptake and regeneration in perennially icecovered Lakes Fryxell and Vanda, Antarctica

The isotope ¹⁵N was used to examine nitrogen dynamics in LakesFryxell and Vanda, two permanently ice-covered Antarctic lakes.Half-saturation constants for NH₄⁺. uptake in the shallow watersof both lakes were <10 µg N l^–1; uptake kineticexperiments on populations forming the deep-chlorophyll layersof these lakes showed zero-order kinetics and could not be fittedwith the Michaelis-Menten equation. Elevated uptake within thefirst few minutes following pulses of NH₄⁺. and NO₃^– occurredin both lakes. NH₄⁺ regeneration, determined from isotope dilutionexperiments, exceeded uptake at 4.6 m in Lake Fryxell, was lessthan uptake at 9 m in Lake Fryxell and was equal to uptake at10 m in Lake Vanda under the experimental conditions. NO₃^–uptake was suppressed by NH₄⁺ levels as low as 2 µg NH₄⁺-N l^–1 in Lake Fryxell; the suppression was strongestin the near-surface populations. Substrate-saturated C:N uptakeratios (g:g) in Lake Fryxell decreased from 8.4 near the surfaceto 1.8 at the bottom of the trophogenic zone. Overall, the nitrogendynamics in these lakes are similar to other lakes and the openocean in that biological productivity during the austral summeris supported by regenerated nutrients.     

Depression of Nitrate and Ammonium Transport in Barley Plants with Diminished Sulphate Status. Evidence of Co-regulation of Nitrogen and Sulphate Intake

When barley plants were grown in a solution with nitrate asthe sole N-source but deprived of sulphate (–Splants)for 1 to 5 d, the capacity for sulphate transport by the rootsincreased very markedly; subsequent measurement of influx using³⁵S-labelled showed increases of > 10-fold compared to plants continuously supplied with sulphate (+S plants).There were only small effects on plant growth over a 5 d periodand yet the influx of , labelled with the short-lived tracer ¹³N, was diminished by approximately 30%.By contrast, the influx of phosphate was little affected bysulphate-deprivation. When a sulphate supply was restored to– S plants, the sulphate influx was quickly repressedover the subsequent 24 h and the nitrate influx was restoredto >90% of the value in +S plants. When plants were grown in a solution with a mixed nitrate andammonium supply and deprived of sulphate for 1 d or 5 d thedepression of nitrate influx was more strongly marked (up to55% depression). The influx of ammonium was also depressed after5 d of sulphate-deprivation, but not at 1 d, nor where the concentrationof ammonium in the uptake solution was lowered to 20 mmol m^–3or less. Additional measurements with ¹⁵N-labelled nitrate and ammoniumover longer periods were used to determine net uptake. Net uptakeof nitrate was depressed to a similar extent to efflux, butnet ammonium uptake was depressed only in unbuffered uptakesolution where the pH decreased to pH 4.9 during the uptakeperiod. The ¹⁵N-tracer experiments showed that the translocationof label to the shoot, from both nitrate and ammonium, was depressedto a greater extent than net uptake in –S plants. Thedepression of nitrate influx, caused by 5 d of sulphate deprivation,could be relieved almost completely by providing plants with1.0 mol m^–3 L-methionine during the day prior to influxmeasurement. This treatment substantially decreased sulphateand potassium (⁸⁶Rb-labelled) influx in both +S and –Splants, but greatly increased total S-status of the plants.This methionine treatment had no effect on ammonium influx ornet uptake in – S plants but increased influx significantlyin +S ones. When plants were grown with sulphate but deprived of nitratefor 4 d there was a marked depression of the sulphate influx(by 48–65%) but a smaller effect on phosphate influx (21–37%of +N). The results are discussed in relation to the effects of sulphate-deprivationon growth rate and the root: shoot weight ratio. It is concludedthat the effects on influx and net uptake of nitrogen are moresevere than could be accounted for by these factors. The decreasedtranslocation of either nitrate, or the products of nitrateand ammonium assimilation from the roots, is suggested as areason for the depression of influx. The restoration of nitrateinflux and net uptake by methionine suggests that, for thision at least, a shortage of S-amino acids within the plant maylead to the accumulation of inhibitory concentrations of non-Samino acids in the transport pool. Key words: ¹³N, sulphate, nitrate, ammonium, ion-uptake, barley     

The size distribution of phytoplankton and participate organic matter in the Equatorial Atlantic Ocean: importance of ultraseston and consequences

The size fractionation of paniculate matter (<200, <35,<3 and <1 µm) has been measured in the EquatorialAtlantic Ocean at different stations. Chlorophyll a, phaeophytin,particulate carbon, nitrogen and phosphorus have been analysed.Primary production by ¹⁴CO₂ uptake was also measured with prescreeningtechnique. It appears from this study, that the pariculate matter has avery small size: 40–60% of the chlorophyll passed through1 µm Nucleopore filter, and 75–90% of the paniculatecarbon and nitrogen passed through 3 µm Nucleopore filterin offshore waters. From the atomic ratio C/N, C/P and C/chla, and primary productionvalues, the <3 µm fraction would be mainly constitutedby inactive photosynthetic organisms or partides of detritus.The 3–35 µm fraction, in contrast, would be principallyactive phytoplankton.     

Ammonium excretion and glutamate dehydrogenase activity of zooplankton in Great South Bay, New York

In Great South Bay, nanoplankton, (<20 sµm) accountedfor the largest fraction (56%) of zooplankton glutamate dehydrogenase(GDH) activity over a one year period. Microzooplankton (20–200µm) and macrozooplankton (>200 µm) accountedfor 20% and 24%, respectively. Total zooplankton ammonium regenerationin Great South Bay could account for 74% of the ammonium requirementby phytoplankton in winter, but in summer when phytoplanktondemand was greater, and zooplankton population was low, it suppliedless than 5%. This study suggests that the smallest zooplanktonfraction, less than 20 µm, can be the most important asregards nitrogen regeneration in estuarine environments. MacrozooplanktonGDH activity in Great South Bay ranged from 0.18 mg atoms NH⁺₄-Nm^–3 d^–1 in winter to 3.34 mg atoms NH⁺₄-N m^–3d^–1 in spring. Over an annual period, the averaged GDH/excretionratio was 20.4 3.5 (n = 10), and this ratio agrees well withobservations by other investigators. Observed macrozooplanktonexcretion rates showed a strong correlation with the excretionrates indirectly estimated from GDH activities. The GDH/excretionratio seems to vary depending on the internal physiologicalstates of zooplankton as well as food availability.     

Carbon dynamics in the 'grazing food chain' of a subtropical lake

Studies were conducted over a 13 month period at four pelagicsites in eutrophic Lake Okeechobee, Florida (USA), in orderto quantify carbon (C) uptake rates by size-fractionated phytoplankton,and subsequent transfers of C to zooplankton. This was accomplishedusing laboratory ¹⁴C tracer methods and natural plankton assemblages.The annual biomass of picoplankton (<2 µm), nanoplankton(2–20 µm) and microplankton (<20 µm averaged60, 389 and 100 µg C 1^–1 respectively, while correspondingrates of C uptake averaged 7, 51 and 13 µg C1^–1h^–1. The biomass of microzooplankton (40–200 µm)and macrozooplankton (<200 µm averaged 18 and 60 µgC 1^–1, respectively, while C uptake rates by these herbivoregroups averaged 2 and 3 µg C 1^–1 h^–1. Therewere no strong seasonal patterns in any of the plankton metrics.The ratio of zooplankton to phytoplankton C uptake averaged7% over the course of the study. This low value is typical ofthat observed in eutrophic temperate lakes with small zooplanktonand large inedible phytoplankton, and indicates ineffectiveC transfer in the grazing food chain. On a single occasion,there was a high density (<40 1^–1) of Daphnia lumholrzii,a large-bodied exotic cladoceran. At that time, zooplanktoncommunity C uptake was <20 µg C 1^–1 h^–1and the ratio of zooplankton to phytoplankton C uptake was near30%. If D.lumholrzii proliferates in Lake Okeechobee and theother Florida lakes where it has recently been observed, itmay substantially alter planktonic C dynamics.     

Purification and Characterization of Assimilatory Nitrate Reductase from the Cyanobacterium Plectonema boryanum

Assimilatory nitrate reductase (NR) was solubilized by acetonetreatment from Plectonema boryanum and was purified 7,700-foldby heat treatment, ammonium sulfate fractionation and chromatographyon DEAE-Sephacel and Sephadex G-150. Purified NR had a specificactivity of 85 µmol NO₂^– formed min^–1 mg^–1protein. The enzyme retained both ferredoxin (Fd)- and methylviologen (MV)-linked NR activities throughout the purificationprocedure. Molecular weight was 80,000. The pH optimum was 10.5in the MV-assay and 8.5 when assayed with enzymatically reducedFd as the electron donor. Apparent Km values for nitrate andMV were 700 µM and 2,500µM in the MVassay and 55µM and 75 µM for nitrate and Fd in the Fd-assay.The enzyme was inhibited by thiol reagents and metal-chelatingreagents. (Received October 1, 1982; Accepted March 8, 1983)