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     

Size structures of microplankton biomass and production in Jiaozhou Bay, China

Seasonal investigations of size-fractionated biomass and productionwere carried out from February 1992 to May 1993 in JiaozhouBay, China. Microplankton assemblages were separated into threefractions: pico- (0.7–2 µm), nano- (2–20 µm)and netplankton (20–200 µm). The biomass was measuredas chlorophyll a (Chi a), paniculate organic carbon (POC) andparticipate organic nitrogen (PON). The production was determinedby ¹⁴C and ¹⁵N tracer techniques. The seasonal patterns in biomass,though variable, were characterized by higher values in springand lower values in autumn and summer (for Chi a only). Theseasonal patterns in production, on the other hand, were moreclear with higher values occurring in summer and spring, andlower values occurring in autumn and winter. Averaged over thewhole study period, the respective proportions of total biomassaccounted for by net-, nano- and picoplankton were 26, 45 and29% for Chi a, 32, 33 and 35% for POC, and 26, 32 and 42% forPON. The contributions to total primary production by net-,nano- and picoplankton were 31, 35 and 34%, respectively. Therespective proportions of total NH₄⁺–N uptake accountedfor by net-, nano- and picoplankton were 28, 33 and 39% in thedaytime, and 10, 29 and 61% at night. The respective contributionsto total NO₃^–-N uptake by net-, nano- and picoplanktonwere 37, 40 and 23% in the daytime, and 13, 23 and 64% at night.Some comprehensive ratios, including C/N biomass ratio, Chla/C ratio, C uptake/Chl a ratio, C:N uptake ratio and the f-ratio,were also calculated size separately, and their biological andecological meanings are discussed.     

Phytoplankton size fractions in a tropical neritic ecosystem near Kingston, Jamaica

Temporal patterns in phytoplankton size fractions were studiedfor >1 year on the shelf south of Kingston, Jamaica. On average,in the East Channel, total chlorophyll was partitioned as 42%netplankton (>20 µm), 30% nanoplankton (2–20µm) and 28% picoplankton (0.2–2 µm); correspondingaverages for size-fractionated primary production were 27, 30and 43%. Netplankton was more variable than the other size fractions,and it predominated at times of high total chlorophyll concentration.When total chlorophyll was low, picoplankton predominated. Regressionmodels using wind and rainfall parameters could explain up to46% of the variation in chlorophyll concentrations. In theseregression models, the variation accounted for was always greatestfor the net- and least for the picoplankton. A suggested mechanismfor meteorological-phytoplankton coupling is that during periodsof easterly winds (90–150°), offshore water (in whichpicoplankton dominates) is advected onto the shelf, either displacingor diluting the nearshore water. At times of weaker westerlywinds (230–290°) there is a persistence of the phytoplanktoncommunity in nutrient-rich shelf waters and net- and nanoplanktonbiomass increases.     

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.     

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.     

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.     

Seasonal uptake and regeneration of inorganic nitrogen and phosphorus in a large oligotrophic lake: size-fractionation and antibiotic treatment

Uptake and regeneration of inorganic N and P in oligotrophicFlathead Lake (Montana) were measured with ¹⁵N and ³²P incorporationand dilution experiments, six times over a seasonal cycle. Theannual mean molar N P uptake ratio at ambient concentrationswas 13 9 (range = 4 8–34.2); uptake of nitrate, ammoniumand phosphate were always below saturation indicating both Nand P deficiency Organisms >280 µm were responsiblefor 0–60% of ammonium and 0–40% of phosphate regeneration,40–100% of the ammonium and 34–98% of phosphateregeneration occurred in the <3 µm fraction The <3µm fraction accounted for 7–70% of the ammoniumand 6–64% of the phosphate uptake. Results from antibiotictreatments indicated that both prokaryotic and eukaryotic ammoniumuptake was important, and that eukaryotes accounted for 53–98%of the ammonium regeneration. During thermal stratification,heterotrophic ammonium and phosphate regeneration by organisms>3 µm supplied much of the inorganic N and P in theepilimnion. Estimated rates of allochthonous and diffusive (i.e‘new’) ammonium, nitrate and phosphate input were<5% of biotic regeneration. These results suggests that (i)both N and P dynamics should be considered when examining nutrientregulation of primary productivity of oligotrophic lakes, (ii)bacteria probably compete with phytoplankton for both ammoniumand phosphate, (iii) biotic regeneration is the main sourceof nutrients to the epilimnion during stratification, and (iv)crustacean zooplankton were relatively unimportant sources ofregenerated ammonium and phosphate.     

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.     

Diurnal and seasonal variation in the contributions of autotrophic pico-, nano- and microplankton to the primary production of an upland lake

An investigation of the diurnal variation in productivity andcontribution to production of populations of autotrophic picoplankton(0.2–2.0 µm), nanoplankton (>2 <20 µm)and microplankton (>20 µm) was carried out at monthlyintervals, from May to October 1989, in Llyn Padarn a mesotrophicupland lake in North Wales. Maximum rates and contributionsto production of the lake by autotrophic picoplankton occurredduring mid-late summer, with the highest average daily contributionfrom picoplankton (64%) recorded in September at 4 m depth.Diurnal variation in contributions from picoplankton was pronounced,with greatest input, recorded at the end of the day, duringthe period of picoplankton dominance in mid-late summer. Maximumcontribution from picoplantkon (86% of total, 9.2 mg C m^–3h^–1) was recorded in September. Nanoplankton primary productionwas of greatest significance in June and July, although levelswere lower than for picoplankton in subsequent months. Contributionsvia nanoplankton increased with depth in the lake at this time,reaching a maximum of 78% of the total at the end of the dayat 9 m depth in early July. At this time, diurnal variationin contributions via nanoplankton was considerable, with maximumphotosynthesis generally at the end of the photoperiod at depthsof 4 and 9 m. Microplankton made the greatest impact on primaryproduction during the mixed water conditions of spring and autumn,and at these times did variation in production was less thanthose of both pico and nanoplankton during summer thermal stratification.Photosynthetic capacity was lower for picoplankton than fornanoplankton and microplankton; the highest values were 5, 33and 51 mg C (mg chl a)^–1) h^–1) for pico-, nano-and microplankton, respectively. The photosynthetic efficiencyof all three size categories of phytoplankton increased withdepth. Maximum values were similar for all phytoplankton groups,between 75 and 131 mg C (mg chl a)^–1) E^–1) m² butmean levels of photosynthetic efficiency for the 6 months werelower for picoplankton than for nano- or microplankton. Ratesof carbon fixation per cell for picoplankton spanned three ordersof magnitude, varied considerably diurnally and reached maximumvalues of 484 fg C(cell)^–1) h^–1) in the afternoonin near-surface waters in the early stages of exponential populationgrowth in July. During the population maximum of picoplanktonin August and September, maximum daily values of carbon fixationper cell, assimilation number and photosynthetic efficiencywere all recorded at the end of the day. The seasonal and diurnalpatterns of production of the three size categories of planktonicalgae in Llyn Padarn were distinct. During spring, microplankton(mainly diatoms) were the dominant primary producers. As thermalstratification developed, nanoplankton were the major contributorsto phytoplanktonic production, particularly in the deeper regionsof the euphotic zone. Picoplankton made the greatest contributionto production in August and September, exhibiting maximum inputtowards the end of the light cycle. Diatoms became the majorphotosynthetic plankton in the mixed water conditions prevalentin Uyn Padarn in October.     

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.     

Autotrophic picoplankton in southern Lake Baikal: abundance, growth and grazing mortality during summer

Autotrophic picoplankton were highly abundant during the thermalstratification period in late July in the pelagic area (waterdepth 500–1300 m) of southern Lake Baikal; maximum numberswere 2 x 10⁶ cells ml^–1 in the euphotic zone ({small tilde}15m). Unicellular cyanobacteria generally dominated the picoplanktoncommunity, although unidentified picoplankton that fluorescedred under blue excitation were also abundant (maximum numbers4 x 10⁵ cells ml^–1) and contributed up to {small tilde}40%of the total autotrophic picoplankton on occasions. Carbon andnitrogen biomasses of autotrophic picoplankton estimated byconversion from biovolumes were 14–84 µg C l^–1and 3.6–21 µg N l^–1. These were comparableto or exceeded the biomass of heterotrophic bacteria. Autotropicpicoplankton and bacteria accounted for as much as 33% of paniculateorganic carbon and 81% of nitrogen in the euphotic zone. Measurementsof the photosynthetic uptake of [^l4C]bicarbonate and the growthof picoplankton in diluted or size-fractionated waters revealedthat 80% of total primary production was due to picoplankton,and that much of this production was consumed by grazers inthe <20 µ.m cell-size category. These results suggestthat picoplankton-protozoan trophic coupling is important inthe pelagic food web and biogeochemical cycling of Lake Baikalduring summer.     

Microzooplankton and macrozooplankton glutamate dehydrogenase as indices of the relative contribution of these fractions to ammonium regeneration in the Gulf of Maine

Ammonium regeneration by micro- (35–153 µm) andmacrozooplankton (> 153 µm) was determined in the Gulfof Maine by measuring the activity of the excretory enzyme glutamatedehydrogenase (GDH) in various size fractions. GDH maxima weregenerally observed to correspond to the depth of the chlorophyllmaximum as previously reported in the Gulf of Mexico and inthe vicinity of the Nantucket Shoals. GDH activity of the microzooplanktonwas considerably lower than the macrazooplankton, suggestingthe microzooplankton made only a minor contribution (1–11%) to the total ammonium regenerated. These results were confirmedby biomass estimates made from counts of individual species.Ammonium excretion by both zooplankton fractions was estimatedto supply 5–31 % of the nitrogen requirements for primaryproduction, with an estimated 59–63% supplied by the verticaltransport of nitrate (new nitrogen) into the euphoric zone.     

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.     

Winter-spring variability of size-fractioned autotrophic biomass in Concepcion Bay, Chile

The temporal variability of size-fractioned autotrophic biomassat three depth levels (1, 8 and 25 m) was studied during thewinter-spring transition at two oceanographic stations in ConcepciónBay. Size spectra were obtained on eight occasions by two differentmethods: (i) determining the biomass of seven autotrophic sizefractions by in vivo fluorescence; and (ii) measuring the filamentlength of chain-forming diatoms through direct microscopy. Aclear vertical gradient of biomass was found in all profiles,with maximum values in the surface layer (1 and 8 m levels).Values of chlorophyll were on average 6.2 (range 1.08–25.67)times higher at 1 m than at 25 m, and 7.4 (range 1.15–26.83)times more at 8 m than at 25 m. On a temporal basis, total biomassincreased from low average values in winter (2.5 mg chl-a m^–3)to high values in late spring (11.6 mg chl-a m^–3). Duringthe whole sampling period (June 8-November 19), the nano- andnet-plankton (1.8–40 µm and 40–335 µmsize fractions respectively) were more abundant near the surface(1 and 8 m depth) than close to the bottom (25 m depth); however,the picoplankton fraction (<1.8 (µm) showed an inverserelationship, with a slight trend to increase near the bottomtoward spring. The highest absolute biomass was concentratedin the net-plankton fraction during the whole period and therelative importance of the picoplankton decreased from winter(6.50 and 15.5% for shallow and bottom levels) to spring (1.5and 10.3% for shallow and bottom levels). This relative effectis caused by the higher absolute values of biomass observedin the net-plankton fraction toward spring. These changing patternsshould have an impact in the size-composition and abundanceof higher trophic levels, mainly through grazing, in particularby modifying food availability to microfJagellates, ciliatesand filter-feeding zooplankton.     

Biomass estimation of components of the marine nanoplankton and picoplankton by the Utermohl settling technique

Estimation of the biomass of some components of the marine nano-and picoplankton is made for several locations using the Utermohlsettling technique. The importance of these small cells in thephytoplankton biomass is confirmed. The proportion of cellsof 2–12 µm diameter in the total phytoplankton biomassis usually lower than the proportion of primary productivityattributed to a similar size fraction in the literature. Dataare presented on geographical and vertical distributions ofsome <2 µm and 2–12 µm organisms. At theoceanic stations and at offshore sites the latter make up ahigher percentage of the total biomass than in the coastal areasbut their numerical abundance is lower. Depth distributionsare variable but oceanic profiles show a decrease in abundancewith depth. The Utermöhl technique is evaluated as a continuingmethod for biomass estimation.     

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.     

Seasonal photosynthetic activity of autotrophic picoplankton in Lake Kinneret, Israel

Autotrophic picoplankton populations in Lake Kinneret are composedof picocyanobacteria and picoeukaryotes. Overall, the ratesof photosynthetic carbon fixed by autotrophic picoplankton duringthis study were low (0.01–1.5 mg Cm^–3 h^–1).The highest chlorophyll photosynthetic activity of the <3µm cell-size fraction was found in spring, when picoeukaryotespredominated and in addition small nanoplankton passed throughthe filters. The maximum cell-specific photosynthetic rate ofcarbon fixation by picocyanobacteria and picoeukaryotes was2.5 and 63 fg C cell^–1 h^–1, respectively. The highestspecific carbon fixation rate of autotrophic picoplankton was11 µg C µg^–1 Chl h^–1 The proportionalcontribution of autotrophic picoplankton to total photosynthesisusually increased with depth. Picocyanobacteria collected fromthe dark, anaerobic hypolimnion were viable and capable of activephotosynthesis when incubated at water depths within the euphoticzone. Maximum rates of photosynthesis (P_max) for picocyanobacteriaranged from 5.4 to 31.4 fg C cell^–1 h^–1 with thehighest values in hypolimnetic samples exposed to irradiance.Photosynthetic efficiency (     

Implications of zooplankton stoichiometry on distribution of N and P among planktonic size fractions

Pooled samples from the upper 20 m at five stations in the Oslofjord(Norway) were size fractionated and analysed for particulatedry weight, carbon, nitrogen and phosphorus. The nano fraction(0.7–20 µm) dominated in biomass throughout thesampling period. The C:N ratios of the fractions did not differmuch from each other. The C:P ratios of the nano- and microfraction(20–200 µm) were considerably higher than the ratioof the mesofraction (200–2000 µm) throughout thesampling period. High C:P ratios and low phosphate concentrationsabove the pycnocline suggest that the system was P-limited.The stoichiometry of mesozooplankton was more constant thanthe stoichiometry of the other fractions, and the zooplanktonconstituted consistently a higher percentage of the phosphoruspool than of the carbon pool. This suggests that the mesozooplanktoncan act as a sink of nutrients due to its invariable stoichiometry.     

The importance of phytoflagellate, heterotrophic flagellate and ciliate grazing on bacteria and picophytoplankton sized prey in a coastal marine environment

The uptake of bacteria and picoplankton sized fluorescentlylabelled beads was measured off the west coast of the SouthIsland of New Zealand in winter. Phytoflagellates and heterotrophicflagellates showed similar grazing rates on 0.49 µm beads,with mean clearance rates of 1.1 and 1.8 nl ind.^–1 h^–1,respectively. Clearance rates for 1.09 µm beads were 0.9nl ind.^–1 h^–1 for heterotrophic flagellates and0.5 nl ind.^–1 h^–1 for phytoflagellates. Non-loricateciliates had clearance rates of 1.5 µl ind.^–1 h^–1for the picoplankton sized particles. The heterotrophic flagellatesshowed no significant difference between clearance rates of0.49 and 1.09 µm particles. Phytoflagellates, however,showed an apparent preference for the smaller particles. Themeasurement of significant grazing by phytoflagellate populationsin the marine environment is important and indicates that weneed to reassess our concepts of food web structure.     

Diurnal variations in the contributions of autotrophic picoalgae and heterotrophic bacteria to planktonic production in an upland lake

An investigation of the diurnal variation in contributions toproduction of the autotrophic and heterotrophic components ofthe picoplankton community was carried out during August andSeptember in Llyn Padarn, a mesotrophic upland lake in NorthWales. The picoplankton was separated using 1 µm pore-sizedfilters into the smaller cell sized fraction (<1 µm),the majority of the bacteria and the larger cell sized picoalgae(<3>1 µm), together with some bacteria. The distributionof bacterial heterotrophic activity between these two fractionsof picoplankton was assessed by uptake of [¹⁴C]glucose and differentialfiltration. Thus, the absolute autotrophic production by picoalgaeand the heterotrophic contribution by bacteria to picoplanktoncommunity production via uptake of extracellular organic carbon(EOC) were determined. Rates of picoplankton community productionexhibited diurnal variation with maximum rates of 19.1 mg Cm^–3 h^–1 recorded at 18.00 h at 4 m depth in September.The bacterial contribution to picoplankton community productionincreased markedly between 15.00 and 18.00 h. Rates of absoluteautotrophic production varied less over 24 h than rates of accumulationin bacteria of ¹⁴C-labelled EOC released from the entire phytoplanktoncommunity. Bacteria contributed up to 86–98% of the neworganic carbon within the picoplankton community at the endof the day. The maximum rate of absolute autotrophic productionin the picoplankton was 1.6 mg C m^–3 h^–1 at 18.00h at 1 m in August, and the maximum rate of bacterial accumulationof new organic carbon was 18.5 mg C m^–3 h^–1 at 18.00h in September at 4 m depth. The diurnal pattern of picoplanktoncommunity production involved increasing rates during the daywith a maximum at 18.00 h. Autotrophic processes were dominantin the first 3–6 h of the light cycle and heterotrophicuptake of ¹⁴C-labelled EOC was the major component from 15.00h onwards. Bacterial uptake of newly released EOC by phytoplanktonwas rapid, comprised the majority of picoplankton production,particularly later in the day, and contributed a maximum of60% of the total pariculate primary production in plankton between15.00 and 18.00 h at 4 m in September with a mean contributionof between 6 and 26% over 24 h in these investigations. Theimportance of autotrophic processes in picoplankton communityproduction has been overestimated in previous investigations.Bacteria play a major role in transferring newly produced EOCrapidly from phytoplankton to the picoplankton community. Atthe end of the day, the majority of newly produced organic carbonis in bacterial cells and this production is significant inthe dynamics of carbon production within the entire planktoniccommunity.