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.     

Response of phytoplankton and bacteria to nutrients and zooplankton: a mesocosm experiment

Although both nutrient inputs and zooplankton grazing are importantto phytoplankton and bacteria in lakes, controversy surroundsthe relative importance of grazing pressure for these two groupsof organisms. For phytoplankton, the controversy revolves aroundwhether zooplankton grazers, especially large cladocerans likeDaphnia, can effectively reduce phytoplankton populations regardlessof nutrient conditions. For bacteria, little is known aboutthe balance between possible direct and indirect effects ofboth nutrients and zooplankton grazing. However, there is evidencethat bacteria may affect phytoplankton responses to nutrientsor zooplankton grazing through direct or apparent competition.We performed a mesocosm experiment to evaluate the relativeimportance of the effects of nutrients and zooplankton grazingfor phytoplankton and bacteria, and to determine whether bacteriamediate phytoplankton responses to these factors. The factorialdesign crossed two zooplankton treatments (unsieved and sieved)with four nutrient treatments (0, 0.5, 1.0 and 2.0 µgphosphorus (P) l^–1 day^–1 together with nitrogen(N) at a N:P ratio of 20:1 by weight). Weekly sieving with 300µm mesh reduced the average size of crustacean zooplanktonin the mesocosms, decreased the numbers and biomass of Daphnia,and increased the biomass of adult copepods. Nutrient enrichmentcaused significant increases in phytoplankton chlorophyll a(4–5x), bacterial abundance and production (1.3x and 1.6x,respectively), Daphnia (3x) and total zooplankton biomass (2x).Although both total phytoplankton chlorophyll a and chlorophylla in the <35 µm size fraction were significantly lowerin unsieved mesocosms than in sieved mesocosms, sieving hadno significant effect on bacterial abundance or production.There was no statistical interaction between nutrient and zooplanktontreatments for total phytoplankton biomass or bacterial abundance,although there were marginally significant interactions forphytoplankton biomass <35 µm and bacterial production.Our results do not support the hypothesis that large cladoceransbecome less effective grazers with enrichment; rather, the differencebetween phytoplankton biomass in sieved versus unsieved zooplanktontreatments increased across the gradient of nutrient additions.Furthermore, there was no evidence that bacteria buffered phytoplanktonresponses to enrichment by either sequestering P or affectingthe growth of zooplankton.     

High abundance of picoplankton-ingesting ciliates during late fall in Lake Kinneret, Israel

Small, aloricate ciliates dominated the biomass of heterotrophicprotists throughout the water column at the end of the periodof stratification in Lake Kinneret, Israel The integrated biomassof cilates was 5–20 times that of heterotrophic flagellatesDuring incubation experiments, ciliate growth rates in cpilimneticwater corresponded to population doubling times of 9.6–19.4h, while flagellate populations showed no growth. Most of thealiates were small forms (10–30 µm long), includingscuticocihates, choreotnchs, Coleps spp. and Colpoda spp., andappeared to be consuming bacteria, coccoid cyanobacteria, and<5 µm eukaryotic algae. Grazing rates of cihate assemblageson picoplankton in the epilimnion, as determined by the uptakeof fluorescently labeled bacteria and cyanobactena, ranged from62 to 86 nl cell^–1 h^–1 Colpoda steini, isolatedfrom lakewater, grew on a cultured freshwater Synechococcussp with a doubling time of 4.5 h, and a gross growth efficiencyof 48% The estimated daily requirements of ciliates for growthapproximately equalled total phytoplankton production. We calculatedthat ciliates in the epilimnion were clearing 4–10% ofthe bacterioplankton and cyanobactenal standing stocks per daySince this would not be sufficient food consumption to meetdaily carbon requirements of the aliates, it is likely thatthese organisms were also grazing a significant amount of autotrophicand heterotrophic eukaryotic cells in Lake Kinneret.     

Role of phosphorus and nitrogen for bacteria and phytopankton development in a large shallow lake

Nutrient (P and N) enrichment experiments in small enclosures (20 l) were carried out to determine P and/or N limitation of bacterioplankton in Lake Võrtsjärv. The specific interest of the study was to test if it is possible to detect nutrient `physiological' or growth (rate) limitation of bacterioplankton and competition for nutrients (N and P) with phytoplankton in generally nutrient rich lake. Thymidine and leucine incorporation; leucine aminopeptidase, -D-glucosidase and alkaline phosphatase activity, total count of bacteria, chlorophyll a concentration and primary production as well as the concentrations of different chemical forms of N and P were followed during 4–5 days of the experiment. To address the question of the interactions between nutrients, bacterio- and phytoplankton, experimental and seasonal data sets were included in the analyses. Phosphorus (P) had a positive effect on bacterioplankton in enclosure experiments in June 1997; no effects of nutrients were found in September 1996, while in May 1996, P affected mainly the phytoplankton. On the seasonal scale, the development of bacterioplankton was connected to primary production, total phosphorus and temperature. In enrichment experiments, bacterioplankton was mainly related with primary productivity but the possible importance of bacterial grazers could be presumed. Thus, no evidence was found for nutrient growth limitation and/or competition for N and/or P, rather bacterioplankton depended on organic food supply originating from phytoplankton.     

Fish-induced changes in zooplankton grazing on phytoplankton and bacterioplankton: a long-term study in shallow hypertrophic Lake Sobygaard

The impact of fish-mediated changes on the structure and grazingof zooplankton on phytoplankton and bacterioplankton was studiedin Lake Søbygaard during the period 1984–92 bymeans of in vitro grazing experiments (¹⁴C-labelled phytoplankton,³H-labelled bacterioplankton) and model predictions. Measuredzooplankton clearance rates ranged from 0–25 ml l^–1h^–1 on phytoplankton to 0–33 ml l^–1 h^–1on bacterioplankton.The highest rates were found during thesummer when Daphnia spp. were dominant. As the phytoplanktonbiomass was substantially greater than that of bacterioplanktonthroughout the study period, ingestion of phytoplankton was26-fold greater than that of bacterioplankton. Multiple regressionanalysis of the experimental data revealed that Daphnia spp.,Bosmina longirostris and Cyclops vicinus, which were the dominantzooplankton, all contributed significantly to the variationin ingestion of phytoplankton, while only Daphnia spp. contributedsignificantly to that of bacterioplankton. Using estimated meanvalues for clearance and ingestion rates for different zooplankters,we calculated zooplankton grazing on phytoplankton and bacterioplanktonon the basis of monitoring data of lake plankton obtained duringa 9 year study period. Summer mean grazing ranged from 2 to4% of phytoplankton production and 2% of bacterioplankton productionto maxima of 53 and 88%, respectively. The grazing percentagedecreased with increasing density of planktivorous fish caughtin August each year using gill nets and shore-line electrofishing.The changes along a gradient of planktivorous fish abundanceseemed highest for bacterioplankton. Accordingly, the percentagecontribution of bacterioplankton to the total ingestion of thetwo carbon sources decreased from a summer mean value of 8%in Daphnia-dominated communities at lower fish density to 0.7–1.1%at high fish density, when cyclopoid copepods or Bosmina androtifers dominated. Likewise, the percentage of phytoplanktonproduction channelled through the bacteria varied, it beinghighest (5–8%) at high fish densities. It is argued thatthe negative impact of zooplankton grazing on bacterioplanktonin shallow lakes is highest at intermediate phosphorus levels,under which conditions Daphnia dominate the zooplankton community.     

Evaluation of bacterivory of Rotifera based on measurements of in situ ingestion of fluorescent particles, including some comparisons with Cladocera

Bacterivory of pelagic rotifers and cladocerans in eutrophicLake Loosdrecht (The Netherlands) was determined by microscopicobservation of in situ tracer particle uptake. Ingestion ratesof rotifer species using 0.51 µm microspheres or fluorescentlylabelled bacteria as tracers differed, with one exception. Theingestion rates depended on both the species and the tracertype. For cladocerans, fluorescently labelled bacteria seemedto underestimate grazing, presumably due to rapid digestionof tracer cells. Comparing results obtained with 0.51 µmmicrospheres, rotifers were much more important grazers on bacteriathan cladocerans in the study period (April-September). Basedon microspheres, the rotifer populations with the highest uptakeof bacteria were Filinia longiseta (May-July) andAnuraeopsisfissa (June-September). According to the uptake of fluorescentlylabelled bacteria, Conochilus unicornis had the highest uptakein June and A.fissa in July.     

Dynamics of microplankton communities at the ice-edge zone of the Lazarev Sea during a summer drogue study

Microzooplankton grazing and community structure were investigatedin the austral summer of 1995 during a Southern Ocean Drogueand Ocean Flux Study (SODOFS) at the ice-edge zone of the LazarevSea. Grazing was estimated at the surface chlorophyll maximum(5–10 m) by employing the sequential dilution technique.Chlorophyll a concentrations were dominated by chainformingmicrophytoplankton (>20 µm) of the genera Chaetocerosand Nitzschia. Microzooplankton were numerically dominated byaloricate ciliates and dinoflagellates (Protoperidinium sp.,Amphisoleta sp. and Gymnodinium sp.). Instantaneous growth ratesof nanophytoplankton (<20 µm) varied between 0.019and 0.080 day^–1, equivalent to between 0.03 and 0.12 chlorophylldoublings day^–1. Instantaneous grazing rates of microzooplanktonon nanophytoplankton varied from 0.012 to 0.052 day^–1.This corresponds to a nanophytoplankton daily loss of between1.3 and 7.0% (mean = 3.76%) of the initial standing stock, andbetween 45 and 97% (mean = 70.37%) of the daily potential production.Growth rates of microphytoplankton (>20 µm) were lower,varying between 0.011 and 0.070 day^–1, equivalent to 0.015–0.097chlorophyll doublings day^–1. At only three of the 10 stationsdid grazing by microzooplankton result in a decrease in microphytoplanktonconcentration. At these stations instantaneous grazing ratesof microzooplankton on microphytoplankton ranged between 0.009and 0.015 day^–1, equivalent to a daily loss of <1.56%(mean = 1.11%) of initial standing stock and <40% (mean =28.55%) of the potential production. Time series grazing experimentsconducted at 6 h intervals did not show any diel patterns ofgrazing by microzooplankton. Our data show that microzooplanktongrazing at the ice edge were not sufficient to prevent chlorophylla accumulation in regions dominated by rnicrophytoplankton.Here, the major biological routes for the uptake of carbon thereforeappear to be grazing by metazoans or the sedimentation of phytoplanktoncells. Under these conditions, the biological pump will be relativelyefficient in the drawdown of atmospheric CO₂.     

Pelagic ciliated protozoa in two monomictic, southern temperate lakes of contrasting trophic state: seasonal distribution and abundance

Two lakes of contrasting trophic state in the central NorthIsland of New Zealand were sampled monthly for protozoan ciliatesand potential food resources. Oligotrichs dominated numbersin both lakes. Subdominants in oligotrophic Lake Taupo includedAskenasia, Pscudobalanion and Urotri-cha, and in eutrophic LakeOkaro Prorodon, Coleps, Urocentrum, Stentor and Spirostomumwere important. Biomass was dominated by large predatory ciliatesand Stentor in Lake Taupo, and Prorodon and Stentor in LakeOkaro. The importance of Prorodon and Stentor to ciliate biomassis unusual and has not been reported for northern hemispherelakes. Small ciliates (<20 µm) capable of consumingparticles <2 µm were a major component of the ciliatecommunity in Lake Taupo. Peaks in ciliate abundance occurredat the same time in both lakes: in autumn, at the beginningof mixis and in spring. Ciliates were vertically stratifiedduring mixis and stratification in both lakes. The effect wasmore pronounced during deoxygenation of the hypolimnion in LakeOkaro which excluded oligotrichs and introduced benthic ciliates.Ciliates were less abundant (mean 40001^–1 in Lake Okaroand 9001^–1 in Lake Taupo) than in comparable northerntemperate lakes. There was no correlation between the seasonaldistribution of ciliates and chlorophyll a, primarily causedby a winter peak in chlorophyll a dominated by large speciesof phytoplankton in Lake Taupo, at a time when ciliate numberswere low. The only consistent, significant correlations weretotal ciliate numbers and individual species of ciliates withbacterial concentrations in both lakes and with picophytoplanktonin Lake Taupo.     

Zooplankton-phytoplankton interactions in a eutrophic lake

Enclosure experiments were made in a cyanobacteria dominatedlake (Lake Rotongaio) to assess the impact of zooplankton (>150µm) grazing on algal growth rates and determine the effectof diel and vertical changes in zooplankton grazing intensityand nutrient (NH₄-N) regeneration upon abundance of phytoplankton.The filamentous cyanobacterium Anabaena minutissima var. attenuataand diatom Cyclotella meneghiniana showed a negative linearchange in abundance with a gradient in zooplankton grazing intensity.Phytoflagellates were not grazed and showed a positive linearchange in abundance with increasing zooplankton biomass. Theseeffects, as well as shortening of filament length of Anabaena,were caused by raptorial feeding by the alanoid copepod Boeckellapropinqua which dominated the zooplankton. Phytoplankton growthwas not stimulated by addition of nutrients, suggesting nutrientregeneration was not important. Diel and vertical changes infeeding and NH₄-N regeneration rates were measured in Marchand June 1988. Diel differences were more pronounced in Marchwhen the water column was stratified. Specific feeding rateswere more important than vertical changes in zooplankton biomassin determining community grazing rates in March, but in Junewhen the water column was mixed, vertical distribution of zooplanktonbiomass was important. Zooplankton grazing was an importantloss process for phytoplankton in the lower part of the epilimnionin Lake Rotongaio.     

Comparison of east and west chlorophyll a standing stock and oceanic habitat along the Transition Domain of the North Pacific

Chlorophyll (Chl) a was measured every 10 m from 0 to 150 min the Transition Domain (TD), located between 37 and 45°N,and from 160°E to 160°W, in May and June (Leg 1) andin June and July (Leg 2), 1993–96. Total Chl a standingstocks integrated from 0 to 150 m were mostly within the rangeof 20 and 50 mg m^–2. High standing stocks (>50 mg m^–2)were generally observed westof 180°, with the exceptionof the sporadic high values at the easternmost station. Thetotal Chl a standing stock tended to be higher in the westernTD (160°E–172°30'E) than in the central (175°E–175°W)and eastern (170°W–160°W) TD on Leg 1, but thesame result was not observed on Leg 2. It was likely that largephytoplankton (2–10 and >10 µm fractions) contributedto the high total Chl a standing stock. We suggest that thehigh total Chl a standing stock on Leg 1, in late spring andearly summer, reflects the contribution of the spring bloomin the subarctic region of the northwestern Pacific Ocean. Thedistribution of total Chl a standing stock on Leg 2 was scarcelyaffected by the spring phytoplankton bloom, suggesting thattotal Chl a standing stock is basically nearly uniform in theTD in spring and summer. Moreover, year-to-year variation inthe total Chl a standing stock was observed in the western TDon Leg 1, suggesting that phytoplankton productivity and/orthe timing of the main period of the bloom exhibits interannualvariations.     

Nitrogen fixation in a large shallow lake: rates and initiation conditions

The fixation of molecular nitrogen (N₂fix) by cyanobacteria in situ and in PO₄-P enrichment experiments was investigated in large shallow Lake Võrtsjärv in 1998–2000. In this lake, N₂fix started when TN/TP mass ratio was about 20, which is much higher than Redfield mass ratio 7. The rate of N₂fix varied between 0.81 and 2.61 gN l^–1 d^–1 and maximum rate (2.61 gN l^–1 d^–1) was measured in 15.08.2000. In L. Võrtsjärv a lag period of a couple of weaks occurred between the set-up of favourable conditions for N₂fix as the appearance of N₂-fixing species and depletion of mineral nitrogen, and the real N₂fix itself. However, if the favorable conditions for N₂fix occurred in the lake, N₂fix started after enrichment with PO₄-P in mesocosms even then when no N₂fix was detected in the lake. N₂fix in mesocosms was also more intensive than in lake water. In our experiments PO₄-P concentrations higher than 100 gP l^–1started to inhibit N₂fix.     

Algal Diet of Small-Bodied Crustacean Zooplankton in a Cyanobacteria-Dominated Eutrophic Lake

Small-bodied cladocerans and cyclopoid copepods are becoming increasingly dominant over large crustacean zooplankton in eutrophic waters where they often coexist with cyanobacterial blooms. However, relatively little is known about their algal diet preferences. We studied grazing selectivity of small crustaceans (the cyclopoid copepods Mesocyclops leuckarti, Thermocyclops oithonoides, Cyclops kolensis, and the cladocerans Daphnia cucullata, Chydorus sphaericus, Bosmina spp.) by liquid chromatographic analyses of phytoplankton marker pigments in the shallow, highly eutrophic Lake Võrtsjärv (Estonia) during a seasonal cycle. Copepods (mainly C. kolensis) preferably consumed cryptophytes (identified by the marker pigment alloxanthin in gut contents) during colder periods, while they preferred small non-filamentous diatoms and green algae (identified mainly by diatoxanthin and lutein, respectively) from May to September. All studied cladoceran species showed highest selectivity towards colonial cyanobacteria (identified by canthaxanthin). For small C. sphaericus, commonly occuring in the pelagic zone of eutrophic lakes, colonial cyanobacteria can be their major food source, supporting their coexistence with cyanobacterial blooms. Pigments characteristic of filamentous cyanobacteria and diatoms (zeaxanthin and fucoxanthin, respectively), algae dominating in Võrtsjärv, were also found in the grazers’ diet but were generally avoided by the crustaceans commonly dominating the zooplankton assemblage. Together these results suggest that the co-occurring small-bodied cyclopoid and cladoceran species have markedly different algal diets and that the cladocera represent the main trophic link transferring cyanobacterial carbon to the food web in a highly eutrophic lake.     

Microzooplankton grazing in a coastal embayment off Concepcion, Chile, (~36{degrees} S) during non-upwelling conditions

The impact of grazing by natural assemblages of microzooplanktonwas estimated in an upwelling area (Concepción, Chile)during the non-upwelling season in 2003 and 2004. Seawater dilutionexperiments using chlorophyll a (Chl a) as a tracer were usedto estimate daily rates of phytoplankton growth and microzooplanktongrazing. Initial Chl a concentrations ranged from 0.4 to 1.4mg Chl a m^–3 and phytoplankton prey biomass and abundancewere numerically dominated by components <20 µm. Phytoplanktongrowth and microzooplankton grazing rates were 0.19–0.25day^–1 and 0.26–0.52 day ^–1, respectively.These results suggest that microzooplankton exert a significantremoval of primary production (>100%) during the non-upwellingperiod.     

Zooplankton community grazing in a hypertrophic lake (Hartbeespoort Dam, South Africa)

Seasonal abundance as total biomass and specific densities ofthe main herbivorous zooplankton (>60 µm) in hypertrophicHartbeespoort Dam from 1981 to 1983 are described. After springzooplankton biomass maxima each year there followed a mid-summerdecline in the Daphnia population and a shift to a smaller bodiedcladoceran community dominated by Ceriodaphnia concomitant witha change from largely edible phytoplankton species to abundantcolonial Microcystis. In situ community grazing rates were measuredthroughout 1983 using a ¹⁴C-labelled unicellular alga. Integratedcommunity grazing rates measured in the aerobic water columnwere highest in December (260.2%/day) when Daphnia was abundantand the edible component of the phytoplankton was diminishing.Lowest integrated community grazing rates occurred in January–February(19.8–35.3%/day) and July (28.3%/day) when the phytoplanktonwas composed almost entirely of Microcysris, and Ceriodaphniadominated the zooplankton community whilst food availabilitywas low. Feeding in Ceriodaphnia was not hindered by abundantlarge Microcysris colonies; total biomass specific grazing ratewas high when Ceriodaphnia dominated and low when Daphnia dominatedthe zoo-plankton community. Results indicate that in hypertrophicconditions it is unlikely that large filter-feeders such asDaphnia are able to retard or limit the development of Cyanophyceaeblooms by high grazing pressure.     

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.     

Primary production of aquatic macrophytes and their epiphytes in two shallow lakes (Peipsi and Võrtsjärv) in Estonia

In shallow lakes with large littoral zones, epiphytes and submerged macrophytes can make an important contribution to the total annual primary production. We investigated the primary production (PP) of phytoplankton, submerged macrophytes, and their epiphytes, from June to August 2005, in two large shallow lakes. The production of pelagic and littoral phytoplankton and of the dominant submerged macrophytes in the littoral zone (Potamogeton perfoliatus in Lake Peipsi and P. perfoliatus and Myriopyllum spicatum in Lake Võrtsjärv) and of their epiphytes was measured using a modified ¹⁴C method. The total PP of the submerged macrophyte area was similar in both lakes: 12.4 g C m^?2 day^?1 in Peipsi and 12.0 g C m^?2 day^?1 in Võrtsjärv. In Peipsi, 84.2% of this production was accounted for by macrophytes, while the shares of phytoplankton and epiphytes were low (15.6 and 0.16%, respectively). In Võrtsjärv, macrophytes contributed 58%, phytoplankton 41.9% and epiphytes 0.1% of the PP in the submerged macrophyte area. Epiphyte production in both lakes was very low in comparison with that of phytoplankton and macrophytes: 0.01, 5.04, and 6.97 g C m^?2 day^?1, respectively, in Võrtsjärv, and 0.02, 1.93, and 10.5 g C m^?2 day^?1, respectively, in Peipsi. The PP of the littoral area contributed 10% of the total summer PP of Lake Peipsi sensu stricto and 35.5% of the total summer PP of Lake Võrtsjärv.     

Macrozoobenthos of Lake Peipsi-Pihkva: long-term biomass changes

Macrozoobenthos of the large (3,558 km²) Lake Peipsi-Pihkva was monitored at 22–24 sampling spots yearly in June 1964–1994 as well as 50 shallow-water profiles in mid-summer 1970, 1980, and 1990. No qualitative changes were observed in the bottom fauna, except for the introduction of a Baikalian gammarid species, Gmelinoides fasciatus. The average total biomass in June (without big molluscs) revealed considerable annual fluctuations, with an insignificant general tendency of increase. The increase was significant in the biomass of several Mollusca (including the highly abundant big clam Dreissena polymorpha), Asellus, Gammaridae, and Hirudinea but not in the most abundant animal groups Chironomidae and Oligochaeta. The low biomass of Ephemeroptera and Hydrachnellae decreased significantly. The mid-summer samples taken from the shallow-water zone in three different years revealed the highest biomass in 1980 and the lowest in 1990 (the latter being probably related to the high water level). A gradual decrease in the biomass of the Ephemeroptera and Hydrachnellae as well as the Oligochaeta, Pisidiidae and Bithynia tentaculata occurred in the shallow-water zone, while the Gammaridae increased on the account of the introduced species. No coincidence was found with the neighbouring Lake Võrtsjärv when comparing the annual fluctuations of biomass. A general tendency to a slow increase in total biomass and a decrease in the biomass of the most vulnerable groups Ephemeroptera and Hydrachnellae, probably due to progressing eutrophication, were common for both lakes.     

Potential grazing impact of the mixotrophic flagellate Ochromonas sp. (Chrysophyceae) on bacteria in an extremely acidic lake

Flagellates are important bacterial grazers in most planktonicfood webs. The prey-size preference of the mixotrophic flagellate,Ochromonas sp. (Chrysophyceae), isolated from an extremely acidiclake, Lake 111 (pH 2.6), was determined using fluorescentlylabelled microspheres (beads). According to grazing experimentswith cultured bacteria, also isolated from Lake 111, the potentialgrazing impact on Lake 111’s single-celled bacterial productionwas calculated. Ochromonas sp. ingested the smallest beads offered(0.5 µm diameter) at the highest rate. Ingestion ratedeclined with increasing bead size. The highest prey volume-specificingestion was measured for Ochromonas sp. feeding on intermediate-sizedbeads (1.9 µm). Ingestion rates were low due in part tothe large fraction of inactive flagellates observed. Accordingto the bacterial ingestion rate, a mean of 88% (epilimnion)and 68% (hypolimnion) of in situ single-celled bacterial productionis potentially grazed daily by Ochromonas sp. In the epilimnionof Lake 111, the heterotrophic carbon gain is three times higherthan the autotrophic production. Alongside carbon uptake, Ochromonassp. also benefits from ingesting bacteria through the uptakeof phosphorus. A biovolume minimum corresponding to the preysize at which Ochromonas sp. feeds most efficiently occurredin the Lake 111 epilimnetic bacterial community, implying top-downcontrol of the bacterial community by Ochromonas sp.     

Production of some oligochaete species in large Estonian lakes

The production of Potamothrix hammoniensis, Lamprodrilus isoporus, and Uncinais uncinata was estimated in the eutrophic lakes Peipsi-Pihkva and Vôrtsjärv. Growth characteristics of these species are briefly described. Calculated production values were 6–32 (P. hammoniensis), 19–79 (L. isoporus), and 1–4 kJ m² (U. uncinata), respectively. The annual P/B ratios for the same species were, 0.8–1.7, 2.5–4.8, and 4.4–5.4, respectively.     

Role of phytoplankton size distribution in lake ecosystems revealed by a comparison of whole plankton community structure between Lake Baikal and Lake Biwa

The influence of the size distribution of phytoplankton on changes in the planktonic food web structures with eutrophication was examined using natural planktonic communities in two world-famous lakes: Lake Baikal and Lake Biwa. The size distribution of phytoplankton and the ratio of heterotrophic to autotrophic biomass (H/A ratio), indicating the balance between primary production and its consumption, were investigated in the lakes of different trophic status. The results revealed that microphytoplankton (>20μm) in mesotrophic Lake Biwa, and picophytoplankton (<2μm) or nanophytoplankton (2–20μm) in oligotrophic Lake Baikal, comprised the highest proportion of the total phytoplankton biomass. The H/A ratio was lower in Lake Biwa (<1) than in Lake Baikal (>1). The low H/A ratio in Lake Biwa appeared to be the consequence of the lack of consumption of the more abundant microphytoplankton, which were inferior competitors in nutrient uptake under oligotrophic conditions but less vulnerable to grazing. As a result, unconsumed microphytoplankton accumulated in the water column, decreasing the H/A ratio in Lake Biwa. Our results showed that food web structure and energy flow in planktonic communities were greatly influenced by the size distribution of phytoplankton, in conjunction with bottom-up (nutrient uptake) and top-down (grazing) effects at the trophic level of primary producers.