Abundance and productivity of heterotrophic nanoplankton in Georgia coastal waters

The population abundances and rates of biomass production ofheterotrophic nanoplankton (HNAN) in Georgia coastal waterswere evaluated by epifluorescence microscopy. HNAN populations(mostly non-pigmented microflagellates <10 µm in diameter)ranged from 0.3 x 10³ cells ml^–1 in shelf waters 15 kmoffshore to 6.3 x 10³ cells ml^–1 in waters 0.25 km fromthe coast. There was a strong correlation (r = 0.83) betweenHNAN and free bacterioplankton population abundances, but noapparent relation (r = 0.38) between HNAN and phototrophic nanopLankton(PNAN) abundances. HNAN biomass production in estuarine andnearshore shelf waters, as estimated from increases in HNANpopulations during laboratory incubations of natural water samples,ranged from 0.10 to 0.79 mg C m^–3 h^–3, with populationgeneration times of 9.7 to 26.5 h. There was a significant linearrelation (r = 0.95) between HNAN biomass and HNAN productivity.We calculated that HNAN may graze at least 30% to 50% of dailybacterioplankton production in Georgia coastal waters.     

Nutrient cycling through phytoplankton, bacteria and protozoa, in selectively filtered Lake Vechten water

The breakdown of organic carbon of dead Synechococcus cell walls,added to selectively filtered Lake Vechten water, was not acceleratedby protozoa. During 4 weeks of incubation at 15°C no significantdecrease of total organic carbon was observed. However, heterotrophicnanoflagel-lates (HNAN) and ciliates strongly increased theremineralization of N and especially P, from both cell wallsand cell extract. Bacterioplankton growth did not result innet P mineralization but in P uptake. P was remineralized onlyin the presence of protozoan grazers. Both HNAN and ciliatesgrazed on bacteria, with ingestion rates estimated at 27–96bad HNAN^–1 h^–1 and 129 bact ciliate^–1 h^–1respectively. Grazers increased N mineralization too, althoughN was also mineralized in the absence of protozoa. The phytoplanktoncell walls yielded less P but more N remineralization than thecell extract. Thus, protozoa can strongly accelerate cyclingof specific nutrients through plankton. Nuclepore filters werefound to cause artificial DOC release during selective filtration.     

Seasonal dynamics of the planktonic microbial community in a maritime Antarctic lake undergoing eutrophication

The abundance, biomass and community structure of phytoplankton,bacterioplankton and protozooplankton in a maritime Antarcticlake were determined at approximately monthly intervals fromDecember 1994 to February 1996 and compared with data from earlierstudies. Heywood Lake has become significantly eutrophic duringthe last three decades because of excreta from the expandingfur seal population in its catchment. Marked seasonal variationsin the abundance, composition and productivity of biota werecorrelated with the seasonality of both physical factors andnutrient levels. Protozooplankton were abundant, diverse andusually dominated by heterotrophic nanoflagellates (HNF), withHNF abundance peaking at 2.35 x 10⁷ l^–1 in summer. Highnumbers of naked amoebae were sometimes present, reaching amaximum of 4.8 x 10³ l^–1 in March. An estimated 89 speciesof protozoa were observed during the study, indicating substantiallymore diversity than is found in continental Antarctic lakes.Diversity was highest in spring and lowest in winter, when theentire water column became anoxic and the plankton were dominatedby bacteria and a few species of relatively large anaerobicflagellates. The current status of the lake is compared withdata for continental Antarctic and lower latitude lakes. Earlierstudies of biota and physical/chemical parameters in HeywoodLake are used to examine the effects of eutrophication overthree decades. Observed changes include increased microbialabundance and changes in both community structure and seasonalpatterns.     

The dynamics of heterotrophic nanoflagellates and bacterioplankton in a large ultra-oligotrophic Antarctic lake

The abundance of both heterotrophic nanoflagellates (HNAN) andbacterioplankton in a large (9km²) ultraoligotrophic Antarcticlake (Crooked Lake) were investigated from December 1992 untilNovember 1993. HNAN abundance peaked in spring, summer and autumn,falling to lowest numbers during the winter. Numbers rangedbetween 0 and 50.9x10⁴ l^–1. Bacterioplankton abundancewas highest during the late summer and then fell progressivelytowards winter and autumn (range 1.19–4.46x10⁶ l^–1)In contrast to numbers, mean cell volumes (MCV) of the bacteriareached their highest in spring, and consequently highest bacterialbiomass occurred at this time. MCV ranged between 0.052 and0.224µm³. Bacterial production measurements followingthe incorporation of [³H] thymidine into DNA and [¹⁴C] leucineinto protein using a doubling-labelling procedure were undertakenin January, June, August, October and November. Rates variedbetween 2.8 and 52 ng C l¹ h¹. On occasions, a significant differencein production rates based on the uptake of leucine and thymidinewas observed, suggesting unbalanced growth. Highest rates ofproduction coincided with times of high dissolved organic carbonlevels in the water column and lowest production with low levelsof DOC. HNAN grazing rates were measured by following the uptakeof fluorescently labelled bacteria and averaged 4.8 bacterialcells individual¹ day¹ at 2 and 4°C. Specific growth rates(h¹) ranged around 0.00070–0.00077 in both the field andlaboratory, giving doubling times of 37.3 and 41.0 days, respectively.These low rates of grazing and growth indicate that there isno adaptation to low temperatures in these freshwater protists.Based on these data, the gross production efficiency is 24%.HNAN removed between 0.1 and 9.7% of bacterial production perday.     

Bacterivory in seawater samples estimated by a dual radioactive-labelling technique

A dual radioactive-labelled bacteria technique using Vibrio(DRLV), developed for laboratory studies on bacterivory, hasbeen refined for use at the concentrations of prey and predatorstypcially found at sea. Experiments with estuarine water collectedin spring and in autumn showed that bacterivorous nanoflagellates(HNF) (concentration 1.38±0.35x10³ HNF ml^–1) ingested2.7±0.96 DRLV flagellate1^–1 h^–1 at concentrationsof 0.8–2.2x10⁶ DRLV ml^–1 in the presence of 2.04±0.68x10⁶natural bacteria ml^–1. The method was also applied tosamples collected in October in the Celtic Sea, when on average1 ml of water from the surface layer contained 1.41±0.16x10⁶natural bacteria, 14.6x10³ cyanobacteria, 530±170 HNF,7.3±3.0x103 phototrophic nanoflagellates (1.5–4µm), 49.0±26.5 phototrophic dinoflagellates, 36.3±12.6heterotrophic dinoflagellates and 21.3±9.5 Leucocryptosmarina. Under these conditions the grazing rate in most samplesdid not exceed the coefficient of variation of the method (2%),although we estimate the grazing rate was -1.6 DRLV HNF^–1h^–1 and on one occasion a rate of 2.45 was recorded. Thegross growth efficiency for protein of -30% displayed by naturalHNF means that they could release about     

The importance of macrophyte bed size for cladoceran composition and horizontal migration in a shallow lake

Cladoceran composition and diel horizontal migration were studiedin 2, 10 and 25 m diameter macrophyte exclosures establishedin the littoral zone of shallow Lake Stigsholm, Denmark. Theexclosures were protected from waterfowl grazing, but open tofish. The macrophyte community cornprized Potwnogeton pectinatus,Potamogeton pusillus and Callitriche hemaphroditica. Cladoceranswere sampled randomly every third hour inside and outside themacrophyte exclosures during a 24 h period. In the 2m exclosure,the pelagic species Ceriodaphnia spp. and Bosmina spp. dominatedduring the day, mean density being as high as 3430 indiv. l^–1During the night, density decreased to 10–20% of the daytimedensity thus indicating diel horizontal migration. In the 10and 25 m exciosures, the daytime mean density of Ceriodaphniaspp. was 865 and 202 indiv. l^–1, respectively, and didnot decrease at night. In contrast to the pelagic species, thedensity of macrophyte-associated species tended to be higherin the 10 and 25 m exclosure than in the 2 m exclosure. In thedaytime, Eurycercus lamellatus density in the 2, 10 and 25 rnmacrophyte exclosures was 7, 28 and 16 indiv. l^–1 respectively,while that of Simocephalus vetulus was 11, 171 and 92 mdiv.l^–1, respectively. There was no thy-night difference inthe density of macrophyte-associated species. We conclude thatcladoceran community composition varies with macrophyte bedsize, and that the edge zone between the bed and open wateris an important daytime refuge for potentially migrating pelagiccladocerans.     

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.     

Feeding of planktonic rotifers on ciliates: a method using natural ciliate assemblages labelled with fluorescent microparticles

A method was developed to allow direct measurements of predationexerted by metazooplankton on ciliates. The method relied onthe use of ciliates labelled with fluorescent microparticles(FMP). Optimal labelling conditions were determined with ciliatesfrom cultures (Tetrahymena pyriformis) and with natural ciliateassemblages sampled in a river. Labelled T. pyriformis wereused as tracer food to determine gut passage time (GPT) andingestion rates of the rotifer Brachionus calyciflorus in thelaboratory. Predation of metazooplankton from the lowland riverMeuse (Belgium) was determined by labelling natural assemblagesof ciliates and using them as tracer food for metazooplankterssampled in the river. Optimal labels of ciliates, i.e. sharpdistribution of FMP in cells, were obtained with short incubations(10 min) and low FMP concentrations (1 x 10⁵ mL^–1). GPTvaried between 30 and 45 min for B. calyciflorus and from 25up to >35 min for rotifers from the river. The ingestionrate of B. calyciflorus fed with T. pyriformis was 3.3 ±0.6 ciliate rot^–1 h^–1, i.e. 1.4 ± 0.3 ngCrot^–1 h^–1. Metazooplankton species for which theingestion of ciliates could be measured were the rotifers Keratellacochlearis, Euchlanis dilatata and Synchaeta spp. Ingestionrates measured ranged from 0.4 to 12.5 ngC rot^–1 h^–1.The method proposed proved to be useful in estimating the predationof microplankton on ciliates in semi- in situ conditions; infurther developments, labelled natural assemblages of ciliatescould be used for in situ incubations with the Haney chamber.     

The effect of detrital addition on the development of nanoflagellates and bacteria in Lake Kinneret

The effect of adding dissolved substrates derived from algalcells on the patterns of nutrient cycling and growth of bacteria,heterotrophic nanoflagellates (HNAN) and photoautotrophs wasdetermined in samples of near-surface waters from Lake Kinneret.Supplementation of substrates always resulted in an increasedpeak of HNAN numbers and had little effect on bacterial numbers.HNAN-mediated nutrient remineralization of nitrogen and phosphoruswas also stimulated. In light-incubated samples the remineralizednutrients were taken up by photoautotrophic cells. Maximum growthrates observed for HNAN ranged from 0.03 to 0.11 h^–1,clearance rates for bacteria 1.1–7.3 nl HNAN^–1 h^–1and remineralization rates 6.4–8.4 µg N mg dry wt^–1h^–1 and 0.37–0.99 µg P mg dry wt^–1 h^–1.     

Grazing on bacteria by flagellates and cladocerans in lakes of contrasting food-web structure

We tested the hypothesis that grazing on bacteria would varybetween lakes with differing plankton community structures.Paul and Tuesday lakes (Gogebic County, MI) are respectivelydominated by piscivorous and planktivorous fish. Consequently,zooplankton in Paul are primarily large daphnids, while zooplanktonin Tuesday are primarily small cladocerans and copepods. Wemeasured flagellate grazing on bacteria using a fluorescentminicell method, while cladoceran grazing was estimated fromthe relationship between body length and filtering rate. Wepredicted that cladoceran grazing on bacteria would be higherin Paul, and flagellate grazing would be higher in Tuesday.Cladoceran grazing on bacteria was important in both lakes contraryto our initial expectation. Large populations of the small cladoceran,Bosmina longirostris, in Tuesday exerted a grazing pressure(0.18–35x10⁶ bacteria 1^–1 h^–1) approximatelyequal to that of the large cladoceran, Daphnia pulex, in Paul(0.34–30x10⁶ bacteria 1^–1 h^–1). Flagellategrazing was higher in Tuesday as predicted (range: Paul, 0.1–6x10⁶bacteria 1^–1 h^–1; Tuesday, 0.2–20x10⁶ bacteria1^–1 h^–1). However, there was not a simple relationshipbetween total abundance of flagellates and total grazing rates.High community grazing by flagellates occurred when attachedchoanoflagellates were present. These flagellates had higheringestion rates than free forms. We find no clear evidence thatdifferences in food-web structure between the two lakes influencethe process of grazing on bacteria. Instead, our results emphasizethe significance of cladocerans and attached flagellates asconsumers of bacteria in freshwater ecosystems.     

Laboratory and field observations on the scuticociliate Histiobalantium from the pelagic zone of Lake Constance, FRG

Histiobalantium sp. was found regularly in the pelagic zoneof Lake Constance, FRG, over five annual cycles. Maxima of upto 6400 cells l^–1 were recorded in late summer, with similarnumbers in the 0–8 and 8–20 m depth intervals. Onan annual average, the population accounted for 10–17%of the total biomass of planktonic ciliates. In the laboratory,Histiobalantium grew well on a diet of the cryptophyte Rhodomonassp. Maximum growth rates obtained in batch cultures were 0.21and 0.33 day^–11 at 9 and 18°C, respectively. In situexperiments using diffusion chambers yielded positive growthrates in autumn and winter. The highest values recorded at theambient temperatures 5, 14 and 17°C were 0.17, 0.32 and0.40 day^–1, respectively. Comparing these results withthe different seasonal distributions and higher measured growthrates of other ciliates from Lake Constance, we conclude thatHistiobalantium is a superior competitor at relatively low algalfood concentrations. ²Present address: Fisheries & Oceans Canada, 4160 MarineDrive, West Vancouver, BC, V7V 1N6, Canada     

The annual cycle of planktonic ciliates in nearshore waters at Signy Island, Antarctica

The abundance and biomass of marine planktonic ciliates in BorgeBay, Signy Island, were determined at monthly intervals betweenApril 1990 and June 1991. At least 24 different ciliate taxawere recorded from samples preserved in Lugol's iodine, includingthe tintinnids Codonellopsis balechi, Cymalocylis convallaria,Laackmaniella naviculaefera and Salpingella sp., and the aloricatetaxa Didinium sp. and Mesodinium rubrum. Ciliate abundance andbiomass exhibited a clear seasonal cycle with high values duringthe austral summer and low values in the austral winter. Abundanceranged from 0.3 10³l^–1 in September to 2.3 10³l^–1in January, while biomass ranged from 0.5 µg C l^–1in October to 12.6 µg C l^–1 in December. Small ciliatesdominated abundance throughout the year, and biomass duringwinter. Larger ciliates contributed most to biomass during summer.Aloricate ciliates were common throughout the year, while tintinnidscontributed substantially to abundance and biomass only duringsummer. Salpingella sp. was the commonest tintinnid, but C.convallariacontributed most to tintinnid biomass. The seasonal patternof ciliate abundance and biomass matched that of chlorophylla concentration and bacterial biomass, suggesting tight trophiccoupling between ciliates and other components of the pelagicmicrobial community. ¹Present address: Scott Polar Research Institute, Universityof Cambridge, Lensfield Road, Cambridge CB2 1ER, UK     

Vertical and temporal heterogeneity of planktonic ciliated protozoa in a humic lake

Seasonal population dynamics and the vertical distribution ofciliates were studied in relation to the particular food resourcesoccurring in a humic and moderately acidic lake (Lake Vassivière).The abundance (1.4 x 10³–20.4 x 10³ cells l^–1 mean= 4.8 x 10³ cells l^–1) and biomass (0.5–34.6 µgC l^–1, mean = 6.0 µg C l^–1) of ciliated protozoawere low and close to values reported for oligotrophic environments.The species composition of the population varied greatly withdepth. Whereas large-sized species of oligotrichs, some of whichwere mixotrophic, dominated at the surface, haptorids were bestrepresented in deep waters. The spatial distribution of thevarious groups of ciliates was largely determined by light andthe vertical distribution of microbial food resources (detritus,bacteria, algae) within the water column of this brown-coloredlake.     

Flagellate nutritional versatility as a key to survival in two contrasting Antarctic saline lakes

1. Seasonal patterns of grazing and photosynthesis were investigated in two saline Antarctic lakes (Highway and Ace) in the Vestfold Hills (68°S). The phototrophic nanoflagellate (PNAN) community was dominated by Pyramimonas gelidicola and two morphological forms of a cryptophyte species that occurred throughout the year. Both species were mixotrophic on bacteria, and in Highway Lake they also exploited dissolved organic carbon as determined by the uptake of fluorescently labelled dextrans. 2. Clearance rates ranged between 0.02 and 0.21 nL h^?1 cell^?1 in Ace Lake and 0.004–1.05 nL h^?1 cell^?1 in Highway Lake. On occasion cryptophyte grazing equalled that of the heterotrophic nanoflagellates (HNAN). 3. Photosynthetic rates showed similar trends in both lakes, but there were differences in chlorophyll a specific rates and photosynthetic efficiency, probably related to the meromictic characteristic of Ace Lake. Primary production was measurable in winter and peaked in summer following the maxima of mixotroph grazing. 4. The HNAN community of Highway Lake achieved clearance rates of 0.02–1.80 nL h^?1 cell^?1 and removing between 50 and 693 ng bacterial carbon L^?1 day^?1, with highest impact in winter when HNAN were most abundant. The HNAN also ingested fluorescently labelled dextrans showing a preference for 4 and 500 kDa molecules. The more diverse HNAN community of Ace Lake had lower clearance rates (0.04–0.37 nL h^?1 cell^?1) and exerted a lower grazing pressure on bacterioplankton. In Highway Lake, where the HNAN community was dominated by the choanoflagellate Diaphanoeca grandis, there was a significant correlation between mean cell volume and clearance rate. 5. The major feature was that the microbial plankton functioned throughout the year by employing nutritional versatility.     

Grazing by ciliates and heterotrophic nanoflagellates on picocyanobacteria in Lago Maggiore, Italy

During the oligotrophication of Lago Maggiore, picocyanobacteria(Pcy) increased in abundance and production. In their bimodal,seasonal cycle, the spring peak was due almost exclusively tosingle cells of Pcy, whereas in late-summer/autumn the varietyof morpho-types increased and larger Aphanothece-like rods appeared.Rates of Pcy cell removal by heterotrophic nanoflagellates andciliates were measured by using fluorescently labelled Pcy (FLC)in four experiments performed during the Pcy population shiftfrom small cocci to larger rods. The ciliate community appearedto be composed mainly of oligotrichs in the first two experiments,and subsequently of scuticociliates; heterotrophic nanoflagellatesdecreased in number from May to September, and there was a sizeshift which might reflect species composition change. Peritrichsemerged as the most efficient Pcy grazers. For the other ciliategroups we observed higher individual ingestion rates duringthe spring experiments than during those performed in late summer/autumn.The heterotrophic nanoflagellates ingested from 0.5 to3 Pcyh^-1 while ciliates ingested from 18 to 80 Pcy h^-1. The grazingimpact of the heterotrophic nanoflagellate community rangedfrom 1.9 x 10³ to 8 x 10³ Pcy ml^-1 h^-1, whereas the ciliatecommunity ingestion rate was one order of magnitude lower (0.2x 10³–0.4 x 10³ Pcy ml^-1 h^-1). A significant inverse correlationbetween Pcy size and the clearance rate of heterotrophic nanoflagellatesand ciliates was found. Our results indicate that protozoa areless efficient in cell uptake when the Pcy are composed of largercells. In Lago Maggiore, the carbon flux from Pcy to protozoadecreased from 29.8 to 10.2 µg C l^-1 day^-1 (May and Septemberrespectively). A tentative balance on an annual basis suggeststhat around 80% of the carbon produced by Pcy is taken up byprotozoa and channelled to metazooplankton.     

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.     

Predation and respiration by the small cyclopoid copepod Oithona similisr: How important is feeding on ciliates and heterotrophic flagellates?

Feeding on natural plankton populations and respiration of thesmall cyclopoid copepod Oithona similis were measured duringthe warm season in Buzzards Bay, Massachusetts, USA. AlthoughO.similis did not significantly ingest small autotrophic andheterotrophic flagellates (2–8 µn), this copepodactively fed on >10 µm particles, including autotrophic/heterotrophic(dino)flagel-lates and ciliates, with clearance rates of 0.03–0.38ml animal^–1 h^–1. The clearance rates increased withthe prey size. O.similis also fed on copepod nauplii (mainlycomposed of the N1 stage of Acartia tonsa with a clearance rateof 0.16 ml animal^–1 h^–1. Daily carbon ration fromthe combination of these food items averaged 148 ng C animal^–1day^–1 (41% of body C day^–1), with ciliates and heterotrophicdino-flagellates being the main food source ({small tilde}69%of total carbon ration). Respiration rates were 020–0.23µl O₂ animal^–1 day^–1. Assuming a respiratoryquotient of 0.8 and digestion efficiency of 0.7, the carbonrequirement for respiration was calculated to be 125–143ng C animal^–1 day^–1, close to the daily carbon rationestimated above. We conclude that predation on ciliates andheterotrophic dinoflagellates was important for O.similis tosustain its population in our study area during the warm season.     

The plankton of a large oligotrophic freshwater Antarctic lake

The planktonic community of Crooked Lake, a large freshwaterlake in the Vestfold Hills, Antarctica was investigated duringthe austral summer in 1990. Very low levels of chlorophyll aranging between mean values of 0 29 and 1.8 µl^–1were recorded. The phytoplankton was largely made up of colouredflagellates, including single species of Chlamydomonas, Ochromonasand Pendimum, which occurred in low concentrations (23.8x 10²–47.3x10² l^–1). Heterotrophic colourless flagellates, includingParaphysomonas vestita, were also relatively sparse (2.1x 10²–21.3x10²l^–1). Ciliated protozoans were particularly poorly represented.Only three species occurred reaching densities of 1001^–1,and among them the mixotrophic species Strombidium vinde wasthe most common. A single species of heliozoan Actinosphaeriumand relatively large numbers of naked amoebae were the sarcodinerepresentatives The protistan community and the bacteria wereconcentrated into microbial consortia associated with floesof paniculate organic matter probably derived from the benthicalgal mat. Of the two microcrustacean zooplankters recordedfrom the lake only Daphniopsis studeri was found breeding inthe plankton in very low numbers. The behavioural and physiologicaladaptations of the organisms inhabiting this extremely oligotrophicenvironment are discussed.     

Microbial communities in saline lakes of the Vestfold Hills (eastern Antarctica)

A detailed survey was undertaken of the microbial communities of 16 saline lakes in the Vestfold Hills (Princess Elizabeth Land, eastern Antarctica), which ranged in salinity from slightly brackish (4–5‰) to hypersaline (maximum: 174‰). Temperatures at comparable sampling depths in the lakes ranged from −12.2°C to +10.5°C. Ranges in the abundances of bacteria, heterotrophic nanoflagellates (HNAN) and phototrophic nanoflagellates (PNAN) were 1.40 × 10⁷ l⁻¹–1.58 × 10¹⁰ l⁻¹, 4.83 × 10⁴ l⁻¹–1.70 × 10⁷ l⁻¹ and 0–1.02 × 10⁷ l⁻¹, respectively. There was considerable variation across the salinity spectrum, though in the case of bacteria and PNAN significantly higher concentrations of cells were seen in two of the most saline lakes. The autotrophic ciliate Mesodinium rubrum occurred in all but five of the lakes and was found at salinity levels up to 108‰. Heterotrophic ciliates were generally scarce. Dinoflagellates, particularly Gonyaulax c.f. tamarensis, Gyrodinium lachryma and Gymnodinium sp., occurred in the majority of the lakes. On the basis of chlorophyll a concentrations, nutrient levels and microplankton concentrations the lakes spanned the spectrum from ultra-oligotrophic to oligo/mesotrophic. The most saline lakes had much reduced species diversity compared with the less saline environments. Isolation from the marine environment has led to nutrient depletion, simplification and a truncated trophic structure. Received: 19 September 1996 / Accepted: 13 January 1997     

Effects of rotifers and ciliates on the growth and survival of Daphnia

Daphnia can suppress ciliates and rotifers through predationand interference competition, but it is not known whether thisproduces any direct benefit to Daphnia. We conducted survivorshipand cohort lifetable experiments to determine whether Daphniacan utilize ciliates and rotifers as food. Three species ofoligotrich ciliates (Halteria grandinella, Strobilidium gyransand Strobilidiumvelox) and one rotifer (Keratella cochlearis)were used. Lifetable experiments were conducted with a basallevel of algae (Cryptomonas sp.), plus either ciliates or rotifers,while survivorship experiments had only the rotifers or ciliates.Densities of 30 H.grandinella ml^–1, 50 S.gyrans ml^–1and 15 S.velox ml^–1 enhanced Daphnia pulex's populationgrowth rate 35–50% over controls with only algae. TenS.gyrans ml^–1 did not produce a significant change inDaphnia's growth rate. Densities of 100 and 300 K.cochlearis^–1 increased Daphnia population growth rates by II and10%, respectively. Both 10 and 50 S.gyrans ml^–1 enhancedDaphnia's survivorship compared to starved controls, but neither100 nor 300 K.cochlearis l^–1 enhanced its survivorship.The amount of enhancement of Daphnia growth rates by rotifersand ciliates is roughly proportional to the death rates imposedby Daphnia. The death rate imposed by Daphnia on rotifers isa function of both algal density and Daphnia size. Per unitbiomass, neither ciliates nor Keratella appear to be as nutritiousfor Daphnia as is Cryptomonas.