Qualitative importance of the microbial loop and plankton community structure in a eutrophic lake during a bloom of cyanobacteria

Plankton community structure and major pools and fluxes of carbon were observed before and after culmination of a bloom of cyanobacteria in eutrophic Frederiksborg Slotssø, Denmark. Biomass changes of heterotrophic nanoflagellates, ciliates, microzooplankton (50 to 140 μm), and macrozooplankton (larger than 140 μm) were compared to phytoplankton and bacterial production as well as micro- and macrozooplankton ingestion rates of phytoplankton and bacteria. The carbon budget was used as a means to examine causal relationships in the plankton community. Phytoplankton biomass decreased and algae smaller than 20 μm replacedAphanizomenon after the culmination of cyanobacteria. Bacterial net production peaked shortly after the culmination of the bloom (510 μg C liter^?1 d^?1 and decreased thereafter to a level of approximately 124 μg C liter^?1 d^?1. Phytoplankton extracellular release of organic carbon accounted for only 4–9% of bacterial carbon demand. Cyclopoid copepods and small-sized cladocerans started to grow after the culmination, but food limitation probably controlled the biomass after the collapse of the bloom. Grazing of micro- and macrozooplankton were estimated from in situ experiments using labeled bacteria and algae. Macrozooplankton grazed 22% of bacterial net production during the bloom and 86% after the bloom, while microzooplankton (nauplii, rotifers and ciliates larger than 50 μm) ingested low amounts of bacteria and removed 10–16% of bacterial carbon. Both macro-and microzooplankton grazed algae smaller than 20 μm, although they did not control algal biomass. From calculated clearance rates it was found that heterotrophic nanoflagellates (40–440 ml^?1) grazed 3–4% of the bacterial production, while ciliates smaller than 50 μm removed 19–39% of bacterial production, supporting the idea that ciliates are an important link between bacteria and higher trophic levels. During and after the bloom ofAphanizomenon, major fluxes of carbon between bacteria, ciliates and crustaceans were observed, and heterotrophic nanoflagellates played a minor role in the pelagic food web.     

Geographical and seasonal variations in taxonomic composition, abundance and biomass of microzooplankton across a brackish-water lagoonal system of Japan

The taxonomic composition, abundance and biomass of microzooplanktonwere studied at eight stations in Lake Shinji–Ohashi River–LakeNakaumi brackish-water system, Japan, monthly from April 1998to March 1999. Over the entire area, naked ciliates numericallydominated the microzooplankton community (annual mean 39.6%)followed, in order, by tintinnids (30.3%), copepod nauplii (24.6%)and rotifers (5.5%). The abundance of each taxonomic group ofmicrozooplankton varied geographically due to large salinityvariations (range 1.5–33.3 p.s.u.). It was notable thatnaked ciliates occurred overwhelmingly in Lake Shinji (54.9%of total microzooplankton) and rotifers were relatively numerousin Lake Shinji (8.8%) and Ohashi River (11.1%), where the salinitywas lower (annual mean 4.1 and 13.6 p.s.u., respectively) thanin Lake Nakaumi and Sakai Strait (26.3 and 29.8 p.s.u., respectively).Owing to large seasonal temperature variation (range 5.4–29.8°C),the abundance of microzooplankton showed marked seasonal variations,being higher in spring and summer than in the remaining seasons.A total of 49 species of tintinnids were identified, and 15of these species reached concentrations >500 individualsl^-l. The occurrence of most tintinnid species was confined tocertain months or locations, closely associated with species-specifictemperature and salinity preference and/or tolerance. In thiseutrophic system, food supply for microzooplankton might besufficient due to extremely high chlorophyll a concentration(annual mean 8.8 µg l^-lin Sakai Strait to 22.6 µgl^-l in Lake Shinji). However, microzooplankton biomass remainedmoderate (range 0.19–18.7 µg C l^-l) due probablyto heavy predation by mesozooplankton, which inhabit this brackish-watersystem at extremely high biomass.     

A survey of ciliates at the long-term sampling station “Helgoland Roads”, North Sea

This study presents a new checklist of ciliates at the long-term sampling station Helgoland Roads. The work is based on a microzooplankton monitoring programme from January 2007 to June 2009 and a ciliate monitoring programme from June 2010 to May 2012. The checklist includes 89 ciliate taxa from 46 different genera. The total abundance of the ciliate community at Helgoland Roads ranged between 0.14 and 67.7 × 10³ cells L^?1 with a distinct peak in June. The total carbon biomass ranged between 0.2 and 234.6 μg C L^?1. The ciliate community showed a clear temporal succession pattern during the monitoring periods. The present study is not only the first detailed and updated list of ciliates at Helgoland Roads, but also provides information on seasonality, i.e. the temporal variation of species composition, abundance and carbon biomass as well as information on the biogeographic distribution of dominant ciliates in comparison with other relevant studies.     

Annual pattern of micro- and mesozooplankton abundance and biomass in a subtropical estuary

The pattern of biomass and abundance of microzooplankton andmesozooplankton were studied over an annual cycle in the NuecesEstuary, Texas. Zooplankton samples and associated hydrographicdata were collected at four locations at biweekly intervalsfrom September 1987 through October 1988. This is a broad, shallowbay system with an average depth of 2.4 m. The concentrationof chlorophyll a in the surface waters averaged 7.4 µgl^–1with 85% passing through a 20 µ mesh. Microzooplankton(20–200 µ in length) were extremely abundant throughoutthis study. Abundances of ciliates (including both aloricateciliates and tintinnids) ranged from 5000 to 400 000 l^–,with a mean of 38 000 l^–1 of seawater over the entirecourse of the study. Mesozooplankton (200–2000 µmin length) abundance averaged 6100 m^–3 for samples collectedduring the day and 10 100 m^–3 for samples collected atnight. Mesozooplankton were dominated by Acartia tonsa whichmade up {small tilde}50% of the total. Biomass estimates formicrozooplankton (based on volume estimates) were often higherthan measured biomass of mesozooplankton. Given the shortergeneration times and higher metabolic rate of microzooplanktoncompared to mesozooplankton, microzooplankton should have agreater effect on the trophic dynamics of the Nueces Estuarythan mesozooplankton.     

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     

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.     

Phytoplankton seasonal dynamics in a Mediterranean coastal lagoon: emphasis on the picoeukaryote community

The dynamics of the phytoplankton community were investigatedin a marine coastal lagoon (Thau, NW Mediterranean) from February1999 to January 2000. Dilution experiments, chlorophyll a (Chla) size-fractionation and primary production measurements wereconducted monthly. Maximum growth and microzooplankton grazingrates were estimated from Chl a biomass fractions to separatepico- from nano- and microphytoplankton and by flow cytometryto distinguish between picoeukaryotes and picocyanobacteria.In spring, the phytoplankton community was dominated by Chaetocerossp. and Skeletonema costatum, which represented most of biomass(B) and primary production (P). Nano- and microphytoplanktongrowth was controlled by nutrient availability and exceededlosses due to microzooplankton grazing (g). Picoeukaryote andcyanobacteria growth was positively correlated with water temperatureand/or irradiance, reaching maximum values in the summer (2.38and 1.44 day^–1 for picoeukaryotes and cyanobacteria, respectively).Picophytoplankton accounted for 57% of the biomass-specificprimary productivity (P/B). Picophytoplankton was strongly controlledby protist grazers (g = 0.09–1.66 day^–1 for picoeukaryotes,g = 0.25–1.17 day^–1 for cyanobacteria), and microzooplanktonconsumption removed 71% of the daily picoplanktonic growth.Picoeukaryotes, which numerically dominate the picoplanktoncommunity, are an important source of organic carbon for theprotistan community and contribute to the carbon flow to highertrophic levels.     

Monsoonal and lunar variability in microzooplankton abundance and community structure in the Terusan mangrove creek (Malaysia)

This study documents the monsoonal and lunar effects on species composition and abundance of microzooplankton in a tropical estuary. We investigated microzooplankton abundance in relation to the various environmental and biotic parameters, sampled in the Matang mangrove (Malaysia) from April 2013 to February 2014. A total of 39 microzooplankton taxa comprising four major groups, i.e. loricate ciliates (37.72%), aloricate ciliates (29.46%), dinoflagellates (24.33%) and meroplanktonic nauplii (8.49%) were identified. The loricate ciliates were the most diverse group with 31 taxa recorded. Four major species of loricate ciliates were identified, i.e. Tintinnopsis beroidea, Tintinnopsis rotundata, Stenosemella avellana and Tintinnidium primitivum, while Strombidiidae and Strobilidiidae dominated the aloricate ciliates. Although small loricate ciliates were ubiquitous, redundancy analysis shows marked shifts in microzooplankton community structure, from one that was dominated by loricate ciliates during the drier SW monsoon, to aloricate ciliates at the onset of the wet NE monsoon, and then to dinoflagellates towards the end of the drier NE monsoon period. These shifts were associated with rainfall, dissolved inorganic nutrients, salinity, temperature and microbial food abundance. There was no clear lunar effect on abundance of microzooplankton except for Favella ehrenbergii and copepod nauplii, which were more abundant during neap than spring tides.     

Planktonic ciliated protozoa: their distribution and relationship to environmental variables in a marine coastal ecosystem

The spatial distribution of planktonic ciliated protozoa, physicalvariables and potential food items were measured at northernand southern sites off the west coast of the South Island, NewZealand, to examine which variables were important in structuringthe ciliate community. Ciliates contributed 30% of total zooplanktonbiomass. Ciliate abundance was greatest within 55 km of thecoast on the northern transect, but on the southern transectthe greatest abundance was close inshore and at the stationsoffshore of the continental shelf. The ciliate community wasdominated by small oligotrichs and abundance (114–1139l^–1 was comparable with other oligotrophic marine environments.The depth of the pycnocline had a major influence on the verticaldistribution of ciliates, which were closely correlated withprofiles of phytoplankton biomass and picophytoplankton abundance,but not primary production. The results of this study suggestthat physical factors were more important than biotic factorsin influencing the distribution of protozoan ciliates. The deepmixed layer prevented the formation of deep chlorophyll a orciliate maxima. Few studies of microzooplankton have incorporatedpicophytoplankton distribution, but the importance of smallciliates and picophytoplankton suggests that these groups mayplay a pivotal role in mediating the transfer of organic matterto higher trophic levels in this marine coastal ecosystem.     

Dinoflagellates and ciliates at Helgoland Roads, North Sea

A monitoring programme for microzooplankton was started at the long-term sampling station “Kabeltonne” at Helgoland Roads (54°11.3′N; 7°54.0′E) in January 2007 in order to provide more detailed knowledge on microzooplankton occurrence, composition and seasonality patterns at this site and to complement the existing plankton data series. Ciliate and dinoflagellate cell concentration and carbon biomass were recorded on a weekly basis. Heterotrophic dinoflagellates were considerably more important in terms of biomass than ciliates, especially during the summer months. However, in early spring, ciliates were the major group of microzooplankton grazers as they responded more quickly to phytoplankton food availability. Mixotrophic dinoflagellates played a secondary role in terms of biomass when compared to heterotrophic species; nevertheless, they made up an intense late summer bloom in 2007. The photosynthetic ciliate Myrionecta rubra bloomed at the end of the sampling period. Due to its high biomass when compared to crustacean plankton especially during the spring bloom, microzooplankton should be regarded as the more important phytoplankton grazer group at Helgoland Roads. Based on these results, analyses of biotic and abiotic factors driving microzooplankton composition and abundance are necessary for a full understanding of this important component of the plankton.     

Zooplankton feeding ecology: clearance and ingestion rates of the salps Thalia democratica, Cyclosalpa affinis and Salpa cylindrica on naturally occurring particles in the Mid-Atlantic Bight

Aggregate stages of the salps Thalia democratica, Cyclosalpaaffinis and Salpa cylindrica collected by SCUBA diving in theMid-Atlantic Bight were fed with naturally occurring food assemblages.This is one of the few studies where salps have been fed withnatural food assemblages. The estimated clearance rate for allspecies based on disappearance of chlorophyll varied from 82to 444 mL individual^–1 day^–1. Cell counts showedthat T. democratica mostly ingested carbon from autotrophicnanoflagellates and autotrophic dinoflagellates. Ingestion byS. cylindrica was primarily on larger prey, such as dinoflagellates,while C. affinis ingested auto- and heterotrophic nanoflagellates.All main prey types ingested by salps corresponded to thosethat contributed most to biomass at each experimental station.Thus, salps fed on naturally occurring particles primarily inproportion to prey biomass and to their mechanical capacityto be retained and ingested. Feeding by salps on dinoflagellatesand ciliates implies that they may act not only as potentialcompetitors with microzooplankton, but also as consumers ofthem.     

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.     

Microplankton and its function in a zone of shallow hydrothermal activity:the Craternaya Bay, Kurile Islands

Biomass and productivity of microplankton were measured in theCraternaya Bay (Kurile Islands), which is influenced by hydrothermalactivity and volcanic heating. The hydrothermal fields are situatedaround its shores and underwater within the 0–20 m depth.A dense ‘bloom’ of photoautotrophic microplanktonwas observed there, dominated by diatoms, phytoflagellates andthe symbiont-containing ciliate Mesodinium rubrum. The biomassof these ciliates attained 3–11 g m^-3 in the upper waterlayer. The total biomass of the phototrophic microplankton reached30–46 g m^-3. The primary production in the water columnwas, correspondingly, enormously high: 6–10 g C m^-2 day^-1.The depth of the euphotic zone was 7 m. Pelagic photosynthesiswas inhibited in the upper 0–1 m by the spreading of alayer of low-salinity hydrothermal water. The numerical densityof bacterioplankton in the upper zone of the water column variedfrom 1 x 10⁶ to 2.9 x 10⁶ cells ml^-1, and its wet biomass from250 to 750 mg m^-3. Its production varied at stations from 70to 390 mg m^-3 day^-1. Chemosynthesis contributed up to 30% ofthis production in the sites neighbouring the hydrothermal vents.Outside their direct impact however, its share was negligible.The biomass of heterotrophic planktonic ciliates varied from30 to 270 mg m^-3. The mechanisms of possible influence of shallowvolcanic activity on development and function of microplanktonin the Craternaya Bay is discussed.     

Assemblage of Ciliated Protozoan Community in a Polluted and Non-polluted Environment in a Tropical Lake of Central Himalaya: Lake Naini Tal, India

In order to obtain information on the assemblage of Protozoain the changing environment in Lake Naini Tal, this study wascarried out for a period of one year from November 1995 to October1996. Samplings were done from the mud–water interfaceat two stations which differed considerably in their magnitudeof pollution. Station I was moderately polluted while StationII was highly polluted. Some physico-chemical parameters ofwater such as temperature, dissolved oxygen, free CO₂, pH, nitrate-nitrogen,phosphate-phosphorus and BOD were also measured. A total of23 ciliates were found at the mud–water interface of thetwo stations during the sampling period. The annual mean ofspecies richness at Station I was significantly greater (19species) than that of Station II (13 species). The species compositionwas also different at the two stations. In general, StationI supported larger ciliated species (1249 x 10³ µm³ taxon^–1)than Station II (348 x 10³ µm³ taxon^–1). Among differentfeeding groups of ciliates, the groups ‘Algivore-Bacterivore’and ‘Bacterivore’ were about twice as common atStation II (116 x 10³ cells^–1) than at Station I (55 x10³ cells^–1). The annual average ciliate community abundancewas more diverse at Station I than Station II. The annual averagebiomass in terms of carbon content for both stations was almostthe same (6.0 mg Cl^–1 for Station I and 6.1 mg Cl^–1Station II). However, different species were responsible forthe contribution to the biomass at the two stations. The valuesfor Shannon-Weiner's diversity indices at Staion I were higherthat those for Station II.     

Plankton composition and cycling of carbon during the rainy season in a tropical coastal ecosystem, Zanzibar, Tanzania

Biomass, species composition and production of the planktoniccommunity were investigated during the rainy season in May andJune 1999 outside Zanzibar Island, Tanzania. In general, theplankton biomass of different organisms was uniform betweendepths as well as over time. The integrated water column primaryproduction ranged from 204 to 4142 mg C m^–2 day^–1.Bacterial production varied between 10 and 72 mg C m^–2day^–1, comprising ~5% of the total bacterial standingstock. The data obtained from these experiments are summarizedin a carbon budget. At the most 77% of the total primary productionchannelled through the heterotrophic flagellates, ciliates andheterotrophic dinoflagellates to higher trophic levels. Of theestimated carbon demand for mesozooplankton, 28% could potentiallybe met by ciliates and heterotrophic dinoflagellates.     

The microbial and metazoan community associated with colonies of Trichodesmium spp.: a quantitative survey

Association with resource-rich particles may benefit a numberof planktonic species in oligotrophic, open-ocean regimes. Thisstudy examined communities of microbes and zooplankton associatedwith colonies of the cyanobacterium Trichodesmium spp. in theSargasso Sea. Trichodesmium colonies and seawater controls werecollected near Bermuda using SCUBA during September 1995, andJune, July and August 1996. Organisms associated with the coloniesand those in the surrounding seawater were enumerated usinglight and fluorescence microscopy. We found that 85% of theTrichodesmiumpuff and tuft colonies examined harbored associated organisms.Associated organisms included bacteria (rod and coccoid), fungi,pennate diatoms, centric diatoms, heterotrophic and autotrophicdinoflagellates, chrysophytes, hypotrich ciliates, amoebae,hydroids, juveniles and nauplii of harpacticoid copepods, andjuvenile decapods. The most common associates (in addition tobacteria) were dinoflagellates (present in 74% of the coloniesexamined), amoebae (50%), ciliates (24%), and diatoms (24%).Numbers of bacteria per colony volume averaged 8.2x10⁸ bacteriaml^-1 (range = 8.1x10⁷ – 3.5 x10⁹ bacteria ml^-1), and thedensity of associated microzooplankton and metazoans averaged6.8x10⁴ organisms ml^-1 (range = 0 – 3.6 x10⁶ organismsml^-1). Associates of Trichodesmium colonies were enriched bytwo to five orders of magnitude over plankton in the surroundingwater. This unique habitat allows for the association of primarilybenthic ciliate, diatom and copepod species and could contributesignificantly to plankton heterogeneity in the open-ocean. Thedistribution of associated organisms was affected by samplecharacteristics such as colony morphology, mucoid matrix structureand colony integrity. The influence of these factors indicatesthat succession or competition between heterotrophic microorganismsultimately determines Trichodesmium microcommunity structure.Similar processes could regulate microbial and metazoan communitiesassociated with other resource-rich microenvironments, suchas marine snow particles.     

Plankton dynamics in the marginal ice zone of the central Barents Sea during spring: carbon flow and structure of the grazer food chain

Plankton community structure was analysed during spring at four stations along a transect from the polar ice into open waters of the Barents Sea. The transect mimicks a time span of months in the biological succession during the Arctic summer. The significance of the microbial food web vs the more classical food web was evaluated using carbon budget models. The standing stocks of diatom-dominated phytoplankton and bacteria were generally high especially in connection to ice. The biomass of microzooplankton, dominated by heterotrophic dinoflagellates was significantly high, with specific growth rates following the in situ temperature. The mean ± SE specific growth rate was 0.40±0.12 d^?1 for ciliates and 0.24 ± 0.05 d^?1 for heterotrophic dinoflagellates, indicating no food limitation. The estimated total carbon requirement for microzooplankton was, at the ice-covered station, approximately 100% of the daily primary production, decreasing to 25% in the open water. Carbon-specific secondary production of the copepodsCalanus finmarchicus (Gunnerus),C. glacialis (Jaschnov),C. hyperboreus (Krøyer) andMetridia longa (Lubbock) were analysed by egg production.C. finmarchicus andM. longa were productive at all stations, including the ice-covered locations, with a maximum at 0.08 d^?1 and 0.035 d^?1, respectively. The other, more Arctic-related,Calanus spp. were virtually outspawned. The standing stock of copepods was only 10–20% of the total microbial grazer biomass. The community growth and grazing by copepods showed significantly less quantitative importance for the pelagic carbon flow than the microbial processes.     

Microzooplankton feeding behaviour: grazing on the microbial and the classical food web of African soda lakes

We investigated the feeding behaviour of the dominant microzooplankton of saline lakes in the East African Rift Valley. A set of grazing experiments revealed high ingestion rates of the two euryhaline rotifers Brachionus dimidiatus and Brachionus plicatilis and of the large-sized omnivorous ciliates Frontonia sp. and Condylostoma magnum reflecting the unique nature of tropical saline systems. The size spectrum of ingested particles was broad and even included filamentous cyanobacteria such as the commonly dominating Arthrospira fusiformis. Feeding selectivity on cyanobacteria, however, was rather low showing higher values for cryptomonads and small ciliates. Bacterial biomass was favoured by the presence of grazers, as small bacterivorous predators were reduced at an average of 13.9%, showing the cascading effect of large zooplankton on the food web structure. Overall, based on this first-time study of the microzooplankton feeding behaviour in East African soda lakes, a strong structuring effect of rotifers and large ciliates on microbial plankton communities is assumed, especially in times of high consumer biomass.     

Trophic coupling of the microbial and the classical food web in Lake Baikal,Siberia

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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.