Top-down effects of crustacean zooplankton on pelagic microorganisms in a mesotrophic lake

A series of single-factor in situ experiments was conductedin a mesotrophic lake in Brandenburg, North Germany, to studythe predatory impact of Eudiaptomus graciloides (adults, copepodites,nauplii), cyclopoid copepods (adult Diacyclops bicuspidatus,Thermocyclops oithonoides) and daphnids (adult Daphnia hyalina,Daphnia cucullata) on the microbial community (bacteria, autotrophicpicoplankton, flagellates, ciliates). All zooplankton speciestested reduced the ciliate community significantly and ingestionrates were always higher for ciliates in the 20–55 µmsize category as compared to smaller ciliates (10–20 µm).Adult E.graciloides, which exhibited the highest predatory impacton ciliates, differed from cyclopoids and daphnids by theirability to decimate ciliates to very low abundances. Ingestionrates of ciliates by the crustacean zooplankton followed thesequence E.graciloides > daphnids = cyclopoids = copepodites.While top-down control was evident for ciliates, top-down effectsdown to the autotrophic picoplankton and flagellates were mostlyrestricted to Daphnia-dominated treatments. Top-down effectswere never strong enough to produce negative bacterial growthrates. For all zooplankton tested, clearance rates for ciliatesexceeded those for phytoplankton. Besides the potential of thecrustacean zooplankton to influence the structure of ciliatecommunities, ciliates may contribute to the energy demands ofcopepods and daphnids, especially when phytoplankton resourcesare limited.     

Dominance of microflagellates over diatoms in the Antarctic areas of deep vertical mixing and krill concentrations

Phytoplankton data obtained during six summer Polish expeditionsto the Antarctic Peninsula area, are compared with concurrentlyrecorded data on water column stabilities and krill abundance.The results show that flagellates (1.5–20 µm) arenumerically dominant over diatoms in the areas of deep verticalmixing and/or extensive krill concentrations. Of 102 stationsdominated by flagellates, 85 (83.3%) are located in a well mixedwater column (>100 m) and correspond to a mean krill densityof 15–346 t Nm^–2. In the same areas, estimated flagellatecarbon biomass exceeds diatom carbon. On the other hand, ofthe 40 stations dominated by diatoms, 36 (90%) are located inareas of increased water column stability (upper mixed layerof 10–50 m) and correspond to a low mean krill biomassof 0.34–4.6 t Nm^–2. Positive correlations of flagellateto diatom (F:D) cell number ratios with the depth of the uppermixed layer suggest light limitation of diatom growth and anincreased sinking rate of diatoms relative to flagellates inthe areas of deep vertical mixing. The relationship of the F:Dratio with krill abundance suggests that krill prefer feedingon diatoms and are less efficient in grazing particles of thesize of microflagellates (<20 µm). Flagellates exceeddiatoms in an unstable water column when the phytoplankton populationsare low; both algal groups increase in numbers with growingstability. The results provide field evidence that deep verticalmixing and krill grazing create conditions for the dominanceof flagellates over diatoms. Both factors acting together arelikely to suppress diatom blooms in the Antarctic.     

Phytoplankton assemblages in the deep chlorophyll maximum layers off the Mediterranean coast of Israel

Phytoplankton assemblages in the deep chlorophyll maximum andnear-surface layers were compared at seven stations in the inshoreand offshore waters of the Mediterranean coast of Israel. Thestudy included the entire spectrum of taxonomic categories overa wide size range, comprising the nano/pico phytoplankton componentsdown to 1 µm and the larger phytoplankters consistingprimarily of diatoms and dino-flagellates. The coccolithophorids<20 µm and the monads constituted the most abundantcomponents of the phytoplankton at the deep chlorophyll maximum(DCM) and near surface layer. Certain individual species, mainlypennate diatoms and smaller dinoflagellates, seemed to adaptto the DCM to form a characteristic association.     

Grazing of planktonic diatoms by microflagellates

Colorless microflagellates (6–14 µm in diameter)were isolated from an oligotrophic lake in Western Canada, andshown to consume diatoms up to six times longer than the diameterof the flagellate. Observations were also made of morphologicallysimilar flagellates attached to diatoms from the Great Lakes,indicating that this phenomenon may occur in nature. It is suggestedthat predation by microflagellates on much larger algal cellscould potentially affect the pathway of nutrient and energytransfer in aquatic foodwebs.     

Phytoplankton composition and cell carbon distribution in Prydz Bay, Antarctica: relation to organic particulate matter and its {delta}13C values

In January-February 1991, in Prydz Bay, phytoplankton bloomwas evident in the inner shelf area with the dominant diatomsbeing represented mainly by pennate species of the Nitzschia-Fragilariopsisgroup. Dinoflagellates and naked flagellates were most abundantin the centre of the bay; however, larger heterotrophic speciesprevailed at the southern stations. Cell carbon values (average317 µg l^–1; range 92-1048 µg l^–1) foundin the bloom in the south were chiefly due to pennate diatomsand larger heterotrophic dinoflagellates. Much lower carbonvalues (average 51 µg l^–1; range 7-147 µgl^–1) in the outer shelf region were mainly contributedby large centric diatoms (70-110 mu;m) and small dinoflagellates(5-25 µm). Wide ranges of algal cell sizes were observedin both southern and northern communities; the overlapping ofsizes of diatoms and flagellates, the latter containing heterotrophs,suggested complex trophic relationships within the planktonand an enhanced heterotrophic activity in the south. North-to-southvariations in surface     

Composition and distribution of the pelagic and sympagic algal assemblages in the Laptev Sea during autumnal freeze-up

The phytoplankton and ice algal assemblages in the SiberianLaptev Sea during the autumnal freeze-up period of 1995 aredescribed. The spatial distribution of algal taxa (diatoms,dinoflagellates, chrysophytes, chlorophytes) in the newly formedice and waters at the surface and at 5 m depth differed considerablybetween regions. This was also true for algal biomass measuredby in situ fluorescence, chlorophyll (Chl) a and taxon-specificcarbon content. Highest in situ fluorescence and Chl a concentrations(ranging from 0.1 to 3.2 µg l^–1) occurred in surfacewaters with maxima in Buor Khaya Bay east of Lena Delta. Thealgal standing stock on the shelf consisted mainly of diatoms,dinoflagellates, chrysophytes and chlorophytes with a totalabundance (excluding unidentified flagellates <10 µm)in surface waters of 351–33 660 cells l^–1. Highestalgal abundance occurred close to the Lena Delta. Phytoplanktonbiomass (phytoplankton carbon; PPC) ranged from 0.1 to 5.3 µgC l^–1 in surface waters and from 0.3 to 2.1 µg Cl^–1 at 5 m depth, and followed the distribution patternof abundances. However, the distribution of Chl a differed considerablyfrom the distribution pattern shown by PPC. The algal assemblagein the sea ice, which could not be quantified due to high sedimentload, was dominated by diatom species, accompanied by dinoflagellates.Thus, already during the early stage of autumnal freeze-up,incorporation processes, selective enrichment and subsequentgrowth lead to differences between surface water and sea icealgal assemblages.     

Changes in the growth rate of phytoplankton in local upwelling around the Izu Peninsula, Japan

The composition and temporal changes in phytoplankton populationswere evaluated in local upwelling off the Izu Peninsula. Japan.in May. 1982. Phytoplankton in the upwelled water was dominatedby diatoms and that in the surrounding water by various flagellatesand monads. Phytoplankton started logarithmic growth almostimmediately after subsurface nutrient-rich water was upwellinginto the euphotic zone and nutrients were depleted within afew days. Growth enhancement was most striking among diatoms.As a result >50% of the total phytoplankton biomass of cells>2 µm was from a centric diatom. Leptocylindrus danicus.The results confirmed many past observations of the dominanceof diatoms in upwelled water. Such dominance is explained bythe rapid growth of diatoms when nutrient concentrations areenhanced.     

Seasonal plankton dynamics in a Mediterranean hypersaline coastal lagoon: the Mar Menor

The seasonal distribution of plankton in a Mediterranean hypersalinecoastal lagoon has been studied through a dataset, comprisingthe taxonomic composition and the size–abundance distributionof both phyto- and zooplankton, measured by image analysis techniquesduring a one-year time series of weekly samplings. The studiedorganisms ranged from small nanoplanktonic heterotrophic flagellates(2 µm diameter) to fish larvae (>2 µm). The phytoplanktonannual succession was characterized by a winter period dominatedby Rhodomonas spp. and Cryptomonas spp. with Cyclotella spp.as the main diatom represented, a spring phase where diatoms(mainly Cyclotella) were the dominant group with some monospecificblooms of other diatoms (mainly of Chaetoceros sp.), a summerphase characterized by diatoms with blooms of Niztschia closterium,and a post-summer phase where dinoflagellates increased withpeaks of Ceratium furca. High densities of the microbial foodweb elements, flagellates and ciliates, indicate the importanceof the microbial loop in the ecosystem. Meroplankton contributedwidely to the seasonal character of the zooplankton distribution.Copepods, represented by Oithona nana, Centropages ponticusand Acartia spp. (mainly latisetosa), remained relatively constantthroughout the year, exhibiting a lower density in the warmerwater period (July–September). At the end of the samplingperiod, a massive proliferation of copepods (>1000 ind l^–1), mainly due to O. nana, took place. The autotrophsto heterotrophs biovolume ratio (A:H) remained lower than 1throughout the year except when, occasionally, large phytoplanktoncells bloomed. Persistent very low values of A:H suggest thatadditional sources of energy, such as the microbial loop ordetrital pathways, would be needed to sustain the high heterotrophicbiovolume found in the lagoon.     

Changes in the pelagic microbial food web due to artificial eutrophication

The effect of nutrient enrichment on the structure and carbon flow in the pelagic microbial food web was studied in mesocosm experiments using seawater from the northern Baltic Sea. The experiments included food webs of at least four trophic levels; (1) phytoplankton–bacteria, (2) flagellates, (3) ciliates and (4) mesozooplankton. In the enriched treatments high autotrophic growth rates were observed, followed by increased heterotrophic production. The largest growth increase was due to heterotrophic bacteria, indicating that the heterotrophic microbial food web was promoted. This was further supported by increased growth of heterotrophic flagellates and ciliates in the high nutrient treatments. The phytoplankton peak in the middle of the experiments was mainly due to an autotrophic nanoflagellate, Pyramimonas sp. At the end of the experiment, the proportion of heterotrophic organisms was higher in the nutrient enriched than in the nutrient-poor treatment, indicating increased predation control of primary producers. The proportion of potentially mixotrophic plankton, prymnesiophyceans, chrysophyceans and dinophyceans, were significantly higher in the nutrient-poor treatment. Furthermore, the results indicated that the food web efficiency, defined as mesozooplankton production per basal production (primary production + bacterial production − sedimentation), decreased with increasing nutrient status, possibly due to increasing loss processes in the food web. This could be explained by promotion of the heterotrophic microbial food web, causing more trophic levels and respiration steps in the food web.     

Effects of Daphnia on microzooplankton communities

Intact phytoplankton and microzooplankton communities from eutrophicStar Lake were incubated for 4 days with and without Daphniapulex, Daphnia galeaia mendotae, or a natural assemblage ofDaphnia species. They were sampled at the onset and terminationof the experiment for bacterial, phytoplankton, ciliate, rotifer,copepod and cladoceran densities. The cladocerans had variedeffects on the rotifers, ranging from significant suppressionof most rotifer species (Keratella cochlearis, Polyarthra remata,Keratella crassa) in the D.pulex jars, to the suppression ofone (K.crassa) or no species in the D.galeata mendotae and StarLake Daphnia assemblage jars, respectively. Small ciliates (<30µm, longest dimension), such as Strobilidium sp. and Pseudo-cyclidiumsp., were adversely affected by most of the cladoceran treatments,while several larger ciliates (>81 µm) were unaffectedin all such treatments. Ciliates were not consistently morevulnerable to cladoceran suppression than similarly sized rotifers.The suppression of ciliates and rotifers was attributable toboth direct effects (predation, interference, or both) and indirecteffects (e.g. resource competition) of the cladocerans. ¹Present address: Department of Biology, University of Louisville,Louisville, KY 40292, USA     

Infection of Coscinodiscus spp. by the parasitoid nanoflagellate Pirsonla diadema: I. Behavioural studies on the infection process

The investigation of successive steps involved in the infectionprocess of the marine diatoms Coscinodiscus granii and Coscuwdiscuswailesii by the host-specific parasitoid nanoflagellate (PNF)Pirsonia diadenw showed that flagellates reacted chemokJinokineticallywith changes of swimming pattern to the presence of a host diatom.Chemosensory stimulation appeared to induce readiness for infection,whereas attachment and penetration of the diatom cell wall wasinduced by a mechanosensory response to morphological featureson the diatom frustules. The mean swimming speed of P.diademaflagellates decreased during their infective lifetime of 3 daysfrom an average of 78 µm s^–1 to 51 µm s^–1while the frequency of small loops in the swimming pattern increasedfrom 0.8 to 6.3 loops min^–1. At high Cgranii densities,an epidemic was delayed. It is suggested that this could becaused by overlapping gradients of extracellular material releasedby the diatoms which impaired the sensing of spatial gradientsby PNF and, therefore, the location of hosts.     

Succession of Protists on Estuarine Aggregates

Abstract Colonization by and succession of bacteria and bacterivorous protists on laboratory-made aggregates were determined over a period of 14 days during winter and spring in 1997. Aggregates were generated from natural water from the limnetic zone of the Elbe Estuary using a tilting tube roller system. Within 1 h after the beginning of the experiments, macroaggregates started to form. Aggregates reached a maximum size of 1 mm with a tendency toward large sizes at the end of the experiment after the 10th day. On the first day, high bacterial densities of more than 10⁹ cells ml⁻¹ were detected within the aggregates. The abundances of flagellates and ciliates within aggregates were also two or three orders of magnitude higher than in the surrounding water. Densities of aggregate associated organisms are comparable to those occuring in sediments. The first protistan colonizers on the aggregates were small heterotrophic flagellates, such as choanoflagellates and small euglenids. Later, beginning on the 4th day, small sarcodines and ciliates became abundant. The most abundant ciliates associated with aggregates were small species of the Hypotrichia, Cyrtophorida, and Hymenostomata. After 9 days, large omnivorous and carnivorous ciliates, such as large members of the Hypotrichia and the Pleurostomatida, occurred. In spring, large heterotrophic flagellates and amebae also appeared at this time. These findings indicated the existence of a succession of protists on newly formed aggregates and a microbial food net within the aggregates based on bacterial production. Additionally, most of the species observed during this study were adapted for living on surfaces. In natural environments they are more common in benthic than in pelagic environments. For them, aggregates are havens in the water column comparable to sediment communities. Received: 7 January 2000; Accepted: 15 May 2000; Online Publication: 28 August 2000     

Microbial Food Webs in Marine Sediments. I. Trophic Interactions and Grazing Rates in Two Tidal Flat Communities

Abstract The role of grazing by marine sediment flagellates, ciliates, and meiobenthic animals in controlling production of their bacterial and diatom prey was investigated. Several novel or modified techniques were used to enumerate prey (bacteria and diatoms), measure bacterial production, quantify proto- and micrometazoan predators, and evaluate rates of bacterivory and herbivory. The results indicated that, in a temperate, marine intertidal flat composed of fine sand, colorless nanoflagellates, ciliates, and nematodes were the most important bacterivores. Together, these organisms were responsible for removing up to 53% of bacterial production, by grazing. The observed rates of bacterivory were high enough to hypothesize that periods of grazing control of bacterial production might occur regularly in similar habitats. Colorless microflagellates, ciliates, and nematodes had high rates of diatom consumption. The combined small diatom consumption rate was equivalent to 132% of diatom standing stock per day. Trophic interactions between diatoms and micro- and meiobenthos might be a factor limiting growth of small (around 10 μm) diatoms. In coarse sands of an open beach, all micrograzers except pigmented nanoflagellates were rare, whereas bacterial and diatom assemblages were rather abundant and active. In this type of sediment, the micrograzers were able to consume only a marginal percentage of bacterial production (<1%) and diatom standing stock (3.8%), thus playing a minor role in controlling the dynamics of their prey. Received: 11 June 1996; Accepted: 13 August 1996     

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.     

Copepod growth and diatoms: insensitivity of <Emphasis Type="Italic">Acartia tonsa</Emphasis> to the composition of semi-natural plankton mixtures manipulated by silicon:nitrogen ratios in mesocosms

The feeding selectivity and the growth and reproductive success of the copepod Acartia tonsa have been studied in mesocosms fertilized at different Si:N ratios (0–1.75:1) and, therefore, at different compositions of the phytoplankton communities. Phytoplankton composition showed a strong response to nutrient ratios, with diatoms comprising >90% at Si:N ratios >1:1 of total biomass as opposed to <20% at the lowest ratio. A. tonsa strongly preferred feeding on motile prey (flagellates and ciliates) to feeding on diatoms. Nevertheless, diatoms comprised a substantial part of the diet at the highest Si:N ratios. A. tonsa egg production and the final (after 4 weeks) abundance of adults and copepodites showed no response to Si:N ratios while nauplii production slightly increased with Si:N ratios. It is concluded that the frequently reported deleterious effect of diatoms on copepod reproduction is rather unusual when copepods are confronted with a naturally diverse phytoplankton assemblage instead of clonal cultures in the laboratory.     

Infection of Coscinodiscus spp. by the parasitoid nanoflagellate Pirsonia diadema: II. Selective infection behaviour for host species and individual host cells

The parasitoid nanoflagellate (PNF) Pirsonia diadema is hostspecific for the marine centric diatom Coscinodiscus spp. Experimentsshowed that flagellates significantly prefer C. wailesii overC.granii as host species (interspecific selectivity). This preferencewas independent of light conditions (dark, irradiance of 10and 70 µmol m^–2 s^–1) and temperature (10 and15C). Among unicellular host diatoms, the infection behaviourwas selective for individual cells: already infected C.graniicells were more attractive for further flagellate attachmentthan non-infected cells (intraspecific selectivity). Individualcells (     

Summer phytoplankton community structure and growth in a regional upwelling area off hachijo Island,Japan

Phytoplankton community structure both in terms of taxonomic composition and size distribution, and growth were examined in a regional upwelling which occurred in a Kuroshio region in summer. Organic carbon abundance of diatoms was markedly enhanced by the upwelling and they accounted for 87% of the total phytoplankton carbon. Nitzschia pungens Grunow was numerically most prominent accompanied by Chaetoceros and Rhizosolenia species. On the other hand, only a small increase was observed in phytoplankton other than diatoms, which consisted of unicellular cyanobacteria, coccolithophorids, gymnodinoids, and so called monads and flagellates. Division rate of diatoms from the upwelled water was twice that of the other forms. This difference in growth response among phytoplankton groups resulted in a shift of community structure to dominance of diatoms from that of the other forms. The time needed for this shift was estimated to be at least ≈ 3 days.     

Phytoplankton and zooplankton seasonal dynamics in a subtropical estuary: importance of cyanobacteria

A seasonal study of phytoplankton and zooplankton was conductedfrom 1999 to 2001 in Pensacola Bay, Florida, USA, to furtherthe understanding of pelagic food webs in sub-tropical estuaries.Monthly measurements included size-fractionated chlorophyll(whole water, <5 µm, <20 µm), net- and picophytoplanktoncomposition analyzed using microscopy, flow cytometry, and HPLCpigment analysis. Additionally, zooplankton abundance and dryweight were determined from net tows. The results show a phytoplanktoncommunity dominated by the small size fraction (<5 µm),especially during the warm periods. The <5 µm chlorophyllfraction was strongly correlated with cyanobacterial abundanceand zeaxanthin. Cyanobacteria (cf. Synechococcus) abundancepeaked during summer in the upper estuary, typically exceeding3 x 10⁹ L^-1, and was strongly correlated with temperature. Cyanobacteriaabundance at the freshwater end of the Bay (in the EscambiaRiver) was very low, suggesting that cyanobacteria were notdelivered via freshwater. Two pigmentation types of cyanobacteriawere observed. Phycoerythrin-containing cells (PE-rich) weremore abundant at the marine end, while phycocyanin-containingcells (PC-rich) were more abundant in the upper estuary. Thelarger algae (>5–10 µm) were predominantly composedof diatoms, followed by chlorophytes, cryptophytes and dinoflagellates.The three most abundant genera of diatoms were Thalassiosira,Pennales and Cyclotella. Zooplankton biomass averaged 12.2 µgC L^-1, with peak biomass occurring during May (     

Arctic Sea ice biota: design and evaluation of a mesocosm experiment

A mesocosm experiment (enclosure volume 220 l) was designed such that sea ice inhabited by Arctic Sea ice organisms was formed and maintained under natural conditions at 66°N in Rovaniemi, Finland. The experiment was run from natural freezing in December 1994 to melting in April 1995. The ice was inhabited by diatoms, chlorophyceae, heterotrophic flagellates, ciliates, nematodes and turbellarians. Biomass in the ice, expressed as Chlorophyll a concentration, was 20–110 μg l⁻¹; total cell densities varied from 5 × 10⁶ to 35 × 10⁶ cells l⁻¹. Amongst phototrophic organisms, a succession from a flagellate-dominated community (Chlamydomonas sp.) to a multi-species diatom-dominated community was observed. Typical Arctic species such as Nitzschia frigida and Melosira arctica were present in the ice. Bacterial concentration varied between 2 × 10⁸ and 7 × 10⁸ cells l⁻¹. At least two trophic levels were present in the ice. Received: 3 April 1997 / Accepted: 9 September 1997