The vitamin B requirement of Phaeocystis globosa (Prymnesiophyceae)

In batch cultures of flagellates and non-flagellate cells ofPhaeocystis globosa, the biomass yield was significantly enhancedby the addition of a mixture of the vitamins thiamine (B1),cyanocobalamin (B12) and biotin (H). A bioassay with B1 andB12 using the non-flagellate cells of P.globosa showed thatthis prymnesiophyte is a B1 auxotroph. The bioassay also indicateda significant difference in growth rate between culture mediumwith 10 nmol l^–1 B1 (µ = 0.80 day^–1) and culturemedium with 10 nmol l^–1 B12 (µ = 0.52 day^–1).These findings are discussed in relation to the hypothesis thatcentric diatoms, through vitamin B1 excretion or B12 depletion,initiate Phaeocystis blooms. It is concluded, however, thatan alternative hypothesis, that diatoms provide a solid substratefor colony initiation, has more experimental support.     

Daily irradiance governs growth rate and colony formation of Phaeocystis (Prymnesiophyceae)

Phaeocystis was cultured at a range of ecologically significantdaily irradiances under nutrient-replete conditions. Below athreshold of 100 W h m^–1 day^–1, the cells were smalland flagellated, and remained solitary. Above this threshold,the cells were larger and able to form colonies. Growth rateand colony formation were maximum at sea surface irradiances(>700 W h m^–2 day^–2). Presumably, colonial growthis a strategy to maintain optimum growth rates in the watercolumn. Sinking, nutrient-stressed colonies reach low irradiancesand colonial cells can transform into small solitary flagellatedcells. These observations are important in understanding theecology and life cycle of Phaeocystis.     

Potential toxicity of chrysophytes affiliated with Poterioochromonas and related 'Spumella-like' flagellates

The chrysophyte genera Poterioochromonas and Ochromonas andtheir heterotrophic analogons, i.e. the ‘Spumella-like’flagellates, account for a significant and often dominatingfraction of the pelagic nanoplankton. Even though several osmotrophicallyand autotrophically grown strains of Ochromonas and Poterioochromonasare assumed to produce toxins, the potential toxicity has beeninvestigated neither for its association with bacterivorousnutrition nor within the related exclusively heterotrophic ‘Spumella-like’flagellates. We investigated the toxic potential of severalflagellate strains using cultures of flagellates, cell extractsand filtrate of flagellate cultures. The effect on potentialpredators was exemplarily tested for the cladoceran Daphniamagna and the rotifer Platyias sp. All tested heterotrophicand mixotrophic flagellate strains were toxic to zooplanktonat abundances exceeding 10⁴ flagellates mL^–1. For therotifers, survival on any of the flagellate strains was significantlylower than that in the control treatment (P < 0.001) alreadyafter 24 h. We conclude that (i) ‘Spumella-like’flagellates can be toxic to zooplankton, (ii) all tested flagellates,i.e. heterotrophic and mixotrophic flagellates, feeding phagotrophicallycan be toxic to zooplankton and (iii) sublethal effects maybe observed at typical field abundances, even though acute toxicityseems to be restricted to flagellate abundances observed onlyat peak events.     

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 (     

Effects of diffusion and upwelling on the formation of red tides

In this paper, records on the timing and location of specificred tides monitored once or twice a week in Mikawa Bay, Japan,are related to horizontal and vertical mixing rates determinedfrom a numerical model. Horizontal (K_h) and vertical (K_z) diffusioncoefficients, and upwelling velocities, were estimated usinga box model analysis. In the wind-mixed period and in the upperlayer during the stratified period, K_h was estimated to be ofthe order of 10² m² s^–1. During the stratified period,K_z was estimated to be of the order of 10^–5 m² s^–1.The upwelling velocity was calculated to be in the range 0.35–5.1m day^–1 with an average of 1.5 m day^–1. Comparisonbetween the literature values of the specific growth rate (µ)of the red tide-forming diatoms and calculated K_h values duringthe red tides show that diatoms which have a low µ cannotform red tides in a strongly diffusive environment, while specieshaving a high µ can form red tides even in a strong diffusiveenvironment. On the other hand, no clear relationship was foundbetween µ of the flagellate group and K_h, although theflagellate group formed red tides even in severe diffusive conditions.From the comparison between the literature values of sinkingrate and swimming speed and the physical parameters associatedwith vertical processes, it was concluded that flagellates willform red tides, even in severe diffusive conditions, by usingtheir swimming ability, while diatoms form red tides by theirhigh growth rates with the aid of vertical diffusion and theupwelling movement of water.     

Effects of temperature and salinity on the growth of the red tide flagellates Heterocapsa circularisquama (Dinophyceae) and Chattonella verruculosa (Raphidophyceae)

Growth responses of the red tide flagellates, Heterocapsa circularisquama(Dinophyceae) and Chattonella verruculosa (Raphidophyceae),were examined with 36 different combinations of temperature(5–30°C) and salinity (10–35 PSU). Heterocapsacircularisquama did not grow at or below a temperature of 10°C.The maximum growth rate of H.circularisquama (1.3 divisionsday^–1) was obtained with a combination of 30°C and30 PSU. In contrast, C. verruculosa did not grow at 10 PSU andat temperatures of 25°C or more. The maximum growth rateof C. verruculosa (1.74 divisions day^–1) was obtainedwith a combination of 15°C and 25 PSU. A significant temperature-salinityinteraction on growth was found by factorial analysis. Basedon the physiological characteristics obtained in the presentstudy, these novel flagellates have a potential for future outbreaksof red tides in pre viously unaffected waters.     

Development, growth and egg production of Centropages abdominalis in the eastern subarctic Pacific

Centropages abdominalis is a neritic, omnivorous, temporallyabundant copepod present throughout the subarctic Pacific andits marginal seas. The two main objectives of this study wereto determine how temperature influences the development of C.abdominalis and whether growth rates of in situ populationsmay be limited by available food. At 6.9°C, median developmenttime from eggs laid to 50% adults was 42 days and the averageweight-specific somatic growth rate was 0.17 day^–1. At4.6°C, median development time to adult was 59 days (projected)and growth rate averaged 0.08 day^–1, suggesting that 4.6°Cmay be approaching the lower temperature for development andgrowth in this species. The functional relationship betweendevelopment time and temperature was established over the temperaturerange in which this species occurs. The in situ adult growthrates between 10 and 13°C averaged 0.14 day^–1 andwere generally lower than the laboratory-reared juvenile growthrates, which may indicate that adult C. abdominalis are foodlimited in the field during summer and autumn.     

Growth and grazing rates of Protoperidinium hirobis Abe, a thecate heterotrophic dinoflagellate

Growth and feeding rates of a laboratory-reared small thecateheterotrophic dinoflagellate, Protoperidinium hirobis Abè,grown on the diatom Leptocylindrus danicus, were measured inbatch cultures. Ingestion rates were determined directly bythe enumeration of empty diatom frustules produced by dinoflagellatefeeding. Both growth and feeding rates saturated at diatom concentrationsof {small tilde} 10⁴ cells ml^–1, and reached maximum valuesof 1.7 divisions day^–1 and 23 diatoms grazer^–1 day^–1,respectively. This rate of cell division is notably high comparedto photosynthetic dinoflagellates, which seldom grow fasterthan 1 division day^–1. A maximal clearance rate of 0.5µl h^–1 was measured. Mean cell size varied proportionallywith food abundance, with food-saturated cells having doublethe mean volume of food-depleted cells. Tuning of cell divisionand grazing rate patterns were also examined; while mitosisoccurred chiefly during the dark period, no diel variationsin feeding rate were detected. These rates represent the firstdirect growth and ingestion measurements to be made for a thecateheterotrophic dinoflagellate. They serve to underscore one functionthese dinoflagellates perform within the microzooplanktonicfood web: that of transforming large diatoms into particlesmore easily ingested by microzooplankters.     

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.     

Feeding and metabolism of the siphonophore Sphaeronectes gracilis

The in situ predation rate of the siphonophore Sphaeronectesgracilis was estimated from gut content analysis of hand-collectedsiphonophores and from laboratory data on digestion rates ofprey organisms. At daytime prey densities of 0.25 copepods 1^–1,S. gracilis was estimated to consume 8.1 – 15.4 prey day^–1siphonophore^–1. From data on abundances of siphonophoresand copepods, S. gracilis was estimated to consume 2–4%of the copepods daily. In laboratory experiments, ingestionrates averaged 13.8 prey day^–1 siphonophore^–1 atprey densities of 5 copepods 1^–1 and 36.9 at 20 copeods1^–1. This was equivalent to a specific ingestion rate(for both carbon and nitrogen) of –17% day^–1 and45% day^–1, respectively, while specific ingestion in situwas only 2% day^–1. Ammonium excretion averaged 0.095 µg-atsiphonophore^–1 day^–1 at 5 prey 1^–1, and 0.162at 20 prey 1^–1. The specific respiration (carbon) andspecific excretion (nitrogen as ammonium) were calculated tobe 3% day^–1 at the lower experimental food level, and5% day^–1 at the higher food level. ¹Contribution from the Catalina Marine Science Center No. 66. ²Present address: Dept. of Biology, University of Victoria,Victoria, B.C., Canada V8W 2Y2.     

The relative importance of food and temperature to copepod egg production and somatic growth in the southern Benguela upwelling system

The fecundity and somatic growth rates of Calanus agulhensisand Calanoides carinatus, the dominant large calanoid copepodsin the southern Benguela upwelling system, as well as the fecundityof several other common copepods, were measured between Septemberand March of 1993/94 and 1994/95. Mean egg production of mostcopepods was low at >30 eggs female^-1 day^-1 {Calanoides carinatus23.7, Calanus agulhensis 19.0, Neocalanus tonsus 16.1 and Rhincalanusnasutus 26.1), whereas the mean fecundity of Centropages brachiatuswas significantly greater (83.6 eggs female^–1 day^-1).This study also presents the first comprehensive field estimatesof the fecundity of Nanno-calanus minor (mean: 26.1 eggs female^–1day^–1, range: 0.0–96.2 eggs female^–1 day^–1)and of somatic growth of N6 and all copepodite stages of Calanoidescarinatus (decreasing from 0.58 day^–1 for N6 to 0.04 day^–1for C5). Somatic growth rates of Calanus agulhensis also declinedwith age: from 0.57 day1 for N6 to 0.09 day1 for C5. Data ongrowth rates were used to assess the relative importance offood [as measured by total chlorophyll (Chi) a concentration],phytoplankton cell size (proportion of cells >10 µm)and temperature to the growth of copepods. Multiple regressionresults suggested that fecundity and somatic growth rates werepositively related to both Chi a concentration and phytoplanktoncell size, but not to temperature. Although it was not possibleto separate the effects of Chi a concentration and phytoplanktoncell size, data from previous laboratory experiments suggestthat copepod growth is not limited by small cells per se, butby the low Chi a concentrations that are associated with theseparticles in the field. Despite growth not being directly relatedto temperature, a dome-shaped relationship was evident in somespecies, with slower growth rates at cool (<13°C) andwarm (>18°C) temperatures. The shape of this relationshipmirrors that of Chi a versus temperature, where poor Chi a concentrationsare associated with cool and warm temperatures. It is concludedthat the effect of food limitation on growth of copepods outweighsthat of temperature in the southern Benguela region. Sourcesof variability in relationships between growth and Chi a concentrationare discussed.     

Dynamics of herbivorous grazing by the heterotrophic dinoflagellate oxyrrhis marina

In a series of batch experiments in the dark the heterotrophicdinoflagellate Oxyrrhis marina grazed three phytoplankton prey(Phaeodactylun tricornutum, Isochrysis galbana and Dunaliellateriolecta) with equal efficiency. Growth rates of the dinoflagellateranged between 0.8 and 1.3 day–1 Maximum observed ingestionrates on a cell basis varied according to the size of the preyfrom about 50 cells flagellate^–1 day^–1 when D.tertiolectawas the prey to 250–350 cells fiagellate^–1 day^–1when the other species were eaten. However, when compared ona nitrogen basis, ingestion rates were independent of prey type.Both ingestion and growth ceased when prey cell concentrationsfell below a threshold concentration of about 10⁵ cells ml^–1.Maximum specific clearance rates were 0.8x10⁴0ndash;5.7x10⁴it day which is considerably lower than that found for heterotrophicdinoflagellates in oceanic waters and may explain why O.marinagenerally thrives only in productive waters. The timing of NHregeneration was linked to the C:N ratio of the prey at thestart of grazing. Regeneration efficiencies for NH₄. never exceeded7%; during the exponential phase and were 45% well into thestationary phase. These results are comparable to those obtainedwith heterotrophic flagellates and demonstrate that the bioenergeticpatterns of grazing and nutrient cycling by different protozoaare very similar. Moreover, they support the notion that toachieve 90+% nutrient regeneration in the open ocean, as iscurrently believed, the microbial food loop must consist ofmultiple feeding steps. Alternatively, nutrient regenerationefficiencies may be considerably lower than 90%.     

Response of ciliates and Cryptomonas to the spring cohort of a cyclopoid copepod in a shallow hypereutrophic lake

The impact of a cyclopoid copepod population on the protozoacommunity (two ciliate categories and Cryptomonas) was assessedweekly during the spring cohort of Cyclops vicinus (one monthduration) in hypereutrophic Lake Søbygård by insitu gradient experiments with manipulation of ambient zooplanktonabundance. As C.vicinus always made up >92% of the zooplanktonbiomass, the response of protozoa is assumed to be a resultof predation by the copepod. Significant effects of copepodbiomass on protozoa net population growth rates were obtainedin the four experiments. Copepod clearance rates were significantlyhigher on oligotrichs than on prostomatids and Cryptomonas butdeclined for all three protozoa categories during the firstthree weeks of the copepod cohort, probably because of the changein developmental instar composition of the copepod population.Grazing impact on protozoa at ambient copepod abundance wasconsiderable (range, 0.05–0.87 day^–1) and could,together with the estimated reproductive potential of protozoans(range, –0.20–0.87 day^–1), account for thedecline in abundance and biomass of protozoa during the cohortdevelopment. Carbon flow from the protozoa to C.vicinus (range,2.8–23.5 µg C l^–1 day^–1) documents thepresence of a trophic link between protozoa and the spring cohortof C.vicinus in Lake Søbygård.     

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.     

Specific growth rates of heterotrophic plankton organisms in a eutrophic lake during a spring bloom

The in situ growth of the dominating pelagic organisms at severaltrophic levels was investigated during a spring bloom characterizedby well-mixed cold water. The study includes primary productionand the carbon flow through the nano-, micro- and mesozooplanktonpopulations based on population dynamics and specific growthrates. The phytoplankton biomass and production were totallydominated by small algae <20 µm. of which {small tilde}5%were <3µm. potentially a food source for the nano-and microzooplankton. The mean carbon-specific primary productionwas 0.15 day^–1 and was regulated solely by light. Themean volume-based specific growth rate of bacterioplankton wasmodest. 0.1 day^–1. and probably controlled by the lowtemperature. The volume-based specific growth rates of heterotrophicnanoflagellates. ciliates. rotifers and copepods were 0.35.0.13. 0.16 and 0.03 day^–1, respectively. The observedgrowth of the heterotrophic plankton was generally not foodlimited, but was controlled by temperature. The stable temperatureduring the experiment therefore allows a cross-taxonomic comparisonof specific growth rates. The b exponent in the allometric relationship(G = aV^th) between volume-specific growth rate (G) and individualbody size (V) was –0.15 ± 0.03 for all filtratingzooplankton. indicating an in situ scaling not far from thephysiological principles onginally demonstrated for laboratorypopulations.     

Impacts of nutrients and zooplankton on the microbial food web of an ultra-oligotrophic lake

In ultra-oligotrophic lakes and the sea, calanoid copepods arethe dominant mesozoo-plankton and cladocerans are generallysparse or absent. To determine the effects of predation andnutrient enrichment on the pelagic microbial food web of anultra-oligotrophic lake, we added copepods and cladocerans atlow biomasses (<60 µg l^–1 to in situ enclosuresin Lake Wakatipu, New Zealand, in the presence and absence ofadded nutrients (nitrogen and phosphorus). In response to nutrientfertilization, the concentrations of phototrophs >3 µmand heterotrophic bacteria increased by 50 and 15%, respectively,over 4 days, but those of cyanobacterial picoplankton decreasedby 68%. The presence of calanoid copepods (Boeckella dilatata)at ambient densities (1 and 4 l^–1) rapidly and severelysuppressed ciliate population growth over 4 days and also loweredthat of flagellates >3 µm, even when microbial growthwas enhanced by added nutrients. The presence of a small cladoceran,Ceriodaphnia dubia, at double the densities, but similar biomasses,to those of copepods, depressed the net growth rates of ciliatesand flagellates to a lesser degree. The net growth rate of heterotrophicbacteria after 4 days declined with flagellate abundance, consistentwith the possibility of regulation by flagellates. Althoughbacteria and algae increased in response to nutrient fertilization(bottom-up control), predation (top-down control) appeared toplay an important role in structuring the microbial food webof this ultra-oligotrophic lake in summer.     

Primary and bacterial production compared to growth and food requirements of Daphnia longispina in Lake Kvernavatnet, west Norway

Primary production, and bacterial production as measured byincorporation of [³H-methyl]thymidineinto ice cold TCA insolublematerial were investigated during 1984 in Lake Kvernavatnet,west Norway. Primary production averaged 222 mg C m^–2day^–1 and bacterial production averaged 163 mg C m^–2day^–1. The bacterial production in the euphotic pelagiczonecontributed -60% of the total pelagic bacterial production.The zooplankton was dominated byDaphnia longispina. From growthexperiments with animals fed only natural food in coarse filteredlake water, the population daily growth increments were calculated.The average production of D.longispina was 151 mg C m^–2day^–1 during the period investigated. The estimated primaryproduction was too low to sustain both the bacterial productionand the zooplankton food requirements. These results imply thatthe carbon cycle of the lake is dependent on the supply of allochtonousmaterial, or that the current methods for measuring productionrates in aquatic environments are systematical erratic.     

Food regulation of growth and maturation in a natural population of Aurelia aurita (L.)

Growth and maturity development of the moon jellyfish. Aureliaaurita, were recorded in Vgsbpollen, a small and semi-enclosedbay on the Norwegian west coast, and compared to those of medusaetransferred to excess food and starving conditions, respectively.Mesozooplankton were extremely scarce in Vgsbpollen. The abundanceand biomass of the medusae in the poll were higher than thosetypicallyfound in open waters, reaching a maximum of 22 ind.m^–3 and 710 mg C m^–3 in June. The average diameterof medusae in the p increased to 8 cm until the last part ofJune, with an instantaneous growth rate between 1.5 and 20%day^–1, thereafter retarding somewhat, giving a negativegrowth rate of up to 2.6% day^–1. Starving medusae showeda negative growth rate ofup to 13.4% day^–1, and all thernedusae were dead after 49 days. Well-fed medusae showed avery stable growth over a 56 day period, diverging from thepollpopulation from early June, and with a growth rate between3.8 and 9.8% day^–1. Medusae from the pollpopulation begancarrying planulae on their oral arms when at least 5 cm in diameter,whereas not even the largest medusa of 15.6 cm diameter amongthose in the well-fed group produced any planulae. For the firsttime, it is thus explicitly shown that thesize and maturityof A.aurita are externally controlled through food availability.Scarcity of food reduces the growth rate, but also changes theenergy allocation towards reproduction, which thus occurs ata smaller size than for well-fed rnedusae. Its plasticity makesit possible for this species to exploit environments with lowadvection of food and develop high abundance in such environments,without losing fecundity.     

Pseudobalanion planctonicum (Ciliophora, Prostomatida): ecological significance of an algivorous nanociliate in a deep meso-eutrophic lake

Pseudobalanion plancioracum was the most abundant ciliate speciesin the pelagic zone of Lake Constance, FRG, over a 3 year period.Annual averages accounted for 30, 35 and 43% of total ciliatenumbers in the uppermost 20 m of the water column in 1987, 1988and 1989 respectively. Highest cell numbers were observed inearly spring, simultaneously with the first phytoplankton maximum.The small ciliate (mean length 15 µm, mean cell volume1300 µm³) is a raptorial feeder and predominantly consumesphytoplankton. In laboratory cultures, P planctonicum grew wellon a diet of Rhodomonas sp. (Cryptophyceae). Maximum growthrates increased from 0.46 day^–1 at 5 5°C to 1.52 day^–1at 18.5° C, while temperatures above 21°C were lethal.Depending on food concentration, 0.2–4.4 Rhodomonas cellswere ingested per ciliate and hour.     

Trophic links in the lowland River Meuse (Belgium): assessing the role of bacteria and protozoans in planktonic food webs

Trophic interactions within the plankton of the lowland RiverMeuse (Belgium) were measured in spring and summer 2001. Consumptionof bacteria by protozoa was measured by monitoring the disappearanceof ³H-thymidine-labelled bacteria. Metazooplankton bacterivorywas assessed using 0.5-µm fluorescent microparticles (FMPs),and predation of metazooplankton on ciliates was measured usingnatural ciliate assemblages labelled with FMPs as tracer food.Grazing of metazooplankton on flagellates was determined throughin situ incubations with manipulated metazooplankton densities.Protozooplankton bacterivory varied between 6.08 and 53.90 mgC m^–3 day^–1 (i.e. from 0.12 to 0.86 g C^–1bacteria g C^–1 protozoa day^–1). Metazooplankton,essentially rotifers, grazing on bacteria was negligible comparedwith grazing by protozoa (1000 times lower). Predation of rotiferson heterotrophic flagellates (HFs) was generally low (on average1.77 mg C m^–3 day^–1, i.e. 0.084 g C^–1 flagellatesg C^–1 rotifers day^–1), the higher contribution ofHF in the diet of rotifers being observed when Keratella cochleariswas the dominant metazooplankter. Predation of rotifers on ciliateswas low in spring samples (0.56 mg C m^–3 day^–1,i.e. 0.014 g C^–1 ciliates g C^–1 rotifers day^–1)in contrast to measurements performed in July (8.72 mg C m^–3day^–1, i.e. 0.242 g C^–1 ciliates g C^–1 rotifersday^–1). The proportion of protozoa in the diet of rotiferswas low compared with that of phytoplankton (<30% of totalcarbon ingestion) except when phytoplankton biomass decreasedbelow the incipient limiting level (ILL) of the main metazooplantonicspecies. In such conditions, protozoa (mainly ciliates) constituted50% of total rotifer diet. These results give evidence thatmicrobial organisms play a significant role within the planktonicfood web of a eutrophic lowland river, ciliates providing analternative food for metazooplankton when phytoplankton becomesscarce.