Limiting effect of abioseston on food ingestion, postembryonic development time and fecundity of daphnids in Lake Balaton (Hungary)

During ice-free periods, seston ranges from 10 to 600 and averages29.2 mg dry wt 1^–1 in Lake Balaton, the largest shallowlake in Central Europe. Most (80–99%) of the seston consistsof 1–10 µm sized mineral particles. The abiosestoncauses permanent food limitation in the ingestion of the ediblephytoplankton (1–22 µm fraction) by Daphnia cucullataand D.galeata. The postembryonic development time of D.galeataincreases and its fecundity decreases with increasing abioseston.The fecundity and the mortality of D.galeata are balanced atan abioseston concentration of 20.0 mg dry wt 1^–1.     

Seasonal variability in the relationship between body length and individual dry weight as related to food abundance and clutch size in two coexisting Daphnia species

Dry weights of individual Daphnia galeata and D. hyalina, whichcoexist in Lake Constance, were determined weekly during a twoyear investigation. From each sample and each species a length–weightrelationship (LWR) was calculated resulting in 54 (65) regressionequations based on N=2466 (3032) in D. galeata (D. hyalina).The constants (Ln(A)) and (B) of the LWRs showed a marked seasonalvariability, ranging from Ln(A)=1.0784–2.4740(1.0152–2.3860)and B=2.14–4.20 (2.11–4.15). The seasonal variabilityof the LWR was estimated with four different models. Condensingthe 54 (65) equations into one seasonal cycle resulted in amodel with an explained variance of r²=0.873 (0.869). Threemore models were established by calculating multiple regressionswith take stratification, average fecundity (E), and food concentrationas determining variates. Differences in LWRs of the two Daphniaspecies were significant (p <0.001). As the most useful approximationduring the period of stratification, LWRs are recommended forD. galeata: Ln(W)=(1.5674+0.0287^*(E))+Ln(L)^*(3.3611+0.0111^*(E)) as a generalized LWR for an epilimnetic daphnid, and for D.hyalina: Ln(W)=(1.5593+0.0613^*(E))+Ln(L)^*(3.2709+0.0017^*(E)) as LWR for a daphnid which migrates diurnally between epi andhypolimnion. The explained variance of this model is r²=0.843(0.826). The influence of food concentration in the epilimnionon LWRs was found to be significant on Ln(A) but not on (B). ^*This research project was supported by the Deutsche Forschungsgemeinschaft     

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.     

Egg size and clutch size in two Daphnia species grown at different food levels

The effect of three different Scenedesmus food concentrations(0.04, 0.2 and 1 mg C l^–1) on maternal investment wasstudied in two cladoceran species of similar size, Daphnia pulicariaand D.hyalina. It was observed that as food concentration decreased(between 1 and 0.2 mg C l^–1), there was an increase insize, protein content, lipid content, carbon and mass of theegg, while, at the same time, the clutch size of the femalesbecame smaller. Such an increase in ‘per offspring investment’was reflected in an increase in body length, body carbon andbody mass of neonates as the food available for females decreased.However, in D.pulicaria this tendency was not maintained downat 0.04 mg C l^–1 in which there was a decrease of theegg characteristics mentioned above. Although, there are notavailable all the egg and neonate parameters of D.hyalina at0.04 mg C l^–1, the body length of the neonates was largerthan at 0.2 and 1 mg C l^–1. These results show that, asfood diminishes, these two cladoceran species are able to respondby decreasing clutch size, but increasing the size of egg, therebyincreasing the probability of neonate survival. This tendencyis probably maintained until the food concentration is too lowand the females have to reduce the energy allocated for reproduction.     

Daphnia populations in three interconnected lakes with roach as the principal planktivore

Data on two co-existing Daphnia species, D.cucullata (G.O.Sars)and D.hyalina (Leydig), from three neighboring and interconnectedlakes in northern Poland, revealed an extremely confined rangeof population density fluctuations throughout summer in eachof the two species in each of the three lakes, with D.cucullatabeing an order of magnitude more abundant than D.hyalina (meanof 11.5 ind. l^–1 for D.cucullata and 1.1 ind. l^–1for D.hyalina with 95% confidence limits of ±2.5 and±0.5 ind. l^–1, respectively), in spite of markedlychanging fecundity. There was no apparent phase of decline orincrease that could be related to the distinct phases of lowor high fecundity resulting from different food levels. Analysisof the gut contents of roach (Rutilus rutilus), the dominantplanktivore in the three lakes, revealed identical selectivityfor each of the two Daphnia species, suggesting that the lowerdensity of the D.hyalina population was compensated for by thegreater conspicuousness of individuals of this species. It isconcluded that the population density of these Daphnia speciesremains far below the carrying capacity of the habitat and doesnot depend on food levels. Food availability merely sets therate of population increase, while the actual population densityin the lakes studied reflects the species' vulnerability topredation by planktivorous fish.     

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.     

Laboratory study of the spawning rate of the calanoid copepod Centropages typicus: effect of fluctuating food concentration

The spawning rate of laboratory-reared Centropages typicus fedHymenomonas elongala increases with food concentration, up toa value of {small tilde}2800 µg C (16 500 cells) ml^–1.An alternation of a low food (1000 cells ml^–1) and highfood concentration (16 500 cells ml^–1) is not favourableto egg release when its periodicity is 1 or 2 days, whereasit may be of advantage if it is longer (3–6 days). Inthe latter case, Centropages typicus will benefit best fromthe rich food diet if this coincides with (or just follows)the last moult.     

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.     

GROWTH OF JUVENILE BITHYNIA TENTACULATA (PROSOBRANCHIA, BITHYNIIDAE) UNDER DIFFERENT FOOD REGIMES: A LONG-TERM LABORATORY STUDY

The growth of juvenile Bithynia tentaculata (Proso-branchia,Bithyniidae) was measured under controlled laboratory conditionsover 18 months. The animals were fed with different concentrationsof suspended food (Chlamydomonas reinhardii at 2, 000 cells.ml^–1and at 10, 000 cells.ml^–1), solid food (lettuce) and combinationsof both (lettuce with 2, 000 cells.ml^–1 and with 10, 000cells.ml^–1 Chlamydomonas). The growth of all animals wasmeasured weekly, and after 1 years final shell sizes, shelldry weights and ash-free dry weights, as well as soft body dryweights and ash-free dry weights were determined. Survival rateof the animals increased from 20% when fed with 2, 000 algalcells.ml^–1 to 75% with lettuce and 10, 000 cells.ml^–1as food. Final shell sizes and shell weights were not influencedby food, but soft body dry weights were significantly reducedwhen animals were fed on suspended food only. Reproduction (i.e.egg laying) was observed at the end of the second summer whenlettuce (with or without algal addition) was given as food.The ecological consequences of these results are discussed. (Received 7 April 1993; accepted 9 August 1994)     

In situ filtration responses of Daphnia galeata to changes in food quality

In the stoichiometric study of phyto–zooplankton interactions,a controversy exists about how Daphnia species regulate theirfeeding rate when submitted to low-quality food (i.e. high dietarycarbon:phosphorus [C:P] ratio). In this study, we gathered dataover 3 years on in situ clearance rates of a Daphnia galeatapopulation, by conducting grazing experiments from April 1998to October 2000 in the Esch-sur-Sûre reservoir (Grand-Duchyof Luxembourg). Observed clearance rates (2.5–13.5 mLindividual^–1 day^–1, mean 7.0 mL individual^–1day^–1) were correlated with population and environmentalvariables. Mean body size of Daphnia individuals was the bestpredictor of clearance rate (r² = 0.639), followed by watertemperature (r² = 0.262) and P concentration in the seston (r²     

Feeding selectivities and food niche separation of Acartia clausi, Penilia avirostris (Crustacea) and Doliolum denticulatum (Thaliacea) in Blanes Bay (Catalan Sea, NW Mediterranean)

Selectivity-size spectra, clearance and ingestion rates andassimilation efficiencies of Acartia clausi (Copepoda), Peniliaavirostris (Cladocera) and Doliolum denticulatum (Doliolida)from Blanes Bay (Catalan Sea, NW Mediterranean) were evaluatedin grazing experiments over a wide range of food concentrations(0.02–8.8 mm³ L^–1 plankton assemblages from BlanesBay, grown in mesocosms at different nutrient levels). Acartiaclausi reached the highest grazing coefficients for large algae>70 µm (longest linear extension), P. avirostris forintermediate food sizes between 15 and 70 µm, and D. denticulatumfor small sizes from 2.5 to 15 µm. Penilia avirostrisand D. denticulatum acted as passive filter-feeders. Acartiaclausi gave some evidence for a supplementary raptorial feedingmode. Effective food concentration (EFC) decreased linearlywith increasing nutrient enrichment for D. denticulatum andfollowed domed curves for A. clausi and for P. avirostris withmaximum values at intermediate and high enrichment levels, respectively.Clearance rates of crustacean species showed curvilinear responseswith narrow modal ranges to increasing food concentration. Clearancerates of D. denticulatum increased abruptly and levelled intoa plateau at low food concentrations. Mean clearance rates were13.9, 25.5 and 64.1 mL ind.^–1 day^–1, respectively.No clearance could be detected for A. clausi at food concentrations<0.1 mm³ L^–1 and for P. avirostris at food concentrations     

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.     

Positive effects of UV radiation on a calanoid copepod in a transparent lake: do competition, predation or food availability play a role?

Zooplankton tolerant to ultraviolet radiation (UVR) could beindirectly affected by UVR through interactions with UV-sensitivespecies in the same ecosystem. In Lake Giles, Pennsylvania,USA, the calanoid copepod Leptodiaptomus minutus is more UVRtolerant than the cohabiting species Daphnia catawba and Cyclopsscutifer. We asked whether L. minutus is affected by UV-inducedmortality of a food competitor (D. catawba) or a predator ofits nauplii (C. scutifer). We conducted two in situ enclosureexperiments with six treatments: L. minutus alone, L . minutus+ Daphnia and L. minutus + Cyclops in the presence and absenceof UVR. There were few differences in survival among treatmentsin Experiment 1, which had enhanced food and a cumulative UVR(320 nm) dose of 9.3 kJ m^–2. In Experiment 2, which hadambient food and a UVR (320 nm) dose of 20.0 kJ m^–2, L.minutus survival and reproduction were higher in the +UVR comparedto –UVR, regardless of competitors or predators. Chlorophylla (Chl a) in Experiment 2 was higher in the +UVR than –UVR.While interactions between zooplankton species of differingUVR tolerances are potentially important, these results insteaddemonstrate that the beneficial UVR effect on L. minutus isindependent of concurrent detrimental UVR effects on competitorsand predators. Further research on the phytoplankton communityis necessary to determine whether UVR alleviates bacterial competition,increases nutrient availability or affects phytoplankton byother mechanisms.     

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.     

Food and suspended sediment influences on the naupliar and copepodid durations of freshwater copepods: comparative studies on Tropodiaptomus and Metadiaptomus

Post-embryonic durations of Tropodiaptomus spectabilis (Kiefer)and Metadiaptomus colonialis(van Douwe) were determined at 20°Cin laboratory factorial experiments involving four algal foodenrichment levels (0, 100, 500 and 2500 µg l^–1 Cof Selenastrum added to 20 µm filtered water from respectivesource-lakes) and three suspended sediment levels (filtered,natural, and 2- to 3-fold sediment-enriched lake water). Foodeffects (30, 75, 225 and 600 (µg ^–1 C of Scenedesmus)were tested alone at 20°C for Metadiaptomus meridianus (vanDouwe). Total naupliar (D_n) and total copepodid (D_c) developmenttimes [summed to give total post-embryonic duration (D_t)] andmetasome lengths at maturity were measured In all taxa, foodsupply maximally affected D_c values 2- to 3-fold, whereas itsmaximal influence on D_n values was relatively slight (generally25%). The measured effect of food supply on D_t, was as strongas the predicted influence of temperature over an appropriateannual range. Food supply influenced size at maturity, and probablythereby fecundity, thus exerting additional demographic influences.Sediment effects were inconsistent, and quantitatively weakerthan food effects Total development of T.spectabilis was 20%raster, and that of M.colonialis 15% slower in sediment-enrichedthan in natural sediment level treatments; contrasting baselinesediment levels (2–3 times higher for the latter species)and different enrichment procedures confound interpretation.Unexpectedly, and inexplicably, development almost invariablyfailed in sediment-free water, implying an apparent dependencyon inorganic particles in these taxa This contrasts with thegenerally adverse influences of high sediment concentrationsupon zooplankton. Minimal male and female D_t values at 20°Cwere comparable and significantly longer in M.colonialis (15.5and 17 7 days) and M meridianus (16.5 and 21.5 days) than inT.spectabilis (11.7 and 12 2 days). These differences in durationare ecologically incongruous in relation to expected r –K life history strategies of genera characteristic of temporaryor semipermanent waters and permanent waters respectively.     

The colonial cyanobacterium Trichodesmium as a physical and nutritional substrate for the harpacticoid copepod Macrosetella gracilis

The pelagic harpacticoid copepod Macrosetella gracilis usesthe colonial cyanobacterium Trichodesmium not only as a physicalsubstrate for juvenile development, but also as a food source.By associating itself with a buoyant colonial cyanobacterium,M.gracilis has developed a successful mode of life for existencein the plankton. Further evidence of M.gracills' dependenceon Trichodesmium as a physical substrate is demonstrated bypreviously undescribed microscopic observations of a gravidM.gracilis female attaching eggs to a Trichodesmium colony.Shipboard experiments investigating the ingestion and assimilationof Trichodesmium carbon (C) were conducted in September 1991and January/February 1992 in waters of the Bahamas and the Caribbean,respectively. Macrosetella gracilis not only ingested, but rapidlyincorporated, cyanobacterial organic matter into its own cellularmaterial. Utilization of ingested Trichodesmium by M.graciliswas investigated by assessing the metabolic partitioning andincorporation of ¹⁴C-labelled Trichodesmium into copepod lipids,proteins, polysaccharides and low-molecular-weight (LMW) compoundsusing sequential biochemical fractionation techniques. Despitevariations in grazing rates between the two sites and times(September 1991,0.017 µg C^* µg^–1 C h^–1;January 1992, 0.134 µg C ^* µg^–1 C h^–1,the partitioning of incorporated C into the different biochemicalfractions was relatively consistent. There was rapid assimilationof ingested C into the LMW ({small tilde}60%) and polysaccharidefractions ({small tilde}30%) in the first few hours, with asubsequent increase in the percent C incorporated into protein.On average, {small tilde}21% of the Trichodesmium C ingestedby M.gracilis was assimilated. Therefore, M.gracilis is an importantsecondary link in the food web of oligotrophic waters whereTrichodesmium is abundant.     

On the causes of interspecific differences in the growth-irradiance relationship for phytoplankton. Part I. A comparative study of the growth-irradiance relationship of three marine phytoplankton species: Skeletonema costatum, Olisthodiscus luteus and Gonyaulax tamarensis

Three marine phytoplankton species (Skeletonema costatum, Olisthodiscusluteus andGonyaulax tamarensis) were grown in batch culturesat 15°C and a 14:10 L:D cycle at irradiance levels rangingfrom 5 to 450 µEinst m^–2 s^–1. At each irradiance,during exponential growth, concurrent measurements were madeof cell division, carbon-specific growth rate, photosyntheticperformance (both O₂ and POC production), dark respiration,and cellular composition in terms of C, N and chlorophyll a.The results indicate that the three species were similar withrespect to chemical composition, C:N (atomic) = 6.9 ±0.4, photo-synthetic quotient, 1.43 ± 0.09, and photosyntheticefficiency, 2.3 ±0.1 x 10^–3 µmol O₂ (µgChl a)^–1 h^–1 (µEinst m^–2 s^–1)^–1.Differences in maximum growth rate varied as the –0.24power of cell carbon. Differences in growth efficiency, werebest explained by a power function of Chl a:C at µ = 0.Compensation intensities, ranged from 1.1 µEinst m^–2s^–1 for S. costatum to 35 forG. tamarensis and were foundto be a linear function of the maintenance respiration rate.The results indicate that interspecific differences in the µ–Irelationship can be adequately explained in terms of just threeparameters: cell carbon at maximum growth rate, the C:Chl aratio (at the limit as growth approaches zero) and the respirationrate at zero growth rate. A light-limited algal growth modelbased on these results gave an excellent fit to the experimentalµ–I curves and explained 97% of the observed interspecificvariability. ¹Present address: Lamont-Doherty Geological Observatory Columbiaof University, Palisades, NY 10964, USA     

Photosynthetic characteristics of Dinophysis norvegica Claparede & Lachmann, a red-tide dinoflagellate

The relationships between photosynthesis and photosyntheticphoton flux densities (PPFD, P-l) were studied during a red-tideof Dinophysis norvegica (July-August 1990) in Bedford Basin.Dinophysis norvegica, together with other dinoflagellates suchas Gonyaulax digitate, Ceratium tripos, contributed {small tilde}50%of the phytoplankton biomass that attained a maximum of 16.7µg Chla 1^– and 11.93 10⁶ total cells I^–1.The atomic ratios of carbon to nitrogen for D.norvegica rangedfrom 8.7 to 10.0. The photosynthetic characteristics of fractionatedphytoplankton (>30 µm) dominated by D.norvegica weresimilar to natural bloom assemblages: o (the initial slope ofthe P-l curves) ranged between 0.013 and 0.047 µg C [µgChla]^–1 h–1 [µmol m^– s^–1]^–1the maximum photosynthetic rate, p^B_m, between 0.66 and 1.85µg C [µghla]^–1 h^–1; l_k (the photoadaptationindex) from 14 to 69 µ,mol m^–2 s^–1. Carbonuptake rates of the isolated cells of D.norvegica (at 780 µmolm^–2 s^–1) ranged from 16 to 25 pg C cell^–1h^– and were lower than those for C.tripos, G.digitaleand some other dinoflagellates. The variation in carbon uptakerates of isolated cells of D.norvegica corresponded with P^B_mof the red-tide phytoplankton assemblages in the P-l experiments.Our study showed that D.norvegica, a toxigenic dinoflagellate,was the main contributor to the primary production in the bloom.     

Experimental studies into the feeding biology of rotifers in brackish water

Mass developments of rotifers of the genus Brachionus, and especiallyof B.quadridentatus, occur regularly in the largely hypertrophicchain of shallow waters (‘boddens’) south of theDarss-Zingst peninsula (Southern Baltic). Interest in the autecologyof the species is, therefore, considerable. Various food sourceswere used in laboratory experiments to ascertain the food requirementsof B.quadridentatus, determine its filtration and ingestionrates, and assess its food particle size-selection ability.Growth experiments showed that the chlorophyceans Nannochlorissp. and Chlorella vulgaris possess considerable nutritionalvalue for the species, whereas abundances declined when Microcystisfirma, detritus from Enteromorpha sp. and only bacteria (Pseudomonas),respectively, were provided as food sources. Filtration ratesvaried between 0.02 and 1.73 µl ind.^–1 h^–1,and ingestion rates between 121 and 5560 cells ind.^–1h^–1, depending on the filtration rate and algal concentration.Investigations into food particle size selection using fluorescentlatex particles revealed that particle size influences foodparticle intake. When particles of different sizes were mixed,the animals showed a preference for the larger particles andingested the smaller ones with a diameter of 1–2 µmless efficiently. The brackish water species Brachionus plicatiliswas studied besides B.quadridentatus in all experiments. Theformer species proved to be superior both in its range of utilizableparticle sizes and its growth rate. The experiments with laboratorycultures were backed up by studies performed with various rotiferspecies taken from natural populations.