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.     

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.     

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.     

Comparison of carbon-specific growth rates and rates of cellular increase of phytoplankton in large limnetic enclosures

Potential carbon-specific growth rates of phytoplankton wereestimated from a series of measurements of photosynthetic radio-carbonuptake over 4- and 24-h exposure periods in the light fieldsof three large limnetic enclosures (‘Lund Tubes’),each providing different limnological and trophic conditions.Photosynthetic behaviour and short-term, chlorophyll-specificcarbon-fixation rates conformed to well-established criteriabut, over 24 h, the net retention represented 23–82% ofthe carbon fixed during the daylight hours. Potential mean growthrates (k'_p, of the photo-autotrophic community were calculatedas the net exponential rates of daily carbon-accumulation relativeto derived, instantaneous estimates of the cell carbon-content.Apparent actual community growth rates (k'_D were calculatedas the sum of the exponential rates of change of each of themajor species present, corrected for probable rates of in situgrazing and sinking, and expressed relative to the fractionof total biomass for which they accounted. The correspondingvalues were only occasionally similar, k'_p generally exceedingK'_D by a factor of between 1 and 30 or 1 and 14, depending uponthe carbon:chlorophyll ratio used. The ratio, K'_p/K'_D was foundto vary inversely both to k'_D and to k_n, the net rate of changein phytoplankton biomass, suggesting that measured carbon fixationrates merely represent a capacity for cellular increase which,owing to other likely limitations upon growth, is seldom realized.Apparent rates of loss of whole cells do not account for theloss of carbon; that the ‘unaccounted’ loss rates(K'_p–K'_D varied in direct proportion to K'_p (i.e., losseswere least when chlorophyll-specific photosynthetic productivitywas itself limited) is best explained by physiological voidingof excess carbon (for instance, by respiration, photorespiration,excretion) prior to the formation of new cells.     

Mixing an enclosed, 1300m3 water column: effects on the planktonic food web

The nutrient, phytoplankton, and zooplankton dynamics in threeenclosed water columns (1300 m³) are described. Two of the enclosureswere mixed using a bubbling chamber at depth. Young chum salmon(Oncorhynchus keta) were added to one of the mixed enclosuresand the unmixed enclosure. No other manipulations were imposed.Copepods appeared in large numbers (e.g. especially Pseudocalanusminutus s.l. and Paracalanus parvus) and population growth rateswere estimated. Ctenophora did not appear in large numbers despitepresumably ideal food environments; it is suggested that inone enclosure this is a consequence of fish predation on thectenophores. The fish experienced high mortalities and low growthrates presumably due to unsuitable prey size. Weekly collectionsof sediment permitted isolation of two major sediment contributors,the first from phytoplankton sinking and the second from biogenkfallout associated with herbivore production. It was found thatthe more oligotrophic enclosure (unmixed) experienced proportionallyhigher utilization of organic carbon. Some of these resultsare explained by our data while others require more sophisticatedexperimentation, both in the design of the containers and inthe types of observations.     

In situ growth rates of marine phytoplankton: approaches to measurement, community and species growth rates

Taxonomic structure and biomass weighting are important determinantsof measurable marine phytoplankton community growth potential.Maximal diel-averaged growth rates of communities appear tofall between 3 and 3.6 doublings day^–1. Mean net growthrates are considerably lower. Ranges of community growth ratesmeasured in tropical, sub-tropical and (summer) temperate ecosystemsare similar. There appears to be a broad dichotomy between thegrowth potential of diatom species as compared to non-diatoms.Doubling rates of small diatoms frequently exceed communitybiomass doubling rates by wide margins. Diel-averaged growthrates of large diatoms, microflagellates and non-motile ultraplanktonpopulations are lower and similar in magnitude to communitygrowth estimates. Maximum growth rates of species measured insitu are in good agreement with maximum growth rates measuredin laboratory cultures. High specific productivity of sub-dominantor rare diatom species or assemblages will be diluted in thelower specific growth rates of microflagellate and non-motileultraplankton assemblages. Specific rates of grazing upon speciesand functional groups remain to be quantified, but stabilityof community size and taxonomic structure implies close linkagebetween growth and mortality rates at the species level overtime intervals of several generations.     

The spatial and temporal dynamics of deep chlorophyll layers in high-mountain lakes: effects of nutrients, grazing and herbivore nutrient recycling as growth determinants

Deep chlorophyll layers (DCL) are a common feature of oligotrophiclakes, yet the mechanisms that form and maintain them are notunderstood fully. These phytoplankton populations occur in themetalimnia of lakes where light levels are moderate to low,and where nutrient levels and zooplankton grazing pressure aredifferent than in the epilimnion. To test the importance ofnutrients and grazing pressure for algal growth in differentlake strata, microcosm experiments and monitoring were conductedin two oligotrophic lakes in the Rocky Mountains of North Americathat contain DCL. In situ microcosm experiments with naturalphytoplankton communities from three depth strata were conductedwith macronutrient additions and with and without the naturalzooplankton grazing communities. Alkaline phosphatase assaysand the in situ microcosm experiments indicated less nutrientlimitation in the metalimnia than in the epilimnia of both lakes.Zooplankton grazing in the experiments decreased algal populationgrowth rates by as much as 6% day^–1, with impacts shiftingto progressively deeper strata over the summer. Zooplanktongrazing losses, however, were partially offset by nutrient recyclingthat increased algal growth rates. Depth-differential nutrientdeficiency and zooplankton grazing and recycling interactedto maintain the DCL in these lakes.     

Continuous plankton records: seasonal variations in the distribution and abundance of plankton in the North Atlantic Ocean and the North Sea

Charts are presented of the seasonal variations in the distributionof four phytoplankton and five zooplankton taxa in the NorthAtlantic and the North Sea. The main factors determining theseasonal variations appear to be the distribution of the mainoverwintering stocks, the current system and, in some instances,temperature control of the rate of population increase. Informationis presented about the variation with latitude (over the rangefrom 34° N to 65 ° N) of the seasonal regime of theplankton. On the assumption that there is a relationship betweennutrient supply and vertical temperature stratification themain features of this variability can be interpreted. In thesouth (to about 43° N) nutrient limitation plus grazingappear to be dominant, resulting in a bimodal seasonal cycleof phytoplankton. North of about 60° N the system appearsto be limited by the size of the phytoplankton stocks beinggrazed primarily by Calanus Finmarchicus and Euphausiacea. Inan extensive zone, from about 44° N to 60° N, it wouldappear that the spring bloom of phytoplankton is under-exploitedby grazing while in summer the zooplankton graze the daily productionof the phytoplankton, the stocks of which are probably maintainedby in situ nutrient regeneration. The implications, for at leastthis mid-latitude zone, that rates and fluxes of processes,as opposed to density dependent interactions between stocks,play a major role in the dynamics of the seasonal cycle is consistentwith previously reported observations suggesting that physicalenvironmental factors play a major role in determining year-to-yearfluctuations in the abundance of the plankton.     

Ecological investigations of blooms of colonial Phaeocystis pouchetti--I. Abundance, biochemical composition, and metabolic rates

A winter bloom of the colonial stage of the prymnesiophyte Phaeocystispouchetit was studied in the 13-m³ mesocosms of the Marine EcosystemResearch Laboratory on Narragansett Bay, Rhode Island The tankswere temperature regulated at 4±2°C but differedin their nutrient concentrations and in situ irradiances. Oneof the tanks was a control without added nutrients, one receiveda temporary nutrient spike and two others received daily N/P/Siinputs. Photosynthesis and growth rates of colonies exposedto a range of natural light levels were measured at weekly intervals.Particulate carbon production and release of dissolved organiccarbon (DOC) by the entire plankton community was determinedconcurrently. Photosynthesis and growth rates of Phaeocystisin tanks receiving daily nutrient additions were asymptoticfunctions of irradiance. Light-saturated rates exhibited asymptoticrelationships with dissolved inorganic nitrogen (N) levels.N-Limited populations showed more variable responses. Althoughirradiance and N availability regulated the population dynamicsof Phaeocystis, the presence or absence of silicate (S₁) influencedits relative importance in each tank. Phaeocystis dominatedcommunity metabolism in the absence of Si, but co-occurred withextensive stands of diatoms when Si was available. A significantpositive correlation was found between the contribution by Phaeocystisto community production and the proportion of photosynthatereleased as DOC In all tanks, Phaeocystis populations exhibitedcycles of abundance in which division of cells within coloniespreceded the multiplication of colonies. The production of newcolonies apparently occurred via two mechanisms: the formationof colonies from solitary cells, and the cleavage of largercolonies into smaller daughter colonies. Phaeocystis in tankswith near undetectable nutrient levels contained C:N, C:Chla, and C:ATP ratios several times higher than colonies in nutnent-repletetanks. Phaeocystis C:Chl a and C:ATP ratios were substantiallygreater than those of non-gelatinous phytoplankton due to carbohydratestorage in colony gelatin In contrast, C:N ratios in Phaeocystisand non-gelatinous phytoplankton were similar, suggesting astorage depot of organic N outside of the cells. The resultssupport the notion that Phaeocystis colonies function as biologicalentities rather than as passive aggregations of cells.     

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

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

Zooplankton growth rates: extraordinary production by the larvacean Oikopleura dioica in tropical waters

The growth of Oikopleura dioica was determined from microcosmsincubated in situ at 29°C in Kingston Harbour, Jamaica.Minimum generation times approached 1 day, with Oikopleura dailyspecific growth rates ranging from 3 to 23 and averaging 10.7-foldincreases in biomass over 24 h. This was an order of magnitudegreater than the copepod Paracalanus crassirostris, whose growthrates ranged from 0.3 to 1.2 and averaged 0.67 day^–1 fromthe same experiments A reassessment of previous data indicatesthat larvacean production approaches, and may exceed, the 693kJ m^–2 annual production of the copepods. Growth rateand recruitment of Oikopleura decreased as the biomass of thezooplankton community increased; both resource and interferencecompetition are probably occurring. The extraordinary productionpotential of the Larvacea emphasizes their understated importancein marine planktonic systems.     

Nutrient regeneration by zooplankton: effects on nutrient limitation of phytoplankton in a eutrophic lake

We tested the hypothesis that excretion of nutrients by zooplanktoncan reduce the severity of nutrient limitation of phytoplankton,and determine whether the phytoplankton community is limitedby nitrogen or phosphorus. In situ experiments were conductedin eutrophic Lake Mendota (Wisconsin, USA) during the summerof 1988, where phytoplankton were limited by N and P, but periodsof nutrient limitation were transitory Increased zooplanktonbiomass and the consequent increased excretion of nutrientsby zooplankton reduced P limitation (as measured by specificalkaline phosphatase activity) in all experiments Excretionof nutrients also reduced N limitation (as measured by ammoniumenhancement response) in one of three experiments. In additionalexperiments in the more highly eutrophic Lake Wingra, excretionof nutrients by zooplankton reduced both N and P limitationThese results support the hypothesis that zooplankton have potentiallyimportant indirect effects on phytoplankton communities throughrecycling of nutrients     

Taxonomic composition and growth rates of phytoplankton assemblages at the Subtropical Convergence east of New Zealand

Off the eastern coast of New Zealand, warm, saline, nutrient-poorSubtropical Waters (STW) are separated from cool, fresher, relativelynutrient-rich Sub-Antarctic Waters (SAW) by the SubtropicalConvergence (STC). The Chatham Rise, a submarine rise, restrictsthe latitudinal movement of the STC as well as mixing of STWand SAW. Due to this restriction, this sector of the STC ischaracterized by sharp gradients in temperature, macro- (nitrate,silicate and phosphate) and micro- (iron) nutrient concentrations.Shipboard incubations were conducted during austral spring 2000and 2001 to test the hypothesis that these gradients affectthe taxonomic composition and/or growth rates of phytoplanktonon either side of and at the STC. Maximum chlorophyll a concentrationsduring 2000 were 0.39 µg L^–1, but were an orderof magnitude higher in 2001. During both years, STC phytoplanktonwere dominated by diatoms (77% of the total chlorophyll a duringaustral spring 2000 and 70% during spring 2001), whereas cryptophytesand prasinophytes dominated STW assemblages (27 and 36% during2000, and 63 and 17% during 2001). Chlorophyll in the SAW wasdominated by procaryotes and photosynthetic nanoflagellatesduring 2000 (17% procaryotes, 68% nanoflagellates), and by diatomsduring the austral spring 2001 cruise (53%). Growth rates ofthe phytoplankton assemblage were determined by ¹⁴C-labelingof chlorophyll a and photosynthetic pigments. During 2000, temperature-normalizedgrowth rates were near maximal at the STC, and decreased onaverage to less than half of the maximum north and south ofthat front, whereas in 2001 both absolute and relative growthrates were low at all stations. Growth rates did not closelyparallel biomass of the various taxa, suggesting that nutrientlimitation and/or grazing were significantly impacting standingstocks. It appeared that growth was strongly influenced by nutrientsand light, but that biomass was more strongly influenced bygrazing. The STC is a globally important region of enhancedbiomass and productivity; however, the phytoplankton assemblagereflects control by both top–down and bottom–upprocesses that makes a predictive understanding of the area'sbiogeochemical cycles extremely difficult.     

A laboratory study of the effect of non-phytoplankton diets on the reproduction of Calanus helgolandicus

The effect of five non-phytoplankton and one phytoplankton dietson reproduction is described for the copepod Calanus helgolandicusand compared to field fecundity estimates. A fecundity increasefollowed ingestion of larvae of the sea urchin Sphaerechinusgranularis (LSU) and the oyster Crassostrea gigas (OYS) by copepodfemales, whereas a decrease followed ingestion of eggs of thecopepods Acartia and Temora spp. (COP), as a function of proteinconcentration in diets. With larvae of the sea urchin Paracentrotuslividus (SSU) and the dinoflagellate Prorocentrum minimum (PM;standard phytoplankton diet), fecundity was identical. Althoughprotein concentrations differed greatly from one food treatmentto the other, mean fecundity values were rather low. For unknownreasons, and despite their very high protein concentrationsin comparison to in situ, none of these diets could improvefemale fecundity or reach maximum egg production rates reportedin C.helgolandicus. Hatching rates were >80%, stable andsimilar with both PM and COP diets. With SSU, LSU and OYS diets,egg viability was lower than in situ and with PM, showing astrong and mild decrease with time with LSU and SSU, respectively.Ingestion of zygotes of the brown alga Fucus spiralis (ZYG)led to the death of females. These results suggest that denseblooms of zygotes of macroalgae and invertebrate embryos andlarvae, occurring in the continental shelf as part of copepoddiets, could have different effects on the demographic responseof C.helgolandicus.     

Interpretation of in vivo fluorescence and cell division rates of natural phytoplankton using a cell cycle model

A natural population of phytoplankton was collected from theMenai Straits and incubated in a continuous culture system undernatural illumination. In vivo fluorescence data were used toderive cell cycle parameters, and the division rates of cellsin the population were analysed using a transition-point modelof the cell cycle. The results indicated that for modellingpurposes the mixed population could be regarded as being composedof a single species having properties equivalent to the averagefor the population. Fluorescence measurements were subsequcntlyrepeated on samples of the in situ population at the collectionsite, and the cell cycle model used to interpret the growthrate of the phytoplankton in the natural environment.     

Growth and production of heterotrophic nanoflagellates in a meso-eutrophic lake

The growth of heterotrophic nanoflagellates (HNF) in mesotrophicLake Constance was measured in situ during a 13 month period.Experiments were conducted with 10 µm pre-filtered lakewater incubated in diffusion chambers at 3 m water depth atthe sampling location for 24 h. Growth rates were calculatedfrom changes in cell numbers occurring during the period ofincubation. Growth rates of all dominant taxa showed pronouncedseasonal variation (–0.13 to 1.76 day^–1 and weregenerally highest in summer at high water temperatures. In situgrowth rates were well below maximum growth rates known forthe respective and similar species from laboratory experiments.While water temperature was a key parameter positively relatedto the growth of all HNF species, the effect of various potentialfood items was taxon specific and less clear. Bacterial abundancewas equally important as temperature for growth in the smallbactenvorous Spumella sp., but was insignificant for growthrates of the larger omnivorous Kathablepharis sp. In Spuniellasp., 84% of the observed seasonal variation of its growth ratecould be explained by temperature and bacterial food supply.Based on these results, a multiple linear regression equationwith temperature and bacterial concentration as dependent variableswas calculated for the growth rate of Spumella. Taxon-specificproduction rates were derived from growth rates and averagebiomass of these two species, and compared to total HNF productionestimated from previously measured community growth rates andbiomass in Lake Constance. Production peaks of Spumella sp.and Kathablepharis sp. alternated seasonally. Total HINF productionranged from –0.01 to 10 mg C m^–3 day^–1. Theaverage seasonal production varied between 1.4 and 33 mg C m^–3day^–1 over 6 consecutive years. These small protozoa thuscontribute a substantial amount to total zooplankton productionin Lake Constance.     

An empirical model of primary productivity (14C) using mesocosm data along a nutrient gradient

The two parameters of the hyperbolic tangent equation, P_m and, were estimated from in situ vertical profiles of primary productionusing mesocosm data along a nutrient gradient. The parameters,derived from 4-h (around noon) ¹⁴C incubations, were used togetherwith the photosynthesis-light curve and hourly solar radiationdata to calculate daily primary production rates (P_d). Approximately40% of the daily production occurred in the 4 h around noon.Considering parameter uncertainty, there was no indication ofan increase in variation in production with increased nutrientloading, nor did biomass-specific P-I parameters increase. Annualproduction ranged from 82 to 901 g C m^–2 year^–1and was highest in the highest nutrient treatment tank. Dailyproductivity ranged from 0.02 to 9.1 g C m^–2 day^–1and was significantly correlated, in all treatments, with acomposite parameter BI₀/k (where B is phytoplankton biomass;I₀ is daily radiation and k is the extinction coefficient).Linear regressions of P_d against BI₀/k indicated that much ofthe variability (86%) in productivity was explained by lightavailability and phytoplankton biomass. Two approaches for predictingproductivity were compared: (i) predicting production directlyfrom environmental variables (i.e. BI₀/k) and (ii) predictingthe parameters of the P-I curve from environmental variablesand using these to calculate daily production.     

Cladoceran and rotifer grazing on bacteria and phytoplankton in two shallow eutrophic lakes: in situ measurement with fluorescent microspheres

Metazooplankton grazing on bacteria and on the phytoplanktonof various sizes was estimated in shallow eutrophic lakes Kaiavereand Võrtsjärv (Estonia) by in situ feeding experimentswith fluorescent microspheres (diameters 0.5 µm for bacteriaand 3, 6 and 24 µm for phytoplankton). Zooplankton communitycomposition, abundance and food density were important factorsdetermining grazing rates in these lakes. Cladocerans and rotifersfiltering rates (FR) and ingestion rates (IR) on bacteria andphytoplankton were several times higher in Lake Kaiavere wherebacterivorous rotifers and Daphnia contributed more to zooplanktonassemblage. While cladocerans were generally the main phytoplanktonconsumers, both lakes differed with respect to the groups ofbacterivores. Based on consumption of fluorescent microspheres,the metazooplankton grazing rates were relatively low and hadlow impact on production and standing stock of bacteria andingestible phytoplankton (<30 µm). On average, 0.5and 0.1% of standing stock of bacteria and 2.6 and 1.0% of standingstock of ingestible phytoplankton was grazed daily by metazooplanktonin lakes Kaiavere and Võrtsjärv, respectively. Thatcorresponded to daily grazing of 4.1% of the bacterial productionand 0.43% of the total primary production (PP) by metazooplanktonin Lake Kaiavere compared with 4.3 and 0.06% in Lake Võrtsjärv,respectively. The results suggest that the majority of consumptionof the bacterial and phytoplankton PP is most likely channelledthrough the microbial loop.     

A methodology to determine primary production and phytoplankton photosynthetic parameters from Fast Repetition Rate Fluorometry

Estimating the primary productivity of phytoplankton as theyare mixed through the surface layer is often hampered by methodologicalor conceptual constraints. Fast Repetition Rate Fluorometry(FRRF) allows some of these constraints to be overcome by providingmeasurements of the instantaneous, depth-dependent rates ofprimary productivity of phytoplankton in situ. Data acquiredby FRR fluorescence is used in this paper to determine the parametersof the photosynthesis–irradiance curve and the instantaneousphotosynthetic rates for phytoplankton from the mixed layerduring a cruise in the Celtic Sea in May 2000. FRR fluorescence-basedestimates of the initial slope of the photosynthesis–lightcurve (     

Zooplankton community grazing in a hypertrophic lake (Hartbeespoort Dam, South Africa)

Seasonal abundance as total biomass and specific densities ofthe main herbivorous zooplankton (>60 µm) in hypertrophicHartbeespoort Dam from 1981 to 1983 are described. After springzooplankton biomass maxima each year there followed a mid-summerdecline in the Daphnia population and a shift to a smaller bodiedcladoceran community dominated by Ceriodaphnia concomitant witha change from largely edible phytoplankton species to abundantcolonial Microcystis. In situ community grazing rates were measuredthroughout 1983 using a ¹⁴C-labelled unicellular alga. Integratedcommunity grazing rates measured in the aerobic water columnwere highest in December (260.2%/day) when Daphnia was abundantand the edible component of the phytoplankton was diminishing.Lowest integrated community grazing rates occurred in January–February(19.8–35.3%/day) and July (28.3%/day) when the phytoplanktonwas composed almost entirely of Microcysris, and Ceriodaphniadominated the zooplankton community whilst food availabilitywas low. Feeding in Ceriodaphnia was not hindered by abundantlarge Microcysris colonies; total biomass specific grazing ratewas high when Ceriodaphnia dominated and low when Daphnia dominatedthe zoo-plankton community. Results indicate that in hypertrophicconditions it is unlikely that large filter-feeders such asDaphnia are able to retard or limit the development of Cyanophyceaeblooms by high grazing pressure.