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     

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.     

Steady-state rotifer growth in a two-stage, computer-controlled turbidostat

Steady-state populations of rotifers (Brachionus calyciflorus)were maintained in twostage, continuous-flow turbidostatic cultureon the green alga Chlorella pyrenoidosa. In this system, themaximum specific growth rate,µ^max of the rotifers wasmaintained by using a computer to control the concentrationof algae, as rotifer food, in the rotifer culture. As rotifersconsumed algae, the turbidity decreased until a set-point wasreached. Then fresh algal suspension (supplied from a steady-statealgal chemostat) was metered into the rotifer culture, whichwas held in the dark. Rotifer and algal populations, as wellas rotifer µ^max entered steady states. These steady-stateresults were consistent with previous data from chemostat studies,but growth transients indicated that the of the µ^maxrotifersmay be subject to selection. The system is unique in providinga means to explore population dynamics of a metazoan maintainednear its µ^max.     

Are bacteria an important food source for rotifers in eutrophic lakes?

In situ grazing measurements using fluorescent particles of0.5, 2.4 and 6.3 µm diameter in eutrophic Lake Loosdrecht(The Netherlands) showed that Anuraeopsis fissa, a small rotifer,filtered the smallest, bacteria-sized particles as efficientlyor more efficiently than the larger particles. In contrast,three other rotifer species (Brachionus angularis, Filinia longisetaand Pompholyx sulcata) filtered the bacteria-sized particlesless efficiently than the larger particles. Both Keratella cochlearisand Conochilus unicornis only ingested the bacteria-sized particles.Anuraeopsis fissa had a higher uptake of fluorescent bacteria-sizedparticles than K.cochlearis, both in 1 µm filtrate oflake water and in lake water. Within both species, uptake didnot differ between juveniles and adults. When cultured on threedifferent size fractions of lake water (1, 3 and 15 µmfiltrate) in July, all rotifer species declined in numbers onthe 1 and 3 µm filtrates, while A.fissa and B.angularisincreased in numbers on the 15 µm filtrate. The high abundanceof small bacteria in the lake water could not support rotiferpopulations. It is concluded that bacteria are not a suitablefood source of high quality for A.fissa because its populationdoes not grow even though the bacterial concentration was higherthan its estimated threshold food concentration. In August,when individually cultured, the mortality was high for all species,but especially for F.longiseta. The lifespan of K.cochleariswas reduced in the 1 and 3 µm filtrates of lake water,compared with in the 15 µm filtrate. The lifespan of A.fissawas similar in all filtrates, but reproduction was reduced inthe 1 and 3 µm filtrates, as in Keratella. On the 15 µmfiltrate, their ages at first reproduction and growth ratesdid not differ. Individuals of A.fissa older than 4 days showeda higher survival in the 15 µm filtrate than in the othertwo filtrates, as did K.cochlearis throughout its life. Hence,bacteria seem to be a more important food source for youngerindividuals of A.fissa than of K.cochlearis.     

Trophodynamics of the scyphomedusae Aurelia aurita. Predation rate in relation to abundance, size and type of prey organism

Ephyra larvae and small medusae (1.7–95 mm diameter, 0.01–350mg ash-free dry wt, AFDW) of the scyphozoan jellyfish Aureliaaurita were used in predation experiments with phytoplankton(the flagellate Isochrysis galbana, 4 µm diameter, {smalltilde}6 x 10^–6 µg AFDW cell^–1), ciliates (theoligotrich Strombidium sulcatum, 28 µm diameter, {smalltilde}2 x 10^–3 µg AFDW), rotifers (Synchaeta sp.,0.5 µg AFDW individual^–1) and mixed zooplankton(mainly copepods and cladocerans, 2.1–3.1 µg AFDWindividual^–1). Phytoplankton in natural concentrations(50–200 µg C I^–1) were not utilized by largemedusae (44–95 mm diameter). Ciliates in concentrationsfrom 0.5 to 50 individuals ml^"1 were consumed by ephyra larvaeand small medusae (3–14 mm diameter) at a maximum predationrate of 171 prey day^–1, corresponding to a daily rationof 0.42%. The rotifer Synchaeta sp., offered in concentrationsof 100–600 prey I^–1, resulted in daily rations ofephyra larvae (2–5 mm diameter) between 1 and 13%. Mixedzooplankton allowed the highest daily rations, usually in therange 5–40%. Large medusae (>45 mm diameter) consumedbetween 2000 and 3500 prey organisms day^"1 in prey concentrationsexceeding 100 I^–1. Predation rate and daily ration werepositively correlated with prey abundance. Seen over a broadsize spectrum, the daily ration decreased with increased medusasize. The daily rations observed in high abundance of mixedzooplankton suggest a potential ‘scope for growth’that exceeds the growth rate observed in field populations,and this, in turn, suggests that the natural populations areusually food limited. The predicted predation rate at averageprey concentrations that are characteristic of neritic environmentscannot explain the maximum growth rates observed in field populations.It is therefore suggested that exploitation of patches of preyin high abundance is an important component in the trophodynamicsof this species. ¹Present address: University of Bergen, Department of MarineBiology, N-5065 Blomsterdalen, Norway     

Zooplankton dynamics in a gypsum karst lake and interrelation with the abiotic environment

Zooplankton species composition and abundance variation was studied in Lake Amvrakia, which is a deep, temperate, gypsum karst lake situated in the western Greece. The two year survey of zooplankton revealed 33 species (23 rotifers, five cladocerans, four copepods and one mollusc larva). The mean integrated abundance of the total zooplankton ranged between 83.6 and 442.7 ind. L⁻¹, with the higher density to be recorded in the surface 0–20 m layer. Small numbers of specimens of almost all species were found also in the hypoxic or anoxic hypolimnion. Copepods and especially the calanoid Eudiaptomus drieschi dominated the zooplankton community throughout the sampling period, followed by Dreissena polymorpha larvae, rotifers and cladocerans. Seasonal succession among the cladocerans and the most abundant rotifer species was observed. The concentration of chlorophyll-a was the most important factor for the variation of total zooplankton, as well as for the rotifers’ community. Dissolved oxygen affected copepods and cladocerans, water level correlated mainly with the molluscs larvae of D. polymorpha, while temperature influenced the variation of several rotifers, the cladoceran Diaphanosoma orghidani and the mollusc larvae. Negative correlation of conductivity with the cladoceran Daphnia cucullata and the copepods E. drieschi and Macrocyclops albidus was found. The differences in species composition found in Lake Amvrakia in comparison to the nearby lakes are probably ought to the geographical isolation and perhaps to its particular chemistry (e.g., elevated conductivity).     

Factors affecting the bacteria-heterotrophic nanoflagellate relationship in oligo-mesotrophic lakes

The coupling between bacteria and heterotrophic nanoflagellates(HNF) was examined in nine lakes of low productivity for evidenceof the effects of various metazooplankton (i.e. rotifers, cladoceransand copepods) on this relationship. We considered the size ofcladocerans and, in contrast to most previous across-systemstudies, the three strata of the water column (i.e. epilimnion,metalimnion and hypolimnion). Rotifers were numerically dominantin all lakes and accounted for 45–84% of total metazooplanktonabundance, while the abundance of large cladocerans was relativelylow, ranging from 0.066 to 15.2 ind. L^–1. The across-lakerelationship between bacteria and HNF was significant in thedeeper strata (meta- and hypolimnion) but not in the epilimnionand in the two groups of lakes separated on the basis of theiraverage number of large cladocerans (<5 and >5 ind. L^–1,respectively). The results confirmed the negative impacts oflarge cladocerans on HNF, but also showed that rotifers, probablythrough grazing on HNF, may be an important factor causing variationin the bacteria–HNF relationship in unproductive waters.Quadratic models best described the relationships between metazooplanktonand the ratio of bacteria to HNF. This ratio seemed to be aresult of complex interactions between several factors, includingthe zooplankton composition and abundance and the depth of thelake. Indeed, this ratio significantly decreased across lakes,with increase in depth. In addition, shallower lakes (having<5 large cladocerans L^–1 and fewer Polyarthra vulgaris)tended to have more bacteria and HNF and a higher ratio of bacteriato HNF than deeper lakes (which had >5 large cladoceransL^–1 and substantial proportions of P. vulgaris). We suggestthat the epilimnion, metalimnion and hypolimnion of lakes betaken into account when analysing the bacteria–HNF relationshipas well as the cascading effects of zooplankton on microbialcommunities.     

Feeding of planktonic rotifers on ciliates: a method using natural ciliate assemblages labelled with fluorescent microparticles

A method was developed to allow direct measurements of predationexerted by metazooplankton on ciliates. The method relied onthe use of ciliates labelled with fluorescent microparticles(FMP). Optimal labelling conditions were determined with ciliatesfrom cultures (Tetrahymena pyriformis) and with natural ciliateassemblages sampled in a river. Labelled T. pyriformis wereused as tracer food to determine gut passage time (GPT) andingestion rates of the rotifer Brachionus calyciflorus in thelaboratory. Predation of metazooplankton from the lowland riverMeuse (Belgium) was determined by labelling natural assemblagesof ciliates and using them as tracer food for metazooplankterssampled in the river. Optimal labels of ciliates, i.e. sharpdistribution of FMP in cells, were obtained with short incubations(10 min) and low FMP concentrations (1 x 10⁵ mL^–1). GPTvaried between 30 and 45 min for B. calyciflorus and from 25up to >35 min for rotifers from the river. The ingestionrate of B. calyciflorus fed with T. pyriformis was 3.3 ±0.6 ciliate rot^–1 h^–1, i.e. 1.4 ± 0.3 ngCrot^–1 h^–1. Metazooplankton species for which theingestion of ciliates could be measured were the rotifers Keratellacochlearis, Euchlanis dilatata and Synchaeta spp. Ingestionrates measured ranged from 0.4 to 12.5 ngC rot^–1 h^–1.The method proposed proved to be useful in estimating the predationof microplankton on ciliates in semi- in situ conditions; infurther developments, labelled natural assemblages of ciliatescould be used for in situ incubations with the Haney chamber.     

Cladoceran filtration rate-body length relations: model improvements developed for a Microcystis-dominated hypertrophic reservoir

Filtering rates were measured for zooplankton species in Situon single-celled Chlorella and on four Microcystis colony sizefractions (5–20, 20–40, 40–60 and 60–100µm) in a hypertrophic reservoir. Natural-log-transformedfiltration rates of five cladoceran species, one copepod andone rotifer were included in an all-food-particle, all-speciesmultiple regression model which explained 43% of the variancein filtration rate as a function of animal body length. An additional14% and 7.6% of the variance was attributable to food type andzooplankton species respectively, with temperature accountingfor <4% of the variance. Restricting the filtration ratemodel to cladocerans alone explained 51% of the variance asa function of animal length, 16% as a function of food type,7.5% as a function of species and only 0.2% as a function oftemperature. In linear filtration rate models for each foodtype, cladoceran body length explained 70% of the variance whenfeeding on Chlorella and between 57 and 67% of the varianceon the four Microcystis colony fractions. Models describingcladoceran filtration rates on Chlorella and the 5–20µm Microcystis colony fraction were significantly differentfrom the three models on larger colonies due to cladoceran responsesto increasing food particle size. Accordingly, a combined modelfor Microcystis colonies >20 µm was developed. Inclusionof food quality factors such as cyanophyte colony size seemsjustified in models aimed at estimating clearance rates, resourceutilization and phytoplankton grazing losses in plankton orecosystem studies when applied to eutrophic or hypertrophiclakes where large cyanophyte particles are abundant.     

Selective predation by herring and mysids, and zooplankton community structure in a Baltic Sea coastal area

In lakes, fish and invertebrate predation are recognized asstrong structuring forces on zooplankton communities. The objectof this investigation was to study whether predation has a similarpotential in a coastal area of the brackish Baltic Sea and ifit could explain observed differences in zooplankton communitystructure between a reference area and an eutrophied area. Speciescomposition and daily vertical migration of zooplankton andzooplanktivores, and the diets of the latter, were studied inJuly and August at two 30 m deep stations differing in primaryproductivity. The biomass of zooplankton >35 µm wasdominated by copepods, but cladocerans and rotifers also occurredin significant numbers. The dominating zooplanktivores wereherring (Clupea harengus) and the mysid shrimp Mysis mixta.They fed almost exclusively on zooplankton, mainly copepods,and their estimated food consumption equalled or exceeded thesummer copepodite production. The structure of the zooplanktoncommunity cannot be explained by effects of predation or byfeeding conditions alone. Increases in rotifer and cladoceranabundances with increased primary production suggests effectsof food supply. However, a generally rapid decline in the annualsummer peak of cladocerans may be caused by predation. The totalabundance of copepods did not increase with improved feedingconditions, but there was a shift in species dominance. Thecopepod Ewytemora affinis hirundoides, which was intensivelypreyed upon, increased with increased phytoplankton production,while Acartia bifilosa and/or A.longiremis, which was less eaten,decreased. Predation may explain a pronounced daily verticalmigration of the most predated copepods. They occurred in deeperwater during the day, when the visually feeding herring wereactive, and moved closer to the surface at night when M.mixtaleft the bottom, to forage in the water column.     

Potential grazing impact of the mixotrophic flagellate Ochromonas sp. (Chrysophyceae) on bacteria in an extremely acidic lake

Flagellates are important bacterial grazers in most planktonicfood webs. The prey-size preference of the mixotrophic flagellate,Ochromonas sp. (Chrysophyceae), isolated from an extremely acidiclake, Lake 111 (pH 2.6), was determined using fluorescentlylabelled microspheres (beads). According to grazing experimentswith cultured bacteria, also isolated from Lake 111, the potentialgrazing impact on Lake 111’s single-celled bacterial productionwas calculated. Ochromonas sp. ingested the smallest beads offered(0.5 µm diameter) at the highest rate. Ingestion ratedeclined with increasing bead size. The highest prey volume-specificingestion was measured for Ochromonas sp. feeding on intermediate-sizedbeads (1.9 µm). Ingestion rates were low due in part tothe large fraction of inactive flagellates observed. Accordingto the bacterial ingestion rate, a mean of 88% (epilimnion)and 68% (hypolimnion) of in situ single-celled bacterial productionis potentially grazed daily by Ochromonas sp. In the epilimnionof Lake 111, the heterotrophic carbon gain is three times higherthan the autotrophic production. Alongside carbon uptake, Ochromonassp. also benefits from ingesting bacteria through the uptakeof phosphorus. A biovolume minimum corresponding to the preysize at which Ochromonas sp. feeds most efficiently occurredin the Lake 111 epilimnetic bacterial community, implying top-downcontrol of the bacterial community by Ochromonas sp.     

Marine pelagic rotifers and tintinnids - important trophic links in the spring plankton community of the Gullmar Fjord, Sweden

During the winter and spring of 1981, the dynamics of a marinepelagic rotifer, Synchaeta vorax Rousselet and tintinnids ofthe genus Tintinnopsis, were studied in the Gullmar Fjord onthe Swedish west coast. Although present in February and March,it was not until early April when the surface water temperatureexceeded 6?C that the rotifer population started to grow rapidly.Thereafter, the population increased exponentially for 6 weeks.Estimates of consumption of phytoplankton by rotifers and tintinnidsin their most dynamic phase approximated to that of the phyto-planktonproduction. Lack of data on the amount of detritus availableand the number of young rotifers passing the 90 µm netare however obvious limitations in such calculations. Nevertheless,the results indicate that the tintinnids and the rotifers arehighly important components in the area's spring plankton community.Through high turnover rates, they are able to utilize effectivelythe spring phytoplankton bloom and thus serve as a link betweenthe primary and the larger secondary producers.     

The importance of crustacean zooplankton in structuring rotifer and phytoplankton communities; an enclosure study

During the spring clear-water phase of 1993, an enclosure experimentwas performed in the mesotrophic Schöhsee (Plön, FRG)in order to assess the impact of crustacean zooplankton on therotifer and phytoplankton community. Among the crustacean plankton,calanoid and cyclopoid cope-pods were abundant, but Daphnia‘longispina’ reached the highest densities in thisexperiment. The colonial rotifer Conochilus unicomis was notaffected by crustacean plankton. The two most abundant species,Synchaeta peclinata and Keratella cochlearis, increased exponentiallywhen macrozooplankton had been excluded from the enclosures,but did not increase when crustaceans were present. Birth anddeath rates of K.cochlearis could be reliably determined inthis field experiment, suggesting that this rotifer specieswas mainly controlled by exploitative competition rather thanby mechanical interference or predation. Daphnia ‘longispina’generally grazed selectively on the smaller ciliates and algae,thus depriving the rotifers of their phytoplankton resources.The dominant alga, the chrysophycean Dinobryon, increased, whethercrustaceans were present or not, but appeared to be grazed uponto a certain extent despite its considerable cell size and colonialorganization.     

Zooplankton of a flooded opencast sulphur mine

The zooplankton community of a flooded opencast sulphur mine was investigated during two years. The complicated physical and chemical relationships in this water-filled pit caused an atypical plankton assemblage. At the time of the investigation 20 species of rotifers, 6 of Cladocera, and 8 species of Copepoda were found. A single Ostracod and Chaoborus flavicans were also noted in the samples. Usually the epilimnion was monopolised by rotifers whereas the hypolimnion had a more diversified assemblage of rotifers, cladocerans and copepods. The result of clustering analysis indicated that each predator was associated with its own group of prey. Discrimination analyses showed weak seasonal differences in the zooplankton and three zones in the vertical profile – epilimnetic, upper hypolimnetic and anaerobic. Principal component analysis reduced the chemical parameters to the main contributors of conductivity (the main anions and cations) plus toxic sulphide and hydrogen sulphide. The animals were classified by the first two components into two groups: cold stenotherms and medium-sized filtrators, such as Bosmina longirostris. A specific analysis of vertical profiles generated five variants of diel vertical migrations. When time–space distributions of the plankters were analysed the highest numbers of different species were usually in different times and depths. The epilimnion was inhabited by small fishes and was dominated by rotifers but was free from C. flavicans which inhabited the cold hypolimnion, together with the remaining invertebrate predators (copepods and Asplanchna).Chaoborus and the other invertebrate predators participated more evenly in the contributions of species to the total density. In the presence of invertebrate predators the structure of the rotifer population shifted from soft-bodied species to spined or fast-moving species. When the predation pressure was too strong, as in the hypolimnion, the soft-bodied species were, in practice, absent. When the pit was in an oligotrophic state the hypolimnion population was composed of up to 60–90% rotifers. When the trophic state shifted to mesotrophy, the proportion of cladocerans increased.     

Clearance rates of bacteria by the rotifer Filinia longiseta (Ehrb.) measured using three tracers

The clearance rates (CRs) of bacteria by Filinia longiseta were measured at 19°C, both in situ in Lake Loosdrecht and in the laboratory during summer. The tracer particles used in the field were: (1) 0.51 µm fluorescent microspheres, and (2) fluorescently labelled bacteria (FLB). A third type of tracer particle, natural [methyl-³H]-thymidine-labelled bacteria (< 1.2 µm), were used as a radiotracer in a laboratory experiment. The uptake of the first two tracer-particle types was measured by microscopic examination of the rotifer guts. In the third case, the uptake of radioactivity was determined by liquid scintillation counting. The rate of uptake of the microspheres decreased 10 min after the start of the experiment, probably because the gut passage time was exceeded. Using a 5 min feeding time, the rate of uptake of microspheres was higher than that of the FLB, though the variation in the uptake in both cases was high. The ingestion rates and CRs of bacteria by F. longiseta based on the fluorescent tracers were: microspheres, 5115 bact.ind^–1 h^–1 and 0.368 µl ind^–1 h^–1; FLB, 2252 bact.ind^–1 h^–1 and 0.162 µl ind^–1 h^–1. The mean CR using the thymidine-labelled natural bacteria and a 10 min feeding time was 0.179 µl ind^–1 h^–1. Thus, the CR based on the microsphere method was twice as high as for the other two methods.     

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.     

Indirectly fluorescently labelled flagellates (IFLF): a tool to estimate the predation on free-living heterotrophic flagellates

A procedure was developed to estimate the direct grazing impacton free-living heterotrophic nanoflagellates (HNF). Culturedflagellates were labelled by feeding on brightly fluorescingbacteria (FLB) and then offered as indirectly fluorescentlylabelled flagellates (IFLF) to potential predators of HNF. Thenumber of FLB in the predators' food vacuoles could be convertedinto IFLF uptake and consumption of HNF. This new techniquewas used to study the HNF-ciliate relationship in the pelagiczone of Lake Constance. Three groups of ciliates were detectedas HNF grazers: small representatives of the genus Strobilidium.a small Haltena-like ciliate (probably Halteria grandinella)and a Codonella sp. Tintinnidium sp. group The ingestion ofHNF by these groups of ciliates ranged between 3 and 15, 3 and39, and 3 and 7 HNF ciliate^–1 h^–1; respectively.The IFLF method allows the direct determination of ingestedflagellate prey in the food vacuoles of their predators. Becauseindigenous living prey organisms were used, tracer discriminationcan be reduced.     

Resource competition between two rotifer species (Brachionus rubens and B.calyciflorus): an experimental test of a mechanistic model

A mechanistic model of competition on a single resource wastested experimentally with two freshwater rotifers, Brachionusrubens and B.calyciflorus, both grown on the alga Chlorellapyrenoidosa. Using open culture systems for each species wemeasured: (i) the resource-saturated exponential growth rate,µ_max, and (ii) the relationship between specific growthrate, µ, at steady-state and the residual algal concentrationover a range of system turnover rates, or dilution rates, D.The µ_max of B.calyciflorus was {small tilde}60% higherthan B. rubens. These results were then used to construct agraphical model for predicting the victor in interspecific competitionbetween the two rotifers. Since the two resource-dependent growthrates crossed, one species, B.calyciflorus, was predicted tobe the victor at a high D while B. rubens was predicted as thevictor at low D. Finally, the outcome of competition was determinedfor two turnover rates. As predicted by the graphical competitionmodels, B.calyciflorus was the dominant species at rapid D (0.029h^–1) and B.rubens was dominant at slow D (0.0044 h^–1).These studies support recent conclusions that mechanistic competitionmodels may be applied to predict dominant species from a prioriinformation on growth potential and resource levels, which isnot possible with traditional Lotka-Volterra models.     

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.     

Zooplankton in the Vasikkalampi pond, a warm water effluent recipient in Central Finland

Seasonal and vertical fluctuations of zooplankton species composition,biomass, and production were monitored by weekly sampling duringa two year period in one eutrophic pond in Central Finland.The study was one part of a more comprehensive study programto investigate the effects of warm water effluents from onesmall thermal power plant (35 MW) on the pond ecosystem. Becauseof the circulation of the pond water through the pumps in thepower plant the crustacean populations were very sparse in planktonduring the seasons the power plant was in operation (late Augustto May). During that time rotifers were dominant and some speciesreached very high densities (e.g., Keratella cochlearis s.l.ca. 15 000 ind. l^–1 in sping). In summer months Asplanchnapriodonta, Ceriodaphnia quadrangula, Bosmina longirostris, Mesocyclopsleuckarti and Thermocyclops oithonoides were dominant. A totalof 96 planktonic and meroplanktonic taxa were identified (26ciliates, 46 rotifers, 21 cladocerans and 3 copepods). The dryweight biomass of total zooplankton was 10 mg m^–3 in wintermonths, 10–100 mg m^–3 in spring and 300–1000mg m^–3 in summer. The total yearly production of zooplanktonwas 8552 mg dry wt m^–3 a^–1 in 1979 and 8440 mg drywt m^–3 a^–1 in 1980, from which the proportion ofrotifers was 33–39%, cladocerans 52–58% and copepods8.6 –9.4%. The winter production was 0.2–0.5% ofthe total yearly production, that of spring and autumn togetherwas 8.1–10.4% and the remainder (89–91%) was summerproduction.