Effects of Daphnia longispina on rotifer populations in a natural environment: relative importance of food limitation and interference competition

To test the relative importance of exploitative and interferencecompetition from Daphnia limiting rotifer populations in Bermejalesreservoir, birth rates of rotifers, food abundance, zooplanktonbiomass and size distribution of Daphnia were analysed fromhigh frequency sampling at two depths. A depletion in food abundanceby Daphnia feeding should be reflected in the birth rates ofrotifers susceptible to exploitative competition, while interferenceshould produce direct negative relationships between the numberof individuals of the interacting species. Results show thatsignificant relationships between food (chlorophyll-a) and birthrates of rotifers are found when the ratio chlorophyll-a/herbivorouszooplankton biomass is used as a measure of food availability.In our study, Anuraeopsis fissa was only affected through exploitativecompetition by Daphnia, while only Synchaeta oblonga showedevidence of being suppressed through interference competitionby this cladoceran.     

Dormant propagule banks integrate spatio-temporal heterogeneity in cladoceran communities

The dynamics of populations of short-lived organisms are very patchy, both in space and time. The production of dormant propagules, however, results in an effective increase in generation time. We hypothesize that prolonged dormancy, together with variable regeneration niches, result in integration of temporal variability in community structure. In addition, in aquatic habitats, mechanisms such as sediment focussing can contribute to the integration of spatial variability. We tested the hypothesis that dormant propagule banks integrate spatial and temporal variation in active zooplankton communities. This was done by comparing cladoceran species richness and the community structure of hatchling assemblages retrieved from propagule bank samples collected on a single occasion with assemblages encountered in active community samples covering spatial variation (littoral and pelagic zone), diel (day and night), intra-year (May–October) and inter-year variation (1996–2000). The egg bank community structure differed significantly from the active community structure, but the dissimilarity decreased as spatial and temporal variation was better covered by the active community samples. Furthermore, the identification of all fully grown hatchlings (n=214) yielded an equally high number of species (n=22) to that occurring in all active community samples together (a total of 1,730 individuals were analysed). We conclude that the analysis of dormant propagules may form a cost-efficient alternative tool to the analysis of active community samples for an integrated assessment of cladoceran communities.     

The relevance of size efficiency to biomanipulation theory: a field test under hypertrophic conditions

The superiority of large zooplankton in suppressingphytoplankton growth has often been inferred from theSize Efficiency Hypothesis (S.E.H.). The S.E.H. hasoriginally been formulated to account for thecompetitive superiority of large to small zooplanktonunder food limiting conditions. Extrapolation of itspredictions to the suppression of phytoplankton byzooplankton under high food availability, should bedone with care. In an attempt to assess the relevanceof the S.E.H. to biomanipulation theory inhypertrophic systems, a fish exclosure experiment wascarried out in which the efficiency of two differentlystructured zooplankton communities in reducingphytoplankton biomass was examined. By inoculatingpart of the enclosures with laboratory grownDaphnia magna, a community dominated by this largecladoceran species could be compared with a communitymainly consisting of Bosmina and smallerDaphnia species. After the exclusion of fish, therewas an exponential increase of total zooplanktonbiomass. Phytoplankton growth was efficientlysuppressed to equal levels in both treatments, thoughthere was a difference in timing: chlorophyll-a levelsin the enclosures inoculated with D. magnadropped one week earlier than in non-inoculatedenclosures. The time-lag was even more pronounced whenlarge phytoplankton was considered. In accordance withthe S.E.H., the time lags could be explained bydifferences in population growth potential as well asby differences in zooplankton grazing rates(indirectly measured as the minimal zooplanktonbiomass needed to suppress phytoplankton growth) andfood particle size range.     

Impacts of climate warming on lake fish community structure and potential effects on ecosystem function

Fish play a key role in the trophic dynamics of lakes, not least in shallow systems. With climate warming, complex changes in fish community structure may be expected owing to the direct and indirect effects of temperature, and indirect effects of eutrophication, water-level changes and salinisation on fish metabolism, biotic interactions and geographical distribution. We review published and new data supporting the hypotheses that, with a warming climate, there will be changes in: fish community structure (e.g. higher or lower richness depending on local conditions); life history traits (e.g. smaller body size, shorter life span, earlier and less synchronised reproduction); feeding mode (i.e. increased omnivory and herbivory); behaviour (i.e. stronger association with littoral areas and a greater proportion of benthivores); and winter survival. All these changes imply higher predation on zooplankton and macroinvertebrates with increasing temperatures, suggesting that the changes in the fish communities partly resemble, and may intensify, the effects triggered by eutrophication. Modulating factors identified in cold and temperate systems, such as the presence of submerged plants and winter ice cover, seem to be weaker or non-existent in warm(ing) lakes. Consequently, in the future lower nutrient thresholds may be needed to obtain clear-water conditions and good ecological status in the future in currently cold or temperate lakes. Although examples are still scarce and more research is needed, we foresee biomanipulation to be a less successful restoration tool in warm(ing) lakes without a strong reduction of the nutrient load.     

Effectiveness of the caudal spine as a defense mechanism in Keratella cochlearis

The temporal distribution of spined and unspined forms of Keratella cochlearis in Cubillas reservoir during an annual cycle, and the stomach contents of Asplanchna girodi, were studied. The results suggest that the length of the caudal spine plays a critical role in the defense against predation by Asplanchna girodi. Short-spined forms showed a higher susceptibility to predation than unspined forms, which may have important consequencies for spine length variation in Keratella cochlearis populations.     

Chemical interference by Daphnia on Keratella: a life table experiment

Daphnia can suppress rotifers through mechanical interference(interference competition) and exploitative competition. Totest chemical interference (interference competition) by Daphniaon the rotifer Keratella, life history characteristics wereanalysed for Keratella cochlearis in the presence and absenceof Daphnia-mediated chemicals. The presence of Daphnia-mediatedchemicals induced a lower fecundity and a decrease in the intrinsicrate of population growth. This type of interference competitionshould affect the composition of natural rotifer communitiesregardless of mechanical interference and exploitative competitiondue to Daphnia.     

Influence of nutrients, submerged macrophytes and zooplankton grazing on phytoplankton biomass and diversity along a latitudinal gradient in Europe

In order to evaluate latitudinal differences in the relationship of phytoplankton biomass and diversity with environmental conditions in shallow lakes, we sampled 98 shallow lakes from three European regions: Denmark (DK), Belgium/The Netherlands (BNL) and southern Spain (SP). Phytoplankton biomass increased with total phosphorus (TP) concentrations and decreased with submerged macrophyte cover across the three regions. Generic richness was significantly negatively related to submerged macrophyte cover and related environmental variables. Zooplankton:phytoplankton biomass ratios were positively related to submerged macrophyte cover and negatively to phytoplankton generic richness in DK and BNL, suggesting that the low generic richness in lakes with submerged macrophytes was due to a higher zooplankton grazing pressure in these regions. In SP, phytoplankton generic richness was not influenced by zooplankton grazing pressure but related to conductivity. We observed no relationship between phytoplankton generic richness and TP concentration in any of the three regions. The three regions differed significantly with respect to mean local and regional generic richness, with BNL being more diverse than the other two regions. Our observations suggest that phytoplankton diversity in European shallow lakes is influenced by submerged macrophyte cover indirectly by modulating zooplankton grazing. This influence of submerged macrophytes and zooplankton grazing on phytoplankton diversity decreases from north to south.     

Reproduction strategies of <Emphasis Type="Italic">Daphnia pulicaria</Emphasis> population in a high mountain lake of Southern Spain

For three consecutive years, a population study of Daphnia pulicaria was undertaken in Río Seco Lake, a small high-mountain lake, in order to elucidate the reproductive strategies adopted by Daphnia in this system. Daphnia appears to colonize this lake every spring by hatching from ephippia and reproduce by means of subitaneous (non-diapausing) and ephippial (diapausing) eggs. D. pulicaria in this lake is an obligate parthenogenetic population. There is a short time period for subitaneous egg production and a much longer period for ephippial egg production. The contribution of subitaneous eggs to Daphnia population density and structure appears to be low. Diapause onset showed a high temporal synchronization in the three studied years in Río Seco Lake, and day-length emerged as the main cue triggering diapause onset and the main explanatory factor for the proportion of ephippial females observed. The development and reproduction of D. pulicaria in Río Seco Lake involves taking a gamble on resting forms to guarantee inter-annual Daphnia persistence in the lake, giving priority to investment in future generations. Guest editor: Piet Spaak Cladocera: Proceedings of the 7th International Symposium on Cladocera     

Emergence pattern and hatching cues of Daphnia pulicaria (Crustacea, Cladocera) in an alpine lake

We developed a methodology to predict brook trout (Salvelinus fontinalis) distribution using summer temperature metrics as predictor variables. Our analysis used long-term fish and hourly water temperature data from the Dog River, Vermont (USA). Commonly used metrics (e.g., mean, maximum, maximum 7-day maximum) tend to smooth the data so information on temperature variation is lost. Therefore, we developed a new set of metrics (called event metrics) to capture temperature variation by describing the frequency, area, duration, and magnitude of events that exceeded a user-defined temperature threshold. We used 16, 18, 20, and 22°C. We built linear discriminant models and tested and compared the event metrics against the commonly used metrics. Correct classification of the observations was 66% with event metrics and 87% with commonly used metrics. However, combined event and commonly used metrics correctly classified 92%. Of the four individual temperature thresholds, it was difficult to assess which threshold had the “best” accuracy. The 16°C threshold had slightly fewer misclassifications; however, the 20°C threshold had the fewest extreme misclassifications. Our method leveraged the volumes of existing long-term data and provided a simple, systematic, and adaptable framework for monitoring changes in fish distribution, specifically in the case of irregular, extreme temperature events.     

Regulation of rotifer species by invertebrate predators in a hypertrophic lake: selective predation on egg-bearing females and induction of morphological defences

Temporal changes in the biomass of rotifer plankton were examinedin a shallow hyper trophic lake during 1 month (April/May),when the predatory rotifer Asplanchna brightwelli develops.The abundance of herbivorous rotifers was regulated by predationfrom A.brightwelli and from the copepod Acanthocyclops robustus.The densities and fecundity rates of Keratella cochlearis andKeratella quadrata were negatively related with the biomassof predators. Stomach analyses showed that Asplanchna fed selectivelyon reproductive females of K.cochlearis, reducing the fecundityof this species. Predators induced longer caudal spines in K.quadrata,which were negatively related to the fecundity of this rotifer,suggesting a reproductive cost associated with spine production.In contrast, spine length of K.cochlearis was not related topredators, but to temperature. These results showed that predatorscan reduce rotifer densities through increasing mortality andthrough decreasing rotifer fecundity rates regardless of phytoplanktonbiomass. We also show that morphological defences of K.cochlearisand K.quadrata are induced in different ways.