A Non-Stressful Temperature Rise and Greater Food Availability Could Increase Tolerance to Calcium Limitation of Daphnia cf. pulex (Sensu Hebert, 1995) Populations in Cold Soft-Water Lakes.

Calcium (Ca) is an important driver of community structure in freshwaters. We examined the combined effects of increased temperatures and variations in food quantity on the tolerance to low Ca of Daphnia pulex. The aim was to predict the impact of climate warming on this keystone zooplanktonic species in cold-climate lakes. We conducted a factorial life-history experiment in a clone of North American Daphnia cf. pulex to analyse the interaction effects of a temperature increase (17.5 °C−21 °C) within their physiological preferred range and expected by climate warming over the next few decades and a narrow Ca gradient (0.25−1.74 mg Ca L−1) under stressful vs. abundant food conditions. We found a striking positive synergistic effect of Ca and temperature on D. pulex reproduction at high food conditions. Although the increase in temperature to 21 °C greatly reduced survival, high energy allocation to reproduction at high food levels allowed the population to succeed in poor Ca (<0.25 mg Ca L−1). Results suggest that climate warming and higher food availability will make the populations of many cold and Ca-limited lakes more tolerant to low Ca levels with higher growth population rates, thereby altering zooplanktonic community structures and inducing potential cascading effects on the food web.

Intraspecific variation in sensitivity to food availability and temperature-induced phenotypic plasticity in the rotifer Keratella cochlearis.

Organisms with wide environmentally induced morphological plasticity and cosmopolitan distribution, e.g. the common freshwater rotifer Keratella cochlearis, are ideal models to study the evolution of plastic polymorphisms and the capacity of zooplankton to adapt to local selection conditions. We investigated population-level differences (population-by-environment interaction) in sensitivity to food availability and temperature-induced phenotypic plasticity between two clones of K. cochlearis isolated from neighboring populations in Ruidera Natural Park (Spain) with different trophic statuses: Tinaja lake (mesotrophic) and Cueva Morenilla lake (eutrophic). Using common-garden experiments, each clone proved to have a different sensitivity to food availability, with substantial phenotypic differences between them. When rotifers grew at moderate temperature (15.6°C), low food levels were more efficiently used by the Tinaja versus Cueva Morenilla clone, whereas high food levels were more efficiently used by the Cueva Morenilla versus Tinaja clone. The posterior spine was much longer and the lorica wider in the Tinaja versus Cueva Morenilla clone, with no difference in lorica length. Phylogenetic analysis based on cytochrome c oxidase subunit I (COI) gene sequences showed that the two populations have the same haplotype. This is the first study to show possible local adaptation by a rotifer species to habitats that consistently differ in food availability. We also detected an intriguing deviation from the expected negative relationship between posterior spine length and temperature. Our experimental results indicate that intermediate temperatures may activate the gene responsible for spine elongation in K. cochlearis This suggests that rotifers in nature could use water temperature as proxy signal of a change in predation risk before defense is needed.

The power of species sorting: local factors drive bacterial community composition over a wide range of spatial scales.

There is a vivid debate on the relative importance of local and regional factors in shaping microbial communities, and on whether microbial organisms show a biogeographic signature in their distribution. Taking a metacommunity approach, spatial factors can become important either through dispersal limitation (compare large spatial scales) or mass effects (in case of strongly connected systems). We here analyze two datasets on bacterial communities [characterized by community fingerprinting through denaturing gradient gel electrophoresis (DGGE)] in meso- to eutrophic shallow lakes to investigate the importance of spatial factors at three contrasting scales. Variation partitioning on datasets of both the bacterial communities of 11 shallow lakes that are part of a strongly interconnected and densely packed pond system <1 km apart, three groups of shallow lakes approximately 100 km apart, as well as these three groups of shallow lakes combined that span a large part of a North-South gradient in Europe (>2,500 km) shows a strong impact of local environmental factors on bacterial community composition, with a marginal impact of spatial distance. Our results indicate that dispersal is not strongly limiting even at large spatial scales, and that mass effects do not have a strong impact on bacterial communities even in physically connected systems. We suggest that the fast population growth rates of bacteria facilitate efficient species sorting along environmental gradients in bacterial communities over a very broad range of dispersal rates.

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.