This study aimed at unraveling the structure underlying the taxon-richness matrix of shallow lakes. We assessed taxon richness of a large variety of food-web components at different trophic levels (bacteria, ciliates, phytoplankton, zooplankton, fish, macro-invertebrates, and water plants) in 98 shallow lakes from three European geographic regions: Denmark (DK), Belgium/The Netherlands (BNL), and southern Spain (SP). Lakes were selected along four mutually independent gradients of total phosphorus (TP), vegetation cover (SUBMCOV), lake area (AREA), and connectedness (CONN). Principal-components analysis (PCA) indicated that taxon diversity at the ecosystem level is a multidimensional phenomenon. Different PCA axes showed associations with richness in different subsets of organism groups, and differences between eigenvalues were low. Redundancy analysis showed a unique significant contribution to total richness variation of SUBMCOV in all three regions, of TP in DK and SP, and of AREA in DK and BNL. In DK, several organism groups tended to show curvilinear responses to TP, but only one was significantly hump shaped. We postulate that the unimodal richness responses to TP that are frequently reported in the literature for many organism groups may be partly mediated by the unimodal response of macrophyte vegetation to lake productivity.

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Insight into how environmental change determines the production and distribution of cyanobacterial toxins is necessary for risk assessment. Management guidelines currently focus on hepatotoxins (microcystins). Increasing attention is given to other classes, such as neurotoxins (e.g., anatoxin-a) and cytotoxins (e.g., cylindrospermopsin) due to their potency. Most studies examine the relationship between individual toxin variants and environmental factors, such as nutrients, temperature and light. In summer 2015, we collected samples across Europe to investigate the effect of nutrient and temperature gradients on the variability of toxin production at a continental scale. Direct and indirect effects of temperature were the main drivers of the spatial distribution in the toxins produced by the cyanobacterial community, the toxin concentrations and toxin quota. Generalized linear models showed that a Toxin Diversity Index (TDI) increased with latitude, while it decreased with water stability. Increases in TDI were explained through a significant increase in toxin variants such as MC-YR, anatoxin and cylindrospermopsin, accompanied by a decreasing presence of MC-LR. While global warming continues, the direct and indirect effects of increased lake temperatures will drive changes in the distribution of cyanobacterial toxins in Europe, potentially promoting selection of a few highly toxic species or strains.

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We examined seasonality, intensity, and level of epibiont infection by the green alga Korshikoviella gracilipes on the zooplankton community over 2 yr in the high mountain Río Seco lake. Daphnia pulicaria was the preferred substrate for the epibiont, whose life cycle was exclusively completed on this taxon. The density of epibiont dispersal stage and the epibiont prevalence and burden on crustacean zooplankton were directly related to D. pulicaria density in both years. Laboratory experiments showed that attached epibionts had a negative effect on Daphnia by increasing the weight and sinking rates of infected animals and a positive effect by increasing its reproductive rates, with a neutral effect on survivorship. On the other hand, the epibiont dispersal stage was actively and intensively grazed by Daphnia, which implies a benefit for Daphnia and a cost for K. gracilipes. Thus, both species derived benefits and costs from this relationship. Our results indicate a positive cost-benefit balance for both epibiont and host in such a way that a mutualistic relationship can be suggested. This epibiont-host interaction may play an important role in population and community regulation in Río Seco lake.The presence of epibionts on marine and freshwater zooplankton is a very common phenomenon. Numerous studies have shown that epibionts can influence zooplankton individual and population dynamics in many ways, although the available data show considerable variability. Negative effects have been described on host reproduction (Green 1974;Xu and Burns 1991;Weissman et al. 1993;Stirnadel and Ebert 1997), survival (Allen et al. 1993;Weissman et al. 1993), swimming abilities (Weissman et al. 1993), and feeding (Green 1974;Kankaala and Eloranta 1987), as well as on increased sinking rates (Herman and Mihursky 1964;Allen et al. 1993) and vulnerability to predation (Willey et al. 1990;Chiavelli et al. 1993;Threlkeld and Willey 1993;Willey and Threlkeld 1993). On the other hand, it has also been reported that there is little or no effect on host reproduction (Allen et al. 1993; Willey 1993), respiration (Ikeda 1977;Allen et al. 1993), excretion (Ikeda 1977, or molting rate (Weissman et al. 1993). Finally, several studies suggested possible benefits for the hosts from the epibiont cleaning up the host surface (Holland and Hergenrader 1981), from epibiont excretion of specific nutrients (Holland and Hergenrader 1981), and from the host feeding on its epibiont (Green 1974;Van Dover et al. 1988;Threlkeld and Willey 1993;Al-Dhaheri and Willey 1996). Field and experimental evidence of this last possibility has only recently been obtained (Bartlett and Willey 1998; Poltz et al. 1998).1 Corresponding author (cperezm@goliat.ugr.es). Present address: Department of Animal Biology and Ecology, Faculty of Sciences, University of Granada, 18071-Granada, Spain. AcknowledgmentsThe authors are grateful to Dr. P. Sánchez-Castillo for helpful comments on K. gracilipes life cycle, to Dr. Julia García-Leal for statistical advice, and to Mr. ...