Relationships among climate variability, Cladocera phenology and the pelagic food web in deep lakes in different trophic states

Achieving a better understanding of the role of climate change in altering population phenology, seasonal cycles in freshwater organisms, and ecosystem structure and function is of high scientific and economic value. The present paper has demonstrated the different food-web responses to teleconnection indices, which are proxy of climate fluctuations, in lakes characterised by different trophic levels. We analysed an 18-year long-term dataset (1998–2015) recorded in the deep eutrophic Lake Iseo and we compared our results to those concerning deep southern-Alpine oligotrophic lakes. Our results confirmed that winter large-scale circulation patterns (for Mediterranean area: East Atlantic Pattern and Eastern Mediterranean Pattern) control a chain of linked causal factors, affecting the winter air temperature, spring water temperature, the resulting water vertical-mixing depth and epilimnetic concentration of total phosphorus. We highlighted that in a lake, characterised by high phosphorus concentration, the spring enrichment in nutrients did not result in either a considerable increase of phytoplankton growth, nor, consequently, in a zooplankton density peak. Whereas in oligotrophic lakes, cascading effects influenced the algal carrying capacity and Daphnia population density. We observed that climatic fluctuations, mediated by, for example, colder water temperature in spring, postponed the timing of population recovery after diapause of both primary consumers (Daphnia and Eubosmina) and secondary consumers (Bythotrephes and Leptodora). The latter being verified for the first time in a eutrophic lake.