Inter-annual climate variability and zooplankton: applying teleconnection indices to two deep subalpine lakes in Italy

Investigating relation between meteo-climatic indices and between-year variation in Daphnia population density and phenology is crucial for e. g. predicting impact of climate change on lake ecosystem structure and functioning. We tested whether and how two teleconnection indices calculated for the winter period, namely the East Atlantic pattern (EA(DJF)) and the Eastern Mediterranean Pattern (EMPDJF) were correlated with Daphnia population growth in two Italian subalpine lakes, Garda and Maggiore. We investigated between-lake temporal coherence in: i) water temperature within the water layer in which Daphnia is distributed; ii) timing of Daphnia initial and spring maximum population density peak and iii) the level of Daphnia spring maximum population density peak over an eleven-year period (1998-2008) of unchanged predation pressure by fish and invertebrates, and of common oligotrophy. Between-lake temporal coherence was high for an earlier start, and earlier, and lower, Daphnia population spring density peak after milder winters. Peak density level was coherently, positively correlated with soluble reactive phosphorus (SRP) concentration. We hypothesized that Daphnia peak densities were related to atmospheric modes of variability in winter and to the degree of late winter mixing promoting replenishment of algal nutrients into upper water layers and phytoplankton growth, enhancing food availability and Daphnia fecundity, promoting and Daphnia peak.