The causes of daphnia midsummer decline in two deep subalpine lakes (L. Lugano and L. Iseo)

Daphnia is a keystone organism in pelagic food webs, acting as a food resource for fish and predatory zooplankton and regulating phytoplankton through grazing. Its population dynamics follows a regular seasonal pattern, where a spring density peak is followed by a population summer decline. This phenomenon (midsummer decline, MSD) shows high inter-annual variation and it has been attributed to different causes; however, the underlying mechanisms remain poorly understood. Our study aims to disentangle the pathways that lead to Daphnia MSDs in two deep peri-alpine lakes, Lugano and Iseo. We hypothesized that: (I) winter air temperature controls MSDs by influencing mixing depth during turnovers and nutrients availability; (II) vernal air temperature directly influences MSD by accelerating the timing of spring population peak; (II) summer temperature indirectly influences MSDs by increasing fish predation. We tested the hypotheses using Structural Equation Modelling on long-term datasets for the two lakes (29 years for Lake Lugano and 19 years for Lake Iseo). Winter air temperature, affecting vertical mixing depth and consequently epilimnetic phosphorus concentration, had the major influence on Daphnia phenology during spring and summer, but the effects were stronger in Lake Lugano, where the lower lake-water P concentrations might have been more limiting.