Seasonal variation in trophic structure and restoration effects in a deep perialpine lake (Lake Lugano, Switzerland and Italy)

Abstract

Using monitoring data from the South Basin of Lake Lugano (Switzerland and Italy), we examined seasonal responses of phytoplankton and herbivorous zooplankton biomass to nearly three decades (1989–2017) of phosphorus (P) management to ask: [1] what is the trophic structure of the lake, [2] whether trophic structure and the effects of nutrient management varied seasonally, and [3] what are the implications of the existent trophic structure for the restoration of the lake. Trophic structure varied seasonally, including a structure consistent with strong consumer control (exploitation food chain) in spring and fall, and an unexpected structure in summer, characterized by a negative correlation between phytoplankton biomass and phosphorus. This structure was explained by accumulation of inedible phytoplankton (mainly cyanobacteria) at low P concentrations. Owing to a lack of apparent resource (P) control, P-management had no detectable effects on phytoplankton biomass. The trophic structures identified in the lake provides explanations for this lack of response to P-management and for the differences between the responses of the South Basin of Lake Lugano and other perialpine lakes. Based on the results, lake restoration practice would benefit from a greater understanding and an increased ability to predict trophic structure within and across lakes.