A paradox of warming in a deep peri-Alpine lake (Lake Lugano, Switzerland and Italy)

Abstract

We investigated the effects of seasonal air temperature on the north basin of Lake Lugano (Switzerland and Italy), a deep peri-Alpine lake that is recovering from eutrophication. A priori ideas concerning the effects of temperature on key ecosystem responses were formalized in a conceptual model, which was tested against observed responses (from 28 years of monitoring data) using structural equation modeling. The results broadly supported our model, and indicated that air temperature had pervasive effects on the lake’s ecosystem. Warmer-than-usual winters restricted the depth of vertical mixing during turnovers and, as a result, reduced the associated replenishment of phosphorus (P) to surface waters. Moreover, although the reduction of P increased the potential for nutrient limitation, warmer winters and summers led to increases in summer chlorophyll a content (an index of phytoplankton biomass) through complex direct and indirect (food-web) effects. This apparently paradoxical effect on chlorophyll mimics a symptom of eutrophication, and suggests that climate warming might impede the recovery of the lake from eutrophication or increase the management interventions that will be required to meet the restoration targets.