The recent trophic decline of Lake Pusiano (Northern Italy) assessed by integrated lake-catchment modelling

Abstract

Climate change is altering the status of lake ecosystems, not only through steady water warming but also with increasing frequency, duration and intensity of droughts and heatwaves. These extreme events, especially if concurrent, have relevant effects on moderately deep lakes, triggering severe stratification periods that lead to acute bottom anoxia. Such conditions notably trigger the release of phosphorus from sediments. Moreover, the anoxic volume in the hypolimnion can extend beyond the central basin towards littoral zones, increasing the sediment area contributing to the release and hence enhancing internal nutrient loading. This promotes cyanobacterial blooms that persist throughout the year, supported by warmer winters brought by climate warming. All these phenomena have been observed in Lake Pusiano (Northern Italy), where a prolonged drought between 2021 and 2023 led to a marked decline in water quality, increased phosphorus release from the sediments and extensive cyanobacterial blooms. To better comprehend these dynamics, a modelling approach was applied using three complementary numerical models: (1) a Soil and Water Assessment Tool (SWAT) + eco-hydrological model of the lake watershed was employed to estimate external loading; (2) a one-dimensional (1D) coupled ecological-hydrodynamic WET model of the lake was used to quantify internal loading; (3) a three-dimensional (3D) Delft3D D-Flow lake hydrodynamic model was applied to evaluate the role of the main tributary in mixing dynamics. Model results confirm that the prolonged drought and summer heatwave conditions experienced by Lake Pusiano during 2021–2023 are the ultimate cause of its recent trophic decline, having triggered an enduring regime shift in lake productivity.

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