Assessment of external and internal nutrient loads to Lake Pusiano (Northern Italy) using watershed eco-hydrological modelling and lake ecological-hydrodynamic simulations

Abstract

Eutrophication is still a serious issue for many lakes in the subalpine lake district of Northern Italy. Significant improvements have been achieved thanks to interventions carried out on their watersheds, whereas the Water Framework Directive of the European Union (EU WFD) has established ecological and chemical water quality targets. Unfortunately, climate change and remaining pollution inputs by human activities play a synergistic role in hampering remediation efforts. In fact, water warming strengthens stratification, leading in shallower holomictic basins to prolonged periods of hypolimnetic anoxia, boosting phosphorus ( P ) release from sediments and promoting phytoplankton blooms in some cases. This study focuses on Lake Pusiano, a 25 m-deep mesotrophic lake with a 5.2 km 2 area in Northern Italy. Lake Pusiano, after recovering from eutrophication in the last two decades, has experienced a recent deterioration in water quality, suggesting that nutrient loads are still excessive. The initial aims and final results of this study were an estimation of the current external nutrient loads from the watershed, assessing their variability in relation to annual rainfall, and evaluating the present internal load from lake sediments. To achieve these targets, an eco-hydrological model of the lake watershed was developed using SWAT+ (Soil & Water Assessment Tool). The obtained external nutrient loads from the basin were then fed into an ecological-hydrodynamic one-dimensional (1D) GOTM-WET (General Ocean Turbulence Model – Water Ecosystems Tool) model of Lake Pusiano, reproducing in-lake dynamics. Additionally, a nutrient loading reduction scenario was developed through the implementation of Nature-Based Solutions (NBS) on the watershed, with the aim of estimating the effect of realistic nutrient load reduction interventions on the lake status, which were deemed insufficient for safeguarding current spring-mixing total phosphorus ( TP ) concentration limits under the present climate-change pressures.

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