Unexpected outcome of an extended drought period on the primary productivity of Lake Pusiano (Lombardia, Northern Italy): the importance of integrated lake-catchment modelling in cause-effect assessments

Water eutrophication is still a reason of concern for lakes at the foothills of the European Alps. Among them, small and shallow basins are the ones exposed to the highest risks. This occurs because they often display anoxic hypolimnions during the stratification period, triggering internal nutrient loading from the sediments. Furthermore, their low volume makes them more vulnerable to alterations in external loading. Interventions on alpine watersheds in the last decades decreased external nutrient pollution for most of these lakes, causing a slow disposal of internal loading as well. However, the results of external load reduction measures can hardly be identified directly. In fact, while the European Union Water Framework Directive (EU WFD) sets ecological status targets for water bodies, it does not require monitoring of nutrient loads delivered to lakes. As a result, when worsening trophic conditions occur, it is difficult to tell whether the cause lies in insufficient external load reduction from the watershed, enduring internal load, short-term random meteorological alterations or long-term climate change effects. Integrated lake-catchment models can significantly help to unravel this issue over each specific case study. In fact, they allow reproducing: 1) the dependence of external loads on rainfalls; 2) the response of lake mixing, nutrient concentrations and primary productivity to present meteorological conditions and to future climate change and nutrient load management scenarios; 3) the release mechanisms of nutrients from sediments.

The case of Lake Pusiano (Lombardia, Northern Italy) is exemplary. Eutrophication peaked in this basin in the mid-1980s, after which nutrient pollution countermeasures in the catchment were implemented, stabilising the lake to mesotrophic conditions and significantly reducing internal load by the early-2010s. However, following an extended drought period lasting from late 2021 to early 2023, the lake trophic conditions rapidly worsened, leading to an exceptionally high primary production, enduring to this date also throughout the winter months. To investigate the causes behind this unexpected deterioration, an integrated lake-catchment model was developed, adopting the Soil & Water Assessment Tool (SWAT+) ecohydrological model to estimate external loads from the watershed and the Water Ecosystems Tool (WET) one-dimensional (1D) coupled ecological-hydrodynamic model for lake mixing and water quality simulations. The extended drought would have been expected to determine a temporary decrease of lake primary production, following the significant decrease in external loads reproduced by SWAT+. However, the WET simulations allowed understanding that the relevant increase in water residence times, also verified through three-dimensional (3D) hydrodynamic simulations with the Delft3D-FLOW model, together with the hot temperatures of summer 2022, led to severe anoxic conditions. These triggered an outstanding phosphorus release from sediments, affecting also shallower depths and thus a much more extended sediment area than usual. The ensuing rainfalls of 2023 and especially of extraordinarily wet year 2024 helped sustain this outstanding lake primary productivity, adding up to nutrient recycling. The warm 2022-23 and 2023-24 winters further made phytoplankton blooms endure through the coldest months.