Hydrological and ecological effects of floating photovoltaic systems: a model comparison considering mussel, periphyton, and macrophyte growth

Floating photovoltaic (FPV) systems are increasingly deployed on gravel pit lakes to generate renewable energy and mitigate land-use conflicts. However, their environmental impacts on hydrological and ecological processes remain insufficiently studied. This study investigates the effects of a 1.5-MWp FPV system covering 8% of a 19-ha gravel pit lake in Germany. The General Lake Model (GLM-AED2) and Delft3D-FLOW were used to simulate FPV-induced changes. Meteorological data—including irradiance, air temperature, wind speed, and relative humidity—were recorded above and below the PV modules. Water quality data—including water temperature, dissolved oxygen, pH, dissolved organic carbon, and chlorophyll-a—were collected beneath the FPV and in open water. Mussel colonisation of the FPV substructure was assessed, and its filtration impact on water quality analysed. Macrophyte distribution was assessed beneath the FPV system and along the shorelines. Results showed a modelled 88% solar irradiance and 57% wind speed reduction beneath the FPV system. Water quality impacts were minimal and primarily influenced by mussels colonising the substructure. Macrophytes occurred in littoral zones up to 5.25 m deep up to 5.25 m deep, but habitat-typical species were scarce due to gravel extraction and herbivorous fish. These findings highlight complex interactions between FPV, mussel filtration, macrophytes, and human activities, suggesting that other anthropogenic factors may outweigh FPV impacts. Model simulations indicated that FPV coverage above 45% could destabilise thermal stratification and alter primary production. This study underscores the need for empirical monitoring and modelling to optimise FPV deployment and inform regulatory frameworks for sustainable development.