Thermal–Ecological Trade-offs of Surface-Water Heat Extraction in a Small Low-flushing Lake

Thermal Energy from Surface Water (TES) can contribute substantially to sustainable heating, yet its ecological implications remain poorly quantified. This study provides a process-based assessment of how thermal energy extraction from surface waters alters aquatic ecosystem functioning, highlighting system-specific trade-offs at the water–energy interface. We used a coupled modelling framework (Delft3D-FLOW, D-Water Quality, PCLake+) to assess how TES-induced cooling (ΔT = 5-10°C) and mechanical filtration (21% mortality per pass) affect a small, shallow, low-flushing freshwater system. This integrated framework allows TES-induced cooling, mechanical filtration, and trophic feedbacks to be assessed jointly rather than in isolation. Across different discharge scenarios (Q = 75-300 m 3 h⁻ 1 ), chlorophyll-a declined by 16-56%, and primary production was reduced by 19-66% during the growing season. Cooling was the dominant driver of ecosystem change, with seasonal relative water temperature reductions, calculated from values expressed in °C, of 6-37% and instantaneous cooling up to 15.6°C. Filtration amplified biomass losses by approximately 5-6 percentage points through direct removal of plankton and reduced grazing. Stressor interactions were modestly sub-additive but resulted in large cumulative impacts and delayed recovery: biomass and production remained around 12-15% below reference levels after system shutdown. These results demonstrate that TES can substantially suppress productivity in vulnerable water bodies and highlight that water–energy interactions must be evaluated system-specifically to avoid ecological degradation while enabling energy extraction.