Combined effects of heatwaves and pollution on freshwater invertebrates: a scoping review of experimental approaches

Freshwater ecosystems are highly vulnerable to climate change and anthropogenic pollution. Climate change is characterized particularly by an increase in average temperatures, with more intense, longer, and more frequent heatwaves (HWs). While the effects of contaminants are well documented, sometimes in the context of global warming, the impact of HWs, either alone or in combination with pollution, remains poorly studied in freshwater invertebrates. A scoping review based on 68 studies was conducted to assess the current state of research. The objectives are to: (1) describe annual publication trends and geographic distribution, (2) analyze the diversity of heatwave (HW) scenarios used in experimental protocols, (3) assess the distribution of experimental approaches and model species, and (4) identify the outcome indicators most studied by authors. The analysis of this corpus showed the scientific community's growing interest in HW effects on freshwater invertebrates. However, only 5 of the 68 studies examined naturally occurring HWs, the rest relying on simulated exposures. Moreover, experimental HW intensities often exceeded those documented in natural lake HWs. Despite the use of robust models such as Daphnia or Lymnaea stagnalis , and the emergence of multi-stress approaches in semi-natural environments, most studies were conducted under laboratory conditions measuring individual-level indicators without necessarily considering the interactions between HWs and pollution. This review highlights the need for more realistic thermal scenarios, multi-stress approaches, and molecular analyses to better predict the resilience of freshwater ecosystems to global changes, particularly in sensitive regions such as the Arctic.