Temporal dynamics of heatwaves are key drivers of sediment mixing by bioturbators

Heatwaves affect tidal flat ecosystems by altering the bioturbating behavior of benthic species, with potential consequences for sediment oxygenation, particle mixing, and erodibility. Although the frequency and duration of heatwaves are expected to increase under global warming scenarios, we lack insights into how heatwaves' temporal dynamics affect bioturbating behaviors. Using the widely distributed bioturbator Cerastoderma edule as model species, we quantified how heatwaves with identical heat-sum but different temporal dynamics (i.e., 3- vs. 6-d heating and normal temperature cycles) affect bioturbating behaviors and the sediment mixing processes in tidal mesocosms. Our results show that short but frequent 3-d heatwave cycles increased the magnitude of bioturbating behaviors, thereby resulting in more bio-mixed sediment than observed under infrequent prolonged 6-d heatwave cycles. This unexpected result could be ascribed to the weakening health condition indicated by a high death rate (47.37%) under 6-d heatwave cycles than in 3-d and no-heatwave control cycles. Present findings reveal that the impact of heatwaves on sediment bioturbation will strongly depend on the temporal dynamics of future heatwaves: bioturbation will be enhanced unless the heatwave duration exceeds species resistance and increases mortality.