Exposure to a gradient of freshwater pulses duration mimicking frequent floodings reveals species-specific vulnerability thresholds in an assemblage of key benthic molluscs

Estuarine communities provide many ecosystem services and are highly vulnerable to global changes. Detrimental changes in land use and intense rainfall events increase the frequency and intensity of flooding, causing abrupt changes in estuarine conditions. As a result, marine organisms are more often exposed to freshwater conditions for several days after flooding. While estuarine organisms are able to thrive under fluctuating salinities, their responses to intense and prolonged hypoosmotic stress may differ. Understanding these responses is crucial as salinity variability is key to species distribution. Changes in hydrological regimes are occurring much faster in high latitudes environments, yet subarctic estuarian communities are rarely studied. Our study investigates the sensitivity of three key marine benthic macroinvertebrates inhabiting subarctic estuaries (Littorina saxatilis, Macoma balthica and Mytilus spp.) to different durations of freshwater exposure. In laboratory conditions, molluscs were exposed during six weeks to a gradient of 12 freshwater pulse durations, ranging from 0 to 9 d, interspersed with 24 h of exposure to sea water. Mortality and shell growth, as well as energy content for Mytilus spp., were compared after two, four and six weeks of exposure using linear mixed models and break-point analyses. All three species demonstrated tolerance to short-duration freshwater pulses (less than 2 d) repeated over several weeks. Littorina saxatilis exhibited the highest vulnerability, with mortality increasing under freshwater pulses longer than 2 d after two weeks. Macoma balthica displayed a comparable threshold but only after four weeks. Mytilus spp. displayed the greatest tolerance, with a threshold only detectable after six weeks of exposure and increasing mortality in treatments with freshwater pulses longer than 5 d. In addition, shell growth decreased with increasing duration of freshwater pulse only in L. saxatilis while it was not affected in M. balthica and Mytilus spp. Energy content in Mytilus spp. varied according to pulse duration and time, with the highest energy content being detected in the longer freshwater exposure periods, suggesting complex physiological responses to hypoosmotic stress. Altogether, our study indicates the existence of highly species-specific differences in freshwater vulnerability among marine benthic macroinvertebrates inhabiting subarctic estuaries. Our findings highlight the importance of using gradients of stress scenarios to identify ecologically relevant thresholds, helping in the prediction and management of extreme climate events like river floods on estuarine communities. Our study provides valuable recommendations for dam management and biodiversity conservation in estuarine ecosystems.