LAUSR

Abstract Sea level rise and more frequent storm and precipitation events associated with climate change are predicted to increase salinity fluctuations in estuarine and inshore areas, where foundation species such as eelgrass (Zostera marina L.) will be exposed to more frequent salinity changes. Effects of acute hyposalinity exposure on seagrasses remain poorly understood compared to the effects of more prolonged, constant salinity. Here, we examined growth and photo-physiological responses of Z. marina to 5 levels of stable salinity (5, 12 19, 25, 33) and compared effects of prolonged (16 days) versus acute (24 and 48 hours) exposure to hyposalinity (salinity 5 and 12) using fluorescence imaging. We also examined if fluorescence kinetics were affected by age differences across leaves. Growth reached an optimum at salinity 19 and was more affected by hyposalinity than hypersalinity. Rapid reduction from salinity 25 to 5 decreased the maximum quantum yield (Fv/Fm) after just 48 h. In contrast with prolonged exposure, non-photochemical quenching processes were not increased at salinity 5 after 48 h. Young leaves were more susceptible to extreme hyposalinity than older leaves (e.g., lower photosynthetic quantum yield), which emphasizes the importance of considering shoot-scale and within-shoot variations in studies of stress response patterns. Differences between hyposalinity and hypersalinity responses were generally replicated in the literature, but we were not able to detect any differences across studies. Overall, these results suggest that eelgrass is tolerant to large fluctuations in salinity, but sudden extreme reductions may act as a severe co-stressor, and contribute to accumulated stress-exposure effects (chronic or lasting effects).