The effect of different patterns of salinity fluctuation on functions and species interactions of an eelgrass (Zostera marina) bed communities

Eelgrass is a species of seagrass that occurs widely in lagoons, estuaries, and coasts of the temperate northern hemisphere. Functioning of eelgrass bed communities are strongly affected by environmental factors such as salinity. Recently, fluctuation in salinity is getting greater in coastal areas due to occurrence of more concentrated heavy rain which is related to the ongoing climate change. This change in disturbance regime (longer decline and more frequent fluctuation in salinity) can potentially change functioning as well as species interactions of eelgrass bed communities. The objectives for this study are to compare the ecological and physiological responses of the major functional groups (eelgrass, epiphytic algae and invertebrate grazer) in an eelgrass meadow in Hokkaido, Japan and to see how the plant-animal interaction between the functional groups is altered by change in salinity due to freshwater discharge. We conducted a 4-week mesocosm experiment with four different salinity fluctuation patterns to simulate different disturbance situations and compared the effects of the fluctuation on eelgrass with and without grazers. We found that survivorship of the grazers decreased with increasing duration and frequency of the disturbance event. Growth rate of eelgrass and the grazer, the amount of epiphytic algae and grazing rate of snails on eelgrass varied among different treatments, showing possible direct and indirect effect of the changes in salinity fluctuation on the response of each organism as well as plant-plant and plant-animal relationships. The results suggest the potential negative effects of more intense freshwater discharge on the functionality of eelgrass bed communities which may become serious in the near future with ongoing climate changes.