Increased salinity stress changes plant productivity and biomass by altering the top‐down controls in eelgrass beds

Abstract

AbstractUnderstanding biological community responses to disturbances such as extreme rainfalls under climate change is crucial when predicting ecosystem changes and functions. However, it is still unclear how species interaction changes with increasing disturbances and how disturbance susceptibility varies among organisms from habitats with different stress levels along stress gradients such as salinity gradients. We examined whether (1) higher frequency (i.e., pulse disturbance) and longer duration (i.e., press disturbance) of salinity change affected the primary producers, animals, and their interactions, and whether (2) organisms from naturally stressful habitats with unstable salinity were more tolerant to salinity changes. We used eelgrass Zostera marina, epiphytic microalgae, and grazer gastropod Lacuna decorata from two sites in each of the two lagoons; Akkeshi with a strong salinity gradient, and Notoro with a weak gradient to do indoor mesocosm experiments for testing the effects of salinity change and grazer presence on eelgrass growth and biomass of epiphytic microalgae, as well as grazer survival, growth, and consumption rates. We established treatments with low, intermediate, and high levels of salinity stress to test the effects of press and pulse disturbances in terms of salinity changes. The findings suggest the presence of a top‐down control of epiphytic microalgae by grazers. Eelgrass growth rate was higher with the presence of grazers but declined with higher salinity stress. The biomass of epiphytic microalgae was higher with the absence of grazers and with higher salinity stress across all sites. Grazer survival varied among individuals raised in habitats with naturally stable or unstable salinity regime and decreased with more frequent and longer salinity changes. Grazer growth rates and grazing rates also decreased with higher salinity stress, but grazing on epiphytic microalgae was highest at the intermediate stress levels across all sites. This study demonstrates that salinity regime shifts that are expected to occur with extreme rainfalls due to ongoing climate change could affect coastal ecosystems such as eelgrass beds, and we call for further investigations on the different stress responses of organisms raised in habitats with stable and unstable salinity to understand the potential effects of the disturbances on ecosystem function.