Effects of salinity on survival of the exotic seagrass Zostera japonica subjected to extreme high temperature stress

Zostera japonica is a non-indigenous seagrass that is expanding along the Pacific Coast of North America. The ecophysiology of this seagrass is poorly studied and management of the species is fragmented. This split-plot mesocosm experiment was designed to evaluate the response of Z. japonica to chronic, extreme temperature and salinity stress to facilitate development of models to predict potential Z. japonica colonization. We collected Z. japonica plants from Padilla Bay, Washington and Yaquina Bay and Coos Bay, Oregon and exposed them to a constant water temperature of 15 degrees C or 35 degrees C at three different salinities (5, 20 and 35). After 7 days exposure, shoot survival ranged between 6% and 42%; after 9 days exposure, only a few plants from the Yaquina Bay population survived. At a ambient temperature (15 degrees C), no differences were detected among the three salinity treatments. However, at a temperature of 35 degrees C, the survival of plants grown at a salinity of 5 was significantly lower than at higher salinities (20 and 35). Although the effect of population was weak, the northern population appeared to be more susceptible to the combined effects of heat stress and low salinity than the southern populations. We suggest that Z. japonica will continue to spread southward along the Pacific coast of North America until it reaches systems that regularly exceed the temperature tolerances of this non-indigenous seagrass.