Synergistic effects of altered salinity and temperature on estuarine eelgrass (Zostera marina) seedlings and clonal shoots

Salinity and temperature are among the most important factors determining eelgrass distribution and performance. Plants in estuarine environments experience large variations in both on a seasonal basis and exceptionally warm summers have caused massive die-backs of eelgrass in many areas. We investigated experimentally how different combinations of salinity and temperature affect the physiological performance of adult eelgrass (Zostera marina) shoots and seedlings. Plants were exposed to different combinations of salinity (salinity 5, 12.5 and 20) and temperature (15, 20 and 25 °C) in a 5-week aquarium experiment. Plants responded in general negatively to decreasing salinity and increasing temperature and the combination of high temperature and low salinity resulted in markedly higher mortality rates and lower leaf production when compared to plants held at more optimal combinations of salinity and temperature. Seedlings had higher absolute mortality, while adult shoots were relatively more sensitive to unfavorable levels of salinity. Leaf tissue sucrose concentrations in both life stages decreased at low salinity, whereas salinity and temperature resulted in contrasting starch concentrations between seedlings and adult shoots. Our results show that altered salinity and temperature may have negative synergistic effects on eelgrass performance. Future climate changes may thus have serious impacts on eelgrass survival and performance.