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Because leaf life span (LLS) is related to cost–benefit balances (such as maximal net gain or efficiency of net gain per individual leaf), factors associated with cost–benefit balances in individual leaves may control LLS. In seagrasses, water temperature and irradiance strongly affect metabolism, and epiphytes can attenuate irradiance reaching the leaves. Therefore, we predicted that seagrass LLS is largely controlled by water temperature, irradiance and epiphyte biomass on seagrass leaves. In the present study, we investigated the relationship between LLS of eelgrass Zostera marina and these parameters over the course of one year. LLS ranged from 34.9 to 89.6 days and was negatively related to water temperature but not related to irradiance, while epiphyte biomass was strongly related to water temperature. Furthermore, path analysis supported a much stronger relationship between LLS and temperature than that between LLS and epiphyte biomass. This sensitivity of eelgrass leaves to increasing water temperature has apparently resulted in a substantial range in LLS and therefore a substantial range in leaf number. Our study indicates that because high water temperature reduces eelgrass LLS, eelgrass and similar plants may acclimate poorly to any future increases in global water temperature.