We tested the effects of salinity and water temperature on the ecological performance of eelgrass (Zostera marina L.) in culture-experiments to identify levels that could potentially limit survival and growth and, thus, the spatial distribution of eelgrass in temperate estuaries. The experiments included eight levels of salinity (2.5, 5, 10, 15, 20, 25, 30 and 35%) and seven water temperatures (5, 10, 15, 20, 25, 27.5 and 30 8C). Low salinity (i.e. 5 and 2.5%) increased mortality (3-6-fold) and had a strong negative effect on shoot morphology (number of leaves per shoot reduced by 40% and shoot biomass reduced by 30-40%), photosynthetic capacity (Pmax-reduced by 30-80%) and growth (production of new leaves reduced by 50-60%, leaf elongation rate reduced by 60-70% and production of side-shoots reduced by 40-60%), whereas eelgrass performed almost equally well at salinities between 10 and 35%. The optimum salinity for eelgrass was between 10 and 25% depending on the response parameter in question. Extreme water temperatures had an overall negative impact on eelgrass, although via different mechanisms. Low water temperatures(5 8C) slowed down photosynthetic rate (by 75%) and growth (production of new leaves by 30% and leaf elongation rate by 80%), but did not affect mortality, whereas high temperatures (25-30 8C) increased mortality (12-fold) and lowered both photosynthetic rate (by 50%) and growth (production of new leaves by 50% and leaf elongation rate by 75%). The optimum water temperature for eelgrass appeared to lie between 10 and 20 8C. These results show that extreme conditions may affect the fitness of eelgrass and, thus, may potentially limit its distribution in coastal and estuarine waters.