Significance of salinity and silicon levels for growth of a formerly estuarine eelgrass (Zostera marina) population (Lake Grevelingen, the Netherlands)

Since the early 1980s, the eelgrass, Zostera marina L., population in the s aline Lake Gevelingen, The Netherlands, is rapidly declining. An earlier st udy, in which long-term data on eelgrass coverage in this former estuary we re correlated with several environmental variables, showed only one signifi cant correlation: coverage was positively related to water column silicon l evels. In addition, a negative correlation with salinity was observed, but this was not significant. In the present study, the effect of silicon and t he effect of salinity on the development of Z. marina were investigated exp erimentally. Enhancement of dissolved silicon concentrations in the water d id not stimulate Z. marina above-ground production or an increase in final above- and below-ground biomass. The highly significant correlation between eelgrass coverage and water column silicon levels, thus, remains to be exp lained. The results of the growth experiments did, however, demonstrate a c lear effect of salinity on Z. marina growth. Plants cultured at 22 psu show ed a higher production of shoots and leaves, resulting in more above-ground biomass, than plants grown at 32 psu. In addition, below-ground biomass wa s also higher at 22 psu. Measurements of chlorophyll a fluorescence, perfor med with a PAM-fluorometer, indicated a reduction of photosynthesis in the high-salinity treatments. Thus, low salinity stimulates development of Z. m arina from Lake Grevelingen. Eelgrass from such a historically estuarine ar ea may be more sensitive to high salinities than other, more marine populat ions. Recovery of the autochthonous eelgrass population is expected to be f avoured when the estuarine conditions of the seagrass area are re-establish ed, or when restoration programmes are carried out with allochthonous ecoty pes that are less sensitive to high salinities.