REPRODUCTIVE STRATEGIES OF EELGRASS (ZOSTERA MARINA L.)

Abstract Geographic variation in the reproductive strategy of eelgrass on the Pacific coast of North America is reflected in differences in flowering frequency, seed production and the effects of salinity on germination. The analysis of variance of temperature and salinity effects on seed germination detected significant salinity effects, but not temperature effects. Low salinities enhanced germination except in seeds from the Gulf of California, which germinated readily in normal strength sea water. Populations at the margins of the geographic range of eelgrass are characterized by high incidence of density-independent mortality, and by an increase in the incidence of sexual reproduction. At the southern margin of the range in the Gulf of California, where high water temperatures kill all plants in summer, there is a 100% flowering response and a high rate of germination success. These populations grow subtidally and arise anew each year from seedlings. At the northern margin of the range in the Bering Sea where ice scouring in winter is an important source of mortality, eelgrass is limited to subtidal areas or intertidal pools. In these perennial populations the sexual reproductive effort is greater than that of subtidal populations in the central portion of the range. In the central portion of the range, eelgrass exhibits two reproductive strategies. In subtidal areas where salinity fluctuation is minimal, dense stands of perennial plants reproduce vegetatively. In intertidal areas where seasonally low salinities enhance seed germination, there is a higher incidence of flowering.