Disturbance by ice and life-history adaptations of the seagrassZostera marina

The formation of winter ice, and its movement with the tides, has had a major influence on the life-history parameters of shallow-water populations of the rhizomatous marine angiosperm eelgrass (Zostera marina) in Nova Scotia, Canada. In this region, and annual form of eelgrass inhabits intertidal and shallow subtidal mudflats and a perennial form is common in subtidal areas. Where lowtide water depth was greater than winter ice thickness (25 cm), ice movement removed much of the aboveground biomass from perennial ramets, but did not influence the density of ramets. Measurements of primary production showed that perennial plants allocated a greater proportion of their total production to below-ground structures than measured in all previous studies on eelgrass. Thus perennials are well anchored in the sediment, are more able to withstand removal by ice, and have reserves available for production of new leaves when ice melts. In the spring, seed germination was greatest in areas where ice had removed whole plants (or their aboveground parts) than among mature ramets which had survived winter intact. Survival of seedlings (new genets) was not affected by shading from the adult canopy, but a shading experiment showed that competition for light with mature ramets had a significant negative effect on morphology, growth and the allocation of seedling biomass to below-ground parts, thus reducing the ability of new genets to survive ice disturbance in the next winter. The annual form ofZ. marina was restricted to areas where low tide water depth was much less than winter ice thickness. Annual plants did not survive winter, had small investment in below-ground parts and high reproductive effort, and overwintered as seeds. The genetic status of the two forms and the restriction of the annual to very shallow water are discussed in the light of previous work on eelgrass