Seasonal shifts in seagrass bed primary producers in a cold-temperate estuary: Dynamics of eelgrass Zostera marina and associated epiphytic algae

The seasonal dynamics of seagrass and epiphytic algal primary production were measured in an eelgrass (Zostera marina) bed in the Akkeshi-ko estuary, Hokkaido, Japan (43°02′N, 144°52′E). During spring and early summer, eelgrass biomass increased, with a high production (maximum: 2.89 g C m⁻² day⁻¹), but the production and biomass of epiphytic algae remained low. In contrast, epiphytic algae bloomed in August, with a high production (5.21 g C m⁻² day⁻¹), but eelgrass production ceased and its biomass subsequently decreased. Therefore, the major primary producers in this eelgrass bed switched seasonally from eelgrass in spring and early summer to epiphytic algae in late summer and autumn. Epiphytic algae maintained similar productivity because of the change of photosynthetic kinetics and the dominant epiphytic diatom changed from highly adhesive species to less adhesive or filamentous small species during the bloom. This suggests that the change of epiphyte density and biomass was due to change of its loss rate, possibly due to herbivorous grazing rate. Moreover, competition between epiphytic algae and eelgrass for nutrients and light may also affect the dramatic seasonal changes in the major primary producers.