We assessed seasonal changes in eelgrass (Zostera marina) functions, i.e., reduction of current velocity, buffering of sediment resuspension, and control of dissolved inorganic nitrogen flux between the sediments and the water column, using field observations and experiments in the Akkeshi-ko estuary, Hokkaido, Japan. We also analyzed the relationships between eelgrass traits and functions. The efficiency of the reduction in current velocity increased with the development of the eelgrass canopy. Sediment resuspension was inhibited from May to August, during which time the eelgrass canopy developed. Eelgrass controlled the NH4+ concentration of sediment porewater through root nutrient uptake, affecting NH4+ flux between the sediments and the water column. Fluctuations in eelgrass functions and coincident changes in dynamics resulted in seasonal changes in the eelgrass environment, which may in turn affect the dynamics of organisms inhabiting eelgrass beds, e.g., mysids and epiphytic algae. Moreover, the developed eelgrass canopy trapped a large amount of material during spring and summer, which was resuspended into the surrounding ecosystem in autumn when the canopy and its functions (i.e., reduction of current and sediment resuspension) diminished. These results suggest that seasonal changes in eelgrass functions also affect communities within marginal coastal ecosystems through the control of allochthonous resources.
