Effects of erosion and macroalgae on intertidal eelgrass (Zostera marina) in a northeastern Pacific estuary (USA)

Eelgrass (Zostera marina) in open-coast northeastern Pacific estuaries is primarily intertidal, yet little research has been done on the natural factors controlling its upper intertidal growth limits. This two-year study in the Yaquina Estuary (Newport, Oregon, USA) evaluated the effects of two factors (erosion and macroalgal accumulations) on populations of eelgrass. Six study sites were located on steep (n=3) and shallow (n=3) intertidal slopes. At each site, triplicate plots (9 m(2)) were placed at five tidal elevations. In the plots, we counted shoots monthly into three categories: vegetative, reproductive, or seedling. Canopy height, blade width, blades per shoot, macroalgal biomass, and an index of erosion were also measured. Yaquina Estuary eelgrass behaves as a perennial in the lower intertidal zone and as an annual in the upper intertidal zone. In the transition between the low and high intertidal zones, there are both perennial (in patches) and annual forms. We suggest that aerial desiccation, macroalgae, and erosion play a role in controlling intertidal eelgrass and that these factors operate in an acute rather than a chronic manner.