Physical and Biological Factors Constraining the Growth and Persistence of a Pacific Northwest Fringing Eelgrass Bed

In this study we examined a small, fringing eelgrass (Zostera marina) bed in Rosario Bay, WA to determine the factors limiting its growth and persistence. The bed is less than 1 ha and entirely subtidal, with around 2 m depth difference between the shallow and deep margins of the bed. Substrate within and seaward of the bed is silty sand, while shoreward and to the north are gravel and rocks. The bay is periodically exposed to heavy wave action which causes disturbance down to eelgrass depth, and light attenuation and large tidal changes limit light levels. Nevertheless, light usually reaches saturation levels for a limited time in all seasons. Sediment nutrient and organic content seem adequate for eelgrass growth within the beds, but not in the gravels on the shoreward side. We determined that the most likely limiting factors are lack of adequate light along the deeper edge of the bed, sediment disturbance by waves along the shoreward margin, and incompatible substrate plus competition with macroalgae along the northern margin. The shallowest depth imposed by wave turbulence appears to be close to the deepest depth imposed by light limitation, so maintenance of low light attenuation in the water column appears to be one of the most important factors contributing to the persistence of this bed. In addition, a previously unreported amensal interaction with anchored but motile bull kelp (Nereocystis luetkeana) may create patches of damage throughout the bed during storms. Key Points The Rosario Bay fringing eelgrass bed appears to be limited in extent by light attenuation, turbulence due to waves, substrate, and competition on different sides. The bed is small and entirely subtidal but has persisted for at least 45 years. This eelgrass bed is being influenced by a novel type of amensalism from bull kelp. Guarding against a decrease in the clarity of the water may be the most important controllable factor that will allow this eelgrass bed and similar ones to persist.