Persistence of eelgrass meadows depends on the balance between rates of plant removal and recovery. During winter in the Pacific Northwest, wind-driven storm waves pound the coastline and can remove large portions of intertidal eelgrass meadows. Additionally, tidal currents peak approximately bimonthly throughout the year, during new and full moon phases, and can add to the hydrodynamic stress that eelgrass experiences. The presence of eelgrass serves to mitigate hydrodynamic energy and stabilize sediment, thus leading to predictions that repeated disturbance could erode the resilience of eelgrass meadows. To date there has been no joint exploration of intertidal disturbance regime and recovery rates to enable tests of this model of resilience. Starting in 2007, I measured eelgrass spread (by seed and clonal branching) into experimental removal plots (5 x 5 m), and natural removal (by waves and currents) in Willapa Bay, Washington, USA. I estimated recent disturbance frequency by combining these local empirical removal rates with available long-term wind speed, wind direction, and tidal elevation data.
Results/Conclusions
The observed disturbance regime at my study site depended on three environmental factors: wind direction (from S), wind speed (exceeding 25 mph), and tidal stage (water’s edge at the elevation of eelgrass, ~0’ MLLW). Extending this relationship to 2000-2009, high-disturbance storms were predicted to affect this intertidal eelgrass yearly in 8 of the 10 years with a 2-year recovery period every half-decade. In some experimental removal plots, eelgrass has increased in abundance via recruitment over 1.5 years, but all plots taken together have not yet reached pre-disturbance levels or undisturbed controls. Overall, this intertidal eelgrass population appears in a precarious balance, in which its current density is just sufficient to mitigate storm waves that would otherwise occur in a regime exceeding patch recovery and population persistence. Understanding factors affecting the balance of disturbance and recovery may help manage this important near-shore habitat in the face of increasing land development by humans and storm frequency with climate change.
