Eelgrass (Zostera marina) Recovery Affected by Disturbance Timing on Mechanically Harvested Oyster Culture Beds

Abstract Amid global seagrass declines and increasing human demands of coastal habitat, it is critical to mitigate the loss of seagrass habitat through understanding seagrass resilience following large-scale disturbance. Although seagrasses often respond to disturbance through increased sexual or asexual reproduction, past research on the cosmopolitan Zostera marina (eelgrass) is highly variable in terms of whether recovery occurs, and whether seeds or clonal growth is the primary contributor. In Willapa Bay, Washington state, we studied eelgrass recovery following large-scale disturbance on six adjacent oyster culture beds (~ 10,000 m 2 in area) that were harvested using mechanical methods (i.e., dredging). We found that recovery potential and mode are heavily affected by the timing of disturbance. In the 200 and 400 days post-disturbance, beds disturbed during the early growing season (EGS; January–April) were estimated to have more than double the vegetative shoot density of beds disturbed during the late growing season (LGS; May–September). We also found a higher contribution of new shoots from seedlings following EGS relative to LGS disturbance, with up to 71% of shoots following EGS disturbance occurring from seedling origin. Consistent with eelgrass life history, spring seedling densities were positively affected by flowering shoot densities the previous summer. Clonal reproduction (i.e., branching) was negatively affected by a disturbance within the past 4 months, likely from physical damage, but also showed negative density dependence, meaning that higher branching rates were observed at lower shoot densities. Overall, this work emphasizes the importance of seedlings to seagrass recovery and demonstrates that Z. marina has higher recovery when disturbed (here by mechanical harvest of shellfish) during EGS versus LGS time periods. These findings provide straightforward guidelines for the management of anthropogenic disturbance on eelgrass beds to reduce permanent habitat loss and can also guide restoration efforts of eelgrass beds.