Relative Impacts of Natural Stressors on Life History Traits Underlying Resilience of Intertidal Eelgrass (Zostera marina L.)

Seagrasses are a critical marine habitat and are in decline worldwide. Previous studies have demonstrated that factors such as sediment conditions, resource availability, and desiccation can influence life history transitions and morphology in intertidal eelgrass (Zostera marina L.) and therefore potential for recovery after a disturbance. We combined these factors in an exploratory path model linking environmental conditions to eelgrass vegetative (shoot size and density) and reproductive traits (branching, flowering, seedling recruitment). In this construction, significant path coefficients reveal factors influencing recovery potential. To test the path model, we collected abiotic and eelgrass data at 17 sites in the southern Salish Sea (Washington, USA) and assessed model fit with structural equation modeling. Significant path coefficients linked sediment organic content to shoot size and seedling recruitment, tidal amplitude to reduced flowering, and shoot size and density were inversely correlated. We found no significant links between any morphological or life history trait and nutrient availability, possibly reflecting consistently high nutrients across sites. Variable rates of asexual reproduction and a trade-off between shoot size and density may reflect light limitation in eelgrass' intertidal range, where light is not expected to be strongly limiting. Overall, structural equation modeling identified organic-rich sediments as relatively more important than desiccation and nutrient conditions for resilience potential of intertidal eelgrass populations in this region. Life history and morphological traits provide eelgrass with recovery mechanisms from disturbance where sediments are muddy, which has implications for both conservation and restoration.