Habitat suitability for eelgrass following large scale tidal restoration on the Nisqually Delta

Large-scale estuary restoration, such as the removal of dams or dikes, can alter nearshore processes and provide an opportunity to study how ecosystems respond to changes in the physical environment. In this study, we quantify changes in the spatial distribution of eelgrass Zostera marina following a 2009 dike-removal project that restored tidal processes to over 350 ha of the Nisqually River delta. We produced high resolution maps of eelgrass percent cover, depth distribution, and nearshore bathymetry from surveys performed in 2012 and 2014 using a Biosonics 430 kHz single-beam echosounder and survey-grade global navigation satellite system. A coupled hydrodynamic and sediment transport model is used to characterize hydrodynamics (water levels, tidal currents, waves) and water column properties (salinity and turbidity) before and after dike removal. Model simulations suggest that removal of the dikes increased the tidal prism and altered sediment transport pathways on the delta. We explore how modeled changes in the hydrodynamics, turbidity, and morphology following restoration relate to eelgrass cover, depth distribution, and habitat suitability for eelgrass on the Nisqually delta. Such relationships provide insight into the physical mechanisms driving ecosystem responses and can improve the capacity to predict changes in nearshore habitats following restoration or other land-scape modifications including climate change.