Multi-decadal prediction of habitat suitability for salt-marsh obligate birds under sea level rise and watershed inputs of sediment

Abstract:

Sediment is a focal point of management of coastal estuaries, which provide important services including recreation, water quality enhancement, flood protection and habitat for threatened and endangered species. Excessive watershed sediment loads can bury coastal marsh habitat and reduce circulation which, in turn, negatively impacts water quality, ecosystems and fluvial flood risks. Conversely, potential inundation of marsh habitat and increased risks of coastal flooding with sea level rise (SLR) may be exacerbated by insufficient loads of watershed sediment. Here, we present an approach to estimate salt-marsh habitat suitability for an endangered bird species in Newport Bay, California, that considers different SLR scenarios and options for sediment management. Our approach builds on the strength of mechanistic modeling for physical processes and empirical modeling for biological outcomes. Estuarine morphodynamics is simulated with a high-fidelity model (Delft3D) in response to episodic sediment loads from watersheds as well as tidal influence. Habitat suitability is based on field observations of nesting sites, vegetation characteristics, and surface elevation. Results indicate that under present-day loads of sediment, suitable bird habitat is likely to increase more under higher SLR trajectories (RCP 8.5) than lower SLR trajectories (RCP 4.5) due to high watershed sediment volumes. Model results also show that the operation of existing sediment basins, designed to capture excess sediment from watersheds, can be adjusted to improve habitat suitability under SLR. The proposed modeling responds to stakeholder needs for planning watershed-based sediment source control measures and sediment dredging activities over decadal time scales.