Shoreline erosion impacts on Chesapeake Bay water clarity: an analysis of effects on light attenuation using a coupled hydrodynamic-biogeochemical model

Over the last several decades, the Chesapeake Bay has experienced a significant net decline in estuarine water clarity in conjunction with rapid human population growth within its watershed. Despite long-term reductions in riverine sediment loading, the main stem of the Bay has shown a lack of improvement in water clarity as measured by Secchi depth. Shoreline erosion may account for part of this discrepancy, as it provides a greater fine sediment loading to the lower Bay than do riverine sources. To better understand the drivers of water clarity variability in the Bay, this study examines the relative importance of shoreline erosion using a numerical model forced with realistic nutrient and sediment inputs for Chesapeake Bay. Specifically, impacts of shoreline erosion on Chesapeake Bay surface water clarity, in terms of both light attenuation and Secchi depth, were explored using the Estuarine-Carbon-Biogeochemistry model embedded in the Regional-Ocean-Modeling-System (ChesROMS-ECB) with watershed nutrient and sediment inputs derived from the Chesapeake Bay Program Watershed Model. Skill assessment was performed using observations from monthly and semi-monthly water quality monitoring cruises. Sensitivity experiments included varying the magnitudes of modeled shoreline erosion, representing different levels of shoreline armoring in the estuary, to investigate the resultant changes in clarity. Counterintuitively, in some regions of the Bay, greater shoreline erosion increased the diffuse light attenuation coefficient, while also resulting in a deeper Secchi depth. Separating out the relative importance of various drivers of water clarity degradation and improvement is essential to policy development and planning in this highly populated, eutrophic estuary.