Relative Roles of Sediment Transport and Localized Erosion on Phosphorus Load in the Lower Susquehanna River and Its Mouth in the Chesapeake Bay, USA

Abstract

Sediments in rivers and lakes have increasingly become phosphorus (P) repositories in recent decades (e.g., Søndergaard et al., 2003). Under specific biogeochemical conditions, such as under high shear stress and low dissolved oxygen conditions, bed sediment P can diffuse up to surface waters, potentially stimulating algal blooms. Due to differences in external P loading, water body morphology, and physical and chemical characteristics of the sedimentary layer, the contribution of bed sediment to the total P load in the overlying water column can vary dramatically from 3% (3%-35% for 22 lakes; Nürnberg, 1984) to 80% (Larsen et al., 1981;Ryding & Forsberg, 1977). Internal loading in smaller lakes may exceed the input from external sources because shallow lakes are more sensitive to localized weather, such as rainfall and wind, and other biogeochemical processes conducive to releasing P from bed sediments (Somlyódy, 1983).Nutrient enrichment in the Chesapeake Bay can be traced back to the late 18th century after European colonization (Bratton et al., 2003). An abrupt decline in dissolved oxygen in the bottom water began between 1934 and 1948(Zimmerman & Canuel, 2000. This seasonal bottom water hypoxia is still present today, even after the implementation of nutrient TMDL mandates by the US Environmental Protection Agency for the past few decades. Point sources of nutrient pollution have decreased since 1992 but nonpoint (agricultural) sources have only marginally changed, with a 9% increase in P load most recently (Ator et al., 2019). The Susquehanna River delivers over one fourth of the P load and half of the freshwater supply to the Chesapeake Bay (Ator et al., 2019;Hargis, 1981). Because the sediment retention of the Conowingo Dam, the largest dam on the Susquehanna River,