Sediment-vegetation interactions on the subaqueous Susquehanna River delta, upper Chesapeake Bay (USA)

Submersed aquatic vegetation (SAV) historically were abundant on the shallow, subaqueous delta of the Susquehanna River (SR), the largest tributary delivering water and sediment to Chesapeake Bay, USA. SAV began to decline in the 1960s on the delta (referred to as the Susquehanna Flats) due to poor water quality and disappeared completely following Tropical Storm Agnes (1972), which resulted in the highest recorded discharge of the SR and delivery of enormous fine-sediment loads to the upper Bay. In response to improved water quality, SAV have returned to the Flats in the last decade and once again are a prominent feature of the upper Bay. While it is well established that SAV promote sediment and nutrient retention, the timing and magnitude of trapping on the Flats is unclear but has important implications for water quality in the Bay. This study evaluates sediment trapping over seasonal to decadal time scales, using naturally occurring radioisotopes (7Be, 210Pb), within the context of fluvial sediment supply and plant biomass, as well as sediment erodibility experiments. Results show that, while average river discharge and suspended-sediment concentration (SSC) were lowest during the summer (plants present), sedimentation rates and mud content were highest, especially in the middle of the plant bed. In contrast, while average discharge and SSC were highest in the spring (plants absent), recent sedimentation was observed only at sites farthest downstream of the SR mouth. Thus, seasonal fluvial sediment supply is out of phase with the maximum deposition on the Flats, suggesting that sediment delivered during the spring freshet bypasses the Flats and enters the upper Bay, while fluvial sediment loads delivered during the SAV growing season can be retained on the Flats. This retention is aided by lower sediment erodibility during summer, relative to spring. This pulsing of seasonal sedimentation is also preserved in the sedimentary record over decadal time scales.