Retention of riverine nutrient and sediment loads by floodplains in the Chesapeake Bay watershed

Few quantitative estimates exist for the percent retention of annual river loads of nitrogen, phosphorus, and suspended sediment by wetlands.  We measured depositional fluxes of nutrients and suspended sediment onto floodplain soil surfaces (g m^-2 yr^-1; 1-6 yrs of accumulation) over a sampling network that included the Coastal Plain portion of five rivers in the Chesapeake Bay watershed.  For each river, the average nitrogen, phosphorus, and sediment depositional flux rates were multiplied by an estimate of floodplain area to calculate floodplain trapping rates (kg yr^-1), and then compared to average river loads.  Average material retention among the rivers was 27% of nitrogen (range 6-70%), 38% of phosphorus (15-82%), and 69% of suspended sediment (5-95%).  Uncertainty in these estimates of retention derive from several assumptions related to adequacy of sampling network, permanency of the sink of deposited nutrients and sediment, and relative importance of the rivers as the source of deposited material.  Coastal Plain floodplains in the Chesapeake Bay watershed likely function as an important long-term sink for material transported by rivers, greatly reducing loading rates to the Bay.  Restoration activities that increase floodplain area or the hydraulic connectivity between floodplains and river channels most likely would enhance nutrient and sediment retention.