Nutrient mass balances for the watershed, headwaters intertidal zone, and basin of the Rhode River Estuary1

Mass balances were determined for a 13‐month period for three parts of the Rhode River ecosystem, a small subestuary of the Chesapeake Bay. Organic matter, nitrogen, and phosphorus budgets were calculated for a 2,300‐ha upland watershed dominated by agriculture and secondary successional vegetation, 88 ha of tidal marshes and mudflats, and an estuarine open water basin. Less organic matter moved out to the estuary (27 t of C) than was discharged into the intertidal zone from the watersheds (71 t of C) and deposited in precipitation (3.4 t of C). Of the 2.7 t of P discharged from uplands into the intertidal zone, only 1.9 t moved into the estuary. Generally, high discharges of P from the watersheds were followed in a week or two by high discharges into the estuary from the intertidal zone. Orthophosphate constituted a significantly lower fraction of the P that moved into the estuary (38%) than of the P that was discharged from the watershed (49%). Of the 10 t of N discharged from the watershed into the intertidal zone, only 7.4 t moved into the estuary. These data indicate overall removal and storage of nutrients from land runoff by the intertidal zone. When land runoff was low the intertidal zone scavenged nutrients from tidal waters. Only when runoff was high, due to storms, were some of the stored nutrients flushed from the intertidal zone.