Nutrients and chlorophyll at the interface of a watershed and an estuary

Abstract

We investigated the fates of nutrients entering the Rhode River estuary from its watershed and from atmospheric deposition. Production or consumption of materials in the upper estuary was calculated from a mixing model with chloride as a conservative tracer. The upper estuary produced chlorophyll and dissolved PO43− (DPO43−) but consumed particulate PO43− (PPO43−), total inorganic N, dissolved organic N, and particulate organic C. These net fluxes were influenced more by shallow, open‐water areas than by the tidal marshes which cover two‐thirds of the area of the upper estuary. Ratios of chlorophyll to organic C, N, and P suggest that most of the suspended particulate organic matter in the upper estuary was produced by phytoplankton rather than derived from watershed inputs. The consumption of nitrate due to phytoplankton production and the production of DPO43− due to release from particulate P after deposition in sediments resulted in low inorganic N : P ratios, contrasting sharply with the lower estuary and adjacent Chesapeake Bay. Dissolved inorganic N and P entered the upper estuary from the watershed at an atomic ratio of 26 but left the upper estuary at a ratio of 2.7. The release of DPO43− from watershed‐derived sediments may be a common feature among estuaries and could promote N limitation of primary production in estuarine and coastal waters.