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

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 PO,‘(DPO,‘-) but consumed particulate PO,“(PPO$), 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 DPO,+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 DPOd3from watershed-derived sediments may be a common feature among estuaries and could promote N limitation of primary production in estuarine and coastal waters. Estuaries function as important sinks and transformers of nutrients, thus altering the quantity and quality of nutrients transported from the land to the sea. The processes that govern the fates of N and P in estuaries differ. Consequently, the ratios of inorganic N to P in estuaries, unlike those in the ocean (Redfield 19 5 8), may vary widely with time and space and may deviate greatly from the ratio of N to P in phytoplankton (e.g. D’Elia et al. 1986). The relative abundance of inorganic N in estuaries may be decreased by denitrification (Seitzinger 1988), by inhibition of N fixation (Howarth and Cole 1985), or by hydrologic factors (Smith 1984). In contrast, abundance of inorganic P may be increased by enhanced release from sediments in the presence of high salinity and SOdz(Caraco et al. 1989). These processes may explain why N is the nutrient that most often limits primary production in estuaries (Boynton et al. 1982). In some estuaries, however, high inputs of N03from waterAcknowledgments This research was supported by the Smithsonian Environmental Sciences Program; by NSFarants BSR 7911563, BSR 82-07212, BSR 83-16948; and BSR 8615902; by EPA grant R80453601; and by a series of awards under the NSF-RANN program. We thank C. Gallegos and P. Rublee for helpful comments on an earlier draft. sheds may increase the ratio of inorganic N to P and lead to P limitation (D’Elia et al. 1986). Thus, the processes governing the fates of nutrients entering estuaries not only determine the ultimate delivery of nutrients to adjacent coastal waters but also influence which, if any, may limit primary production within the estuary. In many estuarine systems, tidal marshes are thought to play an important role in the flux and transformation of nutrients, but the nature and significance of their role is controversial. Some studies suggest that marshes may be important sources of organic matter for coastal ecosystems, while others suggest that marshes are relatively unimportant (Nixon 1980). In addition, marshes have been described both as sinks and sources ofinorganic nutrients (Nixon 1980). Their role in nutrient flux remains obscure due to difficulties in measuring nutrient exchanges and due to real differences among marshes (Jordan et al. 1983). Moreover, it is difficult to evaluate the significance of marshes without comparative knowledge of nutrient fluxes in adjacent subtidal ecosystems, and such comparative information is often lacking. In fact, there is a paucity of information on nutrient exchanges among linked watershed, estuary, and marsh sys-