Ecology: Impact of Nutrient Loading on Estuaries and Marshes

Childs River and Quashnet River, combined with the absence of such spikes at Sage Lot and Flat Ponds, suggests input by plumes from septic tanks. Mean NI&+ concentration in groundwater varied over a relatively small range across the three sites (Fig. 1, middle panels). NH4+ contributed less than 5% of the mean dissolved inorganic nitrogen (DIN) at Childs and Quashnet Rivers. Most of the inorganic nitrogen, therefore, enters these two estuaries in the form of N03-. Nitrogen from septic tanks and soils enters the aquifer in reduced forms, such as NH.,+ [untreated septic water contains up to 8000 PM NH4+ (J. McClelland, unpub. data)], which are oxidized to N09in the aquifer before reaching the estuary. Mean POd3+ concentrations were relatively low and similar in the three estuaries (Fig. 1, right panels). One would not expect POd3+ to vary across the sites, despite a gradient in residential area, because POd3+ is readily adsorbed by soils. Groundwater data from Waquoit Bay suggest that differences in land use can have a major effect on the concentrations of nitrogen entering a shallow estuary. To estimate the loading from groundwater to receiving estuaries and to estuarine systems, such as Waquoit Bay, concentration data such as those reported in this paper must be converted to flux by estimating actual water and nutrient transport. Calculated values of total system loading will be useful in deciding how to manage coastal estuaries threatened by eutrophication. This work was supported by Waquoit Bay Land Margin Ecosystems Research and a Research Experience for Undergraduates grant. Special thanks to Lori Saucy for operating the autoanalyzer.