Freshwater, suspended sediment and nutrient influx to the Little River and Webhannet River estuaries, Wells, Maine

Anthropogenic development in estuarine watersheds accelerates nutrient loading and eutrophication. An understanding of nutrient budgets is essential to minimize water quality deterioration, protecting both current use and future development. Nutrient concentrations were measured in two adjacent southern Maine estuaries in 1994-95. The Little River estuary and watershed are relatively undeveloped. The Webhannet River estuary was modified during harbor construction, and its watershed extensively developed adjacent to its estuary. Comparison of their nutrient budgets helps quantify the impact of development.;Water column nutrient concentrations are low compared to reported concentrations in the literature, but comparable to those in other Maine estuaries. Comparison with data collected three years earlier shows median concentrations of ammonium, nitrate, orthophosphate and total suspended sediment generally increasing, especially in the Webhannet River watershed. Marine water median concentrations of ammonium, orthophosphate, and silica were higher than river water, implying fresh surface water flux is not the primary nutrient source for the estuaries. Nitrate concentrations decreased in summer, but ammonium and orthophosphate concentrations increased. The estuarine water column N:P ratio is generally below the Redfield ratio, suggesting nitrogen is the limiting nutrient. A nitrate sink exists in the estuaries. The sink is strongest in summer, but the absence of a synchronous sink for ammonium and orthophosphate argues against net primary productivity as its cause.;Land use differences produce a nitrogen load in the Webhannet River estuary 1.4 times greater than that in the Little River estuary. The phosphorus load is 2.5 times greater. Observed nutrient concentrations in the rivers and lower estuaries were used with a simple finite element model to predict concentrations in the upper estuaries. Predicted concentrations in the upper Little River estuary were close to those observed. In the upper Webhannet River estuary, predicted ammonium, nitrate, orthophosphate and silica concentrations were 39-83% of those observed. The additional flux needed to account for observed concentrations is probably from smaller streams and/or ground water draining the developed watershed adjacent to the estuary itself, which would be expected to have not only higher nutrient concentrations, but a shorter discharge path to the estuarine water column.