Sediment Phosphorus Cycling in a Managed System Irondequoit Bay, N.Y.

Abstract

The current status of internal phosphorus loading was evaluated in Irondequoit Bay, NY, an embayment of Lake Ontario. Sediment core incubation experiments and phosphorus water column profiles were used to quantify the release rate of phosphorus from the deepwater sediments of Irondequoit Bay during summer periods of low hypolimnetic oxygen. The top 25 cm of sediment cores collected monthly from May 2004 through September 2004 were analyzed for total phosphorus, total manganese, total iron, and total aluminum. Phosphorus fractionation of the sediment cores was determined through sequential extraction. Phosphorus water column data collected during the summer of 2003 and 2004 yielded an average release rate of 8.15 mg P/m2/day. Sediment core incubation experiments yielded an average release rate of 2.89 mg P/m2/day. Predictive models yielded estimates of phosphorus release rates ranging from 0.11 to 31.56 mg P/m2/day. Total phosphorus concentrations in the sediment averaged 1.389 ± 0.150 g P/kg dry wt. Sequential extraction of phosphorus fractions from the top 25 cm of deep-water sediment revealed that approximately 25% of phosphorus was stored in a redox-sensitive form, most likely sorbed onto iron and manganese oxyhydroxides. Iron and manganese profiles from the water column indicated that manganese from the sediment was cycling with phosphorus into the overlying waters, while iron did not demonstrate evidence of cycling. Iron was kept out of solution by low concentrations of oxygen that had been maintained in the hypolimnion through a summer oxygen supplementation program on Irondequoit Bay.