SEASONAL AND LONG-TERM TRENDS IN WATER QUALITY: IMPLICATIONS FOR P CYCLING, IRONDEQUOIT BAY, NEW YORK

Irondequoit Bay, adjacent to Rochester, New York, is one of the larger embayments along the southern coast of Lake Ontario. It has a long history of human use and degraded water quality. Since the 1980s, programs aimed at improving water quality were put in place, including a program of hypolimnetic oxygenation. One goal of the oxygenation is to reduce internal loading during summer thermal stratification. During the summer of 2009, we completed basic water quality parameter profiles and sampling at approximately bi-weekly intervals with four more detailed surveys throughout the season. Results of these surveys showed an increase throughout the season with average hypolimnetic P going from a low of 28.5 mg L^-1 in May to a high of 621.8 mg L^-1 in October immediately preceding turnover. The increase in P is concordant with decreasing dissolved oxygen concentrations ranging from 8.78 mg L^-1 to 0.14 mg L^-1 during the same period. Although the dissolved oxygen levels are persistent in the suboxic range for most of the summer season, the oxygenation program prevents anoxic conditions, limiting the release of P. Similar results are seen for surveys conducted in past years, but a slight upward trend in internal P loading is apparent. Detailed studies in 2003 and 2004 found similar patterns of dissolved oxygen and hypolimnetic P, but the highest average concentration of P found was 535 mg L^-1 in 2004. The length of each season’s stratification is currently being factored to account for some of the variations seen.