Oxygen depletion in Long Island Sound: A historical perspective

A retrospective analysis of available data was conducted to characterize the spatial distribution and temporal trends in dissolved oxygen (DO) concentrations in Long Island Sound (LIS) over the past four decades. A general east-to-west gradient of decreasing bottom DO was evident in all historical data examined. In our review of data from the 1950s, collected by Gordon Riley and colleagues, and from contemporary surveys, we found no evidence of hypoxia (DO≤0.3, mg 1−1) in the Eastern Basin; however, in the deeper waters of the Central Basin, there is some evidence for a recent (1986) emergence of moderate hypoxia. The Western Basin experienced episodes of hypoxia during the 1970s which became more recurrent and possibly more severe in the late 1980s. The most severe, persistent and chronically recurrent hypoxia occurred throughout the water column of the East River and in bottom waters of the Western Narrows. An unprecedented episode of anoxia was observed in both the Western and Eastern Narrows regions of LIS in 1987. Previously, anoxia occurred rarely, was short-lived, and was confined to the East River. Statistical trend analyses revealed a significant increase in the summer minimum bottom DO in the lower and middle reaches of the East River over the past 20 years. Beginning in 1981, however, DO declined markedly in the adjacent Narrows bordering the Nassau County nearshore. The improvements in East River water quality over the previous 15–20 years appear to have been gained at the, expense of poorer water quality in the western sound. Mechanisms potentially responsible for the recent decline in bottom DO in western LIS are suggested.