SEASONAL HYPOXIA IN AN EAST COAST EMBAYMENT: CAUSES & CONSEQUENCES

Seasonal water column hypoxia occurs annually in Western Long Island Sound, NY and in parts of Central Long Island Sound. The primary driver is believed to be excess nitrogen resulting from point and non-point sources within the watershed. Smithtown Bay, the site of this study, is a large embayment located within Central Long Island Sound. One sewage treatment plant discharges treated wastewater into the Bay. Yet, Smithtown Bay often goes hypoxic before any other area within the Sound and has done so relatively consistently since the 1980s. This study undertook an examination of the physical, sedimentary, geochemical, and biological factors operating in Smithtown Bay in order to ascertain the causes of hypoxia within this embayment.

A total of 13 stations were sampled around a tidal cycle following three north-south transects from May-September, 2010 and 2011. Bottom water (< 5 cm above the sediment-water interface) and near-bottom water (~ 1 m above the sediments) samples were taken for chemical analysis (dissolved oxygen, sulfide, ammonium). Grab samples were also taken, serially sieved, and organisms remaining on the sieves were preserved for later identification and enumeration. A sub-sample of sediment was taken for organic carbon analysis. A 600 kHz broadband ADCP attached to the bottom of the research vessel recorded current information continuously during the cruise and a CTD equipped with an oxygen sensor was deployed at every station to obtain depth-profile measurements of temperature, salinity, density, pH, turbidity, fluorescence and PAR.

Analysis of the data shows a counterclockwise gyre within the Bay, affected by winds, which drives many of the spatiotemporal patterning of the environmental variables studied. The duration of vertical thermal stratification was also shown to be a major influence on environmental variables which, in turn, influenced the benthos present within the localized environments within Smithtown Bay.

Thus, the main drivers of hypoxia in Smithtown Bay are natural climatological and meteorological processes, coupled with natural sediment diagenetic processes and biological processes as opposed to anthropogenic ones. Given climate change, it is likely that systems such as Smithtown Bay will continue to experience worsening hypoxia in the future despite management efforts.