Swanson, R.L.; Bauer, C.L.; Wilson, R.E.; Rose, P.S., and O'Connell, C., 2016. Physical processes contributing to localized, seasonal hypoxic conditions in the bottom waters of Smithtown Bay, Long Island Sound, New York.
Summertime hypoxia regularly occurs in the bottom waters of Smithtown Bay, Long Island Sound, New York. Hypoxia is plaguing many coastal estuaries and can be detrimental to aquatic organisms. A generally accepted model for the cause of coastal hypoxia in temperate regions is seen in the introduction of anthropogenic nutrients, which fuels excessive primary production, leading to oxygen depletion exacerbated by seasonal water-column stratification. No major point sources of anthropogenic nitrogen discharge directly into the bay. Groundwater nitrogen discharge is somewhat more than double that of a small sewage treatment plant. Despite great efforts to reduce nitrogen loading into Long Island Sound, summertime hypoxia is a continuing occurrence in Smithtown Bay. Hydrographic cruises accompanied by Acoustic Doppler Current Profiler measurements were conducted during the summers of 2004, 2009, and 2010 to explore physical processes that contribute to hypoxic conditions occurring there. While hypoxia remains seasonally prevalent in the bay, the deeper surrounding bottom waters have considerably higher concentrations of dissolved oxygen. This study examines physical oceanographic contributions to hypoxia in the bay and determines that the problem is more complex than the introduction of anthropogenic nitrogen. Thermally controlled stratification with pycnoclines at middepths inhibit vertical mixing and replenishment of dissolved oxygen to bottom waters. The two headlands that form the bay, Crane Neck and Eatons Neck, create circulation patterns that inhibit lateral advection between the bay and the rest of the Sound. This results in weak currents and bottom stress within the bay and a limited exchange of water masses over a tidal cycle. Furthermore, the headland gyre setting increases the residence time in Smithtown Bay.
