FACTORS AFFECTING THE FORMATION OF HYPOXIC AND ANOXIC BOTTOM WATERS IN NEW ENGLAND COASTAL WATERS

The occurrence of hypoxic/anoxic coastal waters during the summer months along the eastern U.S. coastline has increased over the past two decades and has been tied to increases in nutrient loading, warming temperatures, and water stratification. Bottom water samples [< 5 cm above the sediment-water-interface (SWI)] taken over the course of several years in Long Island Sound (LIS), a large coastal water body located between CT and NY, reveal a complex and dynamic bottom water hypoxic/anoxic zone, dominated during the spring and fall by geochemical exchanges (NH₄⁺, H₂S) across the SWI. These bottom water hypoxic/anoxic events are seasonally decoupled from the annual summertime hypoxic events occurring in the upper water column. The goals of this research were to determine the suite of conditions responsible for the spring and fall bottom water hypoxic/anoxic events in LIS, evaluate whether these conditions are likely to occur in other coastal waters, and identify the potential role of such events in long term carbon storage over geologic time.

A series of controlled laboratory experiments was utilized to collect data on the production and release of NH₄⁺ and H₂S from 3 different sediments from LIS at 3 temperatures, under 3 different dissolved oxygen levels, with/without the addition of fresh plankton, and with/without the presence of bioturbating organisms. The experiments resulted in the collection of 160,000 data points. Analyses of the data prove that bottom water hypoxia/anoxia results from a complex interplay among the factors investigated and that the dominant factors responsible for the spring event differ from those responsible for the fall event. They also confirm the role that temperature exerts on sedimentary biogeochemical processes related to the formation of hypoxic/ anoxic bottom waters and point to the fact that initiation of bottom water anoxic conditions favors preservation of organic carbon, increases sediment oxygen demand and fuels anoxic water formation.

This work has clarified the contribution that sedimentary biogeochemical processes make to hypoxic bottom water formation in coastal waters and further elucidates the important role sediments may play in coastal hypoxic development not just in New England, but in other coastal regions, including the Chesapeake Bay and the Gulf of Mexico.