Physical Processes that Affect Hypoxia in the Thames River Estuary, USA

Norwich Harbor is located at the head of the Thames River, the third largest drainage basin to the Long Island Sound. Hypoxic bottom-water conditions in Norwich Harbor have been documented during previous summers, but the evolution and variability of the low-oxygen levels have not been effectively shown through prior observations. Developing a further understanding of these conditions may help explain seasonal ecosystem patterns in the Thames River and Long Island Sound. An observational program was initiated in June 2008 to study the stratification and oxygen levels in Norwich Harbor. Daily vertical profiles of temperature, salinity, and dissolved oxygen paired with monthly monitoring of tidal cycles provide new insight to the pattern of the anoxic and hypoxic conditions that are present in the harbor during the summer months. Regular monthly low salinity/increased dissolved oxygen events were observed. A significant correlation was found between these low salinity events and the daily tidal range. Additional analysis indicates that persistent stratification in Norwich Harbor prevents vigorous local vertical mixing. Analysis suggests the low salinity/increased dissolved oxygen water parcels are created down estuary and subsequently horizontally advected into the harbor