Long Island Sound (LIS) is an estuary receiving nutrients from a population of >8 million people.  Resulting eutrophication has led to severe summertime hypoxia particularly at its western end.  Despite a ~60% reduction in nitrogen loading, only the beginning of improvements in summertime O₂ has been observed.  Because LIS’s O₂ budget has not been quantitatively well constrained, reliable predictions of the future course of summertime hypoxic have yet to be made. The major terms in LIS’s oxygen budget are 1) local air-sea gas exchange, 2) ventilation through larger scale circulation, 3) near-surface photosynthetic production, 4) water column respiration, 5) sediment respiration.  We have used observation of the ¹⁸O/¹⁶O ratio of O₂ (d¹⁸O) and model analysis to parse the contribution of these terms as they produce distinct isotopic signatures. Photosynthesis produces O₂ with a d¹⁸O 24‰ lower than the atmosphere, respiration in the water column increases d¹⁸O with a fractionation factor of about 20‰ (directly measured by us), whereas sedimentary respiration creates little isotope fractionation (directly measured). Samples were collected from stations throughout LIS but focusing on the western end from monthly to bi-weekly (summer).  In addition, high temporal frequency sampling to capture diel and tidal influences were conducted at several western stations in summer.  Overall, decreasing O₂ was associated with large ¹⁸O enrichments due to the influence of water column respiration but a contribution of photosynthetic O₂ to isotope signals even in bottom waters was also clear.

Primary Presenter: Mark Altabet, University of Massachusetts - Dartmouth (maltabet@umassd.edu)

Authors:

Mark Altabet, UMassD/SMAST  (maltabet@umassd.edu)

Craig Tobias, UConn/Dept Marine Sciences  (craig.tobias@uconn.edu)

James O'Donnell, UConn/Dept Marine Sciences  (james.odonnell@uconn.edu)

Sesha Manning, UMassD/SMAST  (smanning2@umassd.edu)

Alanna Mnich, UMassD/SMAST  (amnich@umassd.edu)