[1] Observations of dissolved oxygen (DO) concentration, salinity, and temperature, during summer of 2004, at three levels on two moorings in the area of western Long Island Sound that is prone to seasonal hypoxia are described. Ship surveys in the area reveal that the DO concentration below the pycnocline decreases at approximately 2.4 mM m À3 d À1 throughout the summer. We show that this is the net result of oscillations in the rate of change of the DO concentration with periods of 3 to 7 days. During intervals of declining DO concentration, the rate of change is consistent with previous estimates of the rate of community respiration. Since there is insufficient light for photosynthesis below the pycnocline, increasing DO concentration (ventilation) must be a consequence of either vertical mixing or horizontal advection from regions of higher concentration. Analysis of the covariation of DO, salinity, and temperature and knowledge of the mean property distributions allow us to associate most ($80%) of the ventilation intervals with increased vertical mixing. Comparison of DO and wind stress measurements suggest that it is the component in the along-sound direction that controls the occurrence of ventilation, perhaps through modification of the rate of stratification by the density-driven circulation. We conclude that the spatial and temporal variability of vertical mixing is crucial to understanding the duration and extent of hypoxia in the Long Island Sound estuary.

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