Abstract
A summer-long monitoring programme investigated mechanisms controlling near-bottom dissolved oxygen (DO) concentrations in New Bedford Outer Harbor, a shallow embayment on the southern Massachusetts coast that receives discharge from a 1.3 m3 s-1 municipal sewage outfall. Continuously recording meters and hydrographic cruises measured DO, temperature, salinity, meteorology, waves and/or currents. The programme quantified the magnitude, spatial extent and duration of oxygen undersaturation in the bottom waters. Summer stratification of New Bedford Outer Harbor reduced reaeration of the bottom waters. Depletion of near-bottom DO was strongly correlated with the presence and duration (though not intensity) of stratification. Stratification is typically thermal, a result of seasonal warming of the water surface, although it can become enhanced (and somewhat salinity driven) owing to the influx of cold saline bottom water following major storm events. Destratification is caused by high wind speed or a drop in 24-h average air temperatures below the water temperature. The decrease in DO concentrations when the water column is stratified is directly related to local sediment oxygen demand. In general, DO dynamics were found to be local and fundamentally one dimensional.
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