STRATIFICATION AND BOTTOM-WATER HYPOXIA IN THE PAMLICO RIVER ESTUARY

Relationships among bottom-water dissolved oxygen (DO), vertical stratification, and the factors responsible for stratification-destratification in this shallow, low tidal-energy estuary were studied using a 15-yr set of biweekly measurements, along with some recent continuous-monitoring data. Hypoxia develops only when there is both vertical water-column stratification and warm water temperature (>15-degrees-C). In July, 75% of the DO readings were <5 mg l-1, and one-third were <1 mg l-1. Severe hypoxia occurs more frequently in the upper half of the estuary than near the mouth. Both the time series data and correlation analysis results indicate that stratification events and DO levels are tightly coupled with variations in freshwater discharge and wind stress. Stratification can form or disappear in a matter of hours, and episodes lasting from one to several days seem to be common. Estimated summertime respiration rates in the water and sediments are sufficient to produce hypoxia if the water is mixed only every 6-12 d. There has been no trend toward lower bottom water DO in the Pamlico River Estuary over the past 15 yr.