Flushing Time Estimates for Estuarine Sub-Regions using Numerical Circulation and Box Models

Hypoxia in estuaries and shelf regions develops when biological oxygen demand exceeds the rate at which oxygen is resupplied by biological and physical processes. An estimate of the timescale for oxygen resupply due to physical processes is the flushing time of the region of interest. The temporal variability of flushing time in various sub-regions of Narragansett Bay, a partially mixed estuary that experiences episodic summertime hypoxia, is investigated using numerical model tracer-based estimates. High-resolution numerical dye simulations, performed using the Regional Ocean Modeling System (ROMS), are used to determine property exchange among the elements of a two-layer ecological box model of the Bay for a two-year period. These results are used to estimate daily flushing times of the ~5 km scale box model elements, and combinations of elements, using several methods. Flushing times of salt and freshwater, while highly correlated, are generally not equal. Median freshwater flushing time for most elements of the Bay is 1-2 days, but in a shallow side embayment in which hypoxic conditions are often observed, the median flushing time is much larger (4-5 days). Fluctuations in river discharge and wind stress are shown to influence the flushing time on timescales of weeks and days respectively. Estimates of the timescale of oxygen drawdown due to respiration suggest that physical processes are an important contributor to the development of hypoxia in this system.