Examining the processes controlling water column variability in Narragansett Bay: Time series data and numerical modeling.

摘要

Narragansett Bay is an important resource to the state of Rhode Island for recreational and commercial use. Eutrophication of coastal waters and its associated oxygen demand are of increasing concern for Narragansett Bay. This project focused on continuous in situ measurements and three-dimensional model simulations to better understand the physical, biological and chemical processes which occur in the Bay.; Measurements of temperature, salinity, dissolved oxygen, pH, chlorophyll and depth were obtained to gain insight into the process that contribute to phytoplankton production and hypoxia in the Bay. Six major hypoxic events were captured during the summers of 2001 and 2002. These events have been linked to surface phytoplankton bloom that occurred during periods of water column stratification associated with freshwater input, heating of the surface waters, low tidal range and northerly winds. Two processes were important in the restoration of oxygen to bottom water: vertical mixing of the water column and advection of oxygenated water. Complete vertical mixing was observed when strong winds, usually from the north, coincided with periods of high tidal range. Northward advection of oxygenated water occurred when tidal range was high, but winds were weak to moderate or there was additional freshwater input.; Dawn to dusk production estimates were made for the data obtained from June to January 2001 and 2002. Average production in the Upper Bay and Providence River was higher during the summer months, (11.86--13.60 g O2/m 2/day at Bullock Reach and 4.45--4.72 g O2/m 2/day at North Prudence) than during the fall (3 g O2/m 2/day). A dramatic decrease in production was observed bay-wide in both years during the fall compared to the summer. This decrease has been attributed to the deepening of the mixed layer below the critical depth as a result of vertical mixing by an episodic event in 2001 and to the gradual decrease in light intensity during the fall of 2002.; The numerical model ROMS was applied to Narragansett Bay to determine the influence of environmental factors on the length of time bottom water remains in the Providence River, where it goes when it leaves the Providence River (East versus West Passage) and stratification in Narragansett Bay. The model results showed that residence times decrease exponentially with increasing river flow. In addition, residence times decrease with wind direction in the following order: west, north, south, and east. The pathways of bottom particles exiting the Providence River were also affected by wind direction and river flow. Under no winds and low discharge more particles exited through the West Passage, however as river flow increases an increase in particles exiting the East Passage was observed. Winds from the north and south show a larger number of particles exiting through the West Passage, while winds from the east and west indicate that the East Passage is the preferred pathway of particles. The model also suggested increased estuarine circulation occurred under increased river flow conditions and was enhanced by winds from the north and west.