Modeling, measurements, and satellite remote sensing of biologically active constituents in coastal waters

Abstract We examine ways of addressing coastal environmental quality concerns through the use of modeling, measurements, and in the future, satellite remote sensing. In a summary of historical trace-metal concentrations in the waters of Narragansett Bay, we partitioned the estuary into sectors that reflect the morphology of the Bay and the transition between freshwater inputs and offshore coastal waters. We constructed a 24-box two-layer model of the Bay. A convenient summary of the chemical variations in the Bay was provided by a schematic diagram which for a constituent such as copper or other metals displays the average concentration, the range, the standard deviation, and the number of observations in our database for each sector. This diagram shows the spatial gradients through the Bay and the variability within a sector. Using a simple two-layer box model with seven transport terms we computed the physical exchanges between boxes using freshwater input and salinity data. The box model approach was applied in greater detail to the upper portion of Narragansett Bay to provide transport terms for use in an oxygen water quality evaluation. A digital bathymetric map of the estuary was compiled to enable volume-weighted calculations of physical and chemical properties. A seasonally variable data set was available to determine the effects of summer/winter and high-/low-runoff conditions on the oxygen concentrations of the waters. Using the freshwater input rates and the observed salinity distribution in the estuary we calculated the transport of waters between boxes and the residence times of water within each box. The model was applied to oxygen concentrations in the estuary incorporating estimates of the effects of air-sea exchange, of sediment oxidation demand, of photosynthetic production and respiratory consumption, and of biochemical oxygen demand from sewage treatment effluents. The model provides a basis to estimate the relative importance of various processes that may cause low oxygen conditions in the waters. An investigation of oxygen variations in coastal waters was conducted with an Endeco/YSI rapid-pulse dissolved oxygen electrode. A 30-day time series was obtained at a depth of 1–2 m in Narragansett Bay. Measurements of oxygen, temperature, and salinity were obtained every 30 min during October 1993. Fourier analyses were used to determine the frequencies in the oxygen, temperature, and meteorological (wind speed and sunlight levels) variables. There was a strong diel signal in oxygen with smaller amplitude variations at the semidiurnal tidal frequency and a large amplitude variation with a period of 3–5 days. High temporal resolution data are needed to detect the events in coastal waters that result in substantial chemical variations of biologically active constituents such as oxygen. In anticipation of possible applications of the next generation ocean color satellite sensor, SeaWiFS, we have been examining the historical CZCS data from the region off the northeastern U.S.A. We have worked with both the 4-km and the 1-km horizontal resolution CZCS data. The scales of variability that are evident are in the range of 5–50 km. For quantitative use of the ocean color data attention must be given to spatial variations in the atmospheric attenuation of the visible radiation, and to the separation of chlorophyll, suspended matter, and possibly blue-absorbing organic matter in the ocean color signal.