Estuarine Processes

Near-Coastal Challenges

Deriving physical processes of rivers and oceans can be achieved using the large-scale viewing capabilities of space-borne sensors such as AVHRR, CZCS, or SeaWiFS. Estuarine processes occur on a much shorter spatial and spectral scale that surpasses the resolution capabilities of such satellite platforms. These smaller scales are due to the mixing of dissolved and particulate constituents, frequent variations in absorptive, scattering and fluorescent spectral properties, and changes in the bottom topography and albedo. AVIRIS is an airborne sensor that can achieve the signal-to-noise resolution required by oceanographers, but at the spatial scales necessary to observe and interpret the near-coastal environment.

In many estuaries, conservative mixing and dilution exist, such that the derivation of the optical properties of dissolved material allows a strong prediction of the salinity of the water. Thus, since colour-dissolved organic matter (gelbstoff) travels with water flow, if the absorption coefficient of such gelbstoff can be derived from the images, so may the salinity, given an accurate absorption-salinity relationship. Such relationships are presently determined by ship-board measurements; it is hoped that future oceanographic buoys moored in the mouth of such estuaries and bays will have the instrumentation required.

An AVIRIS image of Looe Key, an underwater reef structure in the Florida Keys, clearly indicates the effects of bottom reflectance and resuspended sediments. The three line profiles - line 77, line 226, and line 320, illustrate the marked variations in reflectance profiles possible in just one image. The large peak in 226 over a wide spectral range is due purely to strong, fairly nonspectral, bottom reflectance. Strong, sharp peaks at the red wavelengths exist in the three lines, possibly due to phytoplankton fluorescence.

In order to demonstrate the ability of AVIRIS to remotely sense salinity plumes from nearshore processes, an overflight of Tampa Bay was performed in 1990 and 1992. Maps of the model-derived backscattering coefficient at 671 nm and the absorption coefficient at 415 nm are shown. In nearshore environments, variation in the backscattering coefficient is significantly affected by tidally-derived suspended sediments from Tampa Bay, and local resuspensions form shoal areas. At such short wavelengths as 415 nm, the absorption is dominated by gelbstoff, a refractory, rather conservative constituent. The phytoplankton signal that tends to dominate at higher blue and green wavelengths is relatively small at 415 nm compared to the gelbstoff signal. This allows absorption due to gelbstoff to be determined; with a further gelbstoff/salinity relationship, a means of mapping salinity is achieved. Images of salinity, absorption (415 nm), and backscattering (671 nm) were used to depict the distribution of dissolved and particulate constituents, respectively, for the Tampa Bay plume during late, ebb-tidal conditions.

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