Remote sensing of ocean color in the northern Gulf of Mexico

Abstract

Remote sensing of ocean waters, using the Nimbus-7 Coastal Zone Color Scanner (CZCS), in the northern Gulf of Mexico during a 17 month period (November 1978 - March 1980) showed large variability in the spatial and temporal distributions of pigment fronts. Twenty-nine atmospherically- and geometrically-corrected CZCS orbits were processed into total pigment (chlorophyll a plus phaeopigments) images. A correction factor of 1.67 was applied to the pigment concentrations to correct for the tendency of the standard fluorometric method to underestimate chlorophyll a concentrations. Entrainment of coastal waters offshore was a dominant feature throughout the study. Intrusion of the Loop Current and its effect on surrounding waters was easily observed in the pigment imagery. The mean pigment concentration for the 17 month survey was 3.30 [plus or minus] 1.45 mg m^-3. Both the spring (March) and the fall (October) phytoplankton blooms were documented in the imagery, although precise dates for these maxima could not be determined due to the spacings between usuable CZCS orbits. A comparison was made between 7 thermal (CZCS channel 6) and pigment images. At times, the sea surface temperature expressions coincided with pigment gradients. In general, pigment imagery showed structural features not readily discernible in the thermal imagery. Development of a primary productivity algorithm was investigated using historical ground truth data on primary productivity, chlorophyll a concentration, sea surface temperature, and day length. Results indicated that, using these variables, a multiple regressions model could only describe 15% of the variability in production; therefore, a productivity algorithm was not applied to the remotely sensed pigment and thermal data. Conversion of the mean pigment concentration to production was calculated, with results ranging from 237 to 663 g C m^-2 yr^-1 depending on the regressions selected.