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Water from the Caribbean flows through the Yucatan Channel into the Gulf, loops clockwise, and exits through the Florida Straits to become part of the Gulf Stream. This physical behavior is quite constant, though the depth of intrusion into the Gulf and the frequency with which eddies are formed and shed from the main Loop Current vary dramatically from year to year. The biological activity and sea surface temperature (SST), however, change markedly over the course of a single year.
In the winter months, warm water from the Caribbean forms sharp temperature gradients as it intrudes into the colder Gulf of Mexico waters as shown in subsequent AVHRR imagery (Feb'94, Feb'95, Jan'96). This Caribbean water is always low in chlorophyll pigments due to its nutrient-poor source waters. Pigment concentrations in the deep Gulf waters, however, reach a maximum at this time as the winter overturning of the surface waters mixes downwards and brings the nutrient rich waters from below. Later, in the spring, river discharge into the coastal waters increases nutrient supply to the phytoplankton, allowing maximum growth during these months as shown in the CZCS/ocean color images. In the CZCS imagery, the ocean color data have been converted to pigment concentration using standard NASA algorithms, and the purple/blue colors show clear waters (low pigment concentrations) and green, yellow, orange and red show more turbid waters or water with a higher phytoplankton concentration or higher gelbstoff levels (Dec'80, Jan'81, Feb'81). Phytoplankton growth increases simultaneously across the Gulf. Thus, during these colder months, sea surface temperature is more effectively used than chlorophyll pigment concentration as a Loop Current tracer. Pigment concentrations, while high in the winter, do not have the horizontal structure necessary to effectively trace flows in remote-sensing imagery.
During summer, the temperature of the Gulf waters reaches a maximum between July and September. These increases reduce the previous temperature contrast between the Loop Current and Gulf waters, making them indistinguishable using SST imagery (Jul'94, Aug'95). The pigment-poor Loop Current water, however, extends farthest into the Gulf at this time of year. The chlorophyll pigment concentration, while declining from its spring value due to less river discharge (hence lower nutrient availability), is still great enough to be easily distinguishable from CZCS imagery of the very clear Caribbean waters that now extend quite deeply into the Gulf of Mexico (May'80, Jun'80, Jul'80).
Thus, while horizontal gradients in biological processes are most clearly seen during summer from satellites, a combination of SST and chlorophyll pigment concentration imagery allow for year-round observation of near-surface processes caused by flow dynamics within the Gulf of Mexico.
River Plume Biology:
Sea surface temperature and chlorophyll pigment concentration are also excellent indicators of river plumes and discharges. Coastal Zone Color Scanner (CZCS) data clearly show that large amounts of colored material enter the Gulf of Mexico via the Mississippi Delta and Mobile Bay (May'80, Aug'80, Jul'82). River discharge is high in sediment load, gelbstoff, nutrients, and chlorophyll pigments, greatly affecting the light signal emanating from the water and received at the satellite sensor. As a plume extends into the Gulf, the sediment gradually settles out, photodegradation of gelbstoff occurs, and the phytoplankton take up the accompanying nutrients. The phytoplankton are then grazed upon by autotrophic zooplankton. These effects change the reflected (backscattered light) and absorbed light with time, making the mixing patterns nonconservative. All these effects result in a decreased light signal. Additionally, any temperature difference between the river water and Gulf water erodes as one water body mixes with the other. These effects are seen in the monthly composites, as the river plume signal degrades rapidly as it moves away from the coast. The attenuation of light within the water column is greatly affected by the presence of phytoplankton, gelbstoff, and sediment. These effects are readily seen in the K490 images from the CZCS, depicting light attenuation at 490 nm (Dec'82, Jan'83). Temperature, gelbstoff, and chlorophyll pigment concentration are more conservative properties than suspended sediment, and they act as reasonable tracers on 1 to 5 day time scales.