| From 1992 to 1994, Texas A&M University carried out 3 hydrographic survey cruises per year to support the Louisiana-Texas Shelf Physical Oceanography Program (LATEX). These surveys provided fine scale physical and chemical data which when combined with biological data have allowed an ecological characterization of the waters in the region.; Measurements of phytoplankton primary production, standing crop, and species composition were made in an effort to relate them quantitatively to the physical and chemical forcings. Because of the frequent occurrence of Loop Current Eddies (LCEs) in the western Gulf of Mexico, an attempt was made to assess the impact of these mesoscale, high kinetic energy environments in the seasonal and spatial distribution of phytoplankton biomass and primary productivity.; This study describes the primary productivity of the Louisiana-Texas continental shelf on a systematic, multi-season, and interannual basis. Primary productivity was found to be greatly enhanced in the vicinity of the major river deltas over the inner shelf during the spring of 1993. However, there was no increase in productivity of the inner shelf in spring of 1994, when the nutrient flux to the shelf was much lower.; Statistical modeling of the data collected revealed that nutrients and light limitation control productivity of the inner shelf, while nutrients seem to be the factor limiting productivity of the outer shelf. When LCEs enhance the eastward flow of the currents at the continental slope, they induce upwelling at the outer shelf. The shoaling of the nutricline and the resultant increased nutrient flux at the base of the thermocline directly increase primary productivity. Such divergences, however, amount to only about 10% of the riverine nutrient flux from the Mississippi-Atchafalaya River (MAR).; Since the nutrient flux from the MAR seems to be highly dependent on man's agricultural and industrial practices in the river watersheds, it could be argued that these anthropogenic activities now have a controlling influence on primary productivity of the inner shelf, especially near the rivers' deltas. On the other hand, the productivity of the outer shelf is largely determined by wind-driven currents, as well as by the presence of LCEs and their influence on water movement. It would seem, therefore, that anthropogenic practices and hydrodynamic forcing (i.e. winds and currents) are the two main factors that govern the magnitude and distribution of primary productivity in the northwestern Gulf of Mexico. |
