Distribution and controlling mechanisms of primary production on the Louisiana-Texas continental shelf

The northwest (NW) Gulf of Mexico is marked by strong seasonal patterns in regional and mesoscale circulation and variable effects of riverine/estuarine discharge, which influence distributions of nutrients, phytoplankton biomass and primary production. During a series of five cruises in the NW Gulf of Mexico in 1993 and 1994, an extensive data set was collected including nutrients, phytoplankton biomass (chlorophyll a), and photosynthesis-irradiance (P-E) parameters. Primary production was estimated using P-E parameters in conjunction with profiles of biomass and irradiance. Relatively high biomass and primary production were observed in inner shelf waters during spring conditions of high river discharge. This was attributed to the retention of biomass and nutrients on the shelf by the combination of high river outflow and a westward flow along the inner shelf with consequent onshore Ekman component. During summer, when surface currents shifted towards the north and east, values of nutrients, biomass and primary production were relatively high east of Galveston Bay and decreased outward from the coast. This pattern was apparently a consequence of nutrient inputs from riverine, upwelling and benthic sources. Nutrients, biomass and productivity in the western portion of the study area in summer were generally lower as a result of the upcoast Row of oligotrophic offshore water. Inter-annual variability was observed between November 1993 and 1994 with higher biomass and productivity occurring in November 1993. This was partially attributed to higher river discharge prior to November 1993, retention of biomass and nutrients by the downcoast flow along the inner shelf, and possibly, injection of nutrients onto the shelf at the shelf break. Our findings demonstrate that the interaction of circulation and availability of Light and nutrients are largely responsible for variations in primary production. Nitrogen appeared to be the primary limiting nutrient, however, a potential for phosphate limitation was also observed particularly during periods of higher river discharge. Light availability was a critical variable during the fall and winter months, when higher primary production was restricted to shallow waters where vertical mixing was constrained by bottom topography. In deep waters, counteractive changes in nutrient and light availability apparently resulted in minor temporal variation between seasons. The annual carbon production in the Louisiana-Texas (LATEX) continental shelf region was estimated to be 159 g C m(-2) year(-1), which is within the range of prior estimates for this region. Given that the area of the study region was approximately 140,000 km(2), this would be equivalent to an areal carbon production of about 22.2 million metric tons. (C) 2000 Elsevier Science B.V. All rights reserved.