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/estuari ne discharge, which influence distributions of nutrients, phytoplankton bio mass 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-irradi ance (P-E) parameters. Primary production was estimated using P-E parameter s in conjunction with profiles of biomass and irradiance. Relatively high b iomass and primary production were observed in inner shelf waters during sp ring conditions of high river discharge. This was attributed to the retenti on of biomass and nutrients on the shelf by the combination of high river o utflow and a westward flow along the inner shelf with consequent onshore Ek man component. During summer, when surface currents shifted towards the nor th and east, values of nutrients, biomass and primary production were relat ively high east of Galveston Bay and decreased outward from the coast. This pattern was apparently a consequence of nutrient inputs from riverine, upw elling and benthic sources. Nutrients, biomass and productivity in the west ern portion of the study area in summer were generally lower as a result of the upcoast Row of oligotrophic offshore water. Inter-annual variability w as observed between November 1993 and 1994 with higher biomass and producti vity occurring in November 1993. This was partially attributed to higher ri ver discharge prior to November 1993, retention of biomass and nutrients by the downcoast flow along the inner shelf, and possibly, injection of nutri ents onto the shelf at the shelf break. Our findings demonstrate that the i nteraction of circulation and availability of Light and nutrients are large ly responsible for variations in primary production. Nitrogen appeared to b e the primary limiting nutrient, however, a potential for phosphate limitat ion was also observed particularly during periods of higher river discharge . Light availability was a critical variable during the fall and winter mon ths, when higher primary production was restricted to shallow waters where vertical mixing was constrained by bottom topography. In deep waters, count eractive changes in nutrient and light availability apparently resulted in minor temporal variation between seasons. The annual carbon production in t he 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 k m(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.