Distributions and variations of phytoplankton photosynthesis and primary production of the Louisiana-Texas continental shelf

Xiaogang Chen


During a series of five cruises in the Northwest (NW) Gulf of Mexico in 1993 and 1994, an extensive data set was collected. Relatively high biomass and primary production were observed in inner shelf waters during spring conditions of high river discharge. During summer, values of nutrients, biomass and primary production were relatively high east of the study area and decreased outward from the coast and westward. Inter-annual variability was observed between November 1993 and 1994 with higher biomass and productivity occurring in November 1993. 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. 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. Analyses revealed that spatial variations in environmental conditions exhibited strong relationships to photosynthesis-irradiance (P-E) parameters. Within a given period, values of P-E parameters were generally higher in nearshore waters and lower in offshore waters. Values of PB max in the surface layer were generally higher in summer and lower in fall, and were related to temperature variations. Values of α B were less variable spatially and temporally. Regional circulation has a strong influence on the distributions of phytoplankton taxa as determined using HPLC. Diatoms accounted for a high proportion of biomass in nutrient-rich near shore waters, while highest percentages of biomass offshore were associated with Haptophyceae in summer and cyanobacteria in fall. Correlations between the relative biomass of some algal groups and P-E parameters provide the compelling evidence that observed spatial distributions of P-E parameters is an integrated response to the taxa composition and associated environmental conditions. Strong correlations were observed between primary production and the relative biomass of phytoplankton taxa, with distributions that show distinct and consistent patterns similar to that of environmental conditions and primary production. The performance of a remote sensing primary production algorithm, using surface irradiance, biomass, and temperature as input, was evaluated for the region. Analyses demonstrated that realistic simulations of light and biomass, rather than phytoplankton photosynthetic parameters, were most critical to the performance of the model. The performance of the model could be improved by modification of parameters for chromophoric dissolved organic matter absorption.