Phytoplankton distributions and species composition across the Texas-Louisiana continental shelf during two flow regimes of the Mississippi River

Abstract

Phytoplankton abundance and species composition were examined over the Texas-Louisiana continental shelf during May 1992 and May 1993, as part of a phytoplankton diversity study funded by the Office of Naval Research. Phytoplankton distribution data were assessed in relation to the hydrography and physical processes on the shelf, which were studied as part of the Texas-Louisiana Shelf Circulation and Transport Processes Study (LATEX A). Phytoplankton group distributions from 1992 , which was an average flow year for the Mississippi River, were compared with observations from 1993, which was a record flow year. Water samples for phytoplankton determinations were examined at 22 locations on cross-shelf transacts from 90.5' to 94.0'W longitude. Samples were collected at the surface and the chlorophyll maximum from Niskin bottles attached to a Sea-Bird SBE911plus CTD, preserved in 1% glutaraldehyde, and analyzed using the Uterm6hl method and the inverted-microscope technique. Unique phytoplankton distributions and regionspecific hydrography and physical processes were found on the inner, middle, and outer shelf during both flow regimes. Some differences were found in May 1993 due to the record river discharge. In 1992 and 1993, the inner shelf was diatom dominated, and was characterized by the highest nutrient and lowest safety values. River discharge and associated nutrients were focused by the localized downcoast flow predominant on the inner shelf area during the month of May. Water column stability decreased moving from the eastern part of the shelf to the western part in May 1992. The opposite regime was present in May 1993. Inner shelf nutrient concentrations in May 1993 were approximately double those in May 1992. The increased river discharge in 1993 caused a dramatic shift in dominant diatom species to Skeletonema costatum (Grevifle) Grunow, which is found in a range of salinities, temperatures, and depths. Chain-forming diatom and others were predominant in both years. On the middle shelf, the presence of tychopelagic diatoms reflected the possibility of benthic regeneration of nutrients and resuspension into the upper water column. This flux from the benthos supported the phytoplankton community on the middle shelf, where a near-bottom chlorophyll maximum was found. Lower concentrations of phytoplankton were present on the middle shelf than the inner shelf during both years. The upper 30-70 m of the water column on the middle shelf were found to be oligotrophic, so smaller or more motile cers such as dinoflagenates, microflagellates, and coccolithophorids became more dominant. The outer shelf upper water column was nutrient-poor as well during both years, and dinoflageuates, microflagellates, and coccolithophorids were even more dominant than on the middle shelf. The diatom population decreased more moving from the middle to the outer shelf. Effects of a warm core Loop Current eddy were evident on the outer shelf area. Upwelling processes shallower than 100 m may provide a means of supporting the phytoplankton population at the chlorophyll maximum on the outer shelf. The location of the increased volume of river water across the shelf in May 1993 was identified based on the increase in overall phytoplankton abundance in May 1993.