Quarterdeck 3.2
Recent Graduate

Gulf phytoplankton and the Great Flood of 1993

by Paula Bontempi

Phytoplankton are unicellular algae that compose the first trophic level of the oceanic food chain. Most phytoplankton are primary producers, using carbon dioxide, water, and light to carry out photosynthesis. Photosynthesis produces oxygen and organic material used by other marine organisms for their life processes. The crucial role these algae play in the oceanic ecosystem merits a study assessing environmental parameters that affect phytoplankton distributions, abundances, and species composition. My thesis research examines the effects of two flow regimes of the Mississippi River on phytoplankton distributions on the Texas-Louisiana continental shelf.

The Texas-Louisiana shelf in the northern Gulf of Mexico is a dynamic environment. Its ecosystem is subject to the variable outflows of nutrients and suspended particulate material from the Mississippi River and its main tributary, the Atchafalaya River. The two rivers together deliver approximately 300 cubic kilometers of fresh water onto the shelf annually. The first flow regime considered in this study, 1992, represents an average flow year for the Mississippi River while the second, 1993, was a record flow year. The study focuses on the effects of Texas-Louisiana shelf hydrography and physical processes on the phytoplankton in the spring.

[77K] Surface salinity recorded in the Gulf of Mexico during the May 1993 LATEX A hydrography cruise. Compared to 1992, fresher water extends farther out to sea due to the record flows of the Mississippi and Atchafalaya Rivers.

The Texas-Louisiana shelf is divided into the inner, middle, and outer regions based on hydrography, physical processes, and phytoplankton regimes. During the spring of 1992 and 1993, the inner shelf had the highest overall concentrations of nutrients and phytoplankton. These high numbers were due to the influence of river-borne nutrient loads and some advected nutrients from the seafloor. The majority of the phytoplankton population was composed of diatoms.

At the sea surface above the middle and outer regions of the shelf, the water column was oligotrophic, or nutrient-poor. In this environment smaller, more motile cells dominated. Motile dinoflagellates and micro-flagellates can migrate into light-filled or nutrient-rich areas. They may be small cells which have lower nutrient requirements, as are the majority of coccolithophorids found on the shelf in the spring. At the chlorophyll maximum of the middle shelf (40 to 50 meters deep), the depth at which the concentration of chlorophyll is the greatest, regeneration of nutrients from the seafloor and advection of these nutrients into the upper water column probably helped sup-port the phytoplankton population. At the outer-shelf chlorophyll maximum (55 to 70 meters deep), upwelling of nutrient-rich waters into the oligotrophic upper water column may have supported the phytoplankton population.

Major differences were found in the springtime phytoplankton regimes of the flood year (1993) and the average flow year (1992). In May 1993, the area occupied by lower-salinity river water could be identified based on the location of greater phytoplankton abundances at the surface and chlorophyll maximum. There was a shift in dominant diatom species from Leptocylindrus danicus Cleve and Rhizosolenia delicatula Cleve in 1992 to Skeletonema costatum (Greville) Grunow in 1993. This shift was influenced by the radical change in the shelf environment, particularly salinity and nutrients, caused by the increased volume of fresh water. The May 1993 shelf environment favored the dominance of the more cosmopolitan species, Skeletonema costatum.

During both years, tychopelagic diatoms were found in the upper water column on the middle shelf. A tychopelagic diatom spends most of its life cycle attached to a bottom substrate until forcibly torn from it. Their presence in the middle-shelf water column supported the hypothesis that resuspension of nutrients from the seafloor helped maintain the phyto-plankton population there.

Hydrographic and physical data for this research were supplied by the Louisiana-Texas Shelf Physical Oceanography Program (LATEX), funded by the Minerals Management Service. The complementary phytoplankton study is funded by the Office of Naval Research. Researchers at the University of Georgia and the University of Southern Mississippi are developing a coupled biological and physical model to enable prediction of seasonal phytoplankton biomass and species diversity on the Texas-Louisiana shelf based on different flow regimes of the Mississippi River.

Editor's Note: Paula Bontempi graduated with her Master's Degree in May 1995. She currently participates in a study of phytoplankton diversity at Stennis Space Center at the University of Southern Mississippi. Paula plans to return to graduate school next year to pursue a Ph.D.

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Oceanography, Texas A&M University

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Updated September 14, 1995