Cross-shelf changes in phytoplankton community composition in the Gulf of Mexico (Texas shelf/slope): the use of plant pigments as biomarkers
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abstract
Plant pigments were used as tracers to document changes in the composition of phytoplankton assemblages in relation to nutrient distributions across the Texas shelf/slope in the northwestern Gulf of Mexico (GOM) on three cruises in June 1992, January 1993 and October 1993. Diatoms, prymnesiophytes, and cyanobacteria dominated the Texas shelf and shelf/slope break waters in January 1993 (ca. 85%), with diatoms and cyanobacteria as the dominant taxa in October 1993 (ca. 90%). In general, diatoms and cyanobacteria dominated the shelf waters while cyanobacteria, pelagophytes, and prymnesiophytes were dominant in slope waters. The highest concentrations of prymnesiophytes, based on 19' hexanoyloxyfucoxanthin (19-hex) concentrations, were found in slope waters in January 1993 and were likely related to the presence of a warm-core ring at that time. Water column N:P and Si:N ratios in the dissolved phase for the three cruises averaged 11.44 4.46 and 8.67 5.61, respectively. Cyanobacterial abundance was positively correlated with total nitrogen (TN) concentrations in slope waters on all three cruises, indicating that nitrogen-fixing bacteria may have contributed to TN concentrations in Texas slope waters. Based on the range of the ratio of chlorophyll a: diagnostic pigments (F(p) ratios) (0.09-0.34) the overall trophic status of surface waters across the Texas shelf/slope was characterized as oligotrophic. Nutrient concentrations in shelf waters indicated that these waters were not oligotrophic and that the F(p)-derived trophic status was incorrect. One problem with the application of F(p) ratios in nearshore systems is that new production may be represented by classes other than dinoflagellates and diatoms, such as cyanobacteria, which if ignored will underestimate new production resulting in an incorrect characterization of eutrophication.
