Spatial and temporal variability of phytoplankton biomass and community structure over the continental margin of the northeast Gulf of Mexico based on pigment analysis

Chlorophyll and carotenoid pigment distributions are used to describe spatial and temporal variations in phytoplankton biomass in the water column of the northeastern Gulf of Mexico (NEGOM) between the Mississippi River Delta in Louisiana and Tampa Bay in Florida from November 1997 through August 2000 and to infer seasonal changes in phytoplankton taxonomic composition. Chlorophyll a concentration in the water averaged 0.4 μg/l but ranged as high as 12 μg/l on nine cruises in this region during the 3 year period. The spatial distribution of chlorophyll a appeared to be controlled by riverine and estuarine input of nutrient-enriched freshwater. The Mississippi and Apalachicola River systems were the two main controlling features for the spatial distribution of chlorophyll in the NEGOM area. On average, the near-surface as well as subsurface waters near the Mississippi River–Mobile Bay outflow regions had the highest chlorophyll a concentrations of the study area during the 3 years. Subsurface waters near the Apalachicola River outflow region also had consistently high chlorophyll a concentrations during the nine cruises, even though the near-surface waters in this region did not. thmic analyses of pigment composition indicated that prymnesiophytes, prokaryotes, cyanobacteria, diatoms, and pelagophytes were the major phytoplankton taxa on the NEGOM continental margin. On average, prymnesiophytes accounted for 25–40% of total chlorophyll in the water column over this continental margin during the 3 year period. Prokaryotes averaged 14–38% of chlorophyll a in the water column. Diatoms, which contributed 6–13% of chlorophyll a, were primarily found in inner shelf coastal waters. In contrast, prymnesiophytes and pelagophytes reached highest abundances in outer shelf and slope waters. Phytoplankton communities also differed vertically. Pelagophytes occurred in higher abundance in subsurface water, while dinoflagellates reached greater concentrations in near-surface waters.