Phytoplankton Community Structure and Productivity Along the Axis of the Mississippi River Plume in Oligotrophic Gulf of Mexico Waters

The Mississippi River is the largest freshwater input into the Gulf of Mexico (GOM) and contributes a large nutrient load to northern GOM waters. During the summer, the Mississippi River plume is sometimes found to extend into the eastern oligotrophic GOM as far as the Dry Tortugas. The objectives of this study were to determine the relative contribution of the Mississippi River plume to the total surface water production in the oligotrophic GOM and the impact of this feature on the composition of phytoplankton found there. Using Sea-viewing Wide Field-of-View Sensor (SeaWiFS) satellite images, we located and sampled the offshore Mississippi plume along its axis. In situ sampling in combination with remote sensing allowed us to estimate integrated plume primary productivity. Carbon fixation in the northern GOM averaged 0.53 μg C l-1 h-1 for non-plume stations, and 9.3 μg C l-1 h-1 in plume stations. We estimated integrated productivity of the plume at ca. 3.28 × 109 g C h-1, which accounted for 41 and 13% of all surface and total water column productivity in the oligotrophic GOM, respectively, at the time of sampling. Analysis of rbcL cDNA clone libraries and HPLC pigment data indicated that our sampling transect traversed several regions with distinctly different phytoplankton assemblages. Non-plume communities were numerically dominated by Prochlorococcus, and contained prymnesiophytes and eustigmatophytes. Diatoms dominated the most productive inshore station, while Synechococcus dominated in the mid-plume just off the Louisiana shelf. The least productive and most offshore portion of the plume was also diatom dominated. Diatoms were the most diverse algal class observed, accounting for over 42% of all unique rbcL genotypes detected in the plume. Collectively, these results indicate that the Mississippi River plume contributes significantly to oligotrophic productivity in the GOM, resulting from localized blooms of both Synechococcus and diatoms