Influence of the Mississippi River plume on diazotroph distributions in the northern Gulf of Mexico during summer 2011

In the subtropical oligotrophic ocean, nitrogen fixation is an important source of new nitrogen (N) for supporting biological production. Previous studies have found that nitrogen-fixing Diatom-Diazotroph Associations (DDAs) are in high abundance in the intermediate salinity zone of large river plumes such as the Amazon and Mekong rivers, while Trichodesmium spp. becomes more abundant at higher salinities. This recurring pattern in the Amazon River plume suggests that strong salinity and nutrient gradients within the river plumes may lead to a cascade in diazotroph communities. I hypothesized that the Mississippi River, a major source of freshwater, nutrients and sediments to the northern Gulf of Mexico, creates a similar distribution of diazotroph communities. The relationship between large diazotrophs and salinity was examined in samples collected in July 2011 during a flood outflow from the Mississippi River. The dominant DDA, Hemiaulus spp. – Richelia spp., was at greatest abundance (≈31,000 cells L-1) west of the birdfoot delta on the periphery of the plume (≈29 salinity) where bottom water hypoxia was also observed. Trichodesmium spp., a cyanobacterium genus that occurs in both colonial and free trichome morphologies, was abundant at both high (≈35) salinities east of the delta reaching 20+ colonies L-1, as well as in the fresher (≈28) waters of the plume where it reached 3,500 trichomes L-1. Diazotroph distributions were separated east and west of the Mississippi River outflow, with DDAs being most abundant over bottom water hypoxic regions to the west and Trichodesmium spp. in high abundance to the east. The diazotroph – salinity gradient relationships present within the Amazon River plume were not present within the Gulf of Mexico. This study suggests that environmental factors other than salinity, such as nutrients or hypoxia, are influencing the distribution of diazotrophs around the Mississippi River plume. The seasonal hypoxia seen in the Gulf of Mexico with the co-occurring DDA increase could appear in other river systems.