Nitrogen Fixation, it's not just for Nitrogen poor systems : spatial and temporal patterns of N-fixation in the Upper Mississippi river ( UMR)

Nitrogen distribution within the UMR is both spatially and temporally heterogeneous. Sites with high hydrological connectivity to the river's main channel have higher late summer dissolved inorganic nitrogen (DIN), while less connected sites have lower DIN and low nitrogen to phosphorus (N:P) ratios. Biweekly acetylene reduction assays, were conducted at 6 sites, from May to September, 2008, to determine the temporal and spatial extent of N-fixation. In late July, the least connected site had N-fixation rates tenfold higher than observed in the main channel. The onset of fixation coincided with declining N:P ratios and a cyanobacteria bloom, primarily Aphanizomenon flos-aquae or Aphanocapsa sp. The site with the highest N-fixation rates also exhibited the lightest15N signatures in seston and fish indicating that atmospherically derived nitrogen may be an important N source. N-fixation was an early spring and late-summer phenomenon, when DIN and N:P ratio are low favoring the cyanobacteria bloom. Ironically, a reach of the UMR that transports ? 80,000 tons of nitrogen per year can exhibit nitrogen limitation favoring autotrophs capable of N-fixation.