Summer surface waters in the Gulf of California:: Prime habitat for biological N2 fixation

We report significant rates of dinitrogen (N-2) fixation in the central basins of the Gulf of California (GC) during July-August 2005. Mixing model estimates based upon delta N-15 values of particulate matter in the surface mixed layer indicate that N-2 fixation provides as much as 35% to 48% of the phytoplankton-based nitrogen demand in the central Guaymas and Carmen basins. Microscopic analyses identify the responsible genera as the N-2-fixing endosymbiont, Richelia intracellularis, with lesser contributions from the large nonheterocystous diazotroph Trichodesmium. Analyses of remotely sensed chlorophyll a and sea surface temperature indicate that primary production levels are elevated in regions of the GC where oceanographic conditions are ideal in summertime for the growth of N-2-fixing organisms. These findings suggest that biological N-2 fixation must be taken into account when assessing past and present nitrogen dynamics in this environmentally important region.