Abstract
We report significant rates of dinitrogen (N2) fixation in the central basins of the Gulf of California (GC) during July–August 2005. Mixing model estimates based upon δ15N values of particulate matter in the surface mixed layer indicate that N2 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 N2‐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 N2‐fixing organisms. These findings suggest that biological N2 fixation must be taken into account when assessing past and present nitrogen dynamics in this environmentally important region.
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