Trichodesmium in the western Gulf of Mexico: 15N 2-fixation and natural abundance stable isotope evidence

Abstract

Surface aggregations of the diazotrophic cyanobacterium Trichodesmium were encountered at nearly every station along a 300-km transect in the northwestern Gulf of Mexico during July 2000. 15N2-fixation rate measurements for all treatments and locations ranged from 1.3 × 10 -4 nmol N trichome-1 h-1 to 2.6 × 10 -2 nmol N trichome-1 h-1, with a mean of 3.4 × 10-3 ± 7.6 × 10-4 nmol N trichome-1 h-1 (mean ± SE, n = 45). Trichodesmium C-fixation, measured by the incorporation of 13C, ranged from 1.3 × 10-2 nmol C trichome-1 h-1 to 4.5 × 10-1 nmol C trichome-1 h-1, with a mean of 1.2 × 10-1 ± 9.3 × 10-2 nmol C trichome-1 h-1 (mean ± SE, n = 44). Both 15N2-fixation and 13C-fixation rates attenuated with decreasing light intensity from a maximum at 50% surface intensity. Areal Trichodesmium 15N2-fixation rates, based on a 10-h N 2-fixation day, enumerated vertical trichome abundance and corrected for estimated in situ light intensity ranged from 47.2 μmol N m-2 d-1 to 118.6 μmol N m-2 d-1, with a mean of 84.5 ± 17.7 μmol N m-2 d-1 (mean ± SE, n = 4). The light natural abundance isotopic signatures of dissolved inorganic nitrogen, particulate nitrogen, and zooplankton nitrogen within these surface populations are consistent with our limited biological rate measurements and indicate that recently fixed N is moving into the food chain to promote secondary production in these nutrient-impoverished waters. Although our sample size was relatively small, natural abundance isotope data indicate that as much as 60% of macrozooplankton C and N was derived from Trichodesmium in the oligotrophic regions of the Gulf of Mexico. © 2007, by the American Society of Limnology and Oceanography, Inc.