Nitrogen fixation in an anticyclonic eddy in the oligotrophic North Pacific Ocean

Mesoscale physical processes (for example eddies, frontal meanders and planetary waves) can play important roles in controlling ocean biogeochemistry. We examined spatial variations in upper ocean (0–100 m) nutrient inventories, N2 fixing microorganism diversity and abundance, and rates of N2 fixation in an anticyclonic eddy near Station ALOHA (22° 45′ N, 158° 00′ W) in the North Pacific Subtropical Gyre (NPSG). In July 2005, satellite-based sea surface altimetry and ocean color observation revealed an anticyclonic eddy with enhanced chlorophyll in the upper ocean in the vicinity of Station ALOHA. Within the eddy, near-surface ocean chlorophyll concentrations were ∼5-fold greater than in the surrounding waters. Inventories of nitrate and phosphate in the eddy were similar to the concentrations historically observed at Station ALOHA, while silicic acid inventories were significantly depleted (one-way analysis of variance, P<0.01). Quantitative PCR determinations of nifH gene copies revealed relatively high abundances of several N2 fixing cyanobacteria, including Trichodesmium spp., Crocosphaera watsonii and Richelia intracellularis. Reverse transcriptase PCR (RT-PCR) amplified nitrogenase (nifH) gene transcripts were cloned and sequenced to examine the diversity of active N2 fixing microorganisms; these clone libraries were dominated by sequence-types 97%–99% identical to the filamentous cyanobacteria Trichodesmium spp. Near-surface ocean rates of N2 fixation were 2–18 times greater (averaging 8.6±5.6 nmol N per l per day) than previously reported measurements at Station ALOHA. These results suggest that mesoscale physical variability can play an important role in modifying the abundances of N2 fixing microorganisms and associated rates of N2 fixation in open ocean ecosystems.