Trichodesmium Blooms and New Nitrogen in the North Pacific Gyre

Abstract

Trichodesmium blooms are ubiquitous phenomena in tropical and subtropical oceanic waters. Due to their diazotrophic capability, these blooms may play a quantitative role in global new production. However, to date it has been difficult to quantify the importance of diazotrophy due to a combination of undersampling of natural populations and a lack of pure culture physiological studies. In August 1989, we encountered a >1 × 103 km2 Trichodesmium bloom in the North Pacific Ocean near the JGOFS Hawaii Ocean Time-Series (HOT) Station ALOHA. Analyses of particulate matter collected from the densest portion of the bloom showed enrichments of 3,375 to 7,787-fold for carbon (PC), nitrogen (PN), adenosine triphosphate (ATP) and pigments (chl a and phaeo) relative to typical non-bloom concentrations; phosphorus (PP) was enriched only 583-fold. Dissolved nitrogen pools (NH4, [NO3+NO2], DON) were also enriched within the bloom, and in the case of NH4 by 27-fold. Budget estimates based upon seasonally-averaged N imports to and exports from the epipelagic zone at Station ALOHA suggest that diazotrophic cyanobacterial blooms may be an important source of new nitrogen to the ecosystem. Two models are presented and discussed in order to account for the initiation of these open ocean blooms, and to explain the uncoupling of N and P concentrations and the selective enrichments of P which are required to favor the growth of diazotrophic microorganisms.