ASLO/TOS/AGU Ocean Sciences Meeting, Honolulu, HI, February 2006.

Diazotroph abundance and diversity during a large bloom in the oligotrophic North Pacific Gyre

A. A. Fong¹, M. J. Church¹, D. M. Karl¹, R. Lukas¹, C. Mahaffey¹, J. P. Zehr²

¹Department of Oceanography, University of Hawaii, 1000 Pope Road, Honolulu, HI 96822

²University of California Santa Cruz, 1156 High Street, Santa Cruz, CA 95064

The Hawaii Ocean Time-series (HOT) program has monitored biogeochemical inventories and fluxes at Station ALOHA since October 1988. Resolution of unique phenomena such as bloom events are relatively rare due in part to the monthly sampling regime. In July of 2005, a localized region of enhanced chlorophyll in the vicinity of Station ALOHA was observed by ocean color satellite and sampled for a suite of biogeochemical properties. Microscopic analyses revealed the bloom was dominated by a diverse assemblage of diatoms, and unicellular and heterocystous cyanobacteria. Samples were collected for DNA and RNA analyses of nifH gene abundances, expression, and diversity to assess the distributions and activities of nitrogen fixers (diazotrophs) along a transect from the observed center of the bloom. In addition, surface water rates of nitrogen fixation were measured by 15N assimilation into plankton biomass. The nifH gene abundances and diversity of six specific groups of diazotrophs were quantified using quantitative PCR (QPCR). The QPCR analyses suggest significant spatial variability in nifH gene abundances and diazotroph diversity, with unicellular cyanobacteria typically dominating the nifH gene abundances within and outside the bloom feature. Surface water rates of nitrogen fixation were on average 3-8 times greater than rates typically measured at Sta. ALOHA. By merging rate measurements of nitrogen fixation with molecular determinations of the dominant diazotroph assemblages we hope to better constrain the contribution of specific diazotrophs to new production in oligotrophic ocean ecosystems. Interpretation of these data will provide insight into the response and ecological functionality of diazotroph supported blooms in the oligotrophic North Pacific Ocean.