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

Station ALOHA blooms: Biogeochemical characteristics of a large plankton bloom in the oligotrophic North Pacific Ocean

D. W. Sadler¹, K. M. Björkman¹, M. J. Church¹, T. M. Clemente¹, S. E. Curless¹, D. G. Foley², A. A. Fong¹, L. A. Fujieki¹, E. M. Grabowski¹, T. K. Gregory¹, D. M. Karl¹, R. M. Letelier³, P. J. Lethaby¹, R. Lukas¹, C. Mahaffey¹, S. Maenner⁴, P. M. McAndrew¹, C. L. Sabine⁴, F. Santiago-Mandujano¹, B. V. W. Watkins¹

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

²Joint Institute for Marine and Atmospheric Research, 1352 Lighthouse Ave., Pacific Grove, CA 93950

³College of Oceanic and Atmospheric Sciences, Oregon State University, 104 COAS Admin. Bldg., Corvalis, OR 97331-5503

⁴Pacific Marine Environmental Laboratory, National Oceanic and Atmospheric Administration, 7600 Sandpoint Way NE, Seattle, WA 98115

A central objective of the Hawaii Ocean Time-series (HOT) program is to understand how physical processes influence ocean biogeochemistry. Integration of satellite remote sensing technologies and moorings with monthly shipboard measurements is beginning to provide insight into both spatial and temporal dynamics in upper ocean processes. Using satellite derived ocean color we observed and then sampled a ~200 km wide, spatially coherent region of enhanced chlorophyll concentration in the immediate vicinity of Station ALOHA. Analyses of satellite derived sea surface altimetry and surface currents suggested the feature was associated with an eddy traveling south-west towards Station ALOHA. To assess the influence of such events on upper ocean biogeochemistry we conducted a transect from the approximate center of the region of enhanced chlorophyll towards its western edge. Samples were collected from the upper ocean (0-125 m) for determination of soluble reactive phosphorus (SRP), nitrate+nitrite (N+N), silicic acid (Si) and dissolved inorganic carbon concentrations, as well as measurements of plankton community structure based on flow cytometry. Generally these measurements revealed that depth-integrated inventories of SRP and N+N were elevated relative to their respective seasonal climatologies at Station ALOHA, driven by an apparent shoaling of the nutricline associated with the eddy feature. A moored pCO₂ sensor clearly showed the bloom passing through Station ALOHA and revealed this feature to be an unseasonable sink for CO₂. Based on microscopic analyses, the bloom consisted of large diatoms supported in part by increased abundances of nitrogen-fixing bacteria. Similar diatom-diazotroph blooms appear to be a regular occurrence during the summertime months throughout the subtropical North Pacific. As a result, understanding the spatial characteristics of this eddy may provide insight into the physical and biogeochemical conditions that favor the regular, summertime accumulation of similar plankton assemblages in the subtropical North Pacific Ocean.