Letelier, R.M., M.R. Abbott, A. White, D.M. Karl¹ and R.R. Bidigare¹

College of Oceanic and Atmospheric Sciences, Oregon State University, ¹School of Ocean and Earth Science and Technology, University of Hawaii

The role of micro-phytoplankton in bio-elemental cycling in stratified oligotrophic waters of the North Pacific Subtropical Gyre

Under stratified conditions oligotrophic pelagic environments are thought to be dominated by pico-phytoplankton assemblages whose growth are fueled by regenerated nutrients. Implicit in this conceptual model is the tight coupling between rates of photosynthesis and community respiration in the upper water-column. Only the aperiodic supply of nutrients to the euphotic zone due to short lived physical perturbations (e.g., deep-mixing events, cyclonic eddies, and the breaking of internal waves) is considered to induce a temporal or spatial imbalance between these metabolic processes, leading to increased algal biomass and the subsequent export of organic matter.

However, over the past 15 years, field and satellite observations in the vicinity of Station ALOHA (22.75° N, 158°W) have documented extensive blooms of micro-phytoplankton during extended periods of well stratified upper euphotic zone. These events appear to contradict the predictions of the above described conceptual model. They are characterized by the dominance of large cell size phytoplankton, especially diatoms and colony-forming cyanobacteria (e.g., Trichodesmium). To date, all identified algal species responsible for these blooms have been shown to regulate their buoyancy and fix di-nitrogen or host symbiotic di-nitrogen fixing bacteria. These characteristics allow the potential use of nutrient sources that are not available to other phytoplankton taxa.

The Hawaii Ocean Time-series scientists have sampled repeatedly these summer blooms and recorded an increase in elemental fluxes during them. In the present study we summarize the ecological and biogeochemical significance that these distinct events have in the NPSG.