HOT-75 COMMEMORATIVE SCIENCE SYMPOSIUM

Station ALOHA N-Cycle: The Case for N₂ Fixation

David Karl¹, Luis Tupas¹, Dale Hebel¹, Ricardo Letelier², James Christian³ and John Dore¹

¹School of Ocean and Earth Science and Technology, University qf Hawaii, Honolulu, HI 96822

²College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331-5503

³Department of Oceanography, Dalhousie University, Halifax, NS, Canada B3H 4JI

The process of microbiological N₂ fixation in stratified, low-nutrient oceanic habitats is well-documented, but to date it has been difficult to obtain reliable annual estimates, in part, because of an undersampling of the marine environment at any single location. In most open ocean ecosystems, about 90% of the contemporaneous photoautotrophic production of organic matter is supported by nitrogenous compounds that are locally recycled through heterotrophic food webs. New production, defined as the amount of organic matter available for export from the system at steady-state, is thought to be constrained by the upward vertical flux of nitrate across the thermocline. Consequently, most ecosystem models ignore potential fluxes from N₂ fixation.

Since the beginning of the HOT sampling program in Oct 1988, we have encountered several aperiodic "blooms" of the N₂-fixing cyanobacterium Trichodesmium in the vicinity of Station ALOHA. These dense, near surface water accumulations coincide with periods of slack wind that occur most often in summer months (June-Sept). Consequently, we initially hypothesized that new production and particle export may also be phased with the relaxation of upper ocean turbulence. We now recognize Trichodesmium, both the free trichome and colony morphologies, as a normal but temporally-variable component of the Station ALOHA plankton assemblage.

Direct measurements of N₂ fixation at Station ALOHA have not been attempted, but several independent data sets provide evidence supporting the quantitative significance of this pathway in the open ocean N-cycle. A major consequence of N₂ fixation, is a shift in the nutrient dynamics from N to P limitation. Our evidence for significant N₂ fixation and contemporary P-limitation at Station ALOHA includes: (1) the inability to balance measured particulate N export with NO₃^- import using reasonable rate estimates of eddy diffusivity, (2) stoichiometric mass balances for total dissolved and particulate N and P pools in the euphotic zone and anomalously high (i.e., 20-40) molar N:P ratios especially in the DOM pool, (3) distribution and abundance of the N₂-fixing bacterium Trichodesmium, (4) a systematic net decrease in soluble reactive phosphorus (SRP) with time from a 0-100 m integrated value of 10 mmol P m^-2 in 1988 to < 5 mmol in 1995 and (5) ¹⁵N isotopic balance for sedimenting particulate materials. We currently estimate that up to 50% of the total new production at Station ALOHA during the period 1988-1995 was supported by N₂ fixation.