AGU Ocean Sciences Meeting, Honolulu, HI, February 2002.

In vitro measurements of net community production in the euphotic zone of the subtropical North Pacific

P. J. Morris¹, P. J. le B. Williams² and D. M. Karl¹

¹University of Hawaii, Department of Oceanography, Honolulu, HI 96822

²School of Ocean Sciences, University of Wales, Bangor, Anglesey, Menai Bridge, LL59 5EY, United Kingdom

It has been argued that oligotrophic ocean areas are not in oxygen balance. Station ALOHA (22 45N, 158 00W) of the Hawaii Ocean Time-series in the subtropical North Pacific was used as an example of an oligotrophic open ocean location to study net community production in the euphotic zone. A free floating array profiling 6 depth integrals from 5 to 150m was used to make 24-hour in situ, light and dark bottle incubations to measure the changes in dissolved oxygen. Direct measurements of net community production and dark bottle respiration were made as well as calculated estimates of gross primary production. The study was started in May 2001 with subsequent monthly measurements to ultimately compile an annual time series. Relative changes in dissolved oxygen concentration were measured using a computer controlled, high precision, Winkler titration with a photometric endpoint detector. Initial results show both net production and net consumption of dissolved oxygen in the upper 25 m while there is a consistent net consumption of dissolved oxygen at depths below 45 m. The calculated gross primary production rates integrated from 0 to 150 m varied from 68 ± 5 to 86 ± 8 mmol O₂ m^-2 d^-1. Net consumption of dissolved oxygen measured at various depths from 45 to 150 m ranged from 0.14 ± 0.04 to 0.60 ± 0.07 mmol O₂ m^-3 d^-1. Depth integrated values of net oxygen flux have shown a net production, net consumption balance (community compensation point) at a range of depths from 25 to 105 m. These findings primarily show profiles that are different from month to month and only with the ongoing time series of this project will the trends of oxygen flux become apparent. Until such a time series is obtained it remains an open question as to whether oligotrophic areas are, or are not, in oxygen balance.