Rates of primary production, community respiration, and net community production in the ocean are crucial underpinnings to understand the biological pump of carbon from the atmosphere to the deep sea. However, the inherent deficiencies with bottle incubation methods, especially the mismatch between the timescales of bottle incubations and the response of biological communities to changing conditions in the water column, has fueled an ongoing debate whether oligotrophic regions of the ocean are net autotrophic or heterotrophic. Since the in vitro techniques can miss episodes of high productivity, the inherent potential bias in bottle incubation methods is towards heterotrophy. A newly developed method, the oxygen isotope method (OIM) has the advantage of not requiring bottle incubations and thus is not sensitive to the in vitro versus in situ uncertainty inherent to the 14C and O2 bottle incubation methods traditionally used to estimate primary production and net community production in the ocean. Second, the OIM yields rates that integrate over the 10-20 day residence time of O2 in the mixed layer which means that the OIM is much less likely to miss primary production events compared to incubation methods.
In this study, a researcher from the University of Washington will use the OIM to improve our understanding of the temporal variability in primary production and net community production, and test the hypothesis of oceanic heterotrophy by estimating monthly rates of in situ gross primary production, community respiration, and net community production off Hawaii at station ALOHA using measurements of the natural abundance of oxygen isotopes and the ratio of dissolved O2/Ar gases. Based on the investigator's initial oxygen isotope and O2/Ar measurements at station ALOHA, in situ gross primary production measured by the oxygen isotope method was on average more than twice the 14C-based estimates of primary production. Annually integrated in situ gross primary production and net community production rates at ALOHA are therefore expected to exceed the in vitro rates of gross primary production and net community production and indicate net autotrophy. If so, this would be positive test of the hypothesis. Along with the OIM measurements, the PI will compare the OIM-based rates of gross primary production and net community production to independent primary production estimates determined from 14C bottle incubations, Fast Repetition Rate Fluorometry measurements, a bio-optical model, a continuous record of surface layer in situ O2 measured by a moored O2 sensor and satellite-based estimates of variability in chlorophyll and primary production to provide a complete intercalibration with extant techniques for primary production measurements.
Concerning the broader impacts, the proposed research will improve estimates of the ocean's biological pump, which if it should change in response to global warming, would affect the rate of atmospheric CO2 build-up. The proposed research will add a significant dataset of monthly estimates of primary production and net community production in the subtropical N. Pacific that can be used by the broad oceanographic community and compared to the decade-long dataset of 14C-primary production measured at ALOHA. There will be both undergraduate and graduate student involvement in the project. Equipment infrastructure will be enhanced through the automation of the isotope sample preparation system.
