Enhanced New Production During Winter Mixing: A Missing Component of Current Estimates

Bermuda Institute Of Ocean Sciences (Bios), Inc., St. George'S

Investigators

Abstract

ABSTRACT OCE-0241662 Photosynthetic uptake of CO2 by oceanic phytoplankton and the export of the resulting organic carbon to the deep sea comprise a biological pump (Volk and Hoffert, 1985), capable of extracting globally significant amounts of CO2 from the atmosphere. As a consequence, it is important from the perspective of the global carbon cycle to understand both the present efficiency and the main controlling mechanisms of this important carbon pathway. In the open ocean the biological pump is driven by new production of organic matter (production supported by externally supplied nutrients) and export of that organic matter to depth. Many methods have been employed to estimate new production, with varying degrees of agreement. In the Sargasso Sea, for example, geochemical estimates of new production largely exclude the winter mixing period (because their fundamental assumption are valid only during stratified periods). Biological methods suggest that the pre-stratification period can be as important, in terms of new production, as the remainder of the year. Those biological estimates are poorly constrained and based on sparse data. Because of the enormous spatial extent of subtropical gyres similar to the Sargasso Sea, uncertainty in the rate of new production and organic matter export in those systems leads to large uncertainty in biologically-driven carbon fluxes at the global-scale. Recent data suggest that in the Sargasso Sea, the passage of weather fronts leads to increased new production during the winter mixing period. Some oceanographers believe that these events lead to enhanced nitrate input, followed by a rapid biological response and accumulation of biomass, and an equally rapid export of that biomass to depth. In this project, researchers at the Bermuda Biological Station for Research carry out a process-oriented study of new production and its control during the period before formation of the seasonal thermocline in the BATS/BTM/OFP region near Bermuda. Researchers from Oregon State University, Woods Hole Oceanographic Institution, and the University of South Carolina will also participate in some aspects of the study. This sea-going effort will be conducted during two 30-day cruises (one in 2004 and one in 2005) during the winter mixing period when the passage of these fronts is most common and when few data are available to constrain new production estimates. It will be crucial for this study to sample from a fully weather-capable research vessel, which can stay out and continue operations through most winter storms. The team will use direct measurements of nitrate entrainment, nitrate uptake, phytoplankton community structure change, and dissolved and particulate organic matter export to elucidate the linkages between new production and export production as well as determine the main biological responses to short-term physical forcing. Particular emphasis will be placed on biogeochemically critical phytoplankton groups such as diatoms and coccolithophorids, which can exploit transiently favorable conditions of the kind hypothesized to occur in late winter/early spring and which play a disproportionately large role in organic-matter export in many systems. An understanding of ocean function is no longer important just to practicing ocean scientists. This project will provide information critical for biogeochemical modelers seeking to constrain future predictions of changes in the oceanic biological pump, and will also provide information of interest to students, teachers and the general public. If in fact a significant, and previously unmeasured, amount of new production occurs in subtropical gyres during the winter mixing period, then biological processes in the central ocean gyres play a greater role in the global carbon cycle including regulation of atmospheric carbon dioxide than we recognize at present.