Project: The Biological Pump in the Eastern Equatorial Pacific: in situ measurements of Oxygen and Nitrate

NSF Award Abstract:
In the sunlit surface waters of the world's oceans, phytoplankton combine nutrients and carbon dioxide to form organic matter and release oxygen. Some of this surface water biological production ends up in the deep ocean, resulting in a net removal of carbon from the surface ocean and atmosphere. One way to estimate this export of carbon to the deep ocean, also called the "biological pump," is by carefully measuring the amount of oxygen that is produced along with the organic matter. Oxygen dissolved in seawater can be measured very precisely by sensors that can be deployed for years at a time on instrument platforms like moorings, floats, or gliders. In this project, a team of investigators will deploy twelve floats in the equatorial Pacific Ocean to measure ocean profiles of temperature, salinity, oxygen, and nitrate (a nutrient for phytoplankton). The data will be transmitted by satellite and combine with computer models of ocean circulation to understand the biological pump in this productive area of the ocean. The project will support two early-career researchers: a graduate student and a postdoctoral research fellow.

The project will be a combination of in situ measurements of oxygen and nitrate on specially-equipped profiling floats with data interpretation using a high-resolution Global Circulation Model (GCM). The plan is to construct 12 Argo floats with specially-mounted Aanderaa Optode oxygen sensors and ISUS nitrate sensors, and to deploy them in the region between 8 degrees N and 8 degrees S from about 95 degrees W to 140 degrees W; the location where carbon export is greatest in both satellite-based and GCM-based maps of global carbon export. The data will be interpreted in terms of net biological oxygen production, which is stoichiometrically related to Annual Net Community Production (ANCP) and annual organic carbon export, using a Regional Ocean Model System (ROMS) physical transport model with a Biogeochemical Elemental Cycling (BEC)-like ecosystem. Previous NSF research has shown that it is possible to determine the air-sea pO2 difference using oxygen sensors on profiling floats to an accuracy of plus or minus 0.2 % by calibrating the sensors against atmospheric oxygen when the floats surface to transmit data to shore. Since the air-sea flux and net biological production are usually the dominant terms in the upper ocean oxygen mass balance, it has been possible to determine ANCP in subarctic and subtropical gyre locations using a one-dimensional, upper-ocean model. The goal in this project is to determine the ANCP in the strongly advective equatorial region, and compare it to carbon export values determined from satellite-based and GCM-based methods that so far have not been tested with observations.