Project: Oligotrophic ocean metabolism from underwater glider observations

NSF Award Abstract:
Aquatic photosynthesis and respiration rates regulate the flux of organic matter into the ocean’s interior, a process that impacts Earth’s climate by sequestering carbon dioxide from the atmosphere and that provides most of the energy necessary to support the requirements of the organisms inhabiting the dark depths of the ocean. Recent improvements in sensor technology enabled the estimation of photosynthesis and respiration using accurate measurements of the concentration of oxygen dissolved in seawater collected by autonomous underwater vehicles and floats, even in regions of the ocean with low biological activity such as the subtropical gyres.

This project is analyzing data collected in the North Pacific ocean during 7 years using autonomous underwater vehicles in order to obtain an unprecedented number of estimates of metabolic rates for a region of the ocean that is representative of one of the largest oceanic ecosystems. This novel analysis helps constrain the amount of oxygen produced in the sea and improves our understanding of how variations in photosynthesis and respiration influence the flux of organic carbon towards the bottom of the ocean. Two undergraduate students from the University of Hawaii are supported and trained as part of this project. This project is analyzing publicly available observations of temperature, salinity, dissolved oxygen, chlorophyll fluorescence, and optical backscatter collected using underwater gliders in the North Pacific Subtropical Gyre between 2008 and 2014 (>1,000 days of observations). The analyses are used to: (i) quantify in situ rates of gross primary production and respiration in the mixed layer from diel oxygen oscillations, and determine their short-term variability and seasonality; (ii) quantify the net biological oxygen production (both in the mixed layer and in the lower euphotic zone) and determine its seasonality; (iii) quantify annual net community production, from which one can infer the net biological flux of organic C into the ocean’s interior; and (iv) assess how temporal changes in biomass are linked to changes in metabolic rates by comparing oxygen-based metabolic rates with optical proxies of phytoplankton biomass (backscatter and chlorophyll fluorescence). This investigation will better constrain the role of the ocean in regulating Earth’s climate by improved understanding of the mechanisms driving the temporal variability of metabolic rates in the oligotrophic ocean that covers a large fraction of our planet.