University of Rhode Island - Graduate School of Oceanography, USA
Involved scientist: Vitul Agarwal

Oxygen is a by-product of photosynthesis and key as a tracer for marine primary production. Changes in marine oxygen concentrations reflect changes in the balance of photosynthesis and respiration for phytoplankton communities. In Narragansett Bay (Rhode Island, USA), and elsewhere, net community production and respiration rates are poorly characterized in terms of their magnitude and variability.

We collected seawater from the field twice a week and incubated the bottles in a light and temperature-controlled room. The room temperature was close to the water temperature of Narragansett Bay and the bottles were subjected to a 14hr:10hr light:dark cycle. Each bottle was sealed after it was filled and had functioning and calibrated oxygen and temperature sensor spots on the inner wall. This allowed us to measure oxygen concentrations and water temperature (using non-invasive optical fibres) every 60s for 24 hours by minimizing any temperature changes and after letting the seawater equilibrate to room temperature for a few hours.

The high resolution and the replication of measurements yielded a dataset clearly indicating small changes in oxygen concentration over hours. By measuring the rate of change in oxygen over the light and dark periods, we could then estimate the net community production (light – dark) and respiration (dark only) rates, and then the gross oxygen production, for the phytoplankton community at the time of sampling.