We have been conducting phytoplankton experiments on the back deck of the ship throughout the ecosystem monitoring cruise. Pete Bernhardt (Old Dominion University) and his colleagues are using stable isotopes to examine primary productivity (photosynthesis) and other vital rates of phytoplankton to examine productivity on the shelf. Stable isotopes are elements that do not radioactively decay over time. Therefore they are useful in examining biological rates. Pete is using both carbon (13C) and nitrogen (15N) in his phytoplankton experiments with ocean water collected with the Rosette sampler. To measure primary productivity, he adds sodium bicarbonate labeled with 13C by pipette to bottles of ocean water containing phytoplankton.
In other bottles, he injects nitrogen gas labeled with 15N to measure rates of atmospheric nitrogen uptake. These sealed bottles are placed in incubators (water baths) on the back deck of the ship that have seawater continuously pumped through them. This simulates the water temperature and light levels the phytoplankton would experience in the ocean environment. The bottles are allowed to incubate for a period of time, during which the phytoplankton grow, multiply, and use the stable isotopes Pete added to the bottle. Pete then filters the water from the bottles on board the ship. He will analyze the filters after returning to shore, to determine the rates of primary productivity and nitrogen uptake.