Daniel Clark

Physical and Biological Sources and Sinks of Nitrate in the Canary Basin

Thesis Approved April 1997

In the low-nutrient environment of the oligotrophic ocean, new production is thought to be driven by physical mechanisms that transport nitrate to the euphotic zone. This study presents a detailed analysis of the supply to and the uptake of nitrate from the euphotic zone. ,p. During the 1993 North Atlantic Tracer Release Experiment (NATRE) coincident biological and physical measurements were performed. Biological measurements included new primary production estimates and nutrient concentrations. Physical measurements included hydrographic, current velocity and microstructure measurements. These measurements were used to test the hypothesis that: Advective and turbulent fluxes of nitrate balance the uptake of nitrate in the euphotic zone of the study region.

The fluxes considered include both the horizontal and vertical advective and turbulent fluxes of nitrate as well as the uptake of nitrate by new production. The dominant sources of nitrate to the euphotic zone were found to be the vertical component of both the diapycnal and the along-isopycnal fluxes. Note that the vertical component of the along-isopycnal fluxes has been neglected in previous studies. The nitrate uptake measurements were adjusted to take into account biases inherent in our nutrient uptake estimates. As a result, the new production estimates are much lower than previous estimates. A comparison of the sources and sinks of nitrate in the euphotic zone leads to a balance within the uncertainties of the estimates. The results of this study show that the vertical turbulent fluxes meet the demands of new production.