Summary of Abstract Submission

Abstract Submission No. IO50-09-0004PresentationOral

NITROGEN TRANSFORMATION RATES IN THE ARABIAN SEA ODZ

Bess B. Ward*1, S. W. A. Naqvi2, Allan H. Devol3, Amal Jayakumar1

1 Princeton University, USA
2 National Institute of Oceanography, India
3 University of Washington, USA

ABSTRACT :

The strong permanent Oxygen Deficient Zone (ODZ) in the Arabian Sea is a region of intense anoxic nitrogen and carbon cycling linked to the high primary productivity of the region and the resulting depletion of oxygen by remineralization processes. During the process of anaerobic organic matter decomposition, respiration using alternative oxidants sequentially transforms nitrate to nitrite, nitric oxide, nitrous oxide and molecular nitrogen, resulting in the loss of fixed nitrogen. Anaerobic ammonia oxidation also occurs at these anoxic depths, and the genes associated with all these processes, as well as nitrification and nitrogen fixation, have been detected in the ODZ. Over the past decade and half we have investigated N cycling in the ODZ by quantifying the rates of N transformations using stable isotope methods. These approaches allowed us to describe the N dynamics of the system, including denitrfication, anammox, nitrification and nitrous oxide cycling in terms of rates and spatial patterns within the ODZ. The depth distributions of N cycle processes in the Arabian Sea ODZ are fundamentally similar to those in other major ODZs: Nitrification occurs at depths outside the ODZ. Denitrification and anammox are restricted to the ODZ layer but even within that layer are not always detected. The distribution of N loss rates and their relative contribution to the total N loss appears to depend upon the quality and quantity of organic matter supply. We will draw on results from the three major ODZs (the subtropical Pacific and the Arabian Sea) to summarize the N cycle of the system.