(J.P. Montoya and F. Lipschultz)
Denitrification (respiration of oxidized nitrogen to dinitrogen gas) results in the loss from the biosphere of combined nitrogen, a critical nutrient for plant growth. Locally, the magnitude of denitrification affects the rate of decomposition of organic matter in the water column, thereby altering the balance between the amount of carbon that reaches the deep ocean and the amount of CO2 available for ventilation back to the atmosphere. On a global scale, the movement of atmospheric CO2 into the ocean as "new" production is thus directly coupled to the biogeochemical cycling of nitrogen. This project will provide the first direct measurements of the rate of N2 production by denitrification in the ocean and will focus on the environmental factors that link export of surface production to respiratory and decomposition processes within low oxygen zones typified by the waters of the Eastern Tropical North Pacific (ETNP) off the coast of Mexico. This project focuses on two major research questions that are crucial to understanding denitrification and its connections to the cycling of carbon:
1) What are the primary environmental and biological factors that control denitrification in marine systems, and what is the functional response of denitrifiers to these factors? Oxygen concentration and the availability of labile organic matter are known to strongly affect denitrification, but we currently have only a qualitative understanding of the role of these factors in controlling the rate of denitrification in pelagic marine systems.
2) Does the rate of denitrification show significant spatial variability within the ETNP or are rates fairly constant over the region? Surface productivity and subsurface respiration exhibit similar mesoscale features, suggesting tight linkage between the nitrogen cycle in the subsurface waters where denitrification occurs, and the carbon cycle in the overlying surface waters where primary production takes place.
The ETNP is one of three major areas of the world ocean where O2 levels are sufficiently low for denitrification to occur, and it is reasonably well-studied and accessible, simplifying the task of designing and carrying out a realistic field program. Shipboard experiments will include a new, high-sensitivity tracer technique to measure production of N2 directly in short-term incubations. The isotopic composition of N2O will also be measured to provide an accurate measurement of denitrification and to identify whether nitrification or denitrification controls the in situ concentration of this important trace gas. Other rates to be investigated include: a) nitrate respiration
, which can account for a significant amount of organic matter decomposition; and b) NH4+ uptake, which appears to be a major sink for NH4+ produced during decomposition of organic matter. The effect of organic inputs and of variations in O2 concentration on rates of denitrification will be tested in perturbation experiments. These detailed measurements will be
carried out at six "process" stations in the ETNP during two cruises (spring and fall 1997). In addition, denitrification will be assayed at a series of hydrographic stations to identify mesoscale variability and coupling to surface productivity. This study will produce the first direct measurements of spatial and seasonal variability in denitrification activity within one of the major low O2 environments in the world ocean.