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Current CSCOR-Funded Research in Northern Gulf of Mexico 

FY 2008 Projects

Nitrogen, Microbial Food Webs, and Hypoxia

Investigator:  Wayne S. Gardner

Institution:  University of Texas at Austin Marine Science Institute

Abstract/Project Summary:

Nitrate input from the Mississippi-Atchafalaya Basin is the major factor causing large-scale summer hypoxia in the northern Gulf of Mexico (NGOMEX). The effects of new nitrogen (N) on the Gulf of Mexico coastal ecosystem are compounded by the fact that a portion of the nitrate-N assimilated by phytoplankton is regenerated as ammonium (NH4+), which supports food web dynamics by providing available N to microorganisms.

Nitrogen occurs in a variety of oxidation states in the Mississippi River plume (MRP), but its dynamics are not well understood. Rates of fundamental microbial N transformations, such as nitrification (an O2 sink), denitrification (a N sink), N-fixation (a N source), and dissimilatory NO3- reduction to NH4+ (DNRA; a N link), have not yet been measured comprehensively in the MRP. As part of the microbial food web (MFW), protistan grazers (nano- and microzooplankton) consume significant amounts of primary and bacterial production in NGOMEX, but their role in oxygen consumption and N cycling remains to be examined. Studies of N and MFW dynamics relative to oxygen uptake are needed to assess mechanisms and sites of oxygen removal and understand the development of summer hypoxia in shallow regions. Our goal is to provide mechanistic information on N cycling, MFW structure and dynamics, and oxygen depletion in the MRP and thereby support development of accurate models concerned with hypoxia development.

We propose conducting process-rate experiments at seasonal intervals in selected NGOMEX regions to:

(1) Identify the presence, mechanisms, and rates of key N-transformations and O2-uptake.

(2) Examine their temporal and spatial patterns.

(3) Evaluate the structure and dynamics of microorganisms responsible for these processes.        

(4) Develop a conceptual model describing hypoxia development in NGOMEX.

Mid-salinity and other stations in and outside the “dead zone” will be selected based on data from O2 and nutrient profiles. Rates of ammonium regeneration, nitrification, and O2 uptake at selected depths in the water column will be compared to nutrient fluxes and rates of sediment O2 consumption, denitrification, N-fixation, and DNRA at the sediment-water interface. Data produced by this study, combined with results obtained by other investigators, in related monitoring and modeling studies, will provide information needed to develop mechanistic models, and lead to a comprehensive and dynamic picture of hypoxia development in the NGOMEX region.