U.S. Environmental Protection Agency
EPA Science Advisory Board
Background Information

Document NameNitrification induced inhibition of heterotrophic bacterial production in the hypoxic region of the Mississippi River plume.
Document AuthorPohlman, J.W.
Campos, R.
Pohlman, A.M.
Johnson, K
Cifuentes, L.A.
Coffin, R.B.
Short DescriptionAquatic Sciences Meeting, ASLO, Feb 12-16, 2001 Albuquerque, NM
CategorySubgroup 1: Characterization of the Cause(s) of Hypoxia
Publication Year2001

Abstract: The role of nitrification in the carbon cycle and oxygen budget in the hypoxic region of the Mississippi River plume (Nov 97 & Jul 98) was investigated. Nitrification inhibitors (sodium chlorate, allythiourea and methyl fluoride) were used to determine the effect of nitrification on heterotrophic bacterial production, dissolved inorganic carbon production/uptake, oxygen utilization and nutrient cycling. Nitrification was observed at each station sampled in July 1998. The means by which we identified such activity, however, was not expected. Production in nitrification-inhibited samples was twice that of the uninhibited samples, which indicated nitrification might have inhibited heterotrophic production by as much as 50%. We suggest that nitrification diminished the pool of reduced dissolved inorganic nitrogen (DIN, e.g. ammonia) available to heterotrophic bacteria, thus forcing the heterotrophic community to acquire nitrogen by less energetically favorable nitrate assimilation. An oxygen budget calculated for one of the stations in July suggests that nitrification accounted for 83% of the oxygen demand, but such a demand could not have been sustained by the measured bacterial production rates.