Strauss, E. A., Richardson, W. B., Bartsch, L. A., Cavanaugh, J. C., Bruesewitz,
D. A., Imker, H., Heinz, J. A., and Soballe, D. M., 2004, Nitrification in the
Upper Mississippi River: patterns, controls, and contribution to the NO3-
budget: Journal of the North American Benthological Society, v. 23, no. 1, p.
1-14.

Abstract

We measured nitrification rates in sediment samples collected from a variety of
aquatic habitats in Navigation Pool 8 of the Upper Mississippi River (UMR) 7
times between May 2000 and October 2001. We also conducted
nutrient-enrichment experiments and analyzed vertical profiles of sediment to
determine factors regulating nitrification. Nitrification rates were relatively high
compared to other ecosystems (ranging from 0-8.25 mug N cm(-2) h(-1)) and
exhibited significant temporal and spatial patterns. Nitrification rates were
greatest during the summer and spring compared to autumn and winter
(ANOVA, p < 0.05) and were greater in contiguous backwater and impounded
habitats compared to main and side-channel habitats (p < 0.05). Regression
analysis indicated that nitrification rates were weakly (r(2) = 0.18, p < 0.0001)
related to temperature and exchangeable NH4+ of the sediment. However,
nutrient-enrichment experiments showed that NH4+ availability did not limit
nitrification in 3 sediment types with variable organic matter. Vertical profiles
of sediment cores demonstrated that oxygen concentration and nitrification had
similar patterns suggesting that nitrification may be limited by oxygen
penetration into sediments. We conclude that temperature and sediment NH4+
can be useful for predicting broad-scale temporal and spatial nitrification
patterns, respectively, but oxygen penetration into the sediments likely regulates
nitrification rates in much of the UMR. Overall, we estimated that nitrification
produces 6982 mt N/y of NO3- or 7% of the total annual NO3- budget.

Keywords

nitrification, Upper Mississippi River, nitrogen cycle, nitrate budget,
Gulf-of-Mexico, aquatic sediments, dissolved-oxygen, organic-carbon,
hyporheic zone, nitrogen, streams, denitrification, availability, ecosystems