Speakers

Clifford A. Ochs, PhD

My lab has been conducting ecological and biogeochemical research on the Lower Mississippi River since about 2003. Major research emphases include (1) spatial and temporal patterns across the river system in plankton biodiversity and productivity, (2) the roles of hydrologic connectivity with floodplain aquatic habitats on nutrient cycling and sequestration, and in exchanges of materials with the river channel, and (3) relationships of flow and benthic community structure. I love any excuse to visit New Orleans.

Presentation Description

The Mississippi River is a superhighway for nutrient transport, leading to Gulf of Mexico deepwater hypoxia. Efforts to address this problem appropriately focus primarily on reducing nutrient inputs to the river. A supplementary approach might be to enhance conditions that promote elimination and/or trapping of nutrients within the river corridor, preventing them from further downstream flow. One way in which dissolved N can be eliminated from an ecosystem is by denitrification, the anaerobic microbial respiratory conversion of dissolved NO3 to a gaseous form (N2O or N2). We hypothesized that lakes of the extant LMR floodplain (i.e. within the batture) are excellent sites for denitrification to occur because they are highly productive, rich in NO3 by their connectivity to the river, and maintain anoxic hypolimnetic conditions for an extended period. To address this hypothesis, between May and October, 2017, and again in 2022, we conducted an intensive monitoring study examining denitrification rates in Desoto Lake in Coahoma County, MS, a large oxbow lake in the batture. Water samples were collected, and a suite of chemical and biological measurements made, approximately every 7-14 days, mostly along the longitudinal axis of the lake. Denitrification rates were estimated by measuring changes in N2 concentrations over time at different depths in the water column, and from lab-incubated sediment cores. Additionally, for comparison with the pattern observed in Desoto Lake, on a one-time basis we measured nutrient and N2 concentrations in water samples from a set of other lakes in the batture extending over a river distance of 560 km. Results from Desoto and all other lakes indicate that hypolimnetic denitrification is seasonally active in these sites during spring-summer stratification. However, with the current configuration and hydrology of the system, this process does not appear to remove a high proportion of N compared to river transport, especially on an annual basis.

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