Effect of habitat type on in-stream nitrogen loss in the Mississippi River

Abstract

Eutrophic and hypoxic coastal waters are often associated with high nutrient inputs from riverine systems. For example, nitrogen (N) export from the Mississippi River into the Gulf of Mexico has been identified as an important factor causing eutrophication and seasonal hypoxia. Modelling studies of N flux in large rivers, including the Mississippi River, suggest that much of the N that enters rivers remains in solution and is exported downstream. However, patterns of N cycling in the Mississippi River are complex and vary according to habitat type and season. Here we use spatial habitat data and empirically derived denitrification rates to extrapolate N loss to various reaches in 2,400 km of the Mississippi River from Minneapolis, Minnesota to the Atchafalaya diversion. Our results indicate that 9.5 % of the total N load is lost through denitrification in the river and that reaches containing large areas of impoundments and backwater lakes exhibit elevated rates of N loss. The northern 1,041 km reach of the river contains significant areas of impoundments and backwater lakes and yielded a total N loss from denitrification of 89,172 t N y–1. In comparison, total N loss from the southern 1,352 km open river was 69,872 t N y–1. Our results are consistent with high throughput of N in large rivers, but specify that habitat diversity, channel complexity, and retention time are important factors affecting nitrogen loss in rivers.