Sensitivity of stoichiometric ratios in the Mississippi River to hydrologic variability

Abstract

The ratio of key elements such as nitrogen, phosphorus, and silica determines nutrient limitations that are important to regulating primary productivity and species composition in aquatic ecosystems. The flux of these nutrients in streams, as dissolved constituents or as particulate matter, is sensitive to variability in flow conditions. Most previous research on nutrient flux and hydrologic variability has focused on the response of individual elements, especially nitrogen, to changes in flow over time. This study examines how the ratios of total nitrogen to total phosphorus (N:P) and total nitrogen to dissolved silica (N:Si) respond to hydrologic variability in the Mississippi-Atchafalaya River Basin. A doubling of the discharge by the Mississippi and Atchafalaya Rivers to the Gulf of Mexico is found to increase the N:P by 10% and the N:Si by 4%. Analysis of data from upstream stations indicates that the N:P increases with discharge in subbasins with intensive row crop agriculture and high fertilizer application rates but is less predictable in other subbasins. Conversely, the response of N:Si to discharge does not vary predictably with the land use characteristics of the subbasin. The response of the nutrient ratios to variability in flow may be linked to the different sources and sinks of each nutrient, as well as the difference between the dominant transport pathways of each nutrient. High-resolution data and models that describe the dissolved and particulate nutrient cycling are needed to assess the relative contribution of different drivers to these observed patterns and to identify the response of nutrient ratios to hydrologic variability under future land use and climate change. ©2014. American Geophysical Union. All Rights Reserved.