Fluxes of nutrients and primary production between the main channel and floodplain backwaters of the Lower Mississippi River—Development of a simulation model

Abstract

AbstractConnection between rivers and their floodplain is critical to the function of fluvial systems; however, there has been little research quantitatively examining the dynamics of this interaction for large, alluvial rivers. Critical questions include the following: What are the rates and mechanisms of materials and energy exchange, and in what ways does the exchange impact ecosystem functioning? To address these questions, we built a simple model of a hypothetical reach of the Lower Mississippi River (LMR) containing a single backwater. The model is based on empirical data obtained from the LMR system. Our primary objectives for the model were to assess potential backwater impacts on river nitrate transport and in subsidizing phytoplankton biomass to the main channel. Simulations run over a 10‐year period suggest that on an annual basis, (a) LMR backwaters remove NO3–N, and it would require a temporal mean of 34,400 ha functioning like the model backwater, or 2.8 times the current area of oxbow lakes, to eliminate 100% of the river flux of NO3–N of our study region; (b) it would require inputs of phytoplankton from a mean of 5,242 ha of sites functioning like the model backwater to produce observed river flux of phytoplankton biomass; and (c) backwater function is sensitive to the controlling elevation in linking channels hence subject to management. Although simple, this model is a useful first step in quantifying the significance of river–backwater connectivity on ecological processes of the LMR system.