Electronic Theses and Dissertations

Title

Effects of Hydrologic Connection on Physicochemical Conditions and Algal Properties in Backwaters of the Lower Mississippi River

Date of Award

2012

Document Type

Dissertation

Degree Name

Ph.D. in Biological Science

First Advisor

Clifford Ochs

Second Advisor

David H. Reed

Third Advisor

Stephen Brewer

Abstract

In the last 50-100 years, the Mississippi River has been extensively engineered for flood control and navigation purposes. These modifications have substantially decreased the frequency of flooding and degree of contact of the river with its historical floodplain. However, in many areas within the current levee system there remain numerous secondary channels and other backwater sites with which the river is hydrologically connected on a seasonal basis. In this study, the relationships of surface hydrologic connection with the main channel of the Lower Mississippi River to limnological properties and development of phytoplankton biomass in three backwater sites having different patterns of connection to the river were examined. Between November 2007 and September 2009, the depth of the river varied by up to 15 meters. At high river stage, the backwater sites and the main channel were hydrologically connected. As river water flowed into or through these backwater sites, they experienced elevated turbidity, elevated NO3-N and PO 4-P concentrations, and had relatively low chlorophyll concentrations. As the river elevation declined, the backwaters became partly or fully disconnected from the river, resulting in an increase in light as suspended sediments settled out of the water column. Following this decline in turbidity, there was a rapid increase in phytoplankton biomass, and a corresponding decrease in NO3-N and PO4-P concentrations. Comparisons of photosynthetic parameters of algal communities in the Lower Mississippi River floodplain did not suggest photo-acclimation. However, there were differences in phytoplankton community composition between the main channel and backwater sites that depended on the degree of connection. Diatoms dominated in the main channel and backwater sites during connection while cryptomonads dominated in backwater sites during disconnection. Results indicate the importance of these backwater sites for production of phytoplankton organic matter, some of which may be transported into the main channel and contribute to the river food web, and possibly for nutrient immobilization, contributing to a reduction in the flux of nutrients downriver.

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