Relationships of hydrologic connection with the river on physicochemical properties and phytoplankton dynamics of backwaters in the Lower Mississippi River floodplain

Background/Question/Methods

Spatial and temporal variation in ecosystem properties within a river floodplain is influenced by the degree of hydrologic interaction across the floodscape. The Lower Mississippi River (LMR) predictably overtops its banks during the spring flood, resulting in strong but temporary hydrological connection with backwater sites remaining inside the levee system. These backwater sites include numerous secondary channels and backwater lakes. We hypothesized that the physicochemical and biological characteristics of backwater sites in the river floodplain would be related to the timing and duration of hydrologic connection to the river. We tested this hypothesis by collecting samples over a nearly two-year period from the river and three backwater sites that varied in degree, timing, and duration of connection to the channel. Spatial and temporal variation in algal biomass, algal productivity, and the physicochemical environment, as functions of hydrologic connection, were evaluated. In the laboratory, we tested controlling factors for algal productivity in the river and the backwater sites, by performing photosynthesis-irradiance (P-I) incubations, and nutrient limitation experiments.

Results/Conclusions

The types of biological and physicochemical responses in the three backwater sites were similar, although the timing of changes varied, as they were directly linked to the temporal pattern in degree of hydrologic connection with the river. With strong connection in spring, all three backwater sites experienced elevated turbidity and nutrient (N and P) concentrations, and low chlorophyll. In late spring or in summer, there was a reduction in river water flow through the backwaters, resulting in a decline in suspended sediment load, a deepening of the photic zone, a relaxation of light limitation, and a more than 10-fold increase in algal biomass and production. After prolonged separation from the river, there was a decrease in nutrient concentrations to near zero, and nutrient limitation experiments revealed the development of N-limitation of algal production. As seasonally dynamic hot spots of high primary production, these backwater sites warrant detailed study of their roles in foodweb interactions, energy fluxes, and biogeochemical processes in the LMR floodplain.