Effects of Hydrological Connectivity on the Benthos of a Large River (Lower Mississippi River, USA)

The effects of hydrological connectivity between the Mississippi River main channel and adjacent secondary channel and floodplain habitats on macroinvertebrate community structure, water chemistry, and sediment makeup and chemistry are analyzed. In river-floodplain systems, connectivity between the main channel and the surrounding floodplain is critical in maintaining ecosystem processes. Floodplains comprise a variety of aquatic habitat types, including frequently connected secondary channels and oxbows, as well as rarely connected backwater lakes and pools. Herein, the effects of connectivity on riverine and floodplain biota, as well as the impacts of connectivity on the physiochemical makeup of both the water and sediments in secondary channels are examined. Between June 2014 and August 2016, twenty-six sites spanning a gradient of hydrological connectivity to the main channel of the Mississippi River were sampled. First, macroinvertebrate community structure across the floodplain was analyzed, and a strong association between community types present and frequency of connection was found. Next, the effects of hydrologic connectivity on macroinvertebrate communities in secondary channels was investigated. These results indicate the loss of connectivity through seasonal disconnection of secondary channels from the main channel causes a shift from a lotic to lentic environment, and then a legacy effect, a macroinvertebrate void, when connectivity is restored. Finally, connectivity also affects the physiochemical dynamics of water and sediments within secondary channels, transforming disconnected channels into backwater habitats, characterized by warmer temperatures, lower dissolved oxygen concentrations, and sediments rich in organic matter. Colonization of these new pools and slackwater habitats by lentic macroinvertebrate communities occurs, but both the community changes and changes in the nutrient load and sediment makeup are temporary, only persisting until reconnection to the river channel occurs