Effects of Lateral Hydrological Connectivity on Aquatic Invertebrate Assemblages in Floodplains, Oxbow Lakes and the Main River Channel of the Savannah River, Southeastern USA

Within river corridors, hydrological connectivity drives the local abiotic and biotic processes, shapes ecosystem functioning and promotes biodiversity. However, hydrological cycling is changing due to climate change, and drought is intensifying and becoming more frequent in many regions of the world, making it critical to understand the impacts of drought on ecosystems and biodiversity. This study aimed to explore the effects of hydrological connectivity on aquatic macroinvertebrates in lateral river habitats. We hypothesised that during high hydrological connectivity, macroinvertebrate assemblages would be homogenous, but as drying progressed, assemblages would become heterogenous. Aquatic invertebrate assemblages were sampled during a drawdown period during early, mid and late hydrological stages at six sites on the Savannah River, southeastern USA. Three habitats (floodplain, oxbow lake and the river channel) within each of the six sites were sampled for a total of 113 samples. Macroinvertebrate community structure differed between hydrological stages and habitats. We found differences among macroinvertebrate assemblages between habitats within early, mid and late hydrological stages. Our results indicated some overlap of community assemblages within the river corridor, where community assemblages within the three habitats were mostly unique. Drying did not dramatically affect assemblages within habitats, but there was an increasing degree of separation in assemblages among habitats as drying progressed. Hydrological stage had less control over macroinvertebrate assemblages than expected, and changes attributable to drought appeared to be specific to individual taxa or localised within specific habitats. Spatial heterogeneity and a variety of quality habitats are crucial for providing refuges and maintaining biodiversity. Understanding the influence drought will have on aquatic invertebrates in these ecosystems will be critical to anticipate changes and develop strategies to protect freshwater ecosystems. The loss or alteration of river corridor habitats will have consequences for biodiversity and ecosystem services. A mosaic of habitat types with differing levels of connectivity and multiple transition zones between habitats will provide maximum biodiversity and allow for recovery of taxa after hydrological extremes.