Comparison of Leaflitter Processing and Macroinvertebrate Assemblages in Three Coastal Plain Rivers of Southeast Georgia

We compared leaf-litter processing and the concomitant macroinvertebrate assemblages in main-channel habitats of three Coastal Plain rivers in Southeast Georgia: Altamaha, Ogeechee, and Savannah. From mid-September to mid-November (= fall) and from mid-March to mid-May 2015 (= spring), a total of 192 packs of water oak (Quercus nigra) leaves were retrieved at two-week intervals over an eight-week period. Six study sites (two per basin) were chosen to assess the effects of flow regime, particularly the magnitude of discharge, on leaf-litter breakdown and assemblage structure. During the fall study period, a distinct discharge gradient was observed (average Q = 150.8, 75.3, and 8.2 m3/s for the Savannah, Altamaha, and Ogeechee, respectively). Despite differences in discharge, each basin exhibited similar processing coefficients (average k = -0.013). A distinct gradient was also observed in spring (average Q = 499.3, 235.2, and 72.4 m3/s for the Altamaha, Savannah, and Ogeechee, respectively). Processing coefficients remained similar among basins and to those observed in the fall (average k = -0.013, -0.011, and -0.009 for the Ogeechee, Altamaha, and Savannah, respectively). Fall assemblages were similar among basins based on abundance (PERMANOVA, p = 0.085) and biomass (PERMANOVA, p = 0.133). Midges in the subfamilies Orthocladiinae and Tanypodinae were the most abundant taxa in the Ogeechee and Savannah (17 and 13%, respectively). Oligochaetes comprised 31% of the Altamaha’s total abundance. Spring assemblages were also similar among basins based on abundance (PERMANOVA, p = 0.077) and biomass (PERMANOVA, p = .074). Midges in the subfamily Orthocladiinae were the most abundant taxon in all assemblages comprising 56, 32, and 22% of total abundance in the Ogeechee, Altamaha, and Savannah, respectively. These results suggest that discharge is not a determining factor in leaf-litter processing rates and reinforce the usefulness of the ecoregion concept in delineating macroinvertebrate distributions in these systems.