Effects of Flow Regime and Macroinvertebrates on Leaf Decomposition Rates in Three River Basins of the Southeastern Coastal Plain

River systems within the Southeastern Coastal Plain are characterized by low gradients, broad floodplain connections, and limited autochthonous production, and thus derive the majority of their energy from allochthonous sources. The decomposition rate of this imported organic matter is a function of both physical and biological factors. Abiotic factors such as flow rate, water temperature, and pH impact decomposition rates directly (e.g., via physical abrasion and fragmentation) and indirectly, by influencing biotic responses (e.g., microbial activity). Via the consumption, incorporation, and transformation of organic matter, benthic macroinvertebrates play an essential role in stream ecosystem structure and function and, along with aquatic fungi and bacteria, are the major processors of leaves entering the stream. To assess the effects of flow regime, particularly the magnitude of discharge, and colonizing invertebrates on leaf-litter decomposition rates, packs of senesced leaves from Quercus nigra, a red oak native to the southeastern US, were deployed at six sites across three river basins of the Southeastern Coastal Plain (Savannah, Altamaha, and Ogeechee). A total of 96 leaf packs containing 20 g (± 0.15 g) were deployed in mid-September 2014 and retrieved at two-week intervals over an eight-week period. During each deployment and retrieval, a YSI ProPlus multi-parameter water quality meter was used to measure temperature, pH, dissolved oxygen, and specific conductance. Average water temperatures were 19.3, 21.3, and 17.7 °C for the Savannah, Altamaha, and Ogeechee, respectively. Average pH was 7.8 on the Savannah, 8.0 on the Altamaha, and 7.7 on the Ogeechee. The mean level of dissolved oxygen on the Altamaha was 8.1 mg/L, while mean dissolved oxygen on both the Ogeechee and the Savannah was 7.7 mg/L. Mean specific conductance was 109, 154, and 125 µS/cm for the Savannah, Altamaha, and Ogeechee, respectively. During the period of study, we also observed average discharges of 150.8, 75.3, and 8.2 m3 on the Savannah, Altamaha, and Ogeechee rivers, respectively. We predict that decomposition rates will be positively correlated with river discharge, and thus the Savannah will yield the greatest leaf mass losses. Furthermore, we hypothesize that the functional composition of colonizing invertebrate communities will also differ among river basins due to variation in the physicochemical characteristics of each basin