Impacts of channel erosion and floodplain disconnection on primary ecological function in the Avon River catchment, New South Wales

Channel incision and widening has been a common response to European settlement disturbance along many rivers of southeastern Australia. These entrenched streams have perched floodplains that remain beyond the reach of the largest flood events. In essence, floodplains are now disconnected from their channels. Biophysical changes related to channel transformation are often evident in the landscape, however relationships between geomorphic processes and ecological functions are rarely quantified.We tested the hypothesis that channel incision and expansion since European settlement has led to a change in the trophic state of a fine-grained meandering river and its floodplain in the Avon River catchment, New South Wales. Paired channel and floodplain sites were selected in an intact upper reach with small channel capacity and an enlarged lower reach of Craven Creek. Morphometric analysis showed significant differences in channel cross-sectional area (<20 to >120 m²), channel bed slope (~0.011 to ~0.003 m m^-1) and roughness (Manning’s n ~0.08 to ~0.025) between the two reaches. Estimates of bankfull discharge increased from ~55 to ~831 m³ s^-1 between the intact reach and the enlarged reach, resulting in a higher threshold for overbank flooding of >1:100 year ARI event for the enlarged lower reach compared with a <1:5 year ARI event in the intact upper reach.Gross primary productivity (GPP; µg C L^-1 h^-1) of phytoplankton and planktonic respiration (PR; µg C L^-1 h^-1) were determined for channel and floodplain habitats by measuring changes in dissolved oxygen (DO) concentrations of water in biological oxygen demand bottles at the beginning and end of 24 hour in situ incubation experiments. For the intact upper reach, PR exceeded GPP in the channel (GPP 3.02 ± 1.55; PR 9.85 ± 3.12; GPP/PR 0.46 ± 0.24) and on the floodplain (GPP 13.6 ± 4.78; PR 39.43 ± 1.52; GPP/PR 0.35 ± 0.12). For the enlarged lower reach, PR also exceeded GPP in the channel (GPP 7.4 ± 3.31; PR 18.11 ± 2.87; GPP/PR 0.4 ± 0.18) and on the adjacent floodplain (GPP 9.97 ± 2.72; PR 48.91 ± 4.51; GPP/PR 0.2 ± 0.05).Overall, both the intact and enlarged reaches were dominated by respiration (consumption of oxygen and production of carbon dioxide), although the channel sites were more productive than the floodplain sites (evidenced by higher GPP/PR ratios in the channel). However the heterotrophic character of the two reaches was different, with the enlarged downstream reach having higher DO concentrations, GPP and PR. In this case, geomorphic and biological processes have a positive correlation, with channel incision and enlargement and floodplain disconnection leading to greater primary productivity and respiration in the system. This is particularly the case for the perched floodplain in the downstream reach that is acting as the most significant source of carbon. Together, these processes play a critical role in carbon cycling and transformation in channel and floodplain habitats.