Trophic Conditions and Planktonic Processes of Semi-arid Floodplain Lakes Inundated with Environmental Flows

Abstract

Shallow floodplain lakes are critical components of semi-arid floodplain wetland systems. Delivery of environmental flows that aim to sustain ecological processes of semi-arid floodplain wetlands has enhanced inundation of shallow lakes in inland Australia. To maximise environmental flow outcomes to support floodplain productivity and ecosystem functions, environmental managers would benefit from knowing whether semi-arid floodplain lakes function as a sink or source of atmospheric carbon. We investigated abiotic conditions, and rates of planktonic respiration and primary productivity of phytoplankton during summer under environmental flow conditions in three floodplain lakes of the lower Murrumbidgee River, Australia. All lakes showed mesoeutrophic to hypereutrophic characteristics and significant within- and between-lake variability in abiotic conditions, planktonic processes, and associated carbon balance. Nevertheless, the mean net primary productivity of phytoplankton in the lakes (364-1,674 mg C m-2 day-1) were up to about three times greater than in other semi-arid floodplain wetlands of southeast Australia. Therefore, shallow floodplain lakes in semi-arid regions have great potential to function as a sink of atmospheric carbon through planktonic metabolism during summer. A spatial hierarchical framework for lake functional response to inundation is proposed to support decision-making and to maximise the benefits of environmental flow regimes for floodplain lakes.