Many large river-floodplain systems have been disconnected by flow regulation, and the benefits of floodplain inundation to aquatic productivity in regulated river systems remain largely unquantified. Here, we measured and compared primary productivity between the river channel and inundated off-channel areas of a regulated, river-floodplain system in south-eastern Australia – the Edward/Kolety River, and the Ramsar-listed Werai Forest wetlands. Productivity was measured before, during, and after an early summer flow pulse that inundated the forest. We then scaled up these measurements, using spatial models of inundation and spells analysis, to estimate the potential contribution of off-channel inundation to annual primary production across the local network.
Areal rates of gross primary productivity were similar for phytoplankton in river and off-channel environments, but submerged macrophytes and attached algae on the floodplain had rates approximately two times greater than in the water column. When scaled up to the area inundated during the flow pulse, we estimated that off-channel areas were twice as productive as channel environments. The single inundation event (lasting approx. 1 month) may thus have generated a substantial proportion of annual river network production. Our results therefore emphasise the importance of floodplain subsidies to the ecological energetics of large rivers.