Carbon and nutrient subsidies to a lowland river following floodplain inundation

Despite the perceived importance of floodplain inundation to the functioning of lowland rivers, there is limited understanding of the contribution that floodplains make to the main river channel during floods. In 2010, substantial
flooding occurred throughout south-eastern Australia, which provided an opportunity to quantify the export of biological material and nutrients from a floodplain back in to the main river channel. We quantified the amounts of zooplankton, phytoplankton, dissolved organic carbon and nutrients within the main river channel of the River Murray immediately upstream of the Barmah–Millewa Forest, and at two sites immediately downstream of the forest during two flood events in July and October of 2010. Results demonstrated that although a smaller flood event in July did not contribute substantially to
an increase in the measured parameters, a much larger flood in October contributed 0.4 tonnes (t) of phytoplankton; 7 t of zooplankton and 300 t of dissolved organic carbon. This suggests that small floods will provide minimal resource subsidies back into the main channel after the cessation of flooding. In comparison, larger floods that result in large volumes of floodplain water returning to the river will provide substantial subsidies of terrestrially derived resources.