The role of floodplain inundation to riverine carbon dynamics and riverine foodwebs is still of much conjecture in Australian lowland rivers. To date, research suggests that metabolism in regulated lowland rivers in south-eastern Australia, such as the Murray River, is predominantly supported by autochthonous production. We investigated changes in water quality, open water metabolism, biofilm development and biofilm stable isotope (δ13C) signatures upstream and downstream of the Barmah-Millewa Forest (BMF) before and after a major floodplain flooding event.
Prior to flooding, all sites had similar concentrations of dissolved organic carbon (DOC), metabolism and biofilm δ13C signatures. During flooding, DOC increased up to 3 fold downstream of the BMF, gross primary production (GPP) increased at all sites, however, community respiration (CR) increased only at the sites downstream of the BMF by as much as 25 fold, with a corresponding decrease in net production (NP).
Biofilm δ13C signatures became depleted by between 4‰ - 7‰ downstream of the BMF following flooding. We suggest that this reflects the incorporation of terrestrial carbon within the biofilm matrix and subsequent fractionation against 13C. Whereas, biofilm signature upstream of the BMF continued to reflected an autotrophic δ13C with potential carbon limitation and subsequent δ13C depletion.
This study indicated that flooding of the BMF provided an increase to the annual energy budget and processing of the allochthonous DOC into the river biofilms during flooding providing a potential pathway for allochthonous carbon to be incorporated into the metazoan food web.