Short-term changes in zooplankton density and community structure in response to different sources of dissolved organic carbon in an unconstrained lowland river: evidence for food web support

The zooplankton community changes that may occur with increases in concentration of allochthonous dissolved organic carbon (DOC) are not well understood in unconstrained lowland rivers. We examined in mesocoms over 8 days the short-term responses of river zooplankton to amendments of DOC from two different sources, glucose and leachates of a common riparian tree (Eucalyptus camaldulensis; river red gum) both alone and with inorganic nutrients added. DOC additions with and without nutrients increased heterotrophic respiration and led to significant increases in bacterial biomass. These responses varied between glucose and leachate addition. In treatments with added DOC, zooplankton density significantly increased relative to controls. Some zooplankton genera only responded to the leachate as a DOC source, and community structures significantly varied between the control and the glucose and leachate amendments. Zooplankton are particularly important in lowland river systems as they are key organisms for the transfer of carbon to higher trophic levels and this study indicates that allochthonous DOC has the potential to be an important basal resource to lowland river food webs. This may be particularly important in lowland sections of unconstrained flood plain rivers during and immediately following floods when allochthonous DOC is more available.