The role of allochthonous dissolved organic carbon in supporting food webs in Australian lowland rivers

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The flow regime of a river plays a critical role in maintaining the health and processes within a river ecosystem. However, due to high water requirements for human needs such as agriculture, many rivers are regulated with dams and weirs. Regulation greatly reduces large flow events and in-channel flows resulting in a severe impact on the natural flow regime of a river, reducing allochthonous carbon loads otherwise mobilised during flows. Environmental flows are a key tool in mitigating the effects of regulation by reinstating a portion of the natural flow regime. However, there is still a knowledge gap between the role of these flows in mobilising allochthonous dissolved organic carbon (DOC) and the subsequent response of the lower food web. Field monitoring on the Namoi River over 2.5 years revealed discharge was positively correlated with increases in nutrient and DOC concentrations with overbank flows mobilising very high levels of all measured nutrients. Phytoplankton and zooplankton also increased during and/or after flow events indicating flows increased food web production potentially via both heterotrophic and autotrophic production. Carbon stable isotope (13C) analysis revealed allochthonous energy was supporting zooplankton communities even during small flow events which changed to autochthonous sources when the river ceased to flow. Two manipulative studies were run using mesocosm and microcosm techniques. These studies showed pulses of DOC and dissolved organic matter (DOM) can greatly boost food web production with mixotrophic algae and zooplankton increasing significantly following terrestrial floodplain leachate additions in the mesocosm experiment. This was further seen in the phytoplankton communities of the microcosm experiment as mixotrophs and ciliates clearly increased following DOC (as glucose) +nutrient additions, making up for any autotrophic biovolume lost due to competition with bacteria. This thesis contributes to the growing body of evidence suggesting allochthonous carbon may be an important energy source for riverine food webs. In particular, it suggests allochthonous organic matter may be considerably bioavailable and increase production via both heterotrophic and autotrophic production and greatly increase resources for higher consumers. Protecting flows that mobilise allochthonous DOM through environmental flows will be highly beneficial for riverine communities, particularly in highly regulated lowland rivers.
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