Abstract
Rivers around the world are threatened by altered flow due to water resource development. Altered flow can change food webs and impact riverine energetics. The Fitzroy River, in northern Australia, is targeted for development but uncertainty remains about the sources of carbon supporting the food web, particularly in the lowlands—the region most likely to be impacted by water extraction. This study used stable isotopes to investigate if algal biofilm is the main carbon source sustaining fish in lowland habitats. We also sought evidence that large-bodied migratory fish were transporting remote carbon around the system. Our results revealed that local algal biofilm carbon was the dominant source of energy sustaining fish in wet season floodplain habitats, but that fish in main-channel pools during the dry season were increasingly dependent on other carbon sources, such as leaf litter or phytoplankton. We found no evidence that large-bodied fish were transporting remote carbon from the floodplain or estuary into the lower main-channel of the river. We recommend that water planners take a precautionary approach to policy until sufficient food web evidence is amassed.
Highlights
Rivers around the world are threatened by altered flow due to water resource development
Food web studies that use stable isotopes to trace the flow of energy through the food web are useful for understanding riverine energetics because they can identify the sources and locations of energy that underpin consumer p roduction[15]
We focused on two main questions, (1) is algal biofilm the main carbon source supporting food webs in the lowlands of the river? and (2) is there evidence that large-bodied migratory fish are transporting carbon around the system? We analysed data sets collected in the lower river that contained 17 of the 23 known fish species in the river, including Lates calcarifer, Neoarius graeffei, and Macrobrachium spinipes
Summary
Rivers around the world are threatened by altered flow due to water resource development. Previous food web research undertaken in the upper reaches and tributaries of the Fitzroy River found that isotopic ratios of carbon 13C/12C in dry season pools were closely coupled with values of local algal biofilm, indicating that this was the dominant energy source sustaining fish production[16,17]. This suggests that floodplains do not deliver an important energetic subsidy to the river, in contrast to other tropical rivers in A ustralia[18,19,20] and e lsewhere[21,22]. If the findings of previous studies are not broadly applicable, water policy may fail to protect key flow components that maintain primary production and higher order consumers (e.g. fish and prawns)
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