Importance of freshwater flow in terrestrial–aquatic energetic connectivity in intermittently connected estuaries of tropical Australia
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δ13C was used to identify seasonal variations in the importance of autochthonous and allochthonous sources of productivity for fish communities in intermittently connected estuarine areas of Australia’s dry tropics. A total of 224 fish from 38 species were collected from six intermittently connected estuarine pools, three in central Queensland (two dominated by C3 forest and one by C4 pasture) and three in north Queensland (one dominated by C3 and two by C4 vegetation). Samples were collected before and after the wet season. Fish collected in the two forested areas in central Queensland had the lowest δ13C, suggesting a greater incorporation of C3 terrestrial material. A seasonal variation in δ13C was also detected for these areas, with mean δ13C varying from −20 to −23‰ from the pre- to the post-wet season, indicating a greater incorporation of terrestrial carbon after the wet season. Negative seasonal shifts in fish δ13C were also present at the pasture site, suggesting a greater dependence on carbon of riparian vegetation (C3 Juncus sp.) in the post-wet season. In north Queensland, terrestrial carbon seemed to be incorporated by fish in the two C4 areas, as δ13C of most species shifted towards slightly heavier values in the post-wet season. A two-source, one-isotope mixing model also indicated a greater incorporation of carbon of terrestrial origin in the post-wet season. However, no seasonal differences in δ13C were detected for fish from the forested area of north Queensland. Overall, hydrologic connectivity seemed to be a key factor in regulating the ultimate sources of carbon in these areas. It is therefore important to preserve the surrounding habitats and to maintain the hydrologic regimes as close to natural conditions as possible, for the conservation of the ecological functioning of these areas.
We thank R. Johnston, A. Penny and R. Baker for field assistance, and A. Revill and R. Diocares for the analyses of stable isotope samples. We would also like to thank the three anonymous reviewers for their comments, which significantly improved the quality of this manuscript. This work was supported by the Cooperative Research Centre for Coastal Zone, Estuary and Waterway Management, CSIRO Marine Research Laboratories and the Townsville City Council, and was part of K.A’s PhD funded by the Foundation for Science and Technology, Portugal. All research procedures reported comply with the current Australian law and received the approval from the Animal Ethics Committee, James Cook University (Ethics Approval A852_03).
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