The effects of dams on longitudinal variation in river food webs

Abstract

We examined the effects of two dams on longitudinal variation of riverine food webs using stable isotope and gut contents analyses along four rivers in the Hunter Valley in eastern Australia. Longitudinal ¹⁵N enrichment was observed in most invertebrate taxa and food sources but significant longitudinal variation was rare for ¹³C, and composition of gut contents of invertebrate taxa did not vary significantly with longitudinal position. Most invertebrates and food sources were more ¹⁵N-enriched at sites immediately downstream of the dams than expected from their upstream longitudinal position, a result not mirrored by gut contents and ¹³C. Enrichment of ¹⁵N downstream may be attributed to altered water quality as a result of impoundment but further research is necessary to elucidate whether physico-chemical riverine processes or trophic mechanisms are responsible. Our observations regarding the influence of dams on isotope ratios are contrary to the few existing studies, suggesting the small volumes relative to annual inflows of dams in the present study limit downstream impacts by maintaining aspects of flow variability.

Keywords:

• river regulation
• tail waters
• dams
• water resource management
• aquatic macroinvertebrates
• stable isotopes

Acknowledgements

We thank Mark Dahm, David Hoey, David Ryan and Mark Simons for field assistance, and Rene Diocares at Griffith University for analysing the stable isotope samples. Dr Darren Ryder and anonymous reviewers are thanked for comments on the manuscript.

Funding

This study was jointly funded by the Land and Water Australia Innovation Grant Scheme and the New South Wales Offices of Water and Environment and Heritage.