Impact of Didymosphenia geminata on hyporheic conditions in trout redds: reason for concern?
Tobias O. Bickel A B C and Gerard P. Closs AA Department of Zoology, University of Otago, PO Box 56, Dunedin, New Zealand.
B Present address: Ecosystem Management, School of Environmental and Rural Sciences, University of New England, Armidale, NSW 2351, Australia.
C Corresponding author. Email: tobias-bickel@web.de
Marine and Freshwater Research 59(11) 1028-1033 https://doi.org/10.1071/MF08011
Submitted: 18 January 2008 Accepted: 3 September 2008 Published: 27 November 2008
Abstract
Didymosphenia geminata (Lyngbye) Schmidt (commonly called didymo) is an invasive diatom and of concern to fisheries managers in North America and more recently New Zealand. Didymo grows in thick mats in several river systems on the South Island of New Zealand, often smothering entire river beds. Salmonid eggs, deposited in gravel nests (redds), depend on constant water exchange across the riverbed to provide oxygen-rich water for development. Thick didymo mats might restrict the flow of oxygen-rich water into spawning gravels, resulting in increased egg mortality and reduced trout recruitment. The present study measured hyporheic hydraulic conditions in trout redds with varying didymo cover in the Clutha River catchment, South Island, New Zealand. Didymo cover had no significant effects on several hydraulic variables (flow into the substrate, hydraulic conductivity and hyporheic oxygen concentration). However, there was a significant difference in the potential surface water–groundwater exchange between sites, suggesting some effect of didymo on hydraulic conditions. Considering the limited number of replicates, the impact of didymo on trout redds in the Clutha River cannot be excluded. The present study highlights the need for further research on the possible effects of didymo on important surface water–groundwater exchange processes.
Additional keywords: didymo, spawning.
Acknowledgements
We wish to thank Aaron Horrel for assisting with the fieldwork. This research was funded by the Clutha Fisheries Trust, Cromwell, New Zealand. Matt Hickey from the Otago Regional Council provided the discharge data for the sites. The manuscript was considerably improved thanks to the constructive comments of three anonymous reviewers.
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