Impact of Didymosphenia geminata on hyporheic conditions in trout redds: reason for concern?

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.