Canoeing disturbs riverine benthic invertebrates, but effects are small compared to the impact of water power generation

Abstract

Canoeing is increasingly popular and disturbance by canoeists has environmental impacts, e.g. on breeding birds. We investigated how trampling by canoeists affects the benthic invertebrate fauna of a mountain river. In reaches around three weirs, where canoeists are forced to leave the river upstream of the weir and to enter the river in the residual flow section downstream, we sampled benthic invertebrates at the exit and entry sites, and at two undisturbed sites directly up- and downstream of exit and entry. All these were compared to undisturbed, free flowing sections further upstream. Compared to the free flowing sections, we observed a strong effect of stagnation (upstream of the weirs) and of residual flow (downstream of the weirs) on the benthic invertebrate fauna: lower abundances, taxa numbers and shares of sensitive taxa, and higher Saprobic indices. Compared to sites directly up- and downstream, habitat composition and benthic fauna of the exit sites were not modified and characterised by sludge and tolerant taxa. However, at the entry sites downstream of the weirs, benthic invertebrate abundances, taxa numbers and shares of sensitive species were strongly reduced, as compared to undisturbed sites directly up- and downstream. We conclude that trampling and pulling boats causes strong but spatially restricted organismic drift and thus effects on benthic invertebrate abundances at the entry sites. In contrast, it is possible for canoeists to leave the boats upstream of the weirs without causing measurable disturbance. Overall effects of canoeing on benthic invertebrates are small as compared to the effects of stagnation and residual flow.