Impact of invasive crayfish on water quality and aquatic macrophytes in the Netherlands

Several species of invasive crayfish have become established in the Netherlands, the most recent addition being Orconectes virilis. Since crayfish are known to impact water quality and aquatic macrophytes in areas they invade, this study investigated whether this was also the case for this species under Dutch conditions and if so, whether a crayfish density producing “no effects” could be established. We focussed on the potential impact of O. virilis on water quality variables (pH, electrical conductivity, dissolved oxygen and turbidity), as well as their impact on submerged and emergent macrophytes. In a compartment experiment with different densities of O. virilis, statistically significant effects were observed at crayfish densities of 1.25 crayfish/m² on electrical conductivity, turbidity, submersed macrophyte biomass, and the emergent plant Sagittaria sagittifolia, due to crayfish actively severing plants and physically disturbing the sediment. No statistically significant differences with controls were observed at a density of 0.63 crayfish/m². Since densities of 0.03 to 5 crayfish/m² have been found in different water types in the Netherlands, this indicates that the water quality and macrophyte biomass in Dutch waters are being negatively impacted by invasive crayfish. As a consequence, attempts to reach a good ecological status as required in the Water Framework Directive will be frustrated by the presence of this invader.