Net-building behaviour,tolerance and development of two caddisfly species from the river Rhine (Hydropsyche contubernalis and H. pellucidula) in relation to the oxygen content

Summary Tolerance, growth and development of H. contubernalis and H. pellucidula were analyzed at different O₂-concentrations to determine the species specific potency amplitude in relation to O₂-concentration. In addition netbouilding behaviour was compared at different O₂-concentrations and current velocities. Under anaerobic conditions slightly greater tolerance was observed in H. contubernalis than in H. pellucidula. But at a higher O₂-concentration H. contubernalis showed a distinctly higher O₂-deficiency tolerance than H. pellucidula in relation to net-building activity, growth and development. In addition, more than 50% of H. contubernalis instars built capture nets with regular meshes at an O₂-saturation of only 30% (15°C). High growth rates were also found at 50% O₂-saturation and more than half the larvae developed into imagos. In addition to diminished net-spinning activity the mean growth rate started to decrease at an O₂-saturation of 70% to 38% and the development into an imago was not possible in H. pellucidula. Because H. contubernalis corresponds to a regulative type, being able to maintain a high level of netbuilding activity, growth rate and development over a wide range (50% to 100% O₂-saturation). On the other hand H. pellucidula corresponds to a conform type, because physiological efficiency decreases rapidly with falling O₂-concentration, so that development into an imago is no longer possible below 85% O₂-saturation. Positive correlation between net-spinning activity and tested current velocity between 10 cm/s and 25 cm/s (15°C) was found in H. pellucidula. H. contubernalis showed no change in net-building activity, over this range of current velocity. H. contubernalis can be classified as an euryoecious species with a great potency amplitude in relation to O₂-concentration and current velocity. The high tolerance to O₂-deficiency conditions probably allowed the early recolonization of the Mittel-and Niederrhein when a distinct O₂-deficit (mean 29%, max 56%) could still be measured. H. contubernalis can be considered a bioindicator of O₂-deficiency situations. Undisturbed development of H. pellucidula in this section of the river requires a further improvement in O₂-content.