Substrate availability and oviposition success of aquatic insects

Reproduction and active dispersal of many aquatic insects takes place during the short-lived winged adult phase, which is thus crucial for establishment and maintenance of populations. Several aquatic insects are known to have specific oviposition behavior, whereby fertilized females land on protruding boulders to attach eggs on the submersed undersides. Such behavior makes these taxa sensitive to the availability of substrates for oviposition, availability depending on the morphological condition of the river as well as discharge. We conducted a field experiment to study the relevance and sensitivity of this reproductive mode for aquatic insect oviposition. We installed artificial substrates (bricks) in stream reaches having different morphologies (channelized, restored, natural). We monitored oviposition on bricks and natural stones throughout the flight season of most aquatic insects, taking parallel samples of flying adults and benthos. Some 24 taxa (20 aquatic insect taxa), dominated by dipterans, used both stones and bricks for oviposition. We observed clear differences in the phenology of egg masses on artificial and natural substrates that were related to changes in substrate availability due to reach specific differences and seasonal fluctuations in discharge. Results are discussed in respect to potential constraints of restoration measures in running waters.