Drift responses of larval blackflies and their invertebrate predators to short-term flow regulation
Meissner, Kristian; Muotka, Timo; Kananen, Irma
Archiv für Hydrobiologie Volume 154 Number 4 (2002), p. 529 - 542
published: Aug 7, 2002
ArtNo. ESP141015404000, Price: 29.00 €
Outbreaks of larval blackflies were documented after the commencement of short-term flow regulation in river Perhonjoki, western Finland. Previous studies have suggested that reduced predation caused by the poor flow resistance of invertebrate predators might be a key mechanism underlying blackfly outbreaks in regulated rivers. We conducted a field survey and laboratory experiments to assess the responses of larval blackflies and the predatory caddis larva Rhyacophila nubila to flow variability. The drift of blackflies and R. nubila increased in response to peaking flows, both in the field and the laboratory. Substrate type (moss vs. bare stone) had no effect on blackflies, whereas Rhyacophila drifted more in treatments with low substrate complexity. Since no adult emergence occurred during our study, benthic densities of blackflies decreased mainly due to increased drift during the peaking flows. In most years, however, short-term regulation is not initiated until after the emergence of the blackflies. The life cycles of the dominant blackfly taxa (Simulium noelleri; Cnephia pallipes) render their populations safe from regulation damage. In these extremely dense aggregations blackfly larvae escape control by invertebrate predators, which themselves are highly vulnerable to peaking flows. Simplified habitat structure and increased food availability set the stage for blackfly outbreaks, which are enhanced by reduced predation pressure. As a management implication, higher densities of predatory invertebrates might be achieved by enhancing the structural complexity of the streambed.