Long-term studies on aquatic Dance Flies (Diptera, Empididae) 1983-1993: Distribution and size patterns along the stream, abundance changes between years and the influence of environmental factors on the community

Abstract

Twenty nine species of aquatic dance flies (Empididae: Clinocerinae and Hemerodromiinae) were recorded from the Breitenbach (Hesse, Germany) between 1983 and 1993. Although most species emerged in spring and summer, Clinocera (Hydrodromia) stagnalis and C. (H.) wesmaeli were trapped in almost every month. A few species of the subgenus Kowarzia of Clinocera emerged in autumn and early winter. Chelifera pyrenaica had two generations per year in the lower part of the stream, but in the upper reach there was only a spring generation. The distribution pattern along the stream of most of the abundant species was maintained over the entire study period. As an exception, Wiedemannia bohemani was abundant in the middle reach of the stream between 1983-1988, but in the lower reach in 1989-1993. As the amount of Particulate Organic Matter (POM) probably increased the abundance of prey organisms, highest numbers were found in areas with high amounts of allochthonous input. Larvae avoided sandy substrates. Many were found in partly submerged moss carpets on stones, at or below the water level. Females of Hemerodromiinae had higher body weights than males, and specimens in localities near the spring were heavier than those from lower sections of the stream, probably as a result of the temperature pattern of the stream. In Clinocerinae the differences between the sexes, and at different sites, were negligible. Ordination of abundance data, as well as environmental data, showed the dominating influence of 'time' and discharge pattern on the community of aquatic dance flies, and the subordinate influence of water temperature, in a time series of 11 years.