Abundance, size distribution, and predation efficiency of Damselfly larvae (Zygoptera) in the HUMEX Lake Skjervatjern 1989–1992

Abstract

Damselfly (Zygoptera) larvae were studied in the littoral zone of the humic Lake Skjervatjern in western Norway. In 1988, the lake was divided by a plastic curtain into a control half (B) and an experimental half (A). Half A and its catchment have been treated with artificial acid rain since October 1990. Damselfly larvae were sampled in July of two consecutive years before (1989, 1990) and two after (1991, 1992) the start of the acidification. By July 1992, the acidification had caused a small (<0.1 unit) decrease in pH, and in both lake halves the average pH was about 4.6. Coenagrion hastulatum (Charpentier) was the species most frequently caught on both sides throughout the study. In 1991, the first year after acidification, Coenagrion hastulatum, Lestes sponsa (Fabricius) and Enallagma cyanthigerum (Charpentier) were significantly less abundant in the acidified half than in the control half. These differences appear, however, to be due to high abundances on the control side during that year, rather than to lowered abundances on the acidified side. In 1992, the second year after the start of acidification, there was only one significant difference in species abundance between the lake halves: P. nymphula (Sulzer) was found only on the control side. Predation experiments using Heterocope saliens (Lilljeborg) (Copepoda) as prey gave no support for the hypothesis that the sparsely occurring P. nymphula was a less efficient predator than the most common species, C. hastulatum. Both zygopterans had lower feeding rates and longer handling times when feeding on H. saliens in water from the acidified side as compared to when the prey and water were taken from the control side. The results indicate possible food web alterations despite only marginal effects on water chemistry.