Long-term changes in the littoral benthos of a Norwegian subalpine lake following the introduction of the European minnow (Phoxinus phoxinus)

Abstract

The littoral benthos of the subalpine lake, Øvre Heimdalsvatn, has been documented in a series of investigations carried out in 1972, 1976, 1985 and 2000. During this 28 year period there have been major changes in the lake’s fish population following the introduction of European minnow into the lake where brown trout was formerly the sole species. In 1972 Ephemeroptera, Trichoptera, Plecoptera and Gammarus lacustris dominated the macrobenthos, constituting 85% of faunal numbers, while Chironomidae and Oligochaeta made up only c. 6%. However, by 1976 chironomids and oligochaetes had increased in relative abundance, while G. lacustris declined. This trend towards a dominance of chironomids and oligochaetes was confirmed in 1985 and 2000, although absolute numbers of Ephemeroptera, Plecoptera and Trichoptera increased in 2000 relative to 1972 values. Gammarus lacustris had a two-year life cycle in Øvre Heimdalsvatn. In 1972 there were significantly more females than males, but by 1976 and through to 2000 there were greater numbers of males. Despite this reduction in females, numbers of juveniles increased, although mortality, probably due to increased predation from minnows, was higher than previously. There have been major changes in the benthos of Øvre Heimdalsvatn over the 28 years from 1972 to 2000. These changes have occurred at the same time as the European minnow has been introduced to the lake and increased substantially in numbers. The effect of the introduction of minnows on the benthic macroinvertebrates of the lake littoral was clearly seen in the major reduction in the population of G. lacustris and the increased proportions of smaller forms, especially chironomids and oligochaetes in relation to Ephemeroptera and Trichoptera.