Density, depth distribution and richness of emerging insects (Diptera: Chironomidae) before and after experimental watershed deforestation

Abstract

The primary objective o f this study was to document changes in Chironomidae density, depth distribution, and taxon richness, associated with experimental upland and riparian deforestation of L42, a small boreal forest lake, 250 km northwest of Thunder Bay Ontario. Insects were collected in floating emergence traps (0.28 m[squared]) in July and August 1995 before logging, and in May to September 1997 and 1998 following 74 % and 61 % clearcut logging o f the upland catchment and riparian zone. A total of 738 traps set over twenty-three days collected 4,013 insects from 10 families emerging from lake benthos, with the Chironomidae comprising > 95 % each year. Chironomid density declined after logging. Mean chironomid density (no. individuals - 0.28 m[squared] • d* ± S.E.) was higher in 1995 at the pre-logging sites (12.1 ± 1.2) than in 1997 and 1998 (one and two years following clearcut logging, 7.0 ± 0.7 and 5.4 ± 0.5 respectively). Density also differed between sites among years. Mean May through September emergence was lowest in 1998 at the clearcut with riparian buffer strip treatment (west site) (3.3 ± 0.3) and highest during July and August 1995 at the west site (16.2 ± 2.9). No change in non-chironomid aquatic insect density was detected between timber harvest treatments and years. Chironomid depth distribution was variable among sites, between years and may have been affected by logging. Chironomid density declined after cutting at littoral depths (0.5 and 1.0 m) and increased at sublittoral depths (3.0 and 4.5 m), possibly due to a documented increase in littoral zone aeolian sediment deposition which peaked in 1997. Chironomidae taxon richness decreased after watershed deforestation (21 genera (41spp.) vs. 19 (36) and 16 (32)), 1995,1997 and 1998 respectively. Chironominae was the most abundant subfamily in each year. After logging, the density of Tanypodinae increased and Ofthocladiinae decreased. The ratio of male to female emergence was approximately 1:1 each year. Differences in chironomid community composition could be influenced by voltinism, and potentially to climate.