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Keywords:

  • Aquatic biodiversity;
  • biogeography;
  • Chironomidae;
  • lake temperature;
  • lake zoobenthos;
  • mountain lakes;
  • Mount Kenya;
  • Rwenzori;
  • taxonomy

Abstract

Aim  Anthropogenic climate change is expected to result in the complete loss of glaciers from the high mountains of tropical Africa, with profound impacts on the hydrology and ecology of unique tropical cold-water lakes located downstream from them. This study examines the biodiversity of Chironomidae (Insecta: Diptera) communities in these scarce Afroalpine lake systems, in order to determine their uniqueness in relation to lowland African lakes and alpine lakes in temperate regions, and to evaluate the potential of Afroalpine Chironomidae as biological indicators to monitor future changes in the ecological integrity of their habitat.

Location  Mount Kenya (Kenya) and Rwenzori Mountains (Uganda).

Methods  The species composition of Afroalpine chironomid communities was assessed using recent larval death assemblages extracted from the surface sediments of 11 high-mountain lakes between 2900 and 4575 m. Results were compared with similar faunal data from 68 East African lakes at low and middle elevation (750–2760 m), and with literature records of Chironomidae species distribution in sub-Saharan Africa, the Palaearctic region and elsewhere. All recovered taxa were fully described and illustrated.

Results  The 11-lake analysis yielded 1744 subfossil chironomid larvae belonging to 16 distinct taxa of full-grown larvae, and three taxa of less differentiated juveniles. Eleven of these 16 are not known to occur in African lakes at lower elevation, and eight taxa (or 50% of total species richness) appear restricted to the specific habitat of cold lakes above 3900 m, where night-time freezing is frequent year-round. The faunal transition zone coincides broadly with the Ericaceous zone of terrestrial vegetation (c. 3000–4000 m). Snowline depression during the Quaternary ice ages must have facilitated dispersion of cold-stenothermous species among the high mountains of equatorial East Africa, but less so from or to the Palaearctic region via the Ethiopian highlands.

Main conclusions  Chironomid communities in glacier-fed lakes on Africa's highest mountains are highly distinct from those of lowland African lakes, and potentially unique on a continental scale. By virtue of excellent preservation and their spatial and temporal integration of local community dynamics, chironomid larval death assemblages extracted from surface sediments are powerful biological indicators for monitoring the hydrological and ecological changes associated with the current retreat and loss of Africa's glaciers.