Altitudinal distribution of lotic chironomid (Diptera) communities in the Sierra Nevada mountains (Southern Spain)

Pupae and pupal exuviae were collected by drift and hand netting from 27 sites on 10 streams in the Sierra Nevada mountains in 1986 and 1987. The sites represented an altitudinal range from 340 m to 2100 m. The pupal exuviae collections revealed a composite fauna of 143 species. Eukiefferiella (12 spp.) and Tvetenia (3 spp.) were dominant both taxonomically and numerically along most of the altitudinal profile, followed by Cricotopus (11 spp.), Orthocladius (9 spp.) and Diamesa (7 spp.). The altitudinal distribution of the 99 most common species is shown and compared with those obtained in other European streams or rivers, especially in the Pyrenees. The general pattern of altitudinal zonation shows an increase in species richness from the headwaters (< 2000 m : 58 spp.) toward the middle reaches (1000-1600 m : 121 spp.). The theoretically expected highest species richness in the foothill reaches (> 1000 m : 113 spp.) was not obtained. The periods of reduced or intermittent flow, the domestic organic inputs and the narrowness of the channels in the foothill reaches appear responsible for the decrease of the species richness of the chironomid communities.