Longitudinal zonation of macroinvertebrates in an Ecuadorian glacier‐fed stream: do tropical glacial systems fit the temperate model?

Abstract

Summary1. The ecology of glacier‐fed streams at temperate latitudes has been intensely studied in recent years, leading to the development of a well‐validated conceptual model on the longitudinal distribution of macroinvertebrate communities downstream of the glacier margin (Freshwater Biology, 2001a; 46, 1833). However, to our knowledge, the ecology of tropical glacier‐fed streams has not yet been studied.2. We sampled benthic macroinvertebrates and measured environmental variables at nine sites between 4730 and 4225 m altitude along a 4.3 km stretch of a glacier‐fed stream 40 km south of the equator in the Ecuadorian Andes. Our goal was to study the longitudinal distribution of the fauna in relation to environmental factors and to compare this with the conceptual model based on temperate–arctic glacier‐fed streams.3. Total density of invertebrates differed considerably at the two highest altitude sites; 4600 m−2 at a pro‐glacial lake outlet and only 4 m−2 at a site originating directly from the glacier snout. Otherwise, there was a downstream decrease in density to about 825 m−2 at the three lowest sites. Taxon richness increased with distance from the glacier, very similar to the pattern predicted. A total of 28 taxa were collected; two at the glacier snout, seven at the nearby pro‐glacial lake outlet, 13 at site 2 (<400 m from the glacier) and 20 at the lowest sites.4. The numerical percentage of Chironomidae (Diptera) decreased downstream from 100 to 44%. The subfamily Podonominae was numerous at the highest sites but became much less important further downstream. The Orthocladiinae were important both in numbers and species at all sites, while Diamesinae were numerous only in the middle of the reach studied and were completely absent from the upper three sites. The limited importance of the Diamesinae, and its replacement by Podonominae, is different from the pattern typically observed in north‐temperate glacier‐fed streams. This could be because of the fact that the genus Diamesa is missing from the Neotropics.5. Stream temperature and channel stability explained most of the variability in faunal composition and richness, supporting the model. Stability increased systematically downstream while temperature did not. Surprisingly, no classical kryal zone (Tmax < 4 °C) was found, as even the site closest to the glacier snout (50 m) had a Tmax of 15 °C and no site had Tmax < 8 °C. We propose that this might be a general feature of equatorial glacial streams.