Resumen:
Tropical Andean glaciers are retreating rapidly, with possible consequences for trophic
structure and ecosystem processes in high Andean meltwater streams. Here, we measured the
environmental characteristics, quantified pools of particulate organic matter (POM) and periphyton
(Chl. a), sampled benthic macroinvertebrates, determined functional feeding groups (FFG), and
performed mesh bag decomposition experiments with Calamagrostis grass detritus at
17 stream sites along a gradient of glacial influence (GI) with 0–23% glacier cover in the catchment at
4050–4200 m a.s.l. in the Andes of Ecuador. POM was unrelated to GI while Chl. a. showed a weak
(non-significant) negative relationship to GI. The macrofauna abundance decreased while taxon
richness and the number of FFGs per site showed a hump-shaped relationship with increasing GI.
Taxa with an opportunistic and generalist feeding mode generally dominated benthic assemblages
and were related to high GI levels and low Chl. a. Only shredders were negatively related to GI, but
unrelated to POM. Decomposition rates were comparable to those found in temperate alpine streams,
and for both fine (0.0010–0.0065; median 0.0028 d−1
) and coarse (0.0019–0.0088; median 0.0048 d−1
)
mesh bags, peaked at intermediate GI values, while the difference between bag types was small and
almost constant along the GI gradient. This indicates an overall minor effect of macroinvertebrate
shredders compared to that of microbes, in particular at high GI. It also suggests that the relatively
high average temperature of these high-altitude equatorial streams (7–10 ◦C) does not produce higher
decomposition rates than those in comparable but colder streams at temperate latitudes. The results
suggest that, at the lower end of glacier cover, tropical glacier loss will not change the dominant
microbial role in detritus decomposition, but that part of the physical abrasion could be partially
replaced by biological shredding.
