Runoff and the longitudinal distribution of macroinvertebrates in a glacier-fed stream: Implications for the effects of global warming

1. The downstream pattern in benthic macroinvertebrate assemblages along glacier-fed streams is a result of decreasing glacial influence on environmental conditions. However, meltwater run-off shows temporal variation, reflected in differences in, for example, temperature, conductivity and turbidity. Consequently, depending on their run-off patterns, comparable environmental conditions may occur at different distances along glacier-fed streams. Our aim was to assess whether short-term variations in glacial run-off were reflected by changes in longitudinal distribution patterns of macroinvertebrates along a glacier-fed stream in the Ecuadorian Andes. 2. We measured environmental parameters, obtained continuous gauging data, measured macroinvertebrate drift rate with an hourly resolution during glacial floods and sampled benthic macroinvertebrates c. 3-monthly for 30 months at three sites at varying distances (0.1-4.3 km) from the glacier. For each sampling date, we fitted logarithmic equations to plots of taxon richness versus distance from the glacier, calculated similarity in assemblage composition between sites and calculated weighted averages of mid-points of downstream. During glacial afternoon floods, conductivity decreased while temperature and turbidity increased. High flow moved maximal taxon richness downstream (reduced the slopes from logarithmic fits), while low flow made assemblages more downstream like (higher similarity with downstream assemblages). No significant relationships were found between weighted average distributions and flow. Drift rate (ind. h^-1) increased by an order of magnitude at the onset of afternoon floods, and the taxa that contributed most to total drift were those whose benthic densities were most reduced by increases in flow. 4. Our study provides hints as to how biological distribution patterns in glacier-fed streams might be affected by more permanent changes in run-off caused by glacial shrinkage. This study predicts a multidirectional shift in altitudinal/longitudinal species ranges, with a potential downward shift in species ranges as a consequence of global change.