Inedes

Three large rivers have their headwaters in the Patagonian Ice Fields (PIFs) in the Andes Mountains, the
largest mid-latitude ice masses on Earth: Santa Cruz, Baker and Pascua. They are the last large free flowing
rivers in Patagonia, but plans are advanced for building dams for hydroelectric power generation. The
three PIF rivers, with a discharge dominated by ice melt, share a common, unique hydrograph compared
to that of the other eight large rivers in the region: a distinct seasonal cycle, and an extremely stable discharge,
with much lower variability than other rivers. In this study we present the first extensive survey
of habitats and benthic macroinvertebrates in the least studied system, the Santa Cruz River. We assess
how much of the natural capital provided and sustained by benthic invertebrates are expected to be lost
by flooding and discuss how dams would affect riverine habitat and biota. In the Santa Cruz River, we
conducted an intensive field survey during September 2010; a total of 52 sites located at regular 6 km
intervals were sampled along the 310 river-km for macroinvertebrates and seventeen habitat variables.
Although some habitat structure is apparent at the local scale, the Santa Cruz River could be described
as very homogeneous. Macroinvertebrate density and the richness (38 genera) found in the Santa Cruz
River resulted to be one of the lowest in comparison with 42 other Patagonian rivers. Albeit weak, the
structure of the macroinvertebrates assemblages was successfully described by a reduced set of variables.
The reduced flow variation and the lack of bed scouring flows have a direct and negative effect on the
heterogeneity of riverbeds and banks. The high turbidity of the Santa Cruz River may also contribute to
shorter food webs, by affecting autotrophic production, general trophic structure, and overall macroinvertebrate
productivity and diversity. Dams will obliterate 51% of the lotic environment, including the
most productive sections of the river according to our macroinvertebrate data. Since Santa Cruz River has
a naturally homogeneous flow cycle, dams may provide more variable flows and more diverse habitat.
Our data provide critically valuable baseline information to understand the effects of dams on the unique
set of glacial driven large rivers of Patagonia.

The Santa Cruz River is the last free- flowing river in Argentinean Patagonia. Two dams are projected, and no comprehensive pre-impoundment study has been undertaken. The present study inves- tigated macroinvertebrate communities along three different hydrological periods and at three river sections located upstream and downstream of future dams. Fifty-three macroinvertebrate taxa were identi- fied, with the most abundant orders being Ephe- meroptera, Plecoptera, Coleoptera, and Crustacea (particularly amphipods). Ordination methods (CCA) and generalized linear models (GLM) were applied.According to the CCA, the main environmental variables related to macroinvertebrate density were temperature, suspended solids, depth, and substrate size. For the GLM, the main factors associated with macroinvertebrate abundance were location and hydrological period, and variables with the highest influences were temperature, substrate size, current speed, and depth. We anticipate that dam construction will modify in-stream habitat conditions, leading to changes in (i) macroinvertebrate community structure and (ii) local fish abundance due to loss of key prey taxa.