Does elevation increase aquatic-terrestrial exchange in arid mountain streams?

Background/Question/Methods

Streams and their adjacent riparian zones are tightly linked by reciprocal flows of materials and organisms. In mountain streams the strength of this linkage may exhibit altitudinal and seasonal variation. Elevation-specific environmental conditions may also influence exchange rates and modify connectivity across the gradient.  In semi-arid regions where vegetation is restricted to grasses and small shrubs, subsidies such as leaf litter may be less important than terrestrial and aquatic invertebrates. To examine how elevation, seasonal variation and concomitant environmental variables affected lotic-riparian connectivity, we studied two reaches of the Arroyo Tambillos in the Andes Mountains of Argentina (2470 and 2630 m elevation, TD and TU respectively). We present a follow-up to a pilot study carried out during the 2012 austral summer in which no linkage between aquatic and riparian communities was found. The sampling period included spring, summer and fall seasons. The study reaches extended 32 m into the riparian habitat (3 transects/ site and we assessed aquatic (12 Surber samples), terrestrial (36 pitfall and 18 sticky traps) and emergent arthropod assemblages (6 sticky traps above the stream) at each site. Environmental variables were measured for aquatic samples.  Data were analyzed using ANOVA and multiple regression.

Results/Conclusions

Discharge, water velocity and temperature increased with seasonal day length but were generally greater at the lower elevation site. The abundances of aquatic macroinvertebrates also exhibited seasonal change (P = 0.021); however, greater numbers were seen at the high elevation site (P = 0.016).  By contrast, richness did not differ between site or season (P > 0.05).  Emergent aquatic insects caught in riparian sticky traps included (in order of abundance) Diptera, Trichoptera, Ephemeroptera and Coleoptera.  Diptera numbers declined 16 m from the stream, whereas other taxa were absent beyond 4 m.  Mirroring the stream pattern, epigean arthropods were more abundant at the high elevation site (P < 0.01) and most abundant within 4-m of the stream and at 16-m from it.  That no effect of distance on abundance was seen (P = 0.056) is likely due to the second peak in numbers at 16m for both sites.  Despite difference in composition within the aquatic and riparian communities, both exhibited higher abundances at the upper site.  This, however, was not reflected in aquatic insect emergence although we can see an effect of distance from the stream.