Near-bed current velocity structures a benthic community across an elevation gradient in an Andean mountain stream
Near-bed current velocity is a critical feature of stream benthic environments that may be important for establishing community patterns along the stream continuum. Stream community composition shifts from headwater to downstream reaches, but the responses that these communities show to near-bed current across the continuum are unknown. As biotic and abiotic parameters change, the relationship between species richness and abundance to near-bed current could be either idiosyncratic or stereotypic. To test this, we examined the benthic community along a 2800 m elevation gradient in Arroyo Chacay, an Andean mountain stream located on the eastern slope of the Cordillera Frontal in Argentina. Macroinvertebrates were sampled, algal biomass was quantified, and near-bed current velocity was recorded at 15 locations within five sites positioned at 2200, 2400, 2600, 2800 and 3000 m elevation. Macroinvertebrates were identified to lowest feasible taxonomic level, benthic algal abundance was determined using a portable fluorometer, and current velocity was measured 10 mm above the bed at each sampling location.
Across sites, the Arroyo Chacay macroinvertebrate assemblage was comprised of (in terms of % relative abundance) Ephemeroptera (52%), Diptera (22%), Coloeoptera (13%), and Annelida (12%). The algae assemblage was comprised of diatoms (77%) cyanobacteria (21%), and green algae (2%). Temperature increased downstream and correlated with lower macroinvertebrate richness, but showed no relationship to near-bed current. Proportional representation of the macroinvertebrate taxa and algal groups differed among sites, as did near-bed current velocity. Current near the streambed showed a hump-shaped pattern across the gradient, with the fastest velocities occurring at intermediate elevation sites (2400, 2600 and 2800 m) and slowest velocities occurring at the lowest and highest sites (2200 and 3000 m). Neither macroinvertebrates nor algae showed a consistent relationship to near-bed current within any single site; however, clear trends were seen across sites. When site averages were compared, macroinvertebrate and algal abundance showed a positive relationship to near-bed current, as did macroinvertebrate richness. Thus, our data show that near-bed current affected large-scale, but not small-scale community patterns. This was surprising; nevertheless, it suggests that variation in near-bed current could be an important organizing force across the stream continuum.