Near-bed current velocity structures a benthic community across an elevation gradient in an Andean mountain stream

WELLNITZ, TODD; FERNÁNDEZ CAMPÓN, FLORENCIA; SCHEIBLER, ERICA E.

Baltimore

Congreso; ESA 100th annual meeting; 2015

Ecological Society of America

Abstract Text:Background/Question/MethodsNear-bed current velocity is a critical feature ofstream benthic environments that may be important for establishing communitypatterns along the stream continuum. Stream community composition shifts from headwaterto downstream reaches, but the responses that these communities show to near-bedcurrent across the continuum are unknown. As biotic and abiotic parameterschange, the relationship between species richness and abundance to near-bedcurrent could be either idiosyncratic or stereotypic. To test this, we examinedthe benthic community along a 2800 m elevation gradient in Arroyo Chacay, anAndean mountain stream located on the eastern slope of the Cordillera Frontalin Argentina. Macroinvertebrates were sampled, algal biomass wasquantified, and near-bed current velocity was recordedat 15 locations within five sites positioned at 2200, 2400, 2600, 2800 and 3000m elevation. Macroinvertebrates were identified to lowest feasible taxonomiclevel, benthic algal abundance was determined using a portable fluorometer, andcurrent velocity was measured 10 mm above the bed at each sampling location. Results/ConclusionsAcross sites, the Arroyo Chacay macroinvertebrateassemblage was comprised of (in terms of % relative abundance) Ephemeroptera(52%), Diptera (22%), Coloeoptera (13%), and Annelida (12%). The algae assemblagewas comprised of diatoms (77%) cyanobacteria (21%), and green algae (2%).Temperature increased downstream and correlated with lower macroinvertebraterichness, but showed no relationship to near-bed current. Proportional representationof the macroinvertebrate taxa and algal groups differed among sites, as didnear-bed current velocity. Current near the streambed showed a hump-shapedpattern across the gradient, with the fastest velocities occurring atintermediate elevation sites (2400, 2600 and 2800 m) and slowest velocitiesoccurring at the lowest and highest sites (2200 and 3000 m). Neithermacroinvertebrates nor algae showed a consistent relationship to near-bedcurrent within any single site; however, clear trends were seen across sites.When site averages were compared, macroinvertebrate and algal abundance showeda positive relationship to near-bed current, as did macroinvertebrate richness.Thus, our data show that near-bed current affected large-scale, but notsmall-scale community patterns. This was surprising; nevertheless, it suggeststhat variation in near-bed current could be an important organizing forceacross the stream continuum.