Benthic macroinvertebrate distributions in the riffle-pool communities of two east Texas streams

The benthic macroinvertebrate riffle-pool communities of two east Texas streams were sampled monthly for a period of one year. In contrast to previous studies in primarily upland areas, pools in Alazan Creek and Bernaldo Bayou contained significantly higher densities and biomass, as well as a significantly higher diversity and number of taxa. The majority of taxa collected could be characterized as pool adapted organisms, with the Diptera, Ephemeroptera, Coleoptera, and Odonata having greater numbers in pools. Perlesta, Baetis, and Cheumatopsyche were the only genera to have significant numbers in riffles at both sampling sites. A greater number of taxa were restricted to pools at both sampling locations, and although an unstable, sandy substrate was a factor in limiting colonization of riffles, riffles were not as productive as pools even in the presence of suitable substrate conditions. A cluster analysis based on similarity indices indicated that like habitats between streams were more similar to each other, than were adjacent riffle-pool complexes within streams.     

Riffle-pool geomorphology disrupts longitudinal patterns of stream benthos

An Ozark Plateau stream was studied to determine the influence of distinct pool and riffle geomorphology on the longitudinal zonation of macroinvertebrate species assemblages and functional group classification. All study sites were dominated by alluvial pool and riffle channel form and the first two orders became intermittent during summer months. Nine benthos samples were collected seasonally from riffles and pools at each of five sites using a vacuum benthos sampler. Diel temperature pulse and coarse particulate organic matter (CPOM) were measured at each site also. Water temperature was most variable in second order, and CPOM on riffles was not more abundant in upstream reaches. Annual average density and biomass of invertebrates were highest in third and fourth orders, respectively. Diversity was depressed in the intermittent headwaters sites. Macroinvertebrate functional groups did not exhibit strong longitudinal trends as predicted by the river continuum model, with species assemblages apparently more strongly affected by the segment-level physical template, although shredders were more abundant in the headwaters during fall and winter. This study indicates that a reach-level perspective based on channel form is a necessary complement to holistic stream ecosystem models, especially in alluvial gravel streams.