Influence of log additions on physical and biotic characteristics of a mountain stream.

Three pairs of cobble riffle study sites were established in a second-order stream in North Carolina and logs added to the downstream riffle at each site. At log addition transects, stream depth increased, current velocity decreased, cobble substratum was covered by sand and less dramatic effects on uptake lengths of ammonium, nitrate, and phosphate, but they had immediate and significant impacts on invertebrate community structure: abundances and biomass of scrapers and filterers decreased; collectors and predators increased; overall shredder biomass did not change, but biomass of trichopteran and dipteran shredders increased, while that of most plecopteran shredders decreased; and plecopteran predators also decreased despite greater abundances of potential prey. These observations suggest that physiological and morphobehavioral constraints preclude many animals from tracking resources among patches when patches display very different abiotic conditions. Secondary production of scrapers and filterers decreased, whereas that of collectors and predators increased. The shifts in functional group abundances, biomass, and production between reference and debris-dam transects, which differed considerable from those previously reported for low-gradient, sandy-bottom streams, accentuate the importance of localized abiotic factors in structuring invertebrate communities within patches.