Influences of elevation, stream size, and land use on structure, function, and production of benthic macroinvertebrate communities in two southern river ecosystems

Annual abundance, biomass, and production were estimated for benthic macroinvertebrate communities along a longitudinal Appalachian stream gradient and sixth-order Piedmont river reach. Taxonomic composition differed greatly along the river gradient (1st through 7th stream orders), with greatest richness in 5th-order reaches. Densities were highest in cobble-riffles, followed by bedrock outcrops, pebble/gravel run, and depositional areas. Biomass-based functional composition differed between habitat-types but also within habitats along the continuum, likely in response to changes in microscale parameters. Longitudinal patterns of biomass-based functional composition generally matched predictions of the River Continuum Concept (RCC); however, localized stream geomorphology also influenced community structure.; Secondary production in 5th through 7th-order reaches along the gradient varied with habitat-type; production was greatest on substrata covered with hydrophytes (Podostemum ceratophyllum), followed by mineral habitats and depositional areas. Total annual production estimated for 7th-order reaches are amongst the highest reported thus far for lotic systems. Functional composition based on secondary production generally supported RCC tenets; however, as with biomass, habitat availability influenced functional composition independently of the stream-size gradient. Comparison of production to biomass (P/B) ratios for benthic communities vs annual degree-days at 17 sites throughout the catchment indicated community growth rates were strongly correlated to thermal regime.; In the Piedmont river reach, abundance and biomass were influenced by standing crops of Podostemum. Collector-filterers, collector-gatherers, and scrapers dominated functional-group abundance; scrapers and collector-filterers dominated biomass. Benthic invertebrate production consisted primarily of collector-filterer hydropsychid caddisflies and a scraper snail, Somatogyrus sp. Results were compared to those from a previous study conducted 35 years earlier in the same river reach. Physical parameters of temperature and discharge regimes, Podostemum standing crops, and riparian vegetation were similar between studies, but marked changes in land use occurred within the catchment. Community structure was dominated by small, multivoltine collector-gatherers and microfilterers in the previous study; in the present study dominant taxa consisted of larger, longer-lived taxa, macrofilterers and scrapers. Changes in community structure and indices of biotic integrity indicated an improved stream condition in this study relative to the previous one; changing land use practices within the catchment are implicated as the key factor for improvement.