Stream ecosystem function along a land-use gradient

Leaf decay, whole system metabolism, organic particle transport, litter inputs, benthic organic matter storage, and organic matter budgets were measured in similar-sized streams flowing through eight watersheds consisting of different predominant land uses on the Piedmont of Georgia. The gradient of land use consisted of two watersheds each in forest, poultry agricultural, suburban, and urban land use. Forest cover decreased and human population densities increased along this gradient.;Forest streams in the Piedmont have less primary production and litterfall, similar respiration and metabolism, and faster leaf decay than other forest streams of similar size. Poultry agricultural streams on the Piedmont have similar litter inputs and benthic storage to forest streams, but are subsidized by greater primary production as a result of nutrient enrichment. Nutrient enrichment also increased leaf decay. Suburban and urban streams have high litter inputs where riparian vegetation is intact, but storage is very low due to reduced retention of suspended particles. Although nutrient concentrations are higher than forest streams, shifting substrates and pollutants prevent the build-up of algal biomass, reducing the enrichment effect observed in agricultural streams. Instead, many of these urban streams receive subsidies of labile anthropogenic carbon. Leaf decay is faster in urban streams as a result of physical fragmentation due to increased stormflow magnitudes and frequencies.;Several parameters exhibited trends along the land-use gradient: depth decreased, width:depth ratios increased, and ammonium concentration increased. However, the only organic measures that exhibited a trend along the same gradient were those that responded strongly to physical gradients of hydrologic change caused by decreasing forest cover and increasing population growth: benthic storage decreased and organic matter deposition decreased along the land-use gradient. The other measures were affected differently by the nutrient enrichment in agricultural streams and the increased hydrologic stress of urbanizing watersheds. For example, leaf decomposition and primary production were enhanced in agricultural streams, but the trend was not consistent through the suburban and urban streams. The variability in stressors obscured the gradient effect and suggests that for many organic matter processes the gradient of response to urbanization is distinct from that caused by agricultural land use.