Observed relationships between urbanization and riparian cover, shredder abundance, and stream leaf litter standing crops

Leaf litter inputs, an essential driver of river ecosystem structure, are heavily influenced by local and basin wide changes. Urbanization can impact leaf litter inputs at both scales, but the consistency of ecosystem response is unknown. In this study, we investigated relationships between basin land use coverage, reach-scale canopy conditions, and macroinvertebrate shredder metrics in 42 streams across a gradient of landuse in the Piedmont of Georgia, USA. We then evaluated how urbanization affects vertical leaf litter inputs and also autumn and spring leaf litter availability in a subset of 13 streams. Reach-scale riparian conditions controlled canopy cover and vertical litter inputs, but leaf litter standing crop dynamics were more complex and seemed to be controlled at the catchment level by factors beyond land use coverage. Overall, sites with less urbanization had greater litter standing crops during December; however, higher rates of retention occurred in more urbanized areas. We infer that urban streams balance litter export with additional horizontal inputs from storm drains that act to increase the litter source area. Macroinvertebrate shredder taxa richness was negatively affected by watershed landuse, but shredder abundance and percent composition were not. Shredder abundance and composition were not correlated to litter availability. The apparent importance of horizontal leaf litter inputs to urban stream detritus budgets indicates that further studies are warranted to characterize and quantify litter inputs from storm drains and to gain a better understanding of how these inputs effect shredder populations.