| Leaf litter breakdown is a critical ecosystem process in urban streams, but environmental conditions in urban streams may generate confounding effects on breakdown rates. Reduced abundance of macroinvertebrate shredders may slow breakdown, but rates may increase if high nutrient concentrations stimulate microbial decomposers and if flooding enhances leaf fragmentation. We used the litter bag technique to measure the relative importance of multiple environmental drivers on breakdown of eastern cottonwood (Populus deltoides) leaves at 5 sites throughout the North Branch of the Chicago River watershed. Sites spanned a gradient of urbanization, but no specialized macroinvertebrate shredders were present at any sites. However, generalist taxa including isopods ( Ascellus aquaticus) and amphipods (Gammarus sp.) were among the most abundant macroinvertebrates at all sites, and we used large and small mesh bags to test their effect on breakdown rate. In addition, we measured discharge, water chemistry, organic matter standing stock, benthic macroinvertebrate community composition, and sub-watershed land-use at each site. Leaf breakdown was significantly different among sites and between bag types. Discharge and isopod abundance were positively related to leaf breakdown, while nutrient concentrations and land-use categories showed no relationship to breakdown. Litterbags were `hot spots' for isopods and amphipods, as their abundance in litterbags was significantly higher than benthic samples. We conducted a follow-up study in artificial streams to test the individual effects of water velocity and isopods on leaf breakdown using conditions matching the field sites. Increasing water velocity from 0.02 m/s (control) to 0.07 m/s (high velocity) increased leaf breakdown by 33%, and adding isopods at density of 1,034/m2 increased leaf breakdown by 40%. Measuring the combined and individual environmental controls on leaf breakdown is critical for use of breakdown rates as an assessment tool in urban streams. In addition, advances in watershed-scale approaches for stream management and restoration will require studies which examine leaf breakdown across multiple sites within watersheds. Finally, laboratory experiments may be an underutilized tool to measure the role of individual environmental factors on breakdown which are otherwise inextricable in field approaches, and will help parameterize models of stream ecosystem function in urban watersheds. |
