Environmental drivers of leaf breakdown in an urban watershed

Abstract

Leaf-litter breakdown is an important ecosystem process in urban streams, but urbanization may have complicated effects on breakdown rates. Low abundance of macroinvertebrate shredders may slow breakdown, but rates may increase if high nutrient concentrations stimulate microbial decomposers or if flooding enhances leaf fragmentation. We measured the relative importance of multiple environmental drivers on breakdown of eastern cottonwood (Populus deltoides) leaves at 5 sites in the urbanized North Branch of the Chicago River watershed. Few specialized macroinvertebrate shredders were present, but generalist macroinvertebrates, including isopods (Asellus aquaticus) and amphipods (Gammarus sp.) were abundant at all sites. We tested macroinvertebrate effects on breakdown rate in large- and small-mesh bags. We measured discharge, nutrient concentrations, benthic macroinvertebrate community composition, and subwatershed land use at each site. Leaf breakdown differed significantly among sites and between mesh types. Discharge and isopod abundance were positively related to breakdown rates, whereas nutrient concentrations were unrelated to breakdown rates. Abundances of isopods and amphipods were significantly higher in litter bags than in benthic samples. We conducted follow-up experiments in artificial streams to measure the separate effects of water velocity and isopods on leaf breakdown based on conditions from field sites. Increasing water velocity from 0.02 (control) to 0.07 m/s (high velocity) increased leaf breakdown by 33%, and adding isopods (density = 1034 individuals/m2) increased leaf breakdown by 40%. Measuring environmental controls on leaf breakdown throughout urban watersheds is critical to the use of breakdown rates as an assessment tool for urban stream ecosystems. Our study provides input data for models of stream ecosystem function at urban sites and informs management approaches for urban streams at the watershed scale.