Tracing carbon sources in small urbanising streams: catchment-scale stormwater drainage overwhelms the effects of reach-scale riparian vegetation

1. Organic matter provides energy and nutrients to aquatic systems. Alterations to its sources and processing have repercussions for water quality and food-web stability and structure. Despite worldwide recognition of the impacts of urbanisation, there is limited understanding of the relative importance of catchment-scale urban stormwater drainage connection and reach-scale riparian vegetation on organic matter sources. 2. We investigated the effects of catchment-scale urban stormwater drainage connection and reach-scale riparian vegetation cover on organic matter sources in small streams. Using stable isotopes and elemental ratios (i.e. delta C-13, delta N-15 and C:N), we traced the origin of microbially respired carbon and standing stocks of dissolved organic carbon (DOC), suspended particulate organic matter (POM) and benthic coarse particulate organic matter (CPOM). 3. Catchment-scale urban stormwater drainage connection significantly increased the contribution of labile organic matter to POM and DOC standing stocks. Greater POM lability was a product of increased inputs of autochthonous organic matter in more heavily urbanised streams, although the origin of labile DOC was less clear. 4. While reach-scale riparian vegetation was the most likely source of the terrestrially dominated CPOM observed across most sites, increasing cover had no significant effect on the origin of POM or DOC standing stocks. We conclude that catchment-scale stormwater drainage impacts overwhelm the effects of reach-scale riparian vegetation on the sources and lability of POM and DOC in small streams. 5. Our results suggest that the protection or restoration of riparian vegetation, in the absence of modifications to catchment-scale stormwater drainage connection, is insufficient to mitigate the effects of urbanisation on organic matter sources, lability and processing in these small streams.