Spatial and temporal variation in the ecological stoichiometry of aquatic organisms in an urban catchment

Urban land use has increased dramatically over the past few decades. Urban streams are distinguished from forested or agricultural streams, in particular, by their more variable and unpredictable hydrologic pattern. The resulting high variability in nutrient loading is likely to alter the elemental composition of primary producers and, ultimately, to change the elemental composition of other foodweb components. Ecological stoichiometry is a useful framework for improving our understanding of the mass balance of multiple key elements in ecosystems. To this end, the C:N:P of key foodweb components were measured in Oxley Creek, an urban catchment in southeastern Queensland, Australia. Ten stream reaches were sampled to explore the spatial variation of C:N:P of abundant taxonomic groups across the catchment. Four of these sites were sampled weekly (for 8 wk) to examine temporal variation in elemental composition. Our results suggested that spatial and temporal variation in elemental composition of primary producers and some animal taxa were highly dependent on local (i.e., site) conditions. This local dependence makes determination of catchment-wide drivers of stoichiometric variability difficult, but our results do suggest that site-based influences in urban streams can generate substantial variability in the C:N:P content of in-stream biota. In the context of other studies that have been undertaken principally in forested streams, this application of ecological stoichiometry promises to further our understanding of the effects of urbanization on stream food webs and the stability of elemental flow.