Running head: Macroecology of community traits

1. Disregarding few classical ideas, large-scale approaches to understand variation in community traits (i.e. the trait composition of an ecological community) across stream ecosystems are rather recent. Case studies and review papers show clear evidence for the usefulness of trait-based analyses in bioassessment, but how community traits vary along natural gradients at large scales has not yet been synthesised. Here, we attempt to fill this gap by providing a synthesis of trait patterns of stream communities from a macroecological perspective. 2. We argue that although both natural and anthropogenic filters shape community traits, examination of natural filters, including those acting at large-scales, should receive increasing attention. This is because it is inadequately known how natural environmental conditions filter species on the basis of their traits. Such knowledge is vital for reliably inferring anthropogenic impacts on stream communities and ecosystems. 3. We synthesise knowledge of two large-scale spatial patterns of stream communities: among drainage basins (i.e. geographic variation) and within drainage basins (i.e. longitudinal variation). We also examine the temporal dimension of community traits. Our review highlights clear evidence for large-scale influences on the trait composition in stream systems. For example, despite previou