Riparian vegetation and sediment gradients determine invertebrate diversity in streams draining an agricultural landscape

Separation of natural and anthropogenic influences is required to identify land-use impacts on stream ecosystems. We investigated the effects of water quality and riparian condition on invertebrate assemblages along streams draining agricultural land by partitioning out changes in geomorphological characteristics. There was a strong negative relationship between invertebrate richness and distance downstream, driven by a gradient of reducing stream power and substratum particle size along the streams. When substratum particle size was accounted for, richness was reduced by ∼24% when there was limited availability of coarse particulate organic matter, resulting from lower riparian forest cover upstream. High concentrations of fertilizer-derived nitrate boosted invertebrate abundances, but only in mid sections of streams, where coarse substrata (>100 mm) and high insolation were available. Sampling of multiple sites along streams facilitated partitioning of land-use impacts from natural gradients. Invertebrate richness was a good indicator of stream biophysical condition (e.g. nature of the substratum, riparian condition) at the stream scale irrespective of taxonomic resolution (family or higher) or sample size (down to 50 individuals per site), and was therefore a useful monitoring tool. The finding that riparian vegetation is a key determinant of invertebrate diversity should encourage catchment-scale maintenance and rehabilitation of native riparian forest.