LAUSR

Headwater streams drain large proportions of landscapes and represent a large proportion of stream habitat. Understanding their ecology requires more comparative data from the smallest streams. We describe dynamics of benthic organic material (BOM) and invertebrate assemblages in an order-2 Australian tropical stream and compare samples from orders-1 and 2 sites. Litterfall and, especially, stream discharge determined BOM standing stock. Most material was in the > 1 mm fraction, pools stored more than riffles, and order-1 sites more than order-2. BOM increased in the dry season but was depleted by wet-season spates. Retention was greater in years without major spates, when riffle-pool stream sections were largely ecologically isolated. Invertebrate richness was greater in riffles than pools and in order-2 than order-1 sites, with overlap in composition. Current velocity was the primary variable determining distributions in order-2 sites, with particulate organic matter (63–250 µm) the main variable in order-1 sites. However, explanatory relationships were weak because of unusually benign conditions during the sampling period. We need more globally comprehensive spatially explicit assessment of the ecosystem dynamics of headwater streams to predict effects of environmental change. Assessments over extended periods are necessary to capture the variability of even apparently predictable systems.