Ecoregion and land-use influence invertebrate and detritus transport from headwater streams

1. Habitats are often connected by fluxes of energy and nutrients across their boundaries.For example, headwater streams are linked to surrounding riparian vegetation throughinvertebrate and leaf litter inputs, and there is evidence that consumers in downstreamhabitats are subsidised by resources flowing from headwater systems. However, thestrength of these linkages and the manner in which potential headwater subsidies varyalong climatic and disturbance gradients are unknown.2. We quantified the downstream transport of invertebrates, organic matter andinorganic sediment from 60 fishless headwater streams in the Wenatchee River Basinlocated on the eastern slope of the Cascade Range in Washington, U.S.A. Streams wereclassified into four groups (each n = 15) based on their position within two ecologicalsubregions (wet and dry) and the extent of past timber harvest and road development(logged and unlogged).3. Time and ecoregion were significant for all response variables as transport variedacross sampling periods, and dry ecoregion streams displayed significantly highermean values. Logged sites also generally showed higher mean transport, but onlyinorganic sediment transport was significantly higher in logged sites. Both ecoregionand land-use interacted significantly with time depending on the response variable.Differences among stream categories were driven by relatively low levels of transportin unlogged drainages of the wet ecoregion. Interestingly, unlogged dry ecoregionstreams showed comparable transport rates to logged sites in the wet ecoregion.Dominance by deciduous riparian vegetation in all but unlogged streams in the wetecoregion is a primary hypothesised mechanism determining transport dynamics in ourstudy streams.4. Understanding the quantity and variation of headwater subsidies across climateand disturbance gradients is needed to appreciate the significance of ecologicallinkages between headwaters and associated downstream habitats. This will enablethe accurate assessment of resource management impacts on stream ecosystems.Predicting the consequences of natural and anthropogenic disturbances onheadwater stream transport rates will require knowledge of how both local andregional factors influence these potential subsidies. Our results suggest that resourcestransported from headwater streams reflect both the meso-scale land-use surrounding these areas and the constraints imposed by the ecoregion in which they areembedded.