Measuring function and structure of urban headwater streams with citizen scientists

Abstract

AbstractHeadwater streams accumulate, process, and export organic materials for use in downstream environments. Decomposition of organic material, an important ecosystem function, may be sensitive to land cover changes in urbanizing regions since headwater stream processes tend to be tightly coupled with riparian and catchment characteristics. Headwaters represent 70–80% of total stream length in watersheds but are disproportionately converted to drainage infrastructure or buried with urban development. Cumulatively, this loss may result in substantial changes to physical and biological downstream processes. From a monitoring perspective, headwaters are largely ignored compared with fishable/navigable waterways for planning decisions, so their structural and functional variability is not well understood. Here, we engaged citizen scientists to contribute data on this variability and to evaluate the sensitivity of standardized cotton‐strip decomposition rates to multiscale factors across headwaters with varying landscape conditions in the Greater Toronto Area (York Region), Canada. These factors included stream, riparian vegetation, and catchment characteristics. We expected decomposition rates to be similarly sensitive to local‐ and catchment‐scale factors because of the strong links between headwater catchment and stream processes. We also expected a hump‐shaped distribution of decomposition rates across a gradient of urban cover, with stimulating effects at low to moderate cover but deleterious effects at high urban cover. We found that decomposition rate was most sensitive to local‐scale factors (e.g., strip burial, stream velocity, and both local upland riparian vegetation density and topography) rather than whole catchment properties. We did not find the expected hump‐shaped distribution with urban cover and suggest that more mechanistic studies are needed for understanding cotton‐strip decomposition to control for local factors in determining the scale at which decomposition rate is most sensitive to land cover change.