Decomposition and Ecosystem Function in Restored Urban Streams

<strong>Abstract: </strong>As the target of many stream habitat restoration projects, Pacific salmon have the potential to act as umbrella species for ecosystems and communities. Streams that flow through the urban area of Vancouver, B.C., Canada, face symptoms of the urban stream syndrome, including warmer temperatures, higher nutrient loads, and flashier hydrographs due to impervious surroundings, but these streams also support Pacific salmon. We investigated whether habitat restoration undertaken to improve Pacific salmon habitat can naturalize organic matter decomposition rates in urban streams. We deployed cotton strips to measure decomposition, and conducted habitat assessments on factors that may contribute to decomposition including water temperature and chemistry, substrate characteristics, water velocity, and riparian vegetation. We then investigated the relationship between these variables, the amount of surrounding impervious surface, restoration effort, and decomposition rates using structural equation models (SEM’s). We hypothesized that (1) decomposition rates would be either higher (due to temperature, nutrients, and possibly habitat) or lower (due to contaminants or habitat alteration) in more urbanized reaches compared to more natural reaches, and that (2) sites where more effort had been put into restoration would have decomposition rates more similar to un-urbanized sites, in part because restoration would mitigate some changes in environmental conditions associated with urban stream syndrome. We found that streams surrounded by more impervious surface tended to have faster organic matter decomposition rates (standardized path coefficient = 0.58, the second-strongest direct relationship in the SEM), although it was difficult to disentangle the causal effect of urbanization from an underlying altitudinal gradient. Reaches where more effort had been put into restoration showed altered physical habitat characteristics compared to less-restored urban reaches. However, the faster decomposition rates characteristic of more urbanized reaches were only minimally affected by restoration, and ecosystem function remained altered despite efforts to restore salmon habitat. This suggests that human impacts on freshwater ecosystems occur through pathways at larger spatial scales, and that local efforts focusing on physical habitat restoration may not be the most effective strategy for addressing changes to ecosystem functioning.
<br>
Metadata is provided in the attached PDF.