Surprising improvements in stream biotic integrity despite increased urban development and warming temperatures

Multimetric indices, like the Puget Lowland benthic index of biotic integrity (B-IBI), are commonly used to characterize the predictable responses of macroinvertebrate communities to urban development in their contributing basins. As basins are urbanized over time, stream communities (and hence multimetric index scores) are expected to decline accordingly. The effects of climate change are expected to exacerbate this decline via thermal stress on sensitive taxa. We used a regional trend analysis of B-IBI scores (data range 2002–2021) in King County, Washington, USA, to understand whether urban development is related to trends in B-IBI scores over time. We were surprised to find an overall improvement (increase) in B-IBI scores. Over 2 decades, we observed strong evidence of countywide improvement in 8 of the 10 metrics that contribute to the index score. We found some sensitive taxa more frequently and at higher densities over time, whereas densities of several ubiquitous, tolerant taxa declined. When we analyzed trends at individual sites, we found at least weak evidence of positive trends in 78% of the 120 King County sites sampled annually. We observed strong evidence of improvement in 25% of the sites and strong evidence of declines at only 3%. Typical scores increased by 9.1 points (0–100 scale) over the 20-y study. Improvements in B-IBI scores occurred despite a mean increase of 2.0 percentage points (range 0–12) in impervious cover within the contributing basins and a 0.85°C increase in annual daily mean air temperature over the same period. B-IBI scores improved throughout the county and across the urban gradient, including in basins with the largest increase in urban development. We found that trends in B-IBI scores were correlated with trends in instream and climate-related stressors, although not always in the direction we would expect. Although the underlying relationships among metrics of community assemblage and urban development remain strongly negative, long-term monitoring data illustrate that scores can improve, despite increased urban development and some warming.