Macroinvertebrate communities are sensitive to the many stressors associated with the conversion of forests and other natural landscapes to urban lands (e.g., altered hydrology, increased contaminants, excessive fine sediment, increased temperature). In King County, the predictable change in communities across the urban gradient is characterized using the Puget Lowlands Benthic Index of Biotic Integrity (B-IBI). This scoring system grades sites on a 0 to 100 scale; sites scoring 0 are highly degraded and support only the most stressor-tolerant types of macroinvertebrates, whereas sites scoring 100 support a diverse community of macroinvertebrates that require excellent conditions to persist over time.
In this report, we present trend analyses at multiple spatial scales using long-term data from our Ambient Freshwater Benthic Macroinvertebrate Monitoring Program. We explore how macroinvertebrate communities have changed from 2002 to 2021 across 120 sites, and how relationships between biotic metrics and the extent of impervious surface cover in contributing basins in the region have shifted over time. We explore whether changes we observe in Benthic Index of Biotic Integrity (B-IBI) scores and the density and frequency of occurrence of macroinvertebrate taxa were correlated with changing sampling methodology, changing in-stream stressors, changing land cover patterns, or changing climate factors. Surprisingly, we found B-IBI scores improved across the County between 2002–2021. Though we could not definitively relate environmental changes to changes in B-IBI due to the exploratory nature of our investigation, we identified several variables of interest and developed hypotheses for future testing.