Ranking the importance of benthic metrics and environmental stressors from over a decade of bioassessment multiple stressor studies in five California waterbodies

Abstract Comprehensive bioassessment multiple stressor field studies were conducted in five waterbodies in the Central Valley, San Francisco and Central Coast areas of California between 2006 and 2017. Samples were collected over multiple years in Pleasant Grove Creek (PGC – 8 years), Kirker Creek (KC – 2 years), Arcade Creek (ARC – 3 years), Salinas streams (SAL – 3 years) and the lower Santa Maria River watershed (SM – 3 years). In each watershed, metrics indicative of physical habitat quality and the relative health of benthic communities were collected, along with measurements of various potential toxicants in sediment (metals and pyrethroids), total organic carbon (TOC) and sediment grain size characteristics. For all waterbodies, pyrethroids have been reported at concentrations suspected to be toxic based on single-species laboratory toxicity tests. The data from all of these studies were assembled into a single data base which was analyzed by a series of univariate and multivariate statistical analyses. These analyses were designed to examine overall relationships between benthic community health and environmental quality conditions across the watersheds, as well as to address the following two key questions: (1) Which benthic metrics (response indicators) were the most important to relationships with environmental quality conditions and how do these metrics rank from most to least important? and (2) Which environmental variables (stressors) were the most important in terms of relationships to benthic metrics indicative of community health and how do the stressors rank from most to least important? Consensus ranking of 14 benthic metrics showed that stress tolerant (% Tolerant taxa, Tolerance value and Collectors/gatherers) and stress sensitive (EPT taxa and taxa richness) were the most important. The consensus ranking of 38 environmental variables from most to least important showed that habitat metrics, metals and grain size characteristics were most important for shaping benthic communities while pyrethroids were not reported to be an important stressor across the geographic scope of the watersheds.