Water mites and their use as bioindicators of water quality conditions: a Pennsylvania case study

Water mites (Acariformes: Hydrachnidia) are one of the planet's most diverse and abundant groups of aquatic arthropods. With more than 7,500 described species, water mites have been documented in nearly every known freshwater environment. In lotic systems, water mites are commonly found in fast-flowing, riffle-run stream habitats, where one square meter of substrate can contain as many as 5,000 individual mites representing more than 50 species, 30 genera, and all 8 superfamilies. Despite their widespread distribution, relative diversity, and abundance, water mites have been widely neglected in freshwater biomonitoring efforts. However, several studies across the globe have found that water mites can be used as successful bioindicators of water quality conditions. Thus, the following study provides one of the first detailed examinations of lotic water mites and their bioindicator potential in North America. We sampled water mites from sites along "attaining" (unpolluted) and agriculturally "impaired" (polluted) riffle-run stream habitats and compared their assemblages to other physicochemical and benthic macroinvertebrate metrics used to quantify water quality in Pennsylvania. In total, we collected 9,139 individual water mites representing 5 superfamilies, 9 families, and 14 genera from both attaining (n = 13) and impaired (n = 13) streams. Similar to other global studies, we found significant differences between the water mite assemblages of attaining and impaired streams, with attaining streams having greater richness and diversity of water mites. Our data shows that several mite taxa are associated with biological and physicochemical conditions of water quality, where some known sensitive taxa are increasingly dominant in attaining streams and less dominant in impaired streams. Conversely, some pollution-tolerant taxa dominance increases in impaired streams while sensitive taxa abundance declines. These findings illustrate that water mites vary in their sensitivity to pollution and are suitable bioindicator species whose inclusion in biomonitoring assessments will increase assessment accuracy.