Impacts of mercury on benthic invertebrate populations and communities within the aquatic ecosystem of Clear Lake, California

Benthic invertebrates from Clear Lake, site of an inactive mercury (Hg) mine, were analyzed for population and community level parameters in response to a significant point source of sediment-associated Hg. Using multiple regression, at least one taxon (Placobdella leeches) showed a significant decline and another taxon (Procladius midges) showed a significant increase in response to increasing sediment Hg. Responses of invertebrates to sediment Hg levels are complex, likely due to partial confounding between sediment Hg (especially methyl Hg), grain size and depth. Stepwise multiple regression analyses indicate that individual taxa often responded significantly to several environmental factors. Chironomus populations declined with increasing grain size, depth and total Hg; Procladius declined with increasing depth, but increased with increasing sediment grain size and Hg levels; Chaoborus declined with increasing depth; oligochaetes increased with increasing TOC; and Placobdella leeches declined with both increasing depth and sediment Hg levels. Additional multi-variate routines were used to demonstrate more complex relationships than are typically elucidated by standard multiple regression statistics. The complex results presented here may indicate that there are significant population effects above some threshold of sediment Hg concentrations. Community level parameters (diversity and evenness) declined with increasing sediment Hg levels, but with considerable variation at low Hg levels. Simple regression yielded a negative relationship between diversity and evenness versus sediment total Hg that was nearly significant, and one with sediment methyl Hg that was not close to significance. Multiple regression indicated that depth was more important than sediment Hg in describing the variation in diversity.