TOTAL MERCURY AS A SURROGATE FOR METHYLMERCURY IN AQUATIC MACROINVERTEBRATES

Methylmercury (MeHg), the toxic and bioavailable form of mercury (Hg), typically is the Hg species of greatest interest for monitoring and research concerning Hg bioaccumulation. Laboratory analysis of biological tissue for MeHg is, however, much more expensive than analyses for total Hg (THg). Thus, when the MeHg percentage relative to THg (hereafter MeHg%) is high and well-established, as in fish tissue (generally greater than 95%), the less-costly THg analysis can be performed, and the resulting data can be assumed to adequately represent MeHg concentrations. Aquatic macroinvertebrates have great potential to function as MeHg indicator organisms (or "sentinels”) in Hg monitoring, and to inform MeHg cycling and bioaccumulation research. They are important in MeHg trophic transfer from primary producers to fish and wildlife, are often abundant, and can be collected without specialized equipment. Despite these advantages, macroinvertebrates are often not included in MeHg monitoring programs and research studies due to the lack of an empirical basis for the use of THg as a MeHg surrogate, and the resulting need to conduct more-expensive MeHg analyses on macroinvertebrate samples. The U.S. Geological Survey (USGS), with support from the New York State Energy Research and Development Authority, is conducting a study to evaluate the suitability of THg as a surrogate for MeHg in aquatic macroinvertebrates. MeHg, THg, and stable isotope data from more than 400 samples, representing scraper, shredder, collector-gatherer, filterer, omnivore, and predator functional feeding groups (FFGs), were compiled from published USGS stream studies conducted in the central Adirondacks of New York (nine streams), and in the Coastal Plain of South Carolina and Florida (three streams and one stream, respectively). MeHg% in macroinvertebrates collected from Adirondack streams varied significantly among feeding groups; medians ranged from 35% in scrapers to 92% in predators. MeHg% in larval dragonflies (Odonata: Aeshnidae and Libellulidae) was generally high (> 90%) and exhibited little spatial or temporal variation. MeHg% in most of the lower FFGs, however, varied widely among sites and sampling periods. THg may be a suitable surrogate for MeHg in dragonflies and other predatory macroinvertebrates in streams.