The Effectiveness of Total Mercury as a Surrogate for Methylmercury in Aquatic Invertebrates

A. BACKGROUND

Aquatic macroinvertebrates, such as dragonfly larvae or crayfish have great potential for  Mercury (Hg) monitoring, both as indicator organisms (or "sentinels'), and to provide critical  information on Hg in lower food webs to assist in interpreting fish Hg levels. Despite these  advantages, macroinvertebrates are not widely used in Hg monitoring because of the current  need to directly analyze macoinvertebrate tissue for methyl-mercury (MeHg), at much greater  expense than analyzing for total Hg (THg) (i.e. as a surrogate for MeHg), as with fish tissue.  This is because the ratio of MeHg to THg in macroinvertebrates exhibits large taxonomic,  spatial, and temporal variation, and there is no clear empirically-based documentation on which  to base decisions regarding the utility of macroinvertebrate THg levels for monitoring programs.  This also precludes the effective use .of existing macoinvertebrate THg data (collected since the  1980's) from across New York State to examine MeHg trends and spatial patterns.

 

B. PROJECT OVERVIEW

The goal of the proposed study is to provide a tool for the effective use of macroinvertebrates in  Hg monitoring by documenting MeHg:THg statistics and temporal and geographic variation in  these statistics. Specific objectives are 1) calculate MeHg:THg statistics for selected  macroinvertebrate taxa and feeding groups collected from New York State streams; 2) describe  the extent of temporal variation in MeHg:THg ratios for these taxa, and; 3) characterize the  extent to which the observed ratios are geographically robust by comparing results for  Adirondack streams in New York State with those for streams in other states.  A large data set will be analyzed to document ratios (and spatial and temporal variability in those  ratios) of MeHg:THg for a wide variety of aquatic macroinvertebrate taxa. This will provide a  critical scientific basis for decisions regarding the use of THg as a surrogate for MeHg in  macroinvertebrate tissue. The resulting savings in time, complexity, and expense of tissue  analysis will allow for the greater use of macroinvertebrates to determine the extent to which  biota have been affected by, are recovering from, and are responding to changes in Hg deposition  rates.

 

C. STATEMENT OF WORK

The USGS shall compile and analyze Hg species and stable isotope data for macroinvertebrates  that were collected by tl!e USGS during 2004-2008 from nine (9) stream sites in the Adirondack  region of New York State, and from 12 sites in other states. The USGS shall have as the primary  focus of these analyses the documentation and interpretation of MeHg:THg statistics for  macroinvertebrate taxa that were collected in New York State. The USGS shall use stable  isotope data from these samples to estimate trophic levels, which shall be incorporated into the  interpretation of results. The USGS shall incorporate data from sites in other states as needed to  evaluate the geographic robustness of the MeHg:THg patterns observed for taxa from New York  State streams. In addition to the USGS data, the USGS shall evaluate selected macroinvertebrate  datasets from other sources (e.g. University of Maine and the Biodiversity Research Institute)  that were collected in New York State, and will include these, as appropriate, in the analysis and  (or) interpretation of MeHg:THg statistics.  

 

Task I. Data Compilation  

The USGS shall compile existing MeHg, THg, and stable isotope data from the USGS Mercury  Topical Studies in New York, Wisconsin, Oregon, South Carolina, and Florida. The USGS shall  compile these data from published data reports and from the National Water Information System,  and shall communicate with the USGS Mercury Research Laboratory in Wisconsin to obtain any  unpublished data that may be useful for this project. The USGS shall also evaluate relevant  macroinvertebrate MeHg and THg data sets collected from New York State streams, lakes, and  ponds by others, and shall compile appropriate data into the analysis data set for comparison with  the findings from analysis of the USGS-collected data. 

 

Task 2.Data Analysis

The USGS shall calculate the ratios of MeHg to THg for each macroinvertebrate sample, and  shall calculate statistics (mean, median, standard deviation, minimum, maximum) for each  macroinvertebrate taxon. The USGS shall statistically and graphically examine ratios to  document taxonomic, trophic, spatial, and temporal variation in observed ratios for the New  York State data set. The USGS shall analyze nitrogen stable isotope data in order to incorporate  estimates of trophic position to incorporate or control for variation in trophic level within and  among taxa in the analysis of MeHg:THg ratios. The USGS shall statistically and graphically  compare MeHg:THg patterns for New York State with patterns shown by the same taxa from  other states, in order to assess the geographic 'robustness' of MeHg:THg ratios observed in New  York State. 

 

Task 3.Manuscript Preparation

The USGS shall interpret findings from the above analyses and shall summarize these findings  and interpretations in a manuscript for submission to a scientific journal for peer review.