A Probabilistic Risk Assessment of the Effects of Methylmercury on Great Egrets and Bald Eagles Foraging at a Constructed Wetland in South Florida Relative to the Everglades

ABSTRACT

This case study summarizes an assessment of risk of methylmercury (MeHg) exposure to fish–eating birds foraging at Stormwater Treatment Area–2 (STA–2). This assessment was required as a special condition for a permit modification authorizing flow–through operation of STA–2 Cell 1 without it first satisfying formal mercury start–up criteria. The assessment estimates the risks posed by MeHg to the great egret (Ardea albus) and the bald eagle (Haliaeetus leucocephalus). Exposure models were based on literature–derived life history parameters combined with site–specific MeHg concentrations in water, sediment, and fish. To assess risk, daily MeHg intake by females and cumulative MeHg consumed by nestlings were compared to literature–derived effects thresholds. The results indicated the likelihood was low that MeHg exposures to birds foraging throughout STA–2 would exceed effects thresholds at the time of this assessment. Birds foraging exclusively from Cell 1 or the small discharge canal were predicted to experience greater exposures and could be at potential risk. However, this worst–case risk was comparable or lower than risk levels encountered in nearby water conservation areas or the Everglades National Park.

KeyWords:

• methylmercury
• constructed wetland
• bald eagle
• great egret
• ecological risk assessment.

ACKNOWLEDGMENTS

The author acknowledges the support of the South Florida Water Management District in the publication of this document. The author thanks Larry Fink, Tim Bechtel, Ron Bearzotti, Michael Chimney, Gaea Crozier, and three anonymous reviewers for helpful comments on this manuscript. I also thank Ted Lange for making his data sets available and for his efforts in the field.

Notes

^*Normal distributions were truncated at ± 2SD to avoid the simulation of implausibly low or high values. Although assimilation efficiency could be as low as 83% (Fournier et al. 2002), effects thresholds are based on amount ingested not assimilated.

^*Composite of 100–250 mosquitofish.

^a Fishes > 170 mm in length were censored prior to developing PDF for egret exposures due to significant size–Hg tissue concentration relationship.

^b Did not exhibit significant size–tissue Hg concentration relationship and, hence, used in its entirety for egret exposures.

^c Because fish were collected from two different locations in 2001 and 2002 (for details, see Rumbold 2004), both locations were sampled in 2003.

^d Note, 13 sunfish caught at L67A and analyzed as fillets by FFWCC were converted to whole body using equation developed for bass.

^e Data on 2003 fish from this canal site not available at the time of this report. Includes mosquitofish collected from adjacent marsh.

^*Normal distributions were truncated at ± 2SD to avoid the simulation of implausibly low or high values. Although assimilation efficiency could be as low as 83% (Fournier et al. 2002), effects thresholds are based on amount ingested not assimilated.