Response of the mercury cycle in an Adirondack, USA lake watershed to recovery from decreasing acid deposition and lime application

Background/Question/Methods

Acid-impacted lakes in northern Europe and northeastern USA are beginning to recover due to  decreases in acid deposition.  Resultant increases in pH in these lakes have coincided with increases in dissolved organic carbon (DOC), which is positively correlated with total mercury and methylmercury concentrations.  Methylmercury is a neurotoxin, which strongly bioaccumulates and biomagnifies in its methylated form.  Thus, understanding how the mercury dynamics of watershed ecosystems respond to recovery from acid deposition has important implications for wildlife and human health.  Trophic transfer of mercury is of particular concern in Adirondack lakes containing heritage brook trout (Salvelinus fontinalis) populations, (i.e., genetically unique populations unaffected by stocking practices).  Honnedaga Lake is one of seven lakes in the Adirondack Park of New York State with heritage brook trout.  Though this ecosystem was previously impacted by acid deposition, it is beginning to show signs of recovery. However, model projections indicate this process could take decades.  A demonstration watershed-liming project in a chronically acidified tributary of Honnedaga Lake is being conducted to evaluate mitigation of acid deposition in this critical brook trout spawning habitat.  We are utilizing this experiment to investigate the changes in watershed mercury dynamics associated with recovery from acidification. 

Results/Conclusions

During the 12 months after lime addition, the stream draining the treated watershed maintained a pH level above 5, a significant improvement over pre-treatment values and the reference site, which continues to have a pH below this critical level (p<0.001).  Total mercury concentrations in the stream were strongly correlated with DOC concentrations (treatment R²=0.90, reference R²=0.63), indicating a strengthening of this relationship after liming.  Time-series analysis of aqueous mercury concentrations reveals a first flush effect of significantly elevated levels within four weeks after treatment (THg post-treatment peak=5.5ng/L, THg reference peak=2.15). Six months after treatment, there is no significant difference in methylmercury concentrations from reference values (p=0.524), while total mercury remains significantly higher (p=0.004).  This pattern suggests that elevated mercury is leaching from the treated watershed but is not being readily methylated before exiting the tributary.  Further study is required to determine the long-term impacts of reduced acidification on mercury transport and bioavailability.