Mercury and calcium in forest food webs: Potential for synergistic effects on avian reproduction

Background/Question/Methods

Atmospheric deposition of acids and mercury has altered forest and lake ecosystems in the Adirondacks and through the Northeast.  While often studied independently, the impacts of acid rain and mercury deposition have the potential to interact.  We examined the terrestrial food webs of forest songbirds for patterns and interactions between mercury (Hg) and calcium (Ca).  Hg is a well-documented pollutant of northeastern ecosystems and Ca is a nutrient critical for bird reproduction that is depleted from soils with acid rain.  High Hg and low Ca availabilities could co-occur in forest food webs resulting from similar deposition patterns of Hg and acids and associations with soil and water pH.  High Hg and low Ca in songbird diets can both cause thin eggshells and reduced reproductive success.  We collected soils, vegetation, litter invertebrates, and bird eggs from 6 sites throughout the Adirondacks and analyzed samples for Ca and total Hg content. 

Results/Conclusions

Songbird eggs (n=36) had Hg concentrations between 37 and 257 ppb, with highest values from nests near large water bodies.  No eggs showed signs of thin eggshells or reduced hatchability.  Ground spiders had the highest concentrations of Hg (0.161 ppm dry weight), but snails represented the largest pool of invertebrate Hg.  Snails also had the highest concentrations (675 ug/g dry weight) and were the largest pool of Ca.  Hg and Ca in invertebrate prey trended strongly toward higher availabilities in the spring, when birds are beginning to lay eggs.  Invertebrate Hg was highest at sites with high mineral soil Hg and moisture, and was unrelated to Ca availability in soil.  Birds could be exposed to more dietary Hg during egg formation resulting from temporal patterns in invertebrate Hg availability and the potential for increased mercury exposure in Ca-rich food items ingested for eggshell formation.  However, Hg availability was high when Ca availability was also high, providing evidence that the risk of synergistic deleterious effects of high Hg and low Ca on forest songbird reproduction may be less important than overall impacts of Hg pollution.