Understanding the environmental controls on plant mercury from northern Minnesota peatlands.

Abstract

Peatlands are delicate ecosystems that play a crucial role in global biogeochemical cycles, particularly in the storage and cycling of mercury (Hg). Human activities, especially since the Industrial Revolution, have dramatically increased Hg levels in the environment, raising concerns due to health risks associated with its bioaccumulation and biomagnification in food webs. The ability of peatlands to influence the fate, transport, and conversion of Hg is significant, as they are known to act as both sinks and potential sources of this toxic element. Additionally, climate change, which has accelerated due to human activity, further complicates the dynamics of Hg in ecosystems by altering key biogeochemical cycles. A deeper understanding of how climate change affects Hg concentration in peatlands is essential for predicting future environmental conditions and their impacts on human and ecological health. This study investigates THg (total mercury) concentration in peatland vegetation under various climate change scenarios, focusing on how warming, elevated carbon dioxide (eCO2), and peatland type influence Hg uptake by six common peatland species. The research was conducted at the USDA Forest Service Marcell Experimental Forest at the Department of Energy’s SPRUCE (Spruce and Peatland Responses Under Changing Environments) site, a unique climate change experiment designed to simulate future environmental conditions. Samples were collected from six bioindicator species bi-weekly over a 26-week period in 2021, spanning the entire growing season from spring to fall. The samples were then processed and analyzed at the University of Minnesota to quantify THg concentrations.Results from the study showed species-specific responses to warming and eCO2. Sphagnum moss, leatherleaf, and black spruce exhibited significant increases in THg concentrations under warming conditions, with a critical threshold observed at +4.5°C, beyond which THg concentration rates declined. Tamarack displayed a different response, showing decreasing THg concentration with increased temperatures alone, until +4.5°C where concentrations began to increase. The interaction between eCO2 and warming further complicated these dynamics, as eCO2 generally led to a reduction in mean THg concentrations for most species, except for tamarack, which accumulated more THg under these eCO2. The results highlight the complex relationship between plant physiology and climate variables and mercury concentrations in plants. Peatland type (bog vs. fen) did not significantly affect THg concentration for most species, except for Labrador tea, which accumulated more THg in fen environments. The study suggests that hydrology plays a secondary role in climate in THg cycling, but species like Labrador tea may be more sensitive to moisture and nutrient availability. Overall, the findings underscore the importance of species-specific responses and climate thresholds on THg concentration. The results provide a foundation for predicting how peatlands, and the broader ecosystems they support, may behave under future climate conditions. By understanding the role of temperature and CO2 in THg concentration, this study contributes valuable insights to the management of peatland ecosystems, which could inform strategies to mitigate THg pollution and its ecological and human health risks.