Spatial and Temporal Patterns of Bioindicator Mercury in Pennsylvania Oak Forest

Abstract

We monitored spatial and temporal patterns of total Hg in forest bioindicators to assess possible local, regional, and global changes in atmospheric Hg deposition. Total Hg concentrations were monitored in leaves and fresh litterfall of northern red oak ( L.), on an epiphytic moss ( Hedw.) on northern red oak stems, and in surface soil organic matter (O and O horizons) in Pennsylvania oak-dominated forests. Variously configured plots were used to monitor Hg deposition near local coal-fired generating stations and an industrial city and along an extended regional transect. Linearly decreasing temporal trends in Hg concentrations occurred in leaves, litterfall, moss, and soil O and O. Mean annual Hg concentrations were often greater near local emissions sources compared with remote areas, especially in the initial monitoring period. Decreasing time trends for different impact areas tended to converge due to greater rates of Hg decrease where initial bioindicator Hg levels were higher. Fresh litter and soil O showed the greatest overall potential as Hg bioindicators. We conclude that Hg deposition has been significantly decreasing over time throughout the study area as a result of locally and regionally declining Hg emissions. Reductions in Hg emissions are likely a co-benefit of the 1990 Clean Air Act regulations and changing industrial activities. Recent leveling of several bioindicator Hg time trends may foretell a shift in Hg depositional patterns. Mercury monitoring studies such as this fulfill a need for documenting local and regional effects of emissions reduction.

Forest Structure Affects Soil Mercury Losses in the Presence and Absence of Wildfire

“…A similar decline in O-horizon Hg concentration in a Pennsylvania oak forest was attributed to reduced Hg deposition associated with declining regional and local Hg emissions. 3 This is substantiated by trends in Hg concentrations in precipitation, which decreased in the Northeast U.S.A. from 1996 to 2010. 4,5 In the Western U.S.A., however, temporal changes in atmospheric Hg are spatially variable.…”

“…Both atmospheric deposition and parent material are sources of soil Hg . Less atmospheric deposition resulting from a decrease in local and regional anthropogenic Hg emissions is the proposed mechanism of the decrease in soil Hg concentration over a two-decade period in Pennsylvania oak forest . Recent multidecadal decreases in anthropogenic mercury emissions across much of North America − portend possible decreases in atmospheric deposition to forests and the consequent decreases in soil Hg pools.…”