METHYL AND TOTAL MERCURY IN BOREAL WETLAND PLANTS, EXPERIMENTAL LAKES AREA, NORTHWESTERN ONTARIO

A review of Hg concentrations cited in the literature reveals values in the general sequence: grassland herbs < trees and shrubs < aquatic macrophytes < Sphagnum < mosses < lichens < fungi, excluding those samples collected near point sources of environmental Hg. Samples of vegetation collected from wetland and upland catchments in the Experimental Lakes Area contained from 4 to 160 mu g total Hg (THg) kg(-1) and 0.1 to 139 mu g methyl Hg (MeHg) kg(-1). The lowest concentrations of both THg and MeHg were found in the leaves and needles of trees and shrubs, and the highest concentrations occurred in bryophytes, both feather mosses and Sphagnum. Analysis of sections of the mosses revealed larger concentrations of MeHg in the upper live section, whereas THg concentrations increased slightly down the stem into the dead portions. In terms of broad ecological grouping, THg and MeHg concentrations increased the closer the plant's habitat to the water table, though certain upland mosses also contained very large THg and MeHg concentrations. When combined with biomass data, the vegetation of an ombrotrophic wetland contained an average of 30 mu g THg m(-2) and 0.4 mu g MeHg m(-2), dominantly (45 and 73%, respectively) in the mosses. These values are similar to that contained in the peat porewater, but small compared to the estimate of the stock of THg and MeHg contained in the 1.7 m thick peat (12 000 and 200 mu g m(-2), respectively).