Lichens as Monitors of Pollutant Elements at Permanent Sites in Maryland and Virginia

Abstract

Concentrations of sulfur, nitrogen, and seven metal elements are reported for the lichen Flavoparmelia baltimorensis from three monitoring stations in Maryland and Virginia. At Plummers Island, Maryland, 15 km from the center of Washington, D.C., lichens have been collected for nearly 100 years. The other two stations, Bear Island, Maryland (21 km) and Stony Man Mountain, Virginia (120 km), were established in the 1970's by Mason Hale to help document changes in ambient air quality in the Washington, D.C. area. Concentrations of cadmium, chromium, nickel, zinc, and lead were consistently highest at Plummers Island and lowest at Stony Man Mountain; concentrations of the other four elements (nitrogen, sulfur, aluminum, and copper) were also consistently higher at Plummers Island than at Stony Man Mountain. Recent collections, in 1988 and 1992, revealed marked reductions in the concentration of all nine elements except aluminum at all three stations, providing limited, but encouraging, signs of improved air quality in the region. Known primarily for his research on lichen systematics, Mason Hale has nevertheless made a number of important contributions to the study of lichens as environmental indicators. From 1977 until shortly before his death in 1990, Mason and I participated in a number of environmental studies in the Washington, D.C. area designed to monitor changes in regional air quality. One of our principal sites was Plummers Island, Maryland, a small Potomac River island located just 15 km from the heart of Washington, D.C. The studies we carried out there were an extension of Mason's work begun in the late 1950's, and all have been supported by the Washington Biologists' Field Club, a local scientific society that promotes research on Plummers Island and the Potomac River Basin. In initiating his work on Plummers Island, Mason believed it was important to establish permanent study sites for monitoring lichen growth, element accumulation, and floristic composition as the regional environment changed. Mason's early work on the island was involved mainly with the longterm assessment of lichen growth rates (Hale 1970, 1972, 1973; Lawrey & Hale 1977); after 1965, it was expanded to include elemental analysis for pollutant elements, especially lead (Pb). This shift in emphasis was intended to document changes on the island caused by the construction of a bridge (The American Legion Memorial Bridge) supporting a major interstate highway (Interstate 495) over the island. The initial analyses of lichens from Plummers Island during the 1970's (Lawrey & Hale 1979) revealed extraordinarily high Pb values, which were indicative of the high Pb concentrations in automobile exhaust fallout reaching the island. It was at this time that another study site was established on Bear Island, Maryland, 6 km upstream from Plumm rs Island. Bear Island lichens have consistently contained lower concentrations of Pb and other pollut nt elements than Plummers Island lichens (Lawrey 1980; Lawrey & Hale 1981). Floristic analysis of the two islands (much of it unpublished) has also shown that the lichen communities on Plummers Isla d are far less diverse than those found on Bear Island, and that the growth rate of thalli of the dominant lichen species on both islands, Flavoparmelia baltimorensis, is significantly lower in Plummers Island communities. In 1980, we established another long-term study TABLE 1. Lead concentrations in thalli of Flavoparmelia baltimorensis collected from Plummers Island and Bear Island, Maryland and Stony Man Mountain, Virginia at various times. All values are mean concentrations (.tg/g dry weight ? S.E. of the mean). Sample size is two for historical samples and 10 for recent samples. Year Plummers Island Bear Island Stony Man Mt. 1907 82.3 (8.2) 1938 127.8 (14.8) 1958 327.9 (12.6) 197