CAN SITE-SPECIFIC TRENDS BE EXTRAPOLATED TO A REGION - AN ACIDIFICATION EXAMPLE FOR THE NORTHEAST

In the absence of true regional data on changes in the acid/base statu s of lakes in the northeastern United States, we explore the possibili ty of using site-specific trends information from a judgment sample of lakes to assess the efficacy of the Clean Air Act Amendments. A meta- analytical technique is used to combine trends results from 44 Long-Te rm Monitoring (LTM) lakes in the Northeast for the period 1982-1994, w ith the goal of producing estimates of overall trends in the region. T he lakes are subdivided into subpopulations (High ANC, Intermediate Ti ll Drainage, Thin Till Drainage and Perched Seepage lakes) on the basi s of their expected response to changes in acidic deposition, and they appear to represent the most acid-sensitive of these lake classes wel l. While the overall tendencies in the trends are as expected (e.g., m ost of the recovery is observed in the most sensitive subpopulations), there is significant trend heterogeneity among the lakes within most of the subpopulations; this heterogeneity prohibits the summarizing of trends at the regional level (i.e., for all of the Northeast). This h eterogeneity is explained by differences in the responses of lakes in two subregions (Adirondacks vs. New England), and we present trends re sults separately for each subpopulation within these two subregions. A ll subpopulations in both subregions showed decreasing trends in sulfa te concentrations, probably a reflection of decreasing trends in sulfu r deposition in the region. Few trends in nitrate concentrations were observed. Recovery (as evidenced by increasing trends in acid-neutrali zing capacity) was evident in Thin Till and Intermediate Till Drainage lakes in New England, but not in the Adirondacks. Most groups of lake s exhibited downward trends in base cations (Sigma[Ca2+ + Mg2+ + Na+ K+]); the magnitudes of these trends were always greater in Adirondac k lakes than in similar New England lakes. This suggests that the depl etion of soil cation pools in the Adirondacks may be responsible for s ome of the differences in recovery between Adirondack and New England lakes. While export of base cations may be the key difference producin g different trends results in the two subregions, the site-specific na ture of the trends, and their possible lack of regional representation , should be considered in interpreting the overall results.