ENVIRONMENTAL AUDITING - REGIONAL LAKE TROPHIC PATTERNS IN THE NORTHEASTERN UNITED-STATES - 3 APPROACHES

During the summers of 1991-1994, the Environmental Monitoring and Asse ssment Program (EMAP) conducted variable probability sampling on 344 l akes throughout the northeastern United States. Trophic state data wer e analyzed for the Northeast as a whole and for each of its three majo r ecoregions-the Adirondacks (ADI), the New England Uplands (NEU), and the Coastal Lowland and Plateau (CLP)-and inferred to the entire popu lation of lakes greater than or equal to 1 ha (N = 11,076). Results we re compared to a large, nonrandomly sampled data set for the same area compiled by Rohm and others and contrasted with lake trophic state in formation published in the National Water Quality Inventory: 1994 Repo rt to Congress [305(b) report. Lakes across the entire Northeast were identified by EMAP data as 37.9% (+/-8.4%) oligotrophic, 40.1% (+/-9.7 %) mesotrophic, 12.6% (+/-7.9%) eutrophic, and 9.3% (+/-6.3%) hypereut rophic. Lakes in the ADI and NEU generally are at a low, nearly identi cal trophic state (96% oligotrophic/mesotrophic), while those in the C LP are much richer (45% eutrophic). EMAP results are similar to result s of the Rohm data set across the entire region. In the CLP, however, EMAP identified approximately 45% of the lakes as eutrophic/hypereutro phic, while the Rohm data set identified only 21% in these categories. Across the entire Northeast, the 305(b) report identified a much high er proportion (32.2%) or lakes in eutrophic condition and a much small er proportion (19.8%) in oligotrophic condition than did the EMAP surv ey data(12.5% +/- 7.9% and 37.9% +/- 8.5%, respectively). Probability sampling has several advantages over nonrandom sampling when regional resource condition assessment is the goal.