Paleoecological Analysis of Lake Acidification Trends in North America and Europe Using Diatoms and Chrysophytes

Abstract

Analysis of sediment diatom and chrysophyte assemblages is the best technique currently available for inferring past lake water pH trends, and use of this approach is increasing rapidly. Sediment-core-inferred pH data exist for at least 100 lakes in both North America and Europe. This number will approximately double within the next 2 years. The pH inference equations are based on at least 15 calibration data sets for North America and 10 for Europe, involving totals of at least 500 and 300 lakes, respectively.Paleoecological studies indicate that recent acidification has been caused by acidic deposition in the Adirondack Mountains (New York), northern New England, Ontario, Quebec, and Canadian Atlantic provinces in North America; England, Scotland, and Wales in the United Kingdom; and Norway, Sweden, Finland, The Netherlands, and West Germany in Europe. Inferred pH decreases are commonly as much as 0.5 to 1.5 pH units. No acidification trends were observed in regions currently receiving low deposition of strong acids (e.g., Rocky Mountains and Sierra Nevada in the western United States, northwestern Norway, and northwestern Scotland). Slight or no trends toward decreasing pH were observed in study lakes receiving moderately acidic deposition (upper Midwest and northern Florida, United States). The magnitude of pH decline in lakes studied is greater, on the average, in Europe than in North America. The amount of inferred acidification (increase in H+ concentration) correlates with the amount of S and N loading and the ability of watersheds and lakes to neutralize acidic inputs and is generally consistent with current lake acidification models.In most cases, the primary cause of recent acidification trends (post-1850) is atmospheric deposition of acidic material, as opposed to land use changes or natural processes, although these may be contributing factors. Acidic loading has decreased in some regions since 1970 (e.g., northeastern United States, United Kingdom). Some lakes have become less acidic in response, but others continue to lose buffering capacity and are becoming more acidic. Many currently acidic lakes were naturally acidic (pH <5.5) prior to the onset of anthropogenic acidification. These lakes are typically small (<10 ha), located at moderately high elevations, have thin or peaty soils, or are located in glacial outwash deposits. Many of these have acidified further recently.KeywordsDiatom AssemblageAcidic DepositionAcidic PrecipitationLake AcidificationAdirondack LakeThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.