Effects of acidic deposition on North American lakes: palaeolimnological evidence from diatoms and chrysophytes

Abstract Analysis of sediment diatom and chrysophyte assemblages is the best technique currently available for inferring past lake water pH trends. Use of this approach for assessing the ecological effects of acidic deposition is increasing rapidly. As of August 1989, sediment core inferred pH data existed for at least 150 lakes in North America and cores from about 100 more lakes are being analysed. Equations for inferring past pH are based on at least 15-20 calibration data-sets involving about 700 lakes. Palaeolimnological studies indicate that recent acidification has been caused by acidic deposition in the Adirondack Mountains (New York), northern New England, Ontario, Quebec and the Canadian Atlantic provinces. Inferred pH decreases are commonly as much as 0.5-1.0 pH units. With the exception of one lake, 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). Slight or no trends towards decreasing pH were observed in study lakes receiving moderately acidic deposition (upper Mid-west and northern Florida). 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 acid inputs, and is generally consistent with current lake-acidification theory. In most cases, the primary cause of recent acidification (post-1850) is acidic deposition, as opposed to land-use changes or natural processes, though these may be contributing factors. Acid loading has decreased in some regions since 1970 (e.g., northeastern United States). Some lakes have become less acidic in response, but others continue to lose acid neutralizing capacity. Many currently acidic lakes were naturally acidic (pH < 5.5) before the onset of anthropogenic acidification. These lakes are typically small (less than 10 ha) are located at moderately high elevations, have thin or peaty soils, or are located in outwash deposits. Many of these have acidified further recently.