Acidic Deposition and Aquatic Ecosystems

Abstract

Adirondack Mountains The Adirondack Mountain region in northern New York State is a large forested area, underlain by bedrock material primarily composed of granitic gneisses and metasedimentary rocks. The surficial materials covering the region are the result of glacial activity; soils developed from glacial till are typically acidic Spodosols. The Adirondacks receive high deposition of acidic materials (SO42− deposition approximately 500 eq ha−1 yr−1). The combination of large inputs of strong acids from the atmosphere and geologic/edaphic characteristics that limit base cation supply to drainage waters places the aquatic resources of the Adirondacks at risk from acidic deposition. Results from the U.S. EPA Eastern Lake Survey (representing a population of 1,290 lakes) suggest that 14% of the lakes > 4 ha in surface area are acidic (acid neutralizing capacity (ANC) ≤ 0 µeq L−1). However, the number and percentage of acidic Adirondack lakes determined from synoptic surveys are influenced by the boundary designating the region as well as by the minimum surface area of the lakes sampled. For example, the Adirondack Lake Survey Corporation (ALSC) survey, which included 1,469 lakes in the Adirondack ecological zone with a surface area > 0.5 ha, determined that 26% of the lakes are acidic. There is considerable diversity in the acid-base status of Adirondack lakes. Lake-to-lake variation in ANC is largely the result of differences in the supply of base cation concentrations to drainage water, as concentrations of strong acid anions are relatively uniform across the region. Regional patterns in lake chemistry are evident across the Adirondacks. Acidic and low ANC lakes are generally located in the western and southern areas of Adirondack Park. The bedrock geology in this area is principally granitic gneiss and many lakes are situated in high-elevation (> 600 m) basins with shallow glacial till. In the northern and eastern regions of Adirondack Park, lakes occur at lower elevations in watersheds with thick deposits of glacial till or stratified drift. These waters tend to be characterized by higher ANC values. In addition, the bedrock geology of the eastern Adirondacks is calcium-rich anorthosite, which facilitates the supply of base cations to drainage waters. Many lakes along Lake Champlain and outside the Adirondack Park boundary occur at low elevations and are impacted by salt and/or contain carbonate deposits within basins. These waters have high ANC and are generally insensitive to acidic deposition. Process-level watershed studies indicate that hydrologic flow path is the principal factor regulating the supply of base cations to Adirondack lakes. As a result, a classification system based largely on hydrologic flow paths was developed for Adirondack lakes. Drainage lakes or closed lakes are the predominant hydrologic type (84%) of ponded Adirondack waters. Within the population of drainage lakes, thin till basins (< 5% of the basin area covered by thick till and stratified drift; 28% of Adirondack lakes) have the potential to be chronically acidic under current loadings of acidic deposition. Location of Adirondack Park, the Adirondack ecological zone, and the boundary of the U.S. EPA Eastern Lake Survey—Phase I (ELS-I) subregion 1A in New York State. The major watersheds within the Adirondack region are indicated Intermediate till basins (5% to 25% of the basin area covered by thick till and stratified drift; 18% of Adirondack lakes) generally have a baseflow ANC of < 100 µeq L−1. Surface and outlet waters of these lakes have the potential for substantial episodic acidification (loss of ANC > 50 µeq L−1) during snowmelt. Watersheds with thick till or stratified drift (> 25% of the basin area), or containing deposits of carbonate minerals are characterized by lakes with high ANC and are insensitive to acidic deposition (21% of Adirondack lakes). Information is available from historical lake surveys, water quality monitoring studies, and sediment records to assess recent changes in the acid-base status of Adirondack waters. Limitations are evident in all individual approaches, which necessitates compositing observations to verify acidification patterns. Most studies indicate that acidification of Adirondack waters has occurred sometime during this century. Although there is considerable uncertainty as to the exact timing of the recent acidification, the limited data suggest that most changes in surface water acidity probably occurred between 1930 and 1960. Sulfate is currently the dominant anion in acidic waters and this SO42− can be attributed largely to atmospheric deposition. However, the extent of ANC decreases due to acidic SO42− inputs cannot be resolved with existing data. There is limited evidence that base cations and NO3− concentrations have also changed in some waters, contributing to trends in ANC. Simulations from the ILWAS acidification model show that there are large lake-to-lake variations in response to decreases in sulfur loading. Adirondack Lake Survey Corporation observations indicate that 55 species of fish have been collected. There is considerable range in the sensitivity of fish to conditions of acidity and several species of fish are tolerant of acidic conditions. However, the number of fish species caught decreases with decreasing pH in Adirondack lakes. Surveys conducted by the ALSC have shown that no fish were caught in 24% of Adirondack lakes. These waters are generally located in the southwestern Adirondacks. Fishless lakes are significantly correlated with a number of physical and chemical factors, including lake size, lake elevation, pH, and O2 concentrations. Thus, specific reasons for the absence of fish are difficult to identify, although the absence of fish is correlated with lake pH.