Temporal trends in low alkalinity lakes of the Upper Midwest (1983–1989)

The Upper Midwest contains a large concentration of low alkalinity lakes located across a west to east gradient of increasing deposition acidity. We present temporal trends in the chemistry of 28 lakes (4 in Minnesota, 13 in Wisconsin, and 11 in Michigan) representative of the acid-sensitive resource of the region. Lakes were sampled three times per year between 1983 and 1989. Temporal trends in SO₄ ^2? were all negative in direction, consistent with a regional decline in SO₂ emissions and atmospheric SO₄ ^2? deposition. However, these trends occurred predominantly in higher ANC (100 to 225 Μeq L^?1), non-seepage lakes and were associated with increases in ANC and pH in only one of the 8 lakes. ANC decreased in a second group of lakes, usually in concert with decreased [Ca²⁺+Mg²⁺], a response we associate with a severe drought. Disruptions in hydrologic flowpaths caused one lake to acidify rapidly after inputs of ANC-rich groundwater ceased and appeared to cause ANC and [Ca²⁺+Mg²⁺] declines in a second lake by reducing stream-water inflow. Our analysis was thus complicated by hydrochemical effects of climatic variability, which confounded trends related to acidic deposition. Periods longer than 6 yr are needed to transcend climatic signals and verify subtle trends related to atmospheric pollutants.     

Seasonal and long-term temporal patterns in the chemistry of Adirondack lakes

There is considerable interest in the recovery of surface waters from acidification by acidic deposition. The Adirondack Long-Term Monitoring (ALTM) program was established in 1982 to evaluate changes in the chemistry of 17 Adirondack lakes. The ALTM lakes exhibited relatively uniform concentrations of SO₄ ^2?. Lake-to-lake variability in acid neutralizing capacity (ANC) was largely due to differences in the supply of basic cations (Ca²⁺, Mg²⁺, K⁺, Na⁺; C_B) to drainage waters. Lakes in the western and southern Adirondacks showed elevated concentrations of NO₃ ^?, while lakes in the central and eastern Adirondacks had lower NO₃ ^? concentrations during both peak and base flow periods. The ALTM lakes exhibited seasonal variations in ANC. Lake ANC was maximum during the late summer or autumn, and lowest during spring snowmelt. In general Adirondack lakes with ANC near 100 Μeq L^?1 during base flow periods may experience decreases in ANC to near or below 0 Μeq L^?1 during high flow periods. The ALTM lakes have exhibited long-term temporal trends in water chemistry. Most lakes have demonstrated declining SO₄ ^2?, consistent with decreases in SO₂ emissions and SO₄ ^2? in precipitation in the eastern U.S. Reductions in SO₄ ^2? have not coincided with a recovery in ANC. Rather, ANC values have declined in some ALTM lakes. This pattern is most likely due to increasing concentrations of NO₃ ^? that occurred in most of the ALTM drainage lakes.     

Trends and patterns in lake acidification in the State of Vermont: Evidence from the Long-Term Monitoring Project

Twenty-four low acid neutralizing capacity (ANC) lakes in Vermont have been monitored since 1980 to characterize their chemical variability, and to determine if they exhibit temporal trends in acid/base chemistry. Many of the lakes exhibit significant decreasing trends in SO₄ ^2? and base cation (C_B) concentrations, but few exhibit significant changes in pH or ANC. An examination of all trend results (significant and insignificant) suggests a tendency for ANC and pH values in these lakes to be increasing, but either the changes are too small, or the number of observations too small, for these trends to be significant. Data from these lakes suggest that the primary responses of surface waters in this region to declining rates of SO₄ ^2? deposition are decreases in SO₄ ^2? concentrations and rates of cation leaching from watershed soils. Decreasing rates of c_b deposition may combine with lower rates of cation leaching to produce declines in c_B that are very similar to measured declines in SO₄ ^2? concentration. Vermont lakes exhibit their lowest ANC values in spring, attributable, for the most part, to dilution of c_B concentrations during spring snow melt. Concentrations of SO₄ ^2? are also more dilute in the spring, but c_B decreases are greater, and the net effect is a lowering of ANC. One quarter of the Vermont lakes monitored exhibit strong seasonality in NO₃ ^? concentrations, with peak concentrations near 70 Μeq L^?1. In these lakes, spring increases in NO₃ ^? concentrations are more important than C_B dilution in producing minimal spring ANC values.     

Trends in the Chemistry of Acidified Bohemian Lakes from 1984 to 1995: I. Major Solutes

Temporal changes in major solute concentrations in six Czech Republic lakes were monitored during the period 1984–1995. Four chronically-acidic lakes had decreasing concentrations of strong-acid anions (C_SA = SO₄ ^2- + NO^{3 -} + Cl^-), at rates of 3.0 to 9.0 μeq L^-1 yr^-1. Decreases in SO₄ ^2-, NO₃ ^-, and Cl^- (at rates up to 5.1 μeq L^-1 yr^-1, 3.2 μeq L^-1 yr^-1, and 0.6 μeq L^-1 yr^-1, respectively) occurred. The response to the decrease in deposition of S was rapid and annual decline of SO₄ ^2- in lake water was directly proportional to SO₄ ^2- concentrations in the acidified lakes. Changes in NO₃ ^- concentrations were modified by biological consumption within the lakes. The decline in C^SA was accompanied in the four most acidic lakes by decreases in Al_T, increases in pH at rates of 0.011 to 0.016 pH yr^{- 1}, and decreases of Ca²⁺ and Mg²⁺ (but not Na⁺) in three lakes. The acid neutralizing capacity (ANC) increased significantly in all six lakes. Increases in base cation concentrations (CB = Ca²⁺ + Na⁺ + Mg²⁺ + K⁺) were the principal contributing factor to ANC increases in the two lakes with positive ANC, whereas decrease in C_SA was the major factor in ANC increases in the four chronically-acidic lakes. The continued chemical recovery of these lakes depends on the uncertain trends in N deposition, the cycling of N in the lakes and their catchments, and the magnitude of the future decrease in S deposition.     

Long-term trends in the chemistry of precipitation and lake water in the Adirondack Region of New York,USA

Long-term changes in the chemistry of precipitation (1978–94) and 16 lakes (1982–94) were investigated in the Adirondack region of New York, USA. Time-series analysis showed that concentrations of SO₄ ^2–, NO₃ ^–, NH₄ ⁺ and basic cations have decreased in precipitation, resulting in increases in pH. A relatively uniform rate of decline in SO₄ ^2– concentrations in lakes across the region (1.81±0.35 eq L^–1 yr^–1) suggests that this change was due to decreases in atmospheric deposition. The decrease in lake SO₄ ^2– was considerably less than the rate of decline anticipated from atmospheric deposition. This discrepancy may be due to release of previously deposited SO₄ ^2– from soil, thereby delaying the recovery of lake water acidity. Despite the marked declines in concentrations of SO₄ ^2– in Adirondack lakes, there has been no systematic increase in pH and ANC. The decline in SO₄ ^2– has corresponded with a near stoichiometric decrease in concentrations of basic cations in low ANC lakes. A pattern of increasing NO₃ ^– concentrations that was evident in lakes across the region during the 1980's has been followed by a period of lower concentrations. Currently there are no significant trends in NO₃ ^– concentrations in Adirondack lakes.     

Recent Temporal Patterns in the Chemistry of Small,Acid-sensitive Lakes in Central Ontario,Canada

We monitored the chemistry of 603 small water bodies in three acid-sensitive regions of central Ontario, Canada (Algoma n=235, Muskoka n=216, Sudbury n=152) between 1988^-1996 to determine whether they have responded to recent SO2 emission reductions, and whether any chemical changes were related to lake characteristics. During the study, 27-56% of lakes declined in SO₄ ^2- concentrations, 41-57% declined in base cation concentrations, but only 26-28% increased in pH or ANC (acidity status). Increases in pH were greatest in lakes with low ANC, but had weak relationships to lake color or volume. No consistent trends were observed for DOC, NO₃ ^- or TP concentrations. Clearly, the long-term biological recovery of these sensitive aquatic ecosystems will depend on interactions among several environmental stressors, including acidification.     

recent lake acidification and recovery trends in southern quebec,canada

A total of 51 lakes in southern Quebec, Canada, were sampled between 1985 and 1993 to study changes in water chemistry following reductions in SO₂ emissions (main precursor of acid precipitation). Time series analysis of precipitation chemistry revealed significant reductions in concentrations and deposition of SO₄ ^2- from 1981 to 1992 in southern Quebec as well as reductions in concentrations and deposition of base cations (Ca²⁺, Mg²⁺), NO₃ ^- and H⁺ in the western section of the study area. Reductions in atmospheric inputs of SO₄ ^2- have resulted in decreased lakewater SO₄ ^2- concentrations in the majority of the lakes in our study, although only a small fraction (9 of 37 lakes used in the temporal analysis) have improved significantly in terms of acidity status (pH, acid neutralizing capacity – ANC). The main response of the lakes to decreased SO₄ ^2- is a decrease in base cations (Ca²⁺+Mg²⁺), which was observed in 17 of 37 lakes. Seventeen lakes also showed significant increases in dissolved organic carbon (DOC) over the period of study. The resulting increases in organic acidity as well as the decrease in base cations could both play a role in delaying the recovery of our lakes.     

Recent trends in the acid-base status of surface waters in Maine,USA

Data from the EPA Long Term Monitoring Program lakes at the Tunk Mountain Watershed, Maine, indicate that decreases of ≤1 Μeq L^?1 yr^?1 in SO₄, and increases of ≤2 Μeq L^?1 yr^?1 in ANC occurred in the 1980s. The sum of base cations also increased. These changes in aquatic chemistry were coincident with decreased concentrations of all solutes in precipitation during the 1980s. Other data on lakes and streams in Maine collected between the 1930s and 1990 generally confirm these trends and further indicate that larger increases in ANC may have occurred in some lowland lakes since 1940. Paleolimnologic studies indicate that decreases of 0.1 to 0.5 pH units occurred in a few small mountain lakes during the past 20 to 70 yr. However, ongoing acidification of lakes is indicated based on available data. Only lakes that were already at least marginally acidic (pH ≤5.8, ANC approximately 0) appear to have acidified.     

Inter-regional comparison of patterns and trends in surface water acidification across the United States

Temporal trends in acid-base chemistry are reported for surface waters in 6 regions of the United States. The lakes and streams are low ANC, dilute systems, selected to represent acid-sensitive aquatic resources in the 6 regions. The predominant trends observed were decreases in lake and stream SO₄ ^2? concentrations in sites east of the Mississippi River, and increases in NO₃ ^? in the Adirondack lake and Catskill stream sites (both located in eastern New York State). Correlations of trend results from all sites with other factors indicated that trends in precipitation volume were highly correlated with the observed trend patterns. From the surface water trend results, three distinct clusters were identified that corresponded to three trend patterns: ‘dilution,’ ‘recovery,’ and ‘acidification.’ These were distributed across the LTM regions, with no particular geographic patterns.     

Recovery of previously acidified lakes near Coniston,Canada following reductions in atmospheric sulphur and metal emissions

The purpose of the present study was to assess the rate of recovery of acidified lakes located near the town of Coniston following an abrupt reduction in atmospheric SO₂ and metal emissions at the Coniston smelter which closed in 1972. The water chemistry of several lakes was studied over a period of 16 yr (1968–1984). In one extremely acidic lake close to the smelter, the pH increased from 4.05 in 1972 to 5.8 in 1984. Conductivity, as well as concentrations of SO₄, Cu, Ni, Co, Mn, and Zn decreased by 60% to 90% in the lake water during the same period. In another initially less acidic lake nearby, the increase in pH was less dramatic, while the decrease in conductivity, SO₄, and some metals was similar to that of the more acidic lake. Local SO₄ deposition decreased approximately 75 % while Cu and Ni deposition decreased by 90% following closure of the Coniston smelter. These results indicate that even severely acidified lakes can improve within a few years following a substantial reduction in atmospheric S emissions; and that in some regions recovery can occur due to reductions in SO₂ emissions even in the absence of concurrent NO_x control.     

Regional chemical characteristics of lakes in North America: Part I — Eastern Canada

Data defining the major ion chemistry of lakes located in eastern Canada have been compiled for the purpose of evaluating the current status of surface water quality in relation to acidic deposition. A companion paper for lakes in the eastern United States (i.e. Part II, Linthurst et al., 1986) has been prepared also. Data sources in Canada included the National Inventory Survey, the Ontario Lake Sensitivity data set, and the National Aquatic Data base which provided an overall data base of approximately 5700 lakes. Only recently collected data (largely 1980 or later) were used in the analysis. Frequency distribution statistics were obtained for pH, acid neutralizing capacity (ANC), SO₄ and organic anion (A^?) concentrations. Acidic and low ANC waters in eastern Canada occur in a pattern explained by a combination of biogeochemical factors and atmospheric deposition. Nova Scotia contained the highest proportion of acidic and ultralow ANC lakes of any region surveyed in eastern North America; since this region receives approximately 20 kg.ha^?1.yr^?1 wet SO₄ deposition, the proposed target loading may be too high to protect the highly sensitive waters of Maritime Canada. Compared to the rest of eastern Canada, lakes in Ontario have relatively high ANCs due to the influence of CaCO₃ contained in the glacial till of the area. Variation in the SO₄ concentration of lakes approximately follows expected gradients in wet SO₄ deposition. Naturally occurring organic acids do not play a dominating role in the acidification of eastern Canadian lakes.     

Relations between lake acidification and sulfate deposition in Northern Minnesota,Wisconsin, and Michigan

From a level of 1 kg ha^?1yr^?1 in north central Minnesota, emission-related wet SO₄ deposition increases across northern Wisconsin and northern Michigan to about 18 kg ha^?1yr^?1 in south central Michigan. Samples taken from 82 clearwater (low color) lakes across this region in the summer of 1984 showed a pattern of acidification in proportion to deposition. We found a linear increase in the difference between alkalinity and Ca+Mg and in lake SO₄ concentration with increasing deposition. We developed a simple equation to predict the emission-related SO₄ deposition levels that will cause the alkalinity of sensitive clear-water lakes to go to zero.     

Chemical characteristics and temporal trends in eight streams of the Catskill Mountains,New York

Discharge to concentration relationships for eight streams studied by the U.S. Geological Survey (USGS) as part of the U.S. Environmental Protection Agency's (U.S. EPA) Long-Term Monitoring Project (1983–89) indicate acidification of some streams by H₂SO₄ and HNO₃ in atmospheric deposition and by organic acids in soils. Concentrations of major ions in precipitation were similar to those reported at other sites in the northeastern United States. Average concentrations of SO₄ ^2? and NO₃ ^? were similar among streams, but base cation concentrations differed widely, and these differences paralleled the differences in acid neutralizing capacity (ANC). Baseflow ANC is not a reliable predictor of stream acidity at high flow; some streams with high baseflow ANC (>150 Μeq L^?1) declined to near zero ANC at high flow, and one stream with low baseflow ANC (<50 Μeq L^?1) did not approach zero ANC as flow increased. Episodic decreases in ANC and pH during peak flows were associated with increased concentrations of NO₃ ^? and dissolved organic carbon (DOC). Aluminum concentrations exceeding 300 Μg L^?1 were observed during peak flows in headwater streams of the Neversink River and Rondout Creek. Seasonal Kendall Tau tests for temporal trends indicate that SO₄ ^2? concentrations in streamwater generally decreased and NO₃ ^? concentrations increased during the period 1983–1989. Combined acid anion concentrations (SO₄ ^2? + NO₃ ^?) were generally unchanged throughout the period of record, indicating both that the status of these streams with respect to acidic deposition is unchanged, and that NO₃ ^? is gradually replacing SO₄ ^2? as the dominant acid anion in the Catskill streams.     

Experimental acidification of Little Rock Lake,Wisconsin: The first four years of chemical and biological recovery

Little Rock Lake was experimentally acidified in 1984–1990 during which sulfuric acid was added to one basin, decreasing pH from 6.1 to 5.6, 5.1 and 4.7. The lake has been allowed to recover without manipulation since autumn 1990. By the third year of recovery, ～40% of the change necessary to return to pre-acidification values of pH, acid neutralizing capacity (ANC), sulfate (SO₄ ^2?) and calcium (Ca²⁺) had occurred. During recovery years 1–2, ANC was closely predicted by models based on acidification phase observations, but recovery during years 3–4 was slower than predicted. A possible explanation for the slowed recovery is acidification of the upper 0–5 cm of sediment, which acts as a sink for the ANC generated via SO₄ ^2? reduction, the primary recovery mechanism. Trends for Zooplankton did not follow pH recovery very closely. Species diminished by acidification (e.g. Keralella cochlearis, Daphnia dubia) have not recovered, but species that dominated the community at pH 4.7 (e.g. K. taurocephala, D. catawba) have not maintained high populations. The time required for the Zooplankton community to recover to pre-manipulation conditions is uncertain. Delays also have been observed for the mayfly species Caenis, which had disappeared at pH 4.7. In contrast, reproductive success of largemouth bass (Micropterus salmonides) mirrored that observed during acidification; egg hatch and survival of young-of-the-year to autumn recurred when pH exceeded response levels documented during acidification. Overall, recovery has not closely followed the pattern predicted by acidification responses.     

Acid-base chemistry of high-elevation streams in the great smoky mountains

Longitudinal and temporal variations in water chemistry were measured in several low-order, high-elevation streams in the Great Smoky Mountains to evaluate the processes responsible for the acid-base chemistry. The streams ranged in average base flow ANC from ?30 to 28 μeq L^?1 and in pH from 4.54 to 6.40. Low-ANC streams had lower base cation concentrations and higher acid anion concentrations than did the high-ANC streams. NO₃ ^? and SO₄ ^2? were the dominant acid anions. NO₃ ^? was derived from a combination of high leaching of nitrogen from old-growth forests and from high rates of atmospheric deposition. Streamwater SO₄ ^2? was attributed to atmospheric deposition and an internal bedrock source of sulfur (pyrite). Although dissolved Al concentrations increased with decreasing pH in the study streams, the concentrations of inorganic monomeric Al did not follow the pattern expected from equilibrium with aluminum trihydroxide or aluminum silicate phases. During storm events, pH and ANC declined by as much as 0.5 units and 15 μeq L^?1, respectively, at the downstream sites. The causes of the episodic acidification were increases in SO₄ ^2? and DOC.     

Empirical models for lake acidification in the upper Great Lakes Region

A large data base on inland lakes in the Upper Great Lakes Region (UGLR) was used to evaluate assumptions and relationships of empirical acidification models. Improved methods to calculate background alkalinity and background SO₄ ^2? are reported; SO₄ ^2? enrichment factors indicate that terrestrial SO₄ ^2? sources and watershed or lake sinks must be considered for site-specific background SO₄ ^2? estimates. Significant relationships were found between lake acidification estimated as change in SO₄ ^2? and precipitation acidity but not between changes in lake alkalinity and precipitation acidity in this lightly impacted region.     

Human exposure to mercury may decrease as acidic deposition increases

It has been hypothesized that human mercury (Hg) exposure via fish consumption will increase with increasing acidic deposition. Specifically, acidic deposition leads to reduced lake pH and alkalinity, and increased sulphate ion concentration ([SO₄ ^2?]), which in turn should cause increased Hg levels in fish, ultimately resulting in increased human Hg exposure via fish consumption. Our empirical test of this hypothesis found it to be false. We specifically examined Hg levels in the hair of Ontario Amerindians, who are known consumers of fish from lakes across the province, and observed a weak negative association with increasing sulphate deposition. An examination of Hg levels in lake trout, northern pike and walleye, three freshwater fish species commonly consumed by Ontario Amerindians, found a similar weak negative association with increasing sulphate deposition. Further analysis of these fish data found that fish [Hg] was most significantly (positively) associated with lake water concentrations of dissolved organic carbon (DOC), not pH, alkalinity or [SO₄ ^2?]. Lake DOC levels are lower in regions of greater acidic deposition. We propose an alternate hypothesis whereby human Hg exposure declines with increasing acidic deposition. In particular, we propose that increasing sulphate deposition leads to reduced lake DOC levels, which in turn leads to lower Hg in fish, ultimately reducing human Hg exposure via fish consumption.     

The use of calibrated lakes and watersheds for estimating atmospheric deposition near a large point source

We have measured the input and output rates of substances to and from both lakes and watersheds in the Sudbury and Muskoka-Haliburton areas of Ontario. At the former location, we have conducted mass balance studies on 5 lakes and their watersheds for 2½ yrs. At the latter site, we have measured mass balances for 6 lakes and about 30 individual watersheds for the past 5 yrs. Substances studied included SO₄ ^2?, NO₃ ^?, NH₄ ⁺, H⁺, major cations (Ca²⁺, Mg²⁺, Na⁺, K⁺) and HCO₃ ^?. During the course of the investigation at Sudbury we have made several observations that indicate that the inputs of some substances, specifically SO₄ ^2? or SO₄ ^2?-precursors and strong acids, to lakes and watersheds are underestimated when measured as bulk deposition (i.e. by collection in a continuously open container): (a) The output of SO₄ ^2? from the calibrated watersheds was substantially greater than the input measured as bulk deposition. (b) The SO₄ ^2? concentrations of the lakes could not be explained on the basis of the measured inputs. An additional input directly to the lake surface was needed to obtain a mass balance. (c) The net input of acids measured as bulk deposition to the watersheds was much less than the acid consumed, which was estimated by the net output of Ca²⁺, Mg²⁺, Na⁺, K⁺, Al³⁺, and the net retention of NO₃ ^?. (d) The major cation content of the study lakes could be explained on the basis of weathering reactions in the lakes' watersheds only if the input of strong acid had been underestimated. When these observations were quantified, they indicated a major portion of the total input of SO₄ ^2?-precursors and of strong acid was not included in our bulk deposition measurements. Deposition of SO₂ is the most likely explanation for these observations.     

Episodic acidification during snowmelt of high elevation lakes in the Sierra Nevada mountains of California

Atmospheric loads to dilute lakes in the Sierra Nevada mountains of California are very low, and fall almost entirely as snow. When acidic anions preferentially elute from melting snow, these low loads may nontheless be enough to acidify low ANC lakes. Two of the ten lakes included in the Sierra Episodes Study are discussed here: High Lake, the only lake in the study to become acidic during snowmelt; and Treasure Lake, typical of the remainder of the lakes. All lakes exhibited increases in NO₃ ^? concentrations during early snowmelt; these were accompanied by increases in base cations, primarily Ca²⁺. In the first few days of snowmelt, NO₃ ^? concentrations at High Lake increased more rapidly than concentrations of base cations, resulting in ANC values below zero. Export of both NO₃ ^? and SO₄ ^2? from the watersheds exceeded the inputs from the snowpack, suggesting that other sources (e.g., watershed minerals, stored inputs from the previous summer, transformations of other inputs) of these anions are important.     

Pathways of chemical recovery in acidified,metal-contaminated lakes near Sudbury,Ontario, Canada

This paper describes the recovery pathways of two lakes situated near the Coniston nickel-copper smelter. These lakes were exposed to very intense airborne pollution, including SO₂, H₂SO₄, Ni and Cu, during the 60 year operation of the smelter. After the closure of the Coniston smelter in 1972 and the subsequent improvement in air quality, the water quality in both lakes began to improve. Despite their proximity and exposure to similar inputs, the lakes differed both in the rate and magnitude of recovery. This study demonstrates the capacity of lakes to recover from chemical stresses over a very short period. Changes in Cu and Ni concentration could be predicted, while changes in pH, measured as H⁺, could not. The reasons for this discrepancy as well as the processes and lake parameters that control chemical recovery are discussed.