Comparison of paleolimnological with magic model reconstructions of water acidification

In sensitive areas receiving acidic deposition, paleolimnolgical data indicate changes in lake pH over 1 to 3 decades during the past century. Estimates of deposition of SOx and NOx over this same period suggest that deposition rates changed (1) earlier and (2) more slowly than did changes in lake chemistry. Clearly chemical and biological processes in the terrestrial catchment damp, delay, and moderate the response of surface water pH to deposition of acidifying compounds. This response is controlled by key terrestrial processes that include chemical weathering, sulfate adsorption, cation exchange, dissolution and precipitation of Al compounds, and dissolution and dissociation of inorganic C. MAGIC (Model of Acidifcation of Groundwater In Catchments) provides a tool by which these processes can be simultaneously and quantitatively linked to examine the impact of acid deposition on surface water chemistry. We have applied MAGIC to 4 lakes from which paleolimnological reconstructions are available — Big Moose Lake in the Adirondacks, Loch Grannoch in Scotland, Lake Gårdsjøn in Sweden, and Lake Hovvatn in Norway. The results indicate that the processes linked in MAGIC can account for temporal trends in pH and alkalinity such as those obtained from paleolimnological data.     

A protocol for determining lake acidification pathways

The chemistry of 282 sampled low pH (<6.0) lakes in the U.S. E.P.A. Eastern Lake Survey (ELS) was evaluated in an attempt to assess why these systems have low pH. Evaluations were made using a decision protocol for classifying lakes according to several hypothesized acidifying mechanisms: acidic deposition, presence of wetlands and organic soils, acid mine drainage, watershed S sources, salt driven acidification, and changes in land use. The algorithm evaluates lakes in three steps: (1) initial exclusion criteria exclude from consideration lakes with pH greater than 6.0 or subject to strong confounding influences (e.g., road salt); (2) a general classification discriminates between lakes according to anion dominance; and (3) a secondary classification of lakes within each anion dominant class determines the most likely acidification pathway, using preliminary quantitative criteria designed to discriminate among competing hypotheses. Results computed for sampled lakes were scaled-up to produce regional population estimates, using the statistical framework of the ELS. Acidic deposition appears to be the most likely cause of low pH conditions in about two-thirds of the non-excluded lakes in the ELS low pH target population. Organic acidity arising from wetlands or land use changes appears to be primarily responsible for the low pH status of one quarter of these lakes. Watershed S sources and acid mine drainage appear to be of negligible importance, though further information on dry deposition rates and/or watershed soils is required to confirm this.     

Biological indicators of lake acidification

Indicator taxa are identified, based on both synoptic surveys and whole lake acidification experiments, for lake acidification in the pH 6.0 to 5.0 range. Acidobiontic diatoms (e.g Asterionella ralfsii, Fragillaria acidobiontica, etc.), periphyton (Mougeotia and related species), macroinvertebrates (e.g. Hyalella azteca, Orconectes sp., etc.), leeches, and cyprinid fishes (e.g. Pimephales promelas, Notropis cornutus, etc.) are identified as target organisms during early phases of lake acidification.     

Regional monitoring of lake acidification in Finland

The primary objective of this monitoring is to detect long-term Long-Range Transboundary Air Pollution (LRTAP) induced changes in the water quality of small lakes, throughout Finland, with low conductivity. The monitored lakes (n=171, sampled every autumn since 1990 and in 1987) have a smaller watershed (usually headwater or seepage lakes), a larger lake/catchment ratio, and lower base cation concentrations, alkalinity and pH than Finnish lakes on average. The monitoring network provides background data for air pollution dose/response studies, critical load calculations and for the modelling of acidification scenarios. The declining sulphate deposition seems to be reflected in small headwater lakes all over southern and central Finland as a lowering of the sulphate concentration in the waters. Nitrate concentrations in these lakes have been typically low in the autumn. The base cation concentration is not generally declining, as it is in deposition in many areas. The sulphate concentration in lakes has declined more than base cations. Hydrologically, the recent years have been quite variable because of varying annual precipitation. The variation in alkalinity and pH in typical Finnish lakes is dependent on the content of humic material derived from catchments. The monitoring period is too short to reveal consistent trends in major ion chemistry. However, signs of improvement in recent years can be seen; in comparing 1993 to 1987, years with similar organic acidity and base cations, it seems that the sulphate decline in lakes monitored is compensated by a significant rise in both alkalinity and pH.     

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.     

Chrysophyte scales in lake sediments provide evidence of recent acidification in two Quebec (Canada) lakes

Several lake surveys have indicated that many lakes in Quebec are sensitive to acidification, but no historical data are available to provide conclusive evidence of lake acidification. Paleolimnological studies can provide such evidence. Chrysophycean algal microfossils were analyzed from the sediments of Lake Bonneville and Lake Truite Rouge in Quebec. The chrysophyte flora in the study lakes was stable until 40 yr ago, but since then the taxa characteristic of acidic or slightly acidic waters increased in abundance. The study provides evidence of recent lake acidification in Quebec.     

Diatoms as indicators of the rate of lake acidification

Autecological studies of diatoms as pH indicators have opened the way to estimating a lake's past pH on the basis of its diatom species composition and relative abundance. Estimating the rate of lake acidification from its sediment subfossil diatoms is possible when these subfossils can be identified and accurately enumerated in the surface sediments of 20 to 30 lakes. Once this is done the diatoms down the length of the sediment core of one or more of these lakes can be enumerated and the pH inferred at each depth. This technique holds considerable promise in assessing the temporal impact of acid precipitation for acid-sensitive lakes. When loga values were regressed against observed pH for 28 lakes located north of Lake Superior, a significant (P < 0.01) correlation (r = 0.89) resulted. Downcore diatom stratigraphy for one of these lakes indicated that its pH had dropped from 6.2 to 5.2 over the last 20 yr while a second lake had dropped from a pH of 7.1 to 5.2 over the last 30 yr.     

Effect of lake acidification on the adsorption of phosphorus by sediments

The retention properties of acidic and non-acidic lake sediments were determined in order to assess the effects of lake acidification on the immobilization of P from solution by sediments. The adsorption of P by solids was described by the Langmuir model which was used to determine the sorption parameters, e.g. sorption maxima and equilibrium constant of adsorption. The pH of solution and the chemical and mineralogical characteristics of sediments affect mainly the magnitude of adsorption maxima. The binding strength of the adsorbed complex is similar for all the investigated sediments (Δ=?25.3 to ?28.5 kJ mol^?1) and it is affected little by variation in pH or by chemical and mineralogical composition of sediments. The results indicate that the magnitude of P removal is determined more by sediment chemistry and mineralogy (amorphous Al/Fe oxy-hydroxides, carbonate content) than by pH of the water.     

Simulated mobilization of metals from sediments in response to lake acidification

The regeneratable forms of pollutant metals in lake sediments were estimated using different chemical extractants and the effects of lake acidification on the mobilization of such metal fractions studied in laboratory microcosm. The release of each metal was found to increase exponentially below a threshold pH value, which was about 4.0 for Cu, Ni, Zn, Cd and Fe. The data suggest that a large percentage of pollutant metals are mobile as determined by chemical extraction techniques and long-term release experiments.     

Influence of organic acids on model projections of lake acidification

We employed three mathematical models to make quantitative estimates of the pH of 33 statistically-selected lakes in the Adirondack mountains, New York (USA) prior to the Industrial Revolution (1840). The models included 1) the MAGIC watershed acidification model, 2) a paleolimnological model of diatom-inferred pH, and 3) the MAGIC model modified to incorporate an empirically-based model of natural organic acidity. Application of approaches 2) and 3) yielded consistent estimates of pre-industrial Adirondack lakewater pH. However, when the organic acid model was not included, MAGIC calculations and diatom-inferred values showed poor agreement. MAGIC projections of lakewater pH 50 years into the future, under differing atmospheric deposition scenarios, were also sensitive to inclusion of the organic acid model. MAGIC predicted greater recovery in response to reduced deposition when organic acids were not considered. These results suggest that failure to consider the pH buffering of naturally-occurring organic acidity will often result in biased projections which overemphasize the response of lakewater pH to changes in atmospheric inputs of strong acid.     

An update on the lake acidification mitigation project (LAMP)

Limestone addition is a management tactic to mitigate acidic conditions in lakes. The Lake Acidification Mitigation Project is a long-term integrated project to study the ecological effects of liming and develop the technical information necessary for an effective liming program. Three Adirondack lakes have been limed: Cranberry Pond, a small, 60-day hydraulic residence time fishless lake; Woods Lake, a larger fishless lake with longer residence time (214 days); and Little Simon Pond, a large lake (160 ha) containing populations of brook trout, lake trout and other fishes, with a residence time of 450 days. Woods Lake has been limed twice and the stocked brook trout fishery has been maintained; Cranberry has been limed once and then allowed to reacidify with loss of the stocked fishery. Little Simon Pond showed no ill effects of liming on the extant populations. In general, short term evaluation of ecological effects has shown no major deleterious effects of liming. Methods for predicting reacidification are extremely accurate and use of different limestone particle-size fractions is recommended to achieve longer term in-lake neutralization.     

Sulfur constituents of sediments and their relationship to lake acidification

Sulfate is a major component of acid precipitation in the Northeastern United States. Transformations and fluxes of S may be important indices of the effect of acidification on limnetic systems. Sulfur constiuents and respiration rates were compared in sediments among Oneida, Deer and South Lakes in New York which exhibit a range of buffering capacity and pH from high to low, respectively. Total S in Deer and South was higher near their major inlets. Sulfide was highest in Oneida sediment, which had a lower redox potential (Eh). Ester sulfate was a major portion of the S in the three lakes. Oxygen consumption was highest in Oneida and lowest in South sediments. The low ester sulfate and an elevated C/N ratio of South sediment may indicate that acidification inhibits decomposition     

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.     

Examination of the oligotrophication hypothesis: Phosporus cycling in an acidic Florida lake

Phosphorus cycling processes were studied in an acidic lake to test the oligotrophication hypothesis. P storages and inputs were measured; in situ and laboratory experiments measured pH effects on planktonic uptake and release, macrophyte release, and sediment-water exchange of SRP. Planktonic rates showed little response to pH; release from macrophytes initially was slowest at highest pH (5.5), but after 225 days was not affected over the pH range 3.7 to 5.5. Maximum SRP sorption by sediments occurred at pH 4.0 to 5.5; decreasing sorption above pH 5.5 provides some support for the oligtrophication hypothesis.     

Role of precipitation chemistry versus other watershed properties in Wisconsin lake acidification

Data for over 100 watershed properties, including aspects of topography, hydrology, geology, soils, vegetation, lake morphometry and input precipitation chemistry, have been developed since 1980 for 316 watersheds in northern Wisconsin. The hypothesis being evaluated for this lake population is that the observed water chemistry, can be accounted for as a function of antecedent water and chemical inputs, after considering exchange processes in the lake and watershed and the lake/groundwater interactions. The variables found by regression analysis to explain observed variability in color, sulfate, and acid neutralizing capacity (ANC) levels in Wisconsin lakes are: for color, vegetative characteristics, mean depth, and water renewal times; for sulfate, precipitation concentration of sulfur, evaporative concentration, and lake water renewal time; ANC appears to be controlled by the size of the watershed, lake depth or water renewal time, and the intensity of anthropogenic inputs and cultural developments in the watershed. These results differ from previous studies in Wisconsin and nearby areas of Michigan and Minnesota by indicating that in some lakes acidity may not be in equilibrium with current precipitation chemistry.     

Controlled experimental acidification of lake sediments and resulting trace metal behavior

Severe stream water acidification occurs at higher altitudes (> 600 m a.s.l.) in the Western Harz mountains in Northern Germany. Since 1986 an interdisciplinary research team has followed the fate of pollutants in the 50 km² catchment of an important drinking water reservoir (Lake Söse). An acidification experiment has estimated the role of the remobilization of selected elements from the lake sediments via acidification. Aquaria were used to monitor the effects of a stepwise acidification (from the natural pH of 6.5 to 5.0, 4.0 and 3.0) of the water column over a reconstituted sediment layer. The sediment chemistry has been analyzed before and after the acidification by XRF. The water chemistry was sampled at time intervals and analyzed by ICP-MS. With a pH drop from 6.5 to 3.0, many elements increase in concentration in the water of the acidified basins. Enrichment factors were: Al (5000), Ba (10), Cd (220), Co (800), Cu (170), Ni (90), Pb (5000), and Zn (400). This corresponds fairly well with the field data. Al, Cd, Fe, Mn, and Pb exceed German drinking water limits at pH 4.0. The combined high concentrations (μg L^?1) of Al (1000–2600), Cd (2–4), Cu (4–7), Pb (30–60), and Zn (100–300) in the water column of the acidified streams are not only toxic for fish but also for many other aquatic organisms. Chemical changes in the sediment are not significant within the experimental setup.     

The PIRLA project (paleoecological investigation of recent lake acidification): Preliminary results for the Adirondacks,New England,N. Great Lakes states,and N. Florida

The PIRLA project is an interdisciplinary paleoecological study designed to provide reconstructions of the recent acidification histories of a representative set of lakes in four acid-sensitive regions in North America. We are trying to determine if lakes in the study regions have acidified, and if so, to what extent, over what time period and why. Sediment cores from 5 to 15 lakes in each region are being analyzed for several characteristics. Diatoms and chrysophytes are being used to reconstruct lakewater pH. Results for three Adirondack lakes with current pH of 4.8 to 5.0 indicate a decrease in pH beginning in the 1930's–1950's. Increased atmospheric deposition of strong acids appears to be the primary factor responsible for the pH decline. Two lakes (pH 4.4 and 4.7) in New England show clear evidence of acidification probably due to acidic deposition. Preliminary reconstructions for two lakes in Michigan (pH 4.4 and 5.6), one in Wisconsin (pH 5.3), and one in Minnesota (pH 6.8) suggest no recent pH decrease. For, the one Florida lake (pH 4.4) analyzed, inferred pH decreases by about 0.5 unit, beginning in the 1950s; the cause has not been determined.     

Influence of pollution and acidification on metal concentrations in Finnish Lapland lake sediments

Fifteen Finnish Lapland lakes have been investigated to study pollution levels and possible acidification effects on nickel (Ni), copper (Cu), cobalt (Co), zinc (Zn), cadmium (Cd), lead (Pb), manganese (Mn), iron (Fe), potassium (K), sodium (Na), calcium (Ca), magnesium (Mg) and aluminium (Al) concentrations in sediments. Four lakes have average water pH lower than 6.0 and alkalinity lower than 0.050 meq/1. Contamination factor (C_f, ratio of metal concentrations in the uppermost to the deepest layers for a given lake sediment core) of Pb is high, particularly for acidic and acidifying lakes (C_f=5.2–10.4). Ni, Cu, Co, Zn and Cd concentrations increase insignificantly towards sediment surface of some lakes (with a neutral pH) with the rare exception. The influence of passible lake acidification consists of decreasing Cu, Cd, Al, Zn concentrations and organic material contents (loss on ignition) towards the sediment surface. The buffer capacity index (BCI), determined as the ratio of the sum of alkaline and alkaline-earth metals (K, Na, Ca, Mg) to Al, is lower for acidic lakes (from 0.12 to 0.36), whereas for the other lakes the BCI values are higher (from 0.42 to 1.34). Thus, BCI-values, decreased contents of Al, Cd, Zn and Cu, as well as organic matter contents (OMC in the upper lake sediment suggest acidification of freshwater environments.