Finnish lake survey: the role of organic and anthropogenic acidity

A lake survey consisting of 987 randomly selected lakes was conducted in Finland in autumn 1987. The survey covered the whole country, and the water quality of the lakes can be considered as representative of the approximately 56 000 lakes larger than 0.01 km² in Finland. The median TOC concentration is 12 mg L^-1 and the median pH 6.3. The proportion of lakes with TOC concentrations > 5 mg L^-1 in the whole country is 91 %. Organic anion is the main anion in the full data set (median 89 μeq L^-1). The high organic matter concentrations in Finnish lakes are associated with catchment areas containing large proportions of peatlands and acid organic soils under coniferous forest. The survey demonstrated that organic matter strongly affects the acidity of lakes in Finland. The decreasing effect of organic matter on the pH values was demonstrated by both regression analysis and ion balances. At current deposition levels of ^*SO₄ the pH of humic lakes in Finland is determined to a greater extent by high TOC concentrations than by ^*SO₄ in most areas. In lakes with pH values lower than 5.5 the average organic anion contribution is 56 % and non-marine sulfate contribution 39 %. However, in the southern parts of the country, where the acidic deposition is highest, the minerogenic acidity commonly exceeds the catchment derived organic acidity.     

Natural and anthropogenic acidity sources for Finnish Lakes

The headwater lakes in peat-rich areas in Finland commonly have high organic matter concentrations and are surrounded by soils and bedrock poor in bases. As a result the organic acid load on the watercourses is generally high. The significance of the organic vs anthropogenic acidity in 78 moderately acid lakes (pH range 4.3 to 7.1, TOC range 0.6 to 33.9 mg L^?1) was evaluated in areas receiving different levels of acid deposition. The lakes were sampled during autumn overturn, and their water quality (mean TOC = 10.9 mg L^?1, mean pH = 5.9) represents rather well the average water quality in small lakes in Finland. According to the correlation and regression analyses, TOC is a better predictor of pH than non-marine sulfate. Base cation concentrations are important in determining the sensitivity of the lakes to organic and anthropogenic acidity. The highest minerogenic acid contribution is found in high-deposition areas, while the percent organic acid contribution is greater in low deposition areas although TOC levels are comparable in both areas. It would appear that the pH of humic lakes is determined to a greater extent by individual watershed characteristics than by their geographical location at the present atmospheric loadings received in Finland.     

Brook Water Quality and Background Leaching from Unmanaged Forested Catchments in Finland

The water chemistry of 21 outlet brooks in undisturbed, forested catchments (0.074–38 km²) in Finland was monitored during three years (1997–1999) in order to estimate the background levels and leaching of total organic carbon (TOC), iron, nitrogen and phosphorus, and examine the relationships between catchment characteristics and brook water quality in pristine forested areas. The studied catchments are located throughout Finland except the northernmost parts of the country. The concentrations of TOC were relatively high, on average 20 mg L^-1. Annual leaching of TOC ranged from 3000 to 10 000 kg km^-2. The average total nitrogen concentration and annual leaching were 430 μg L^-1 and 140 kg km^-2, respectively. The average total phosphorus concentration and annual leaching were 15 μg L^-1 and 5.4 kg km^-2, respectively. On a national level temperature and discharge conditions were the most important variables for predicting total phosphorus, total nitrogen and TOC concentrations. Both total nitrogen and total phosphorus concentrationsdisplayed positive correlation with temperature, and thus the concentrations were higher in the south than in the north. Nitrogen was mainly organic and showed strong correlation with TOC. On the catchment level, both TOC and total nitrogen concentrations and export had a strongpositive correlation with the abundance of Norway spruce (Piceaabies Karsten) and a strong negative correlation with the abundance of Scots pine (Pinus sylvestris L.). Nitrate concentrations and leaching were related to average site type. The more fertile the average site type was in the catchment, the higher the nitrate concentrations and export were.     

Groundwater acidification at Birkenes,southern Norway: Comparing trends of chemical composition of precipitation,throughfall, soilwater and groundwater

Chemical time trends for precipitation, throughfall, and soilwater (1986–1992), and groundwater (1980–1993) at Birkenes, southern Norway, are compared to gain insights into possible causes for the recent increase in groundwater acidification there. Precipitation and throughfall trends do not show evidence for an increase in anthropogenic acids (e.g. sulphate), but seasalt deposition (e.g. chloride) has been marginally greater in 1990–1992 than in most previous years on record. Soilwater composition partly indicates increasing acidification in recent years (pH, Al and ANC), but hardness and sulphate content are decreasing. Soilwater ANC became negative in 1989, revealing a lasting deficit in its potential to buffer acidity. Groundwater shows clear signs of intensifying acidification (pH, Al, ANC, hardness and sulphate), and this may result partly from climatic conditions (mild winters, seasalt episodes) and partly from the deterioration of an acid buffering system within the soil cover. Acidification via sulphate deposition certainly is not a direct cause. The declining hardness of soilwater suggests that the ion-exchange buffer in the soil may have ceased to function properly. The necessity for obtaining long-term time-series of water chemistry is underscored by this study.     

The decline of fauna in small streams in the Swedish mountain range

Investigations in the southern part of the Scandinavian mountain range have shown a direct correlation between snowpack pH and the lowest pH in small streams. In streams with catchments <100>² a snowpack pH below 4.8 resulted in a stream pH below 5.5. As the snowpack pH in the southern mountain range is 4.0–4.6, lower values to the south and at high altitudes, large areas are affected by acid deposition. The acidity of the snowpack is released almost directly into the streams during thaw, due to the large snowpack, rapid thaw, steep terrain and thin soils. The acidification of the snow has lead to an extensive fauna depletion, especially in smaller streams without upstream lakes. The abundance of benthos in acidified streams in the municipality of Härjedalen (11.000 km²) is today only one tenth of the abundance before acidification. Fish populations have declined to the same extent, and several acid-sensitive species have been lost. It is concluded that mountain streams and fauna are extremely sensitive to acidification, and that even large reductions of emissions will be insufficient.     

Organic versus anthropogenic acidity in tributaries of the kejimkujik watersheds in Western Nova Scotia

Dilute waters (Ca = 0.3 to 0.8 mg L^?1) respond by depressed pH levels throughout the year to existing atmospheric wet deposition of sulphate (20 kg ha^?1yr^?1).This occurs in southwest Nova Scotia particularly during the cold, wet season when runoff is high. Colored waters of similar Ca levels receiving runoff from peaty catchments exhibit pH values one unit lower (4.7 to 4.1) than those of catchments of similar Ca levels but free from peat deposits (pH > 5.3). In colored streams sulphate and organic anions show opposing cyclic patterns while the negative gran alkalinity is the near mirror image of sulphate. Sulphate anion peaks during the high spring discharge when the organic anion concentration is lowest. Sulphate suddenly drops during the summer months during periods of high evapotranspiration, low water table and runoff, when the concentration of organic anions is highest. Both Al and Fe follow cyclic patterns similar to that of the organic anions while H⁺ reflects that of the sulphate anion. Hydrogen ion concentration is always higher than that of sulphate in very colored waters, particularly during the summer months when organic anion concentrations are very high. Analysis of data in the tributaries in the Kejimkujik watersheds indicates that while considerable organic acidity is present in colored waters, anthropogenic sulphate further increases the free acidity of these waters, particularly at times of high discharge.     

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.     

Covered catchment experiment at Gårdsjön: changes in runoff chemistry after four years of experimentally reduced acid deposition

Atmospheric deposition was almost entirely excluded from the forested headwater catchment G1 ROOF, by means of a 7000 m² plastic roof that prevents rain and throughfall from reaching the ground. Under the roof an irrigation system was installed to simulate a natural precipitation regime. The intercepted throughfall was substituted by the same amount of clean pre-industrial throughfall. The experiment started in April 1991. During the four years of the treatment, 2960 mm (18 600 m³) of sprinkled solution was applied under the roof, which is about 15 times the mean water storage of the catchment. After four years of treatment major changes in runoff chemistry were observed. The exclusion of all non-marine sulphate (SO₄ ^2–) input to the catchment (i.e. ca 75 % of total SO₄ ^2– input excluded) resulted in significant decline of sulphate in runoff through all four years of treatment. During the fourth year, annual volume weighted SO₄ ^2– concentration was 46 % lower than the two years prior to the treatment. Concentrations of inorganic aluminium Al³⁺ declined 52 % and Mg²⁺ declined 54 %. No change of H⁺ concentration was detected. As the treatment proceeds there seems to be a trend towards less negative acid neutralising capacity in runoff.     

Leaching of Nutrients,Organic Carbon and Iron from Finnish Forestry Land

This study provides an assessment of the spatial variability of the long-term leaching of nutrients, total organic carbon (TOC) and iron (Fetot) from 22 forested catchments (0.69-56 km2), distributed across all but the northenmost areas of Finland. The natural, unmanaged Kruunuoja catchment is located in a national park, while the other catchments represent Finnish forestry land. The average leaching of Ptot (4.2 kg km-2 yr^-1) from the Kruunuoja catchment was small compared to the catchments representing forestry land (on average 10 kg km-2 yr^-1). Moreover, P fertilization was the most important predictor for the spatial variation in Ptot leaching (r2=0.45). Leaching of TOC, Fetot and N compounds was not closely related to forestry practices. Median C/N ratio in the study streams was high (range 34-66). The average inorganic N proportion and leaching of P_tot were lowest in the Kruunuoja catchment (7.3 % and 2.8 kg km-2 yr^-1, respectively) and highest in the southernmost Teeressuonoja catchment (54 % and 100 kg km-2 yr^-1, respectively) located in the highest N deposition area. The most important forestry practices since the 1960's have affected about 2.4 % of the area of study sites per year (cf. 2% in the entire country in 1991). Moreover, the mean annual runoff from the catchments (230-430 mm yr^-1) agrees with the mean annual runoff from Finland (301 mm yr^-1). Consequently, the results of the study catchments can be used to estimate average total annual leaching from Finnish forestry land: 2,700 t of Ptot, 48,000 t of Ntot, 110,000 t of Fetot and 1.5 million t of TOC.     

Temporal variation in aluminum speciation and concentration during snowmelt

Precipitation samples in Alberta were collected and analyzed monthly from six Alberta Environment stations. Samples were collected with Sangamo samplers and analyzed for the major ions, pH and acidity. The data were tabulated and analyzed for spatial distribution, seasonal variation, temporal trends, ionic character and wet sulphate deposition. The major ionic species in Alberta precipitation are Ca²⁺, SO _inf4 ^sup2? , NH _inf4 ^sup+ and N0 _inf3 ^sup? . The spatial distribution shows a slight decrease in pH from southern Alberta (pH 6.0) to northern Alberta (pH 5.4). The seasonal variation shows higher hydrogen ion content in the summer months (pH 5.4 in summer and pH 5.8 in winter). Temporal trends are not apparent over the five year period investigated. The five year average wet sulphate deposition rate in Alberta is 9.1 kg ha^?1 yr^?1.     

Interactions between anthropogenic sulphate and marine salts in the Bs horizons of acidic soils in Scotland

In laboratory adsorption experiments, the comparison of podzol Bs horizons from coastal and inland moderately-impacted catchments with those from a severely-acidified inland region has demonstrated the effect of marine inputs on SO₄ ^2– -retention. Moderate sea-salt inputs and low acid deposition leads to the retention of most of the SO₄ ^2– and the release of soluble Mg²⁺; increasing the marine salt loading causes the development of a selectivity towards retention of acidic SO₄ ^2– and the retention of Mg²⁺. In the highly-impacted soil, the marine input caused a decrease in SO₄ ^2– retention in open moorland soils. The opposite occurred under forest, due to the ion-exchange of marine Mg²⁺ for soil Al³⁺, increasing soil acidity towards the pH₀ (Gillman and Uehara, 1980), which is depressed below that of its moorland equivalent.     

Characterization of ester sulphate in a gypsum-amended podzol using an immobilized sulphatase reactor

An immobilized sulphatase reactor column was successfully used to determine the biochemical stability of ester sulphate in soil organic matter extracted from a podzol amended with gypsum. The sulphatase from Helix pomatia was covalently attached to controlled-porosity glass beads, and the immobilized enzyme was packed into a small glass column. The optimum pH, the time required to reach equilibrium, and the percentage of substrate consumed for the enzymatic hydrolysis of soil ester sulphate (pH 7.7, 90 min, 23–59%) were substantially different from those of p-nitrophenyl sulphate at similar concentrations of substrate (pH 7.0, 40 min, 99%). The striking difference in the biochemical stability and kinetic behaviour between soil ester sulphate and the simple synthetic substrate reflected their different chemical nature and structural features. The amounts of enzymatically hydrolysable (labile) ester sulphate in soil organic matter extracted from the podzol amended with gypsum at rates of 0, 50, and 200 kg S ha^-1 were significantly different (P=0.0004), being 0.5, 1.1, and 1.4 g S ml^-1 soil extract (or 5, 11, and 14 g S g^-1 soil), respectively. The labile ester sulphate was not correlated with the total hydriodic acid-reducible organic sulphate with the soil organic matter extracts but with the hydriodic acid-reducible organic sulphate: organic C ratio, which increased as a result of gypsum amendment. This study revealed that input of inorganic sulphate as gypsum substantially increased the accumulation of labile ester sulphate in a podzol.     

Airborne transport of sulphur: Impacts on chemical composition of rivers on north shore of the St. Lawrence River (Québec)

The St. Lawrence North Shore region (Québec) is subject to acid precipitation entailing sulphate deposition (17 to 22 kg SO₄ ^2? ha^?1 yr^?1) which poses a threat to sensitive aquatic ecosystems. Physicochemical surveys conducted in 1982–1983 revealed the extreme sensitivity of the region owing to weak mineralization of the waters (mean alkalinity of 55 μeq L^?1 and conductivity of 17 μS cm^?1). Calculation of the annual loads of S discharged from 21 rivers throughout the region shows atmospheric deposition as the principal source of sulphate. A decreasing west-east gradient in the concentration is interpreted in terms of the impact of long-range airborne transport, although certain local sources of S emission are not to be overlooked. Analysis of the seasonal variation in the sulphate load balance, conducted in a small drainage basin (40 km²), revealed that the sulphate anion plays a part in lowering the water pH in spring. The spring pH depression is apparently intensified by an additional input of sulphate stemming from the release of this element subsequent to accumulation in the drainage basin during summer and fall. Organic acids play a measurable role in the chemical equilibrium of surface waters in the region, particularly in the eastern sector where there is less S fallout. Low pH levels in this sector (5.5 to 6.0) point to some degree of organic acidification.     

Soil sensitivity due to acid and heavy metal deposition in East Central Europe

Simultaneous soil acidification and deposition of heavy metals is a major concern for forest and agricultural soils of the Black Triangle region of East Central Europe including southern former East Germany, northern Bohemia of the Czech Republic, and southern Poland. The objective of this project was to develop historical and future projections of acid and heavy metal deposition to soils (As, Cd, Pb, Zn) and to produce a preliminary map of soil sensitivity to cadmium pollution and uptake by crops. Ultimately, we wish to assess the relative hazard and recovery times of soils to metals deposition in the region. Emission and deposition data bases obtained from several models developed at IIASA were linked using the Geographical Information System ARC/INFO to produce soil maps of sensitivity to cadmium mobility based on metals deposition, soil type, soil texture, organic matter content, and acid deposition. RAINS 6.1 (Alcamo et al., 1990) was utilized to produce maps of acid deposition for EMEP grids (150 km x 150 km). The largest amount of acid load is deposited in southern East Germany. Sulfur deposition in that area was 10–12 gS/m²/yr in 1990, and S+N deposition exceeded 8000 eq/ha/yr. But the hot spot for metals deposition is further to the east, in the Silesia area of southern Poland. The TRACE2 trajectory model of Alcamo, Bartnicki, and Olendrzynski (1992) was used to estimate cumulative metals deposition since 1955 with scenarios to 2010. Pb has improved over Europe since 1970 when depositions in the Ruhr River Valley of West Germany exceeded 60 mg/m²/yr. But cadmium deposition in southern Poland (Katowice and Krakow) has now accumulated to 60–70 mg/m² by atmospheric deposition alone. During base case simulations from 1955–87, approximately 1.8 mg/kg Pb and 0.12 mg/kg Cd have been added to the mixed plow-layer of 30 cm. If these emissions continue indefinitely, the accumulation of metals will become problematic for agriculture and the food chain.     

Precipitation chemistry in Japan 1989–1993

Precipitation chemistry in Japan was discussed on a wet-only sample database obtained in a nationwide survey from April 1989 to March 1993. Wet-only samples were collected at 29 stations over Japan on a biweekly basis. Commonly determined chemical parameters were measured in laboratories. The volume-weighted annual mean pH at each site ranged from 4.50 to 5.83 with a mean of 4.76. Concentration ranges and means (parenthesized) on an equivalent basis for major ions were as follows: nss-SO₄ ^2–; 5.2–58.9 (38.6), NO₃ ^–; 1.8–25.0 (14.1), NH₄ ⁺; 0.55–29.8 (18.3), nss-Ca²⁺; 2.0–34.5(14.2), Na⁺; 6.4–275.3 (49.1), Cl^–; 13.7–322.4 (63.5) eq L^–1. Acid-base relationships for Phase-II records were quantitatively discussed in terms of three measures: pH, fractional acidity, and our proposed pA_i.     

Labile ester sulphate in organic matter extracted from podzolic soils

Summary We studied the effect of soil pretreatment, molecular-weight fractionation, and K₂SO₄ addition on the concentration and biochemical stability of ester sulphate in soil organic matter. A labile ester sulphate fraction (8.1 g S g^–1 soil) was detected in the organic matter extracted from a sulphate-rich podzolic sandy loam. This fraction was susceptible to loss during soil pretreatment with water and KCl solution and subsequent extraction of organic matter from the soil. The low-sulphate loam was low in labile ester sulphate (0.6 g S g^–1 soil) and the pretreatments had little effect. The addition of K₂SO₄ to the organic matter extracted from the low-sulphate soil resulted in the formation of appreciable amounts of labile ester sulphate. Newly formed ester sulphate tends to be biochemically less stable than indigenous ester sulphate in soil humic polymers and the ester sulphate associated with the low molecular-weight fractoin of soil organic matter appears to be more susceptible to loss by enzymatic hydroylsis. The results were interpreted in terms of steric effect. Ester sulphate groups bound to external surfaces of soil humic polymers may be easily accessible to sulphatase enzyme and thus readily mineralizable during incubation or extraction of soil organic matter at low soluble-sulphate levels. Sulphate groups on inner surfaces of the organic polymers are shielded from the enzyme due to size exclusion and hence more stable.     

Ion mass budgets for small forested catchments in Finland

Ion mass and H⁺ budgets were calculated for three pristine forested catchments using bulk deposition, throughfall and runoff data. The catchments have different soil and forest type characteristics. A forest canopy filtering factor for each catchment was estimated for base cations, H⁺, Cl^? and SO ₄ ^2? by taking into account the specific filtering abilities of different stands based on the throughfall quality and the distribution of forest types. Output fluxes from the catchments were calculated from the quality and quantity of the runoff water. Deposition, weathering, ion exchange, retention and biological accumulation processes were taken into account to calculate catchment H⁺ budgets, and the ratio between external (anthropogenic) and internal H⁺ sources. In general, output exceeded input for Na⁺, K⁺, Ca²⁺, Mg²⁺, HCO ₃ ^? (if present) and A^? (organic anions), whereas retention was observed in the case of H⁺, NH ₄ ⁺ , NO ₃ ^? and SO ₄ ^2? . The range in the annual input of H⁺ was 22.8–26.3 meq m^?2 yr^?1, and in the annual output, 0.3–3.9 meq m^?2 yr^?1. Compared with some forested sites located in high acid deposition areas in southern Scandinavia, Scotland and Canada, the catchments receive rather moderate loads of acidic deposition. The consumption of H⁺ was dominated by base cation exchange plus weathering reactions (41–79 %), and by the retention of SO ₄ ^2? (17–49 %). The maximum net retention of SO ₄ ^2? was 87% in the HietajÄrvi 2 catchment, having the highest proportion of peatlands. Nitrogen transformations played a rather minor role in the H⁺ budgets. The ratio between external and internal H⁺ sources (excluding net base cation uptake by forests) varied between 0.74 and 2.62, depending on catchment characteristics and acidic deposition loads. The impact of the acidic deposition was most evident for the southern Valkeakotinen catchment, where the anthropogenic acidification has been documented also by palaeolimnological methods.     

Status and distribution of soil sulphur fractions,total nitrogen and organic carbon in camp and non-camp soils of grazed pastures supplied with long-term superphosphate

Summary Topsoils (0–75 mm) from four different soil types were collected from stock camp and non-camp (main grazing area) areas of grazed pastures in New Zealand, which had been fertilised annually with superphosphate for more than 15 years, in order to assess the effects of grazing animals on the status and distribution of soil S fractions and organic matter. These soils were analysed for organic C, total N, total S, C-bonded S, hydriodic acid-reducible S, 0.01 M CaCl₂, and 0.04 M Ca(H₂PO₄)₂-extractable S fractions, and soil pH. Soil inorganic and organic S fractions extracted by NaHCO₃ and NaOH extractants were also determined. The results obtained showed that camp soils contain higher soil pH, organic C, total N, total S, organic (C-bonded S and hydriodic acid-reducible S) and inorganic S fractions, NaHCO₃-and NaOH-extractable soil S fractions but a lower anion retention capacity than non-camp soils, attributed to a higher return of plant litter and animal excreta to camp soils. In both soils, total S, organic S, C-bonded S, and hydriodic acid-reducible S were significantly correlated with organic C (r0.90***, ***P0.001) and total N (r0.95***), suggesting that C, N, and S are integral components of soil organic matter. However, C: N : S ratios tended to be lower in camp (60: 5.6: 1–103: 7.2: 1) than in non-camp soils (60:6.1:1–117:8.3:1). Most (>95%) of the total soil S in camp and non-camp soils is present as organic S, while the remainder is readily soluble and adsorbed S (i.e. Ca(H₂PO₄)₂-extractable S). C-bonded S and hydriodic acid-reducible S constituted 55%–74% and 26%–45% of total S, respectively, reflecting a regular return of plant litter and animal excreta to the grazed pastures. NaHCO₃, and especially NaOH, extracted significantly higher amounts of total soil S (13%–22% and 49%–75%, respectively) than Ca(H₂PO₄)₂ or CaCl₂ (<5%). In addition, NaHCO₃ and NaOH-extractable soil S fractions were significantly rorrelated with soil organic S (r0.94***), C-bonded S (r0.90***) and hydriodic acid-reducible soil S (r0.93***). Differences between soils in either camp or non-camp areas were related to their sulphate retention capacities, as soils with high sulphate retention capacities (>45%) contain higher levels of C-bonded and hydriodic acid-reducible S fractions than those of low sulphate retention soils (<10%). Long-term annual superphosphate applications significantly increased the accumulation of soil organic and inorganic S fractions, and organic C and total N in the topsoil, although this accumulation did not occur when the superphosphate application rates were increased from 188 to 376 kg ha^-1 year^-1.     

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.     

Export of Dissolved Inorganic Nitrogen in a Partially Cultivated Subtropical Mountainous Watershed in Taiwan

A spatial and temporal investigation of dissolved inorganic nitrogen (DIN; NO₃, NO₂ and NH₄) was conducted under various water discharge conditions in Lanyang-Hsi, a subtropical mountainous stream, which drains through distinct degrees of agriculture-influenced sub-watersheds. In both the cultivated and non-cultivated sub-watersheds, NO₃ was the most abundant species accounting for >80% of total DIN, while NH₄ and NO₂ accounted for <15% and=" 5%=" of=" din,=" respectively.=" agricultural=" activities=" along=" the=" riverbank=" led=" to=" significantly=" higher=">₃ concentrations (13–246 M) and DIN yields (1300–3800 kg N km^–2 yr^–1) in main channel when compared to those of non-cultivated tributaries (9–38 M for NO₃ and 550–740 kg N km^–2 yr^–1 for yield). The much lower and less variable DIN yields observed in tributary stations (mean = 660 ± 120 kg N km^–2 yr^–1) are considered as the present day background of DIN yield, which is significantly higher than those of most natural watersheds in other regions. Elevated atmospheric DIN deposition is likely the cause for the high background DIN yield. Human activity within the watershed results in additional DIN yield, which accounted for 49% of total N export. However, the reported atmospheric DIN input in northern Taiwan (1800 kg N km^–2 yr^–1) is much higher than the background DIN yield implying that a major fraction (70%) of atmospheric inputs are retained or processed within the watershed. A dilution pattern occurred in the main channel where high NO₃ concentrations from the upstream sources decreased significantly in downstream direction due to inputs of NO₃-diluted water from non-cultivated areas. We adopted a two-source mixing model to predict the NO₃ dilution pattern. This model revealed a third yet not recognized N source in the lower part of watershed. Model results also indicated the importance of water discharge rate in regulating the relative contribution to total DIN export among these sources.