Impacts of warm winters and extreme rainstorms on the base consumption in a limed lake, southern Norway

The Science of the Total Environment 313 (2003) 127–139

Impacts of warm winters and extreme rainstorms on the base consumption in a limed lake, southern Norway Dag O. Andersen* Agder University College, Department of Natural Sciences, Serviceboks 422, N-4604 Kristiansand, Norway Received 28 February 2002; accepted 12 April 2003

Abstract The chemical composition of a limed lake, the two main inlets and the outlet was monitored during a period of 3 years. The winters of 1991–1992 and 1992–1993 were unusually warm while the winter of 1993–1994 was more normal. The lake surface water was wind exposed in the warm winters and as a consequence of frequent turnovers the acid input from the catchment mixed with the whole lake water body. In the winter of 1993–1994, the lake was ice-covered for approximately 4 months. During this period the drainage water from the catchment flowed to the outlet of the lake in the upper 2–3 m of the water column and only some of the acid input was neutralised. This is compared to a complete neutralisation in the winter of 1992–1993. The in-lake loss of alkalinity during this warm winter was approximately 29 meqyl (November–June) compared to approximately 7 meqyl lakewater in 1993–1994. Acid drainage from the catchment induced by an extraordinary rainstorm with heavy sea-salt deposition contributed to the in-lake alkalinity consumption in spring 1993. As winter temperatures above 0 8C and more frequent rainstorms may be common due to expected global warming, future increased lime consumption in-lakes may be projected in acidified areas as southern Norway. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Acidification; Liming; Global warming; Warm winters; Rainstorms

1. Introduction Surface water acidification is a significant environmental problem in many regions of the world (Bresser and Salomons, 1990; Steinberg and Wright, 1994). To mitigate acidification extensive liming of lakes and watercourses has been carried out, especially in Sweden (Svenson et al., 1995) and Norway (Hindar, 1997). The technique has *Tel.: q47-38-14-1064; fax: q47-38-14-1063. E-mail address: [email protected] (D.O. Andersen).

also been practised in UK (Howells and Dalziel, 1990), Canada and the US (Olem, 1991). Lakes have been the most commonly treated part of the water system (e.g. Henrikson and Brodin, 1995; Hindar, 1997). The rate of re-acidification depends on the hydrological input (e.g. Sverdrup et al., 1986; Driscoll et al., 1989) and on the in-lake mixing and neutralisation of the acid tributary water. Climate warming with increasing air temperature and probable shift in hydrological flows may influence these parameters substantially.

0048-9697/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0048-9697(03)00264-X

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Global mean air temperature is projected to increase by 1.4–5.8 8C over the period 1990– 2100 (IPCC, 2001), and the annual mean air temperature in Norway is expected to increase by 0.2–0.5 8C per decade in the next 50 years (Regclim, 2003). Higher air temperatures may lead to increases in the ratio of evaporation to deposition. Observations in the Experimental Lakes Area (ELA) (1971–1990) indicate consequences as decline in lake levels, water renewal rates, stream flows, the extent of and water levels in wetlands, soil moisture and groundwater levels (Schindler et al., 1996; Schindler, 1997). As chemical exports from the catchments are controlled by hydrology, reduced water flows may decrease the annual export of chemical substances including Al and DOC (Seip et al., 1989; Schindler et al., 1996). However, during severe droughts reduced sulphur in the catchments may, by exposure to air, be oxidised resulting in elevated sulphate concentrations and acidity in the streams during subsequent wet periods (Dillon et al., 1997). In addition, oxidation of reduced sulphur compounds in air exposed littoral zones may cause further acidification of lakes (Schindler et al., 1996; Yan et al., 1996). On the other hand, increased in-lake sulphate reduction as water renewal decreases may cause increased alkalinity (Schindler et al., 1996; Schindler, 2001). Besides, in areas characterised by cold winters, climate warming may result in lack of snow and ice-cover andyor frequent snowmelt episodes occurring during the winter rather than the typical pattern of accumulation in the winter and melting in the spring (e.g. Wright and Schindler, 1995). Other predicted consequences of climate warming are increased winter deposition (Watson et al., 1998), increased frequency of extreme precipita¨ ¨ tion events (IPCC, 2001; Palmer and Raisanen, 2002) and increased number of strong wind events (Haugen, 2002). In coastal areas rainstorms may cause high deposition of sea-salts. Ion-exchange processes in the soil may result in extreme acid drainage from catchments affected by acidification (Andersen and Seip, 1999). During a 3-year monitoring period in the Lake Terjevann catchment in southern Norway, the area experienced two unusually warm winters and one

more normal and cold. In the winters of 1991– 1992 and 1992–1993, the catchment was covered with snow only for a few days and the limed lake had poor ice-cover. The winter of 1993–1994 however, was cold and nival and the lake was icecovered for approximately 4 months from December 16. From January 4 to 24 1993, extremely low pressure over the North Atlantic gave strong southwesterly winds and large amounts of deposition (149 mm) heavily loaded with sea-salts. Effects of these deviating weather conditions on the acid– base chemistry of Lake Terjevann are reported here. The observations may preview probable effects of climate warming on limed lakes in acidified catchments. 2. Materials and methods 2.1. Site description Lake Terjevann is situated approximately 8 km west of Kristiansand in the southernmost part of Norway (Fig. 1). The catchment area is 1.09 km2 with an altitude range 19–119 m above sea level. The bedrock consists mainly of augen-gneiss. The soil cover is thin and frequently interrupted by outcrops of bedrock. The total catchment, which is divided into four sub-catchments (A, 0.50; B, 0.24; C and D 0.35 km2; Fig. 1), was naturally dominated by coniferous (mainly Scots pine) stands mixed with some deciduous species (oak, birch). Approximately 50% of the monitored subcatchment A was forested with Norway spruce 40 years ago. There are well-defined brook channels in the two sub-catchments A and B while discharge from sub-catchment C and D is observed only during storm-flow conditions. In sub-catchment A, there is a small pond and several small bogs without defined outlets while in sub-catchment B almost the whole brook passes through small bogs. Approximately 1.4=106 m3 water enters the lake yearly. There are no human activities in the catchment besides the liming of the lake water. The lake (Table 1, Fig. 2) was limed for the first time in 1980 and then every autumn during the period 1984–1991. Finely ground ((0.2 mm) limestone with a CaCO3 content of approximately 77% (limestone SR from NORCEM, Norway) has gen-

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erally been used except in 1980 when the lake was treated with rough ground seashells. The limedoses were 2.7 mg calcium carbonate per litre lakewater in 1980, 20.5 mgyl in the autumn of 1984 and then 7.6, 7.6, 7.3, 7.1, 11.5, 7.2 and 12.9 mgyl in the period 1985–1991. In January 1993, a gauge (Crump overflow) for water flow recording was installed at the outlet of the lake. 2.2. Climate A maritime climate prevails on the southern coast of Norway. The monthly mean temperature is commonly below 0 8C in periods during December, January and February and generally highest in July (Table 2). The annual mean deposition is approximately 1300 mm and the distribution is characterised by deposition maxima during late autumn and winter (Table 2). Deposition is generally higher than evapotranspiration except for a period from early June to late August as indicated

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Table 1 Morphometric data of Lake Terjevann Maximum length (l) Maximum depth (zm) Mean depth (z) Surface area (A) Volume (V) Theoretical retention time

618 m 35 m 12 m 0.09 km2 106 m3 0.72 year

by lack of discharge from the catchments and the lake (Fig. 3) during this period. 2.3. Deposition The area receives a high atmospheric deposition of sea salts as well as long-range transported acidic sulfur and nitrogen compounds (cf. Table 3). 2.4. Fieldwork Samples were collected for water chemistry analyses at the two inlets and the outlet of the

Fig. 1. The Lake Terjevann catchment consists of four sub-catchments A, B, C and D. Light grey areas denote bogs, dark grey areas the pond and the lake. The brooks are marked with dark lines while filled circles denote the water sampling points (InA, InB, the lake and the outlet).

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Fig. 2. Bathymetric map for Lake Terjevann. Open circles indicate the points sampled from ice March 1994. Arrows indicate water flow direction.

lake almost weekly in the period February 1992 to December 1994. The lake was sampled 21 times in 1992, 13 times in 1993 and 8 times in 1994 at several depths (0.5, 1.0, 2.0, 3.0, 5.0, 7.0, 10.0, 15.0, 20.0, 25.0, 30.0, 31.0 and 32.0 m) over the

deepest part (Fig. 2). On March 14 1994, the lake was sampled from the ice at 33 different points spread over the whole surface area (Fig. 2). Temperature of the lake samples was measured with a build-in thermometer in the water sampler.

Table 2 Air temperature (8C); mean values 1961–1990 and mean values in 1992 through 1994 at Kjevik airport approximately 16 km east of the Lake Terjevann catchment. Deposition (mm); mean values 1961–1990 at Kjevik and mean values in 1992 through 1994 in Søgne approximately 1 km west of the catchment Temperature 1961–1990 January February March April May June July August September October November December Yearly meanytotal a b

a

Deposition 1992

a

1993

a

1994

a

1961–90a

y1.7 y1.8 1.0 4.6 9.9 14.0 15.5 14.8 11.5 7.9 3.1 y0.1

2.6 2.8 3.9 4.6 12.3 16.9 15.4 14.0 11.7 4.7 3.6 1.9

2.7 2.2 2.8 6.5 12.6 13.7 14.2 13.3 9.7 6.0 1.3 y0.6

0.2 y4.2 1.7 5.8 10.4 13.1 18.6 16.3 10.9 7.3 5.2 3.4

121 80 87 59 86 75 88 118 141 164 164 116

6.6

7.9

7.0

7.4

1299

Kjevik (Norwegian Meteorological Institute, DNMI). Søgne (Norwegian Institute for Air Research, NILU).

1992b 62.8 85.6 145.9 118.8 26.3 9.4 93.0 151.1 100.0 131.6 199.2 156.5 1280

1993b 149.1 31.7 52.1 66.1 24.7 17.7 70.0 77.7 129.1 104.6 176.5 212.4 1112

1994b 176.0 67.0 173.1 87.1 25.7 70.7 16.7 141.9 184.4 145.0 144.8 208.5 1441

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Fig. 3. Deposition and discharge at Lake Terjevann. (Each bar represents the total deposition during the week before).

2.5. Chemical analyses Conductivity (Radiometer CDM83 Conductivity Meter; CDC 304 cell) and pH (Radiometer Ion85; G202C pH and K201 reference electrodes) as well as UV-absorbance at 254 nm (Abs254) (1 cm quartz cuvettes; Hitachi Model V-2000 UVyVIS Spectrophotometer) were measured in the laboratory on unfiltered samples within 2–4 h after collection. Total organic carbon (TOC) was measured in selected brook samples as CO2 after digestion with peroxidisulphate and UV light (Astro Model 2001 TOC-analyzer) within 2–3 days. The samples were kept at 4 8C until analyses. The equation Abs254s 0.0349TOCq0.0413 (ns45, R 2s0.90) was developed for the brook waters and used to cal-

culate TOC concentrations. Dissolved oxygen content was determined by the Winkler method (Andersen and Føyn, 1969). Aluminium species in unfiltered samples were fractionated using the procedure of Driscoll (1984). The samples were passed through a strongly acidic cation-exchange column (DOWEX-50Wx8, 20–50 mesh, Naqform). The operationally defined fractions, total acid reactive aluminium (Alt) and non-labile monomeric (mainly organically complexed) aluminium (Alo) which passed the resin, were analysed by flow injection analysis (FIA) using the pyrocatechol-violet method (Tecator, 1985). The labile aluminium fraction, mainly monomeric, inorganic aluminium (Ali), was calculated as the difference between Alt and Alo. Calcium was

Table 3 Volume-weighted mean pH and concentrations of main ions (mgyl) in deposition 1989–1994 in Søgne approximately 1 km west of Lake Terjevann (SFT 1991a; SFT, 1991b, 1992, 1993, 1994, 1995)

1989 1990 1991 1992 1993 1994 a

pH

Naq

Mg2q

Ca2q

Kq

NHq 4 –N

Cly

SO2y 4 –S

a SO2y 4 –S

NOy 3 –N

4.34 4.33 4.30 4.33 4.33 4.39

3.27 4.39 3.69 2.94 4.42 2.55

0.43 0.52 0.47 0.34 0.53 0.31

0.31 0.25 0.23 0.19 0.26 0.19

0.27 0.25 0.21 0.14 0.26 0.15

0.91 0.48 0.58 0.49 0.63 0.54

6.53 7.94 6.67 4.96 8.22 4.35

1.39 1.17 1.25 1.04 1.33 0.98

1.12 0.79 0.94 0.79 0.95 0.76

0.93 0.60 0.66 0.59 0.71 0.62

Non-marine.

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analysed by atomic absorption spectroscopy (Perkin Elmer AAS Mod. 1100). Lanthanum was added to samples before calcium analyses to a concentration of 0.5 gyl. Alkalinity was determined by titration with 0.05 M HCl with a conductometric (Radiometer CDM83 Conductivity Meter; PP1042 Conductivity Cell) end point determination (Golterman et al., 1978). Chloride was determined by FIA using the thiocyanate method (Tecator, 1983). 3. Results 3.1. Atmospheric conditions The mean monthly air temperatures were above 0 8C in January and February 1992 and 1993 while the winter of 1993–1994 was more normal (Table 2). Total amounts of deposition were about normal in 1992 compared to the mean values 1961–1990, lower than normal in 1993 (86%) and higher than normal in 1994 (111%) with a somewhat different distribution the 3 years (Table 2, Fig. 3). The importance of the wind direction on the quality of the deposition is indicated by the strong fluctuations in chloride concentration in the deposition observed during 1992–1994 (Fig. 4). The anomaly of the sea-salt event in January 1993 is illustrated by the volume-weighted mean concentration of 36.2 mgyl Cly in deposition these 3 weeks (cf. Fig. 4) compared to the average of the volumeweighted annual means from 1989 to 1994 of approximately 6.4 mgyl (cf. Table 3). Almost 60% of the annual wet deposition of Cly in 1993 occurred during these 3 weeks. The pH of the deposition during the sea-salt event was 4.28 in the first 19.4 mm while during the next 2 weeks, when the deposition amounted to 129.7 mm it was approximately 4.50. The volume-weighted mean annual pH of the deposition was 4.33 both in 1992 and 1993 while it was somewhat higher in 1994 (Table 3). 3.2. The tributaries and the outlet Due to the unusual warm winters in 1991–1992 and 1992–1993, there was no accumulation of snow in the catchment. Consequently, the dis-

charge fluctuated with deposition. In the winter of 1994, the catchment was more or less covered with snow, at most approximately 60 cm, from late November to early April. The tributary water (Inlet A and B) was generally soft and acidic with low or moderate concentrations of TOC (Table 4). The pH was generally higher than the mean pH of the deposition (4.33, cf. Table 3). In dry periods (mainly June–August) with very low flow and most probably contribution of groundwater, the pH was much higher. This was particularly evident for the drainage from sub-catchment B (Fig. 4). The seasonal fluctuations in pH were very similar in the drainage from the two sub-catchments (Fig. 4). Generally, decreases in pH correlated with increased flow. The lowest measured pH (4.18, in Inlet A) occurred on January 19 1993 (Fig. 4), 3 days after the heaviest load of sea-salts. Yearly mean total aluminium (Alt) concentrations were between 465 and 777 mgyl and 305 and 432 mgyl in the drainage from sub-catchments A and B, respectively, with the highest concentrations in 1993 (Table 4). The non-labile (Alo) fraction constituted on average 54 and 73% of the Alt in drainage in 1992 and 1994, respectively, while the Alo fraction constituted less than 35% in 1993 (cf. Table 4). Generally hydrology and, in particular, water pathways (cf. Seip et al., 1989) influence the aluminium chemistry. Inorganic aluminium (Ali) increased with flow only during rain episodes preceded by low flow periods while during subsequent rain episodes Ali remained nearly constant or even decreased. The sea-salt event set in after a 3 weeks’ period without wet deposition (Fig. 3). As a consequence, the Cly concentration more than doubled (Fig. 4) and the labile aluminium (Ali) concentration almost quadrupled to unusually high concentrations in the drainage from both catchments, from 274 to 901 mgyl and from 129 to 480 mgyl in Inlet A and B, respectively (Fig. 4). It should be noted that it took 3– 4 months until the Ali concentrations were back to the lower pre-event levels (Fig. 4). Andersen and Seip (1999) discuss the effects of the sea-salt loading on the labile inorganic aluminium chemistry in more detail and ascribe the increased

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Fig. 4. Chloride in deposition and chloride, pH and inorganic aluminium (Ali) in the drainage from sub-catchments A and B and the outlet of Lake Terjevann in 1992–1994.

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Table 4 Chemistry of the inlets (InA, InB) and the outlet of Lake Terjevann in the period 1992–1994 1992 Mean

1993 Range

Mean 12.5 11.3 8.5

1994 Range

Mean

Range

Conductivity (m Sym)

InA InB Out

7.8 6.8 8.0

6.6–9.7 5.5–8.7 7.3–9.1

UV-absorbance (A254, 1 cm)

InA InB Out

0.25 0.18 0.13

0.17–0.48 0.13–0.27 0.09–0.17

TOCa (mg Cyl)

InA InB Out

6.0 3.9 2.6

pH

InA InB Out

4.7 5.1 6.6

Aluminium (Alt) (mg Alyl)

InA InB Out

504 305 237

334–705 102–437 138–310

777 432 249

407–1152 121–747 150–337

465 317 307

337–800 185–567 162–435

Aluminium (Alo) (mg Alyl)

InA InB Out

274 193 213

223–385 84–259 43–291

211 150 211

113–345 66–278 123–256

291 231 225

224–448 153–353 151–274

Calcium (mg Cayl)

InA InB Out

1.4 1.3 3.5

7.7–15.6 6.7–14.0 6.9–9.4

6.8 5.6 7.4

5.2–9.4 4.1–7.6 6.2–8.2

0.20 0.14 0.13

0.11–0.35 0.08–0.30 0.08–0.20

0.28 0.22 0.17

0.19–0.50 0.14–0.43 0.08–0.25

3.8–12.5 2.6–6.5 1.5–3.7

4.4 2.9 2.6

2.0–8.8 1.1–7.3 1.0–4.6

7.0 5.2 3.6

4.2–13.2 2.7–11.1 1.1–6.1

4.3–5.4 4.6–6.3 6.2–7.1

4.5 4.9 6.1

4.2–4.7 4.4–6.2 4.9–6.7

4.7 5.1 5.5

4.3–5.1 4.5–6.2 4.6–6.4

0.8–2.5 0.8–3.3 2.8–5.9

1.8 2.1 2.8

0.8–3.0 0.6–4.7 1.7–4.4

1.0 0.9 2.0

0.4–1.8 0.5–2.4 0.8–3.1

Number of samples (n): InA, ns32, 36, 39, InB, ns35, 42, 41, Out, ns36, 37, 39 in 1992, 1993, 1994, respectively. a Calculated from Abs254s0.0349TOCq0.0413 (ns45, R 2s0.90).

concentrations of aluminium to ion exchange with sodium. The outlet water from the lake was generally less acid and contained lower concentrations of aluminium and organic matter than the inlets (Table 4). In-lake processes influenced apparently the labile aluminium (Ali) fraction, as non-labile aluminium (Alo) constituted approximately 90% of the Alt concentration in the outlet. However, during the ice-covered period in the winter of 1993–1994 the pH and the Ali concentration in the outlet were similar to those in the inlets (Fig. 4). 3.3. Lake The lake was rarely ice-covered in the winters of 1991–1992 and 1992–1993 due to air temperatures generally above 0 8C. Uniform temperature

and oxygen saturation in the epi- (1–5 m) and the hypolimnion (25–31 m) in these periods (Fig. 5) indicate frequent lake turnovers. During the icecovered period (December 16–April 4) in the winter of 1993–1994 the lake was stratified with colder water above warmer (Figs. 5 and 6). In the autumn of 1991, prior to the present monitoring period, the lake was treated with a relatively high amount of lime. A significant decrease in lake water alkalinity was observed during the winters 1991–1992 and 1992–1993 (Fig. 5). In the winter 1993–1994 until ice-out, the drainage from the catchment acidified only the upper 2–3 m of the lake water column and it should be noted that the through-flowing water acidified the whole lake surface area under the ice (Fig. 6). The increase in alkalinity during the summer stratification (Fig. 5) may most probably be ascribed to CO2 induced dissolution of settled

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Fig. 5. Water temperature, oxygen saturation and alkalinity of Lake Terjevann in 1992–1994. (Ice-cover: December 16–April 4 1994).

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Fig. 6. Temperature, pH and inorganic aluminium (Ali) in the region 0–4 m under the ice at 33 points at Lake Terjevann March 14 1994.

lime particles. The general decreasing dissolution of CaCO3 in the summers from 1992 to 1994 (cf. Fig. 5) was probably due to a general depletion and less availability of the lime pool in the sediments. 4. Discussion The yearly mean air temperatures in 1992–1994

in the southernmost Norway were from 0.4 to 1.3 8C higher than mean values for the period 1961– 1990 (Table 2). Whether or not this was due to climate warming, the observations from this period may preview how projected climatic changes (IPCC, 2001) may affect acid–base chemistry of limed lakes in acidified catchments. Generally, the frictional movement of wind will

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Fig. 7. Alkalinity measured in Lake Terjevann before and after the winter seasons of 1992–1993 and 1993–1994.

set the surface water into motion and in periods with water temperatures near the temperature of maximum density only small amounts of wind energy are required to mix the water column (Birge, 1916). During the ice-free winters of 1991–1992 and 1992–1993 the lake had turnovers several times as indicated by the uniform temperature and oxygen saturation of the whole water column (Fig. 5). The quality of the outlet water in these periods (Fig. 4) indicates that the acid input from the catchments was neutralised in the lake. The general low Ali concentration in the outlet (Fig. 4) and the in-lake loss of alkalinity (Fig. 7) suggest that the Ali hydrolysed when the acid input mixed with the limed water. Sedimentation of aluminium also occurred as indicated by the generally lower Alt concentration in the outlet compared to the inlets (Table 4). Co-precipitation with organic matter could be one possible mechanism (cf. Andersen et al., 2000) as the concentration of organic matter was generally lower in the outlet than in the inlets (Table 4). Andersen and Pempkowiak (1999) reported increased accumulation of aluminium in the Lake Terjevann sediment after lime treatments. After the formation of ice in winter of 1993– 1994 the inlet waters passed the lake almost unchanged and decreased pH and increased Ali concentrations were evident in the outlet water during this period (Fig. 4). However, the inverse

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thermal stratification restricted this re-acidification to the upper 2–3 m of the lake (Fig. 6) at least until a few weeks before ice-out. The duration of the ice-cover and consequently, the thermal stratification inevitably influence the in-lake lime consumption. This is indicated by the loss of approximately 29 meqyl of alkalinity in the warmer winter (November–June) of 1992–1993 compared to approximately 7 meqyl lakewater in 1993–1994 (Fig. 7). The sea-salt event in January 1993 (Andersen and Seip, 1999) caused most probably some extra loss of alkalinity. However, frequent lake water circulation in the ice-free winter and mixing with the inlet waters were probably most important for the difference in lime consumption between these two winters. Even settled lime particles seems consumed during this ice-free period due to the limited release of alkalinity from the sediment during the following summer (1993) stratification (Fig. 5). Overflow and re-acidification of the upper strata of ice-covered limed lakes and downstream waters are well-known phenomena (e.g. Driscoll et al., 1989; Abrahamsson, 1993). However, global warming and mean air temperatures above 0 8C during winter will prevent this common wintery spring acidification of both the upper portions of the lake water column (Fig. 6; cf. Gubala et al., 1991) and the outlet water (Fig. 4). More stable environments with less fluctuation in labile inorganic aluminium (Ali) and pH may be developed in the littoral zone (e.g. Gunn and Keller, 1986) and for downstream dwelling organisms. 5. Summary and conclusions Monitoring of water chemistry in the limed Lake Terjevann during the unusual warm winters of 1991–1992 and 1992–1993 and the more normal cold and nival winter of 1993–1994 showed a significant higher in-lake alkalinity consumption during the warm winters. This was most probably due to frequent turnovers of the ice-free lake and neutralisation of the acid input from the catchment. During the cold winter of 1993–1994 the acid drainage from the catchment flowed to the outlet and only the upper few meters of the water column under the ice were acidified.

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In January 1993, a heavy deposition of sea-salts induced ion-exchange processes in the catchment soil and a substantial export of acidity (hydrogen and aluminium) that contributed to the in-lake lime consumption. Global warming is expected to result in both increased winter mean air temperatures and deposition and besides to cause more intense deposition events (rainstorms) over northern mid- to highlatitudes. Input of increasing amounts of acidity to wind exposed lakes may increase the in-lake lime consumption and consequently, the future demand for lime if current water quality is desired maintained. Acknowledgments The Norwegian Directorate of Nature Management (DN) supported this work. Grants from Andreas and K. Ludvig Endresens legate are appreciated. I am grateful for comments on the manuscript from Egil T. Gjessing, Hans M. Seip and an anonymous referee. References Abrahamsson I. Impact of overflows on acid–base chemistry in limed lakes. Vatten 1993;49:24 –33. Andersen AT, Føyn L Jr. Dissolved oxygen and hydrogen sulphide. In: Lange R, editor. Chemical Oceanography. Oslo: Universitetsforlaget, 1969. p. 123 –131. Andersen DO, Pempkowiak J. Sediment content of metals before and after lake water liming. Sci Total Environ 1999;243y244:107 –118. Andersen DO, Seip HM. Effects of a rainstorm high in seasalts on labile inorganic aluminium in drainage from the acidified catchments of Lake Terjevann, southernmost Norway. J Hydrol 1999;224:64 –79. ´ M. Nature of natural organic Andersen DO, Alberts JJ, Takacs matter (NOM) in acidified and limed surface waters. Water Res 2000;34:266 –278. Birge EA. The work of the wind in warming a lake. Trans Wis Acad Sci 1916;18:341 –391. Bresser AHM, Salomons W. Acidic precipitation. International overview and assessment, vol. 5. New York: Springer, 1990. p. 344. ˜ Dillon PJ, Molot LA, Futter M. The effect of El Nino-related drought on the recovery of acidified lakes. Environ Monit Assess 1997;46:105 –111. Driscoll CT. A procedure for the fractionation of aqueous aluminum in dilute acidic waters. Int J Environ Anal Chem 1984;16:267 –283.

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