Phosphorus transfer from agricultural areas and its impact on the eutrophication of lakes—two long-term integrated studies from Norway

Journal of Hydrology 304 (2005) 238–250 www.elsevier.com/locate/jhydrol

Phosphorus transfer from agricultural areas and its impact on the eutrophication of lakes—two long-term integrated studies from Norway M.E. Bechmanna,*, D. Bergeb,1, H.O. Eggestada,2, S.M. Vandsemba,2 a

Jordforsk—Norwegian Centre for Soil and Environmental Research, FA. Dahls vei 20, 1432 A˚s, Norway b NIVA—Norwegian Institute for Water Research, P.O. Box 173, 0411 Oslo, Norway Received 30 November 2003; revised 1 May 2004; accepted 1 July 2004

Abstract Eutrophication of most fresh water systems is limited by phosphorus (P) concentration. High P concentrations originate from external and internal sources. In most Norwegian lakes, agriculture is a main external contributor of P. Two long-term, integrated studies of the relationship between agricultural management, transfer of P and suspended sediments (SS) from agricultural areas and the total P (TP) and chlorophyll-a (Chl-a) concentrations of the receiving lake were carried out in Norway. The Grimestad subcatchment/Aker Lake system (1993–2000) represents a cereal-growing area with mixed livestock production, while the Time subcatchment/Frøyland Lake (1986–2000) system represents a grass and dairy cow production system. A comparison of the two systems showed that the mean annual concentration of SS in the Grimestad Stream was 20 times the corresponding concentration in the Time stream. The difference in transparency (secchi depth) of the two lakes reflected this difference. The losses of TP and SS from the Grimestad subcatchment increased significantly during the monitoring period. In the Time stream, there was a significant downward trend in concentrations of TP. Corresponding to the measured inputs, the TP concentration of the Aker Lake (recipient of Grimestad Stream) increased slightly during the monitoring period, while the TP concentration of the Frøyland Lake (recipient of Time Stream) showed a slightly decreasing trend. Loads of TP from the Grimestad subcatchment during spring (March–April) described 70% of the variation in TP concentration of the Aker Lake the following summer. The TP concentration in the Time stream in November–December also were correlated (r2Z0.6) to the TP concentration in the Frøyland Lake the following summer. The annual TP concentrations of the lakes were not very well correlated to the measured Chl-a in the lakes, partly because of bio-manipulation, which was performed in both lakes during the monitoring period. q 2005 Elsevier B.V. All rights reserved. Keywords: Eutrophication; Agriculture; Stream; Lake; Phosphorus; Chlorophyll-a

* Corresponding author. Fax: C47 64948110. E-mail address: [email protected] (M.E. Bechmann). 1 Fax: C47 22185200. 2 Fax: C47 64948110. 0022-1694/$ - see front matter q 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2004.07.032

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1. Introduction Water quality is the main focus in the EU Water Framework Directive. One of the major problems regarding water quality in Norway is eutrophication, which tends to occur in low-lying areas, particularly near settlements and agricultural land (SFT, 2003). Point and non-point sources within the catchments of these lakes contribute to high nutrient concentrations. Algal growth in nearly all eutrophic lakes is limited by the concentration of phosphorus (P) rather than nitrogen (Schindler, 1977; Berge, 1987; Faafeng and Hessen, 1993). Since 1972 there has been a six-fold increase in the capacity of waste water treatment plants and at the same time they have improved their ability to remove P by introducing chemical treatment (SFT, 2003). The contribution of P from point sources decreased from 1985 to 2001 to less than half (Borgvang et al., 2002). This reduction in point sources leaves agriculture as the main external contributor of P in many Norwegian lakes (Borgvang et al., 2002). The effect of agricultural P losses on eutrophication of lakes is not a straightforward relationship. Vollenweider (1976) describes the effect of TP contribution from the catchment on the eutrophic status of the recipient. This model has been adapted to Norwegian conditions by Berge (1987), who found that a non-logarithmic approach for the theoretical residence time gave a better estimate for shallow Norwegian lakes than the Vollenweider-model. In addition to external contributions of TP, internal processes within a lake may significantly influence the concentration and availability of P and hence algal growth (Kauppi et al., 1993; Graneli, 1999; Schernewski, 2003). Furthermore, not only the dissolved P contributes to eutrophication, since when the stream water enters a lake, the particulate P (PP) in the SS of the stream water begins to re-equilibrate with the standing water’s dissolved P (Correll, 1998). Hence, much of the PP inputs become available to the phytoplankton and bacteria (Krogstad and Løvstad, 1989; Ekholm, 1994; Reynolds and Davies, 2001). Haraldsen et al. (1995) described the regional differences in P loss processes and concluded that erosion and related losses of PP were the most important process of P loss on arable land in south-eastern Norway, while losses of dissolved P related to manure application and point

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sources were more important in the south-western part of the country where livestock density is generally high. The different pathways and forms of P transported have implications for the required measures to reduce P transfer, as underlined by, e.g. Haygarth and Jarvis (1999). The seasonal variations in erosion influence the risk of eutrophication, since light and temperature conditions are important factors for algal growth. In addition, once the particles have settled to the bottom of the receiving water other processes become important (Correll, 1998). The complex relationship between external and internal loading of P makes it difficult to link measures taken in the catchment to improved water quality of the receiving lake (Daniel et al., 1998). This paper presents an integrated and comparative study of two sites, including the relationships between agricultural management, transfer of P and suspended sediments (SS) from the agricultural areas to the stream and the concentration of total P (TP) and chlorophyll-a (Chl-a) in the lakes. The objective is to link agricultural activities and losses from agricultural areas to TP concentration and eutrophication of lakes.

2. Material and methods 2.1. Study areas The two areas that were studied are located in south-eastern (Grimestad subcatchment/Aker Lake and south-western (Time subcatchment/Frøyland Lake) Norway (Fig. 1). The time series comprise long-term monitoring data for the Grimestad (1992–2001) and Time (1986–2000) Streams and the Aker (1992–2000) and Frøyland (1984–2000) Lakes. The systems represent two contrasting production systems in Norwegian agriculture, namely cereal and mixed livestock (swine, sheep and poultry) production in south-eastern Norway, and dairy and grass production in the south-western part of Norway (Table 1). The Grimestad catchment is a subcatchment of the Aker Lake catchment and constitutes 12% of the total catchment area of Aker Lake. Out of 11 streams contributing to the Aker Lake, the Grimestad Stream is the largest. This subcatchment has a high livestock

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Fig. 1. Map showing the two integrated study areas, Aker Lake with its catchment and subcatchment Grimestad and Frøyland Lake with its catchment and subcatchment Time (different scale for study areas).

density (1.5 animal manure units (AMU) haK1 agricultural land. 1 AMU corresponds to 14 kg P yrK1), compared to the average livestock density of the agricultural areas in the catchment of Aker Lake (0.6 AMU haK1). Hence, the potential risk of P losses from manure is higher in the Grimestad subcatchment than on average for the entire Aker Lake catchment. Therefore, the Grimestad Stream is one of the main contributors of P and SS to the Aker Lake. The Time subcatchment constitutes 2% of the total catchment area of Frøyland Lake. The livestock

density of the subcatchment is 2.3 AMU haK1 agricultural areas and consists mainly of dairy cows. In the catchment, dairy production is combined with a high percentage (75% of the total agricultural area) of grassland. The production system and intensity of livestock in the Time subcatchment is representative for the agricultural areas of the Frøyland Lake catchment. However, the percentage of agricultural land is much lower in the total catchment of Frøyland Lake (49%) than in the Time subcatchment (85%) (Table 1). Variations in concentrations of TP and SS

Table 1 Characteristics of the Grimestad, Aker Lake, Time and Frøyland Lake catchments Catchment

Size (ha)

%Agric. area

Dominating production

Livestock density (AMU haK1 agric. area)

Area ploughed (% of agric. area)

P-AL (mg 100 gK1)

Soils

Cereal/swine/ sheep/poultry Cereal

1.5

57

15

0.6

90

–

2.3

21

17

Sand, silty sand Sand, silty sand Silty sand

2.1

35

–

Silty sand

Grimestad

169

47

Aker Lake

1411

41

114

85

5500

49

Time Frøyland Lake

Grass/dairy cattle Grass/dairy cattle

Animal manure units (AMU haK1 agric. area) (SSB, 2003), plant-available P in soil by Ammonia-Lactate extraction (P-AL) (JordforskLab, 2003).

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in the Time Stream are expected to be representative for concentrations in runoff from most agricultural areas in the Frøyland Lake catchment. The geological conditions of both the Grimestad and Time subcatchments are representative for the catchments of the Aker Lake and Frøyland Lake, respectively. The Grimestad subcatchment has sandy beach sediments with varying silt contents (Table 1). Most of the area has slopes with gradients of less than 6%. The soils in the Time subcatchment developed in moraine material of Precambrian origin and slopes are gentle (2–6%). In the Frøyland Lake catchment, only 35% of the agricultural area is ploughed each year, as opposed to the Aker Lake catchment, where 90% is ploughed (Table 1). Accordingly, the risk of soil loss is lower in the Frøyland Lake catchment than in the Aker Lake catchment. Plant available soil P (Ammonium-Lactate extraction, P-AL) of the 0–20 cm topsoil in the catchments (area-weighted average of farmers’ samples) are 15 and 17 mg 100 gK1 for the Grimestad and Time subcatchments, respectively (Table 1). According to Norwegian classification this corresponds to high to very high levels. The mean annual temperature is 7.4 8C for both areas. Annual precipitation is high in both, Grimestad /Aker Lake (1154 mm) and Time/Frøyland Lake (1672 mm) for the respective monitoring periods. Most fields in both studies are artificially drained at a spacing of 8 m. In Frøyland Lake, 20 constructed wetlands were built between 1992 and 2000. Within the catchment of Aker Lake, 11 constructed wetlands were built in 1995–1997. About 48% of the discharge from agricultural areas passes through these wetlands. Since the subcatchments constitute only a minor part of the lake catchments, they may not give an exact estimate of the actual P loading in the lake. However, the two most important factors for the transfer of TP and SS from agricultural areas are the conditions for agricultural production and the weather. Since these conditions are expected to vary simultaneously for the subcatchments and the rest of the lake catchments, the variations in transfer of TP and SS from the subcatchments are expected to reflect the main variations in transfer of TP and SS for the total catchments of the lakes.

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2.2. Monitoring The data from the Grimestad subcatchment included time series from 1993 to 2001, while the Aker Lake data covered the period from 1992 to 2000. The Time subcatchment monitoring data included from 1986 to 2000, and for the Frøyland Lake, data from 1984 to 1988, 1990, and 1992 to 2000 were available. Monitoring of the Grimestad and Time subcatchments consisted of discharge measurement, water quality analysis and field scale data on agricultural management. Discharge measurements were carried out using a V-notch in the Grimestad Stream and a Crump weir in the Time Stream. The flow depths were measured with pressure transducers. Data loggers recorded flow data every hour. Data quality control was carried out every 2 weeks based on field visits. The discharge measurements in the Time Stream during summer and autumn were disturbed by vegetative growth in the stream, leading to submerged flow conditions. Hence, flow data for the Time Stream are not reliable. Reconstruction of a valuable dataset was not possible. Losses and concentrations of SS, TP and DRP in runoff for the agricultural areas were calculated on the basis of standard losses from nonagricultural areas of 0 g SS haK1, 60 g haK1 TP and 60 g DRP haK1. In both streams, flow proportional composite samples were collected downstream of the monitoring stations throughout the entire monitoring period. The error in discharge, as described above, for the Time Stream has not biased the flow-proportional sampling, since sampling only covered short periods with similar influence of in-stream vegetation (Deelstra, pers. comm.). The composite samples from the Grimestad Stream were analysed once a week in spring and autumn and once every 2 weeks in winter and summer. Samples were analysed approximately once every 2 weeks in the Time Stream throughout the entire year. Results presented here include concentrations of SS, TP and DRP. Annual data for the streams are presented as mean values from 1 September to 1 September the following year. In both lakes, samples were collected twice per month from about 20 May to about 10 October (9–10 times) each year. In Aker Lake the samples

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were taken as a mixed sample from 0 to 6 m depth. In Frøyland Lake samples were mixed from 0 to 3 m. Layered samples were collected once in late winter from four depths in each lake. The presented annual data for lake samples included concentration of TP and Chl-a. Information on agricultural management within the Grimestad and Time subcatchments was derived from annual questionnaires to farmers for each field within the catchments. For the Aker Lake and Frøyland Lake catchments, the information on agricultural management was obtained from Statistics Norway (2003) for the two main municipalities within the catchments. The Aker Lake catchment was represented by data from Stokke and Tønsberg municipalities, and the Frøyland Lake catchment was represented by data from Time and Klepp municipalities. 2.3. Analyses Samples from the streams and lakes were analysed by the following procedures. Filtered samples (!0.45 mm) were analysed for the concentration of dissolved reactive P (DRP). Unfiltered samples were used to determine the TP concentration, by digestion with K2S2O8. The P concentration of all samples was analysed spectrophotometrically by the ammonium molybdate method of Murphy and Riley (1962) with ascorbic acid as a reducing agent. PP was calculated as TP minus DRP. The SS concentration was determined by filtering an exact sample volume of 25–250 ml (containing at least 5 mg SS) through a pre-weighed Whatman GF/A filter. Analyses were carried out by certified laboratories (Food control agency; Holt Research station). The samples were stored at 5 8C for some weeks before analysis, therefore they may not give a true estimate of the DRP concentration at sampling time. Braskerud (2001) studied the effect of storage of water samples from similar areas in Norway and showed that the DRP concentration changed significantly during storage. Hence, this paper does not place much emphasis on the DRP concentrations. Chl-a concentrations were determined after filtration (Whatman GF/C) by extraction with acetone/DMSO and absorption measured at 665, 645, 630 and 750 nm calculating a tricromatic function, which corrects for chlorophyll b and c (Klaveness, 1984).

3. Results and discussion 3.1. Concentrations of SS, TP and DRP in agricultural runoff The intensively studied subcatchments in the two different regions in Norway showed marked differences between farming systems and between the concentrations of SS, TP and DRP. The Grimestad Stream (south-eastern Norway), with its combination of cereal and mixed livestock production had a higher mean annual concentration of TP (270 mg TP LK1) than the Time Stream (south-western Norway) (177 mg TP LK1), with its grassland and dairy production. These concentrations represent runoff from both agricultural and non-agricultural areas within the catchments. Table 2 presents the calculated concentrations in runoff from the agricultural areas within the subcatchments. Forty-seven percent of the Grimestad subcatchment is agricultural land, compared to 85% in the Time subcatchment (Table 1). The difference in percentage of agricultural land is important for nutrient concentrations in the streams (e.g. Ekholm et al., 2000). However, the Grimestad and Time Streams do not reflect this, since other factors are far more important within these catchments. Concentrations of TP in runoff from the agricultural areas in the Grimestad subcatchment are 2–3 times higher than concentrations from agricultural areas in the Time subcatchment (Table 2). In the Grimestad Stream, the mean SS concentration from agricultural areas was 314 mg LK1, which is 24 times higher than for the Time Stream (Table 2). Grassland areas, as in the Time subcatchment, are known to have low losses of SS (e.g. Uhlen (1989) and Daniel et al. (1998)). TP concentrations in the Time Stream consist of 66% PP and 33% DRP and hence may be influenced both by erosion and the soil P status as well as the P application rate. In the Grimestad Stream only half of the TP is PP (Table 2). Since DRP concentrations are much lower in the Time Stream than in the Grimestad Stream, the relationships between TP and SS in Fig. 2 indicate that eroded soil particles contain more P in Time than in Grimestad. The content of PP in SS in the Time Stream is 10‰, while in the Grimestad Stream the corresponding figure is only 0.8‰. The relatively low PP in the Grimestad Stream is, at least partly, due to

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Table 2 Mean annual flow-weighted concentrations of suspended sediments (SS), total phosphorus (TP) and dissolved reactive phosphorus (DRP) in runoff from agricultural areas in the Grimestad and Time catchments Year (1 Sept.–1 Sept.)

Grimestad

Time

Runoff (mm)

SS (mg LK1)

TP (mg L

1986/1987 1987/1988 1988/1989 1989/1990 1990/1991 1991/1992 1992/1993 1993/1994 1994/1995 1995/1996 1996/1997 1997/1998 1998/1999 1999/2000 2000/2001 Waterweighted average

– – – – – – 729 1394 1332 379 405 909 1240 994 1452 982

– – – – – –

– – – – – – 249 258 368 328 361 482 760 1072 916 577

26 30 129 122 280 357 358 652 668 314

)

K1

DRP (mg LK1)

Precip. (mm)

SS (mg LK1)

TP (mg LK1)

DRP (mg LK1)

– – – – – – 143 163 252 269 261 365 573 560 357 340

1209 1276 1358 1274 1172 1425 1237 1096 1180 958 1469 1285 1187 1331 – 1247

13.5 8.2 10.5 6 17.9 12.5 9.6 14.8 13.7 13.7 19.2 12.7 9.7 16.4 – 13

335 233 245 155 208 196 173 234 187 208 214 158 160 202 – 207

122 69 92 59 64 50 61 55 57 86 75 61 55 65 – 69

Annual runoff (Grimestad) and annual precipitation (Sola-station, DNMI, 2004).

evident channel erosion. The plant available soil P content for the two catchments is about equal; 15 and 17 mg P-AL 100 gK1 for Grimestad and Time catchments, respectively (Table 1). The transport of P in erosion is known to be selective (Øygarden, 2000). Because of particle size and origin of particles in the erosion process, SS generally contain more P, than the soil from which it originates. P-AL was analysed for 0–20 cm soil depth. On grassland, plant available soil P in the upper few centimetres may be much higher, than in deeper soil layers (Sharpley et al., 2001), and since transport of PP may be related only to the upper few centimetres of the soils, this may explain the high P content of SS in the Time Stream. The mean annual concentrations of SS in the Grimestad Stream are high compared to other monitored Norwegian catchments (Vandsemb et al., 2002). Notably, annual mean concentrations of both SS and TP increased dramatically in the Grimestad Stream during the monitoring period (Table 2). Field data, provided by farmers, showed that in 1998, 70 sheep grazed in a field just upstream of the monitoring site. In 1999 and 2000, 165 and 80 sheep, respectively, grazed in this field. Grazing along the stream banks

was also observed, devegetation and erosion were evident. In other studies, grazing has been shown to give a significant contribution to erosion (e.g. Heathwaite and Johnes, 1996), while P application in manure adds additional P directly to the stream. The grazing has contributed to high concentrations of SS and TP in the Grimestad Stream during the years 1998–2001. Conversely, no such incidental activities were observed in the Grimestad subcatchment before

Fig. 2. Relationship between total phosphorus (TP) and suspended sediment (SS) concentrations (mg LK1) in samples taken in the Grimestad and Time Streams during the monitoring period.

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1998, and the mean annual concentration of TP was 312 mg TP LK1 in 1993–1997, which is 50% higher than for the Time Stream. The comparison of SS concentrations for the period 1993–1997 in the Grimestad and Time Streams may illustrate the effect of different soil tillage systems. The annual mean concentration of SS for 1993–1997 was 117 mg LK1 in the Grimestad Stream, compared to 13 mg LK1 in the Time Stream (Table 2). Information from farmers showed that 57% of the agricultural area in the Grimestad catchment was ploughed every year (26% of the ploughing was carried out in the autumn), while in the Time catchment, 90% of the agricultural area was grassland and only 21% was ploughed annually, nearly all in spring (Table 3). Ploughing, especially in autumn, has been shown to significantly increase the risk of erosion (Øygarden, 2000; Lundekvam et al., 2003). Farmers in Norway have been encouraged to reduce autumn ploughing by means of subsidies. In the Grimestad subcatchment, the autumn-ploughed area has decreased slightly during the monitoring period. Nonetheless, the water quality deteriorated as a result of the specific activities mentioned above. In contrast, a reduction in the TP concentration during Table 3 Autumn ploughed area in percent of agricultural area, mean phosphorus application rate on agricultural areas in the Grimestad and Time catchments during the monitoring period Year

1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 Mean

Grimestad

Time

Autumn ploughed area (%)

P application (kg haK1)

Autumn ploughed area (%)

P application (kg haK1)

– – – – – – 21 22 15 17 9 11 2 15 18 15

– – – – – – 41 32 40 32 31 38 38 33 34 35

– – – – – – 0 0 0 0 0 2.4 1.8 0 0 0.5

47 40 36 – – – 27 28 27 35 39 36 34 38 38 35

the monitoring period was measured in the Time Stream. Mean P application rates are quite similar for the two catchments, 3.5 kg P haK1 for Time (average for 1985–2000) and 3.5 kg P haK1 for Grimestad (average for 1993–2000). However, in the Time subcatchment there was a reduction in the rate of P application in fertilizer and manure (Table 3). Furthermore, manure spreading in autumn was reduced and methods of fertilizer/manure application improved (Bechmann and Sta˚lnacke, submitted) during the monitoring period. This may contribute to reduced risk of incidental losses (Withers et al., 2003) and hence, reduced TP concentration in the stream. A systematic seasonal pattern for TP concentrations was not found for any of the two streams. However, winter concentrations of TP were somewhat lower than summer concentrations in both the Grimestad and Time Streams. Concerning TP loads from the Grimestad subcatchment, these were significantly lower in summer than during autumn and winter (Fig. 3). Hence, to reduce the total annual TP loads it is necessary to focus on measures which are efficient during autumn and winter, e.g. no autumn tillage and application of manure in growing season. 3.2. Total phosphorus and chlorophyll-a in Aker Lake and Frøyland Lake The mean annual TP concentrations of the two lakes showed marked differences (Table 4). Mean TP concentration of the Aker Lake in south-eastern Norway was 57 mg LK1, while in the Frøyland Lake (south-western Norway) the concentration was 35 mg TP LK1. In the Aker Lake, the concentration of TP increased during the 1990s. The mean summer concentration was 48 mg LK1 in 1992 and increased to 61 mg LK1 in 2000 (Fig. 4a). The TP concentration in the Frøyland Lake showed a significant (p!0.01) downward trend from the start of the monitoring in 1984 to the end in 2000 (Fig. 4b). This downward trend may partly be due to extensive establishment of constructed wetlands. The Chl-a concentration did not show clear trends in the same period for any of the catchments (Fig. 4a and b). The transparency (secchi depth) is lower (1.4 m) in the Aker Lake than in the Frøyland Lake (2.1 m). Also, transparency in the Aker Lake is not correlated (r2Z0.2) to the Chl-a. This may be due to suspended sediments, which reduce secchi

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Fig. 3. The Grimestad subcatchment: losses of total phosphorus (kg TP haK1) from agricultural areas in autumn (Sept.–Dec.), winter (Jan.–Apr.) and summer (May–Aug.) on the left-axis and annual discharge (mm) on the right-axis.

depth. In the Frøyland Lake, secchi depth and Chl-a show a better relationship (r2Z0.5), since the catchment of Frøyland Lake contributes less SS (Table 2). For both lakes, bio-manipulation measures were implemented in the beginning of the monitoring period. These have lowered both the P and Chl-a concentration in the free waters of the lakes in the first part of the monitoring period (Berge et al., 2001). Aker Lake was stocked with pike-perch (Stizostedion lucioperca) in the late 1970s. In the late 1980s this population increased dramatically, and the population of planktivorous fish was strongly reduced. The zooplankton grazed down the algal biomass and the water cleared up. This lasted for some years, but in 1994 an opportunist algae, which was not grazed by

the zooplankton (the large and spiny Ceratium hirundinalla) invaded the lake and has since then dominated the phytoplankton. Each year late summer and autumn, the blue greens return. Most likely, the bio-manipulation measures no longer had any effect in the last part of the monitoring period. In Frøyland Lake, about 35 tonnes of planktivorous fish were caught in the years 1990–91, an estimated 50% of the population. There was still some fishing for some years afterwards, but after 1999 it ceased due to high costs and lack of demand for the fish. It is believed that the low Chl-a-values from 1992 and throughout the 1990s were partly due to this fishing. Bio-manipulation reduces both the P concentration and the Chl-a concentration in lakes, the latter being reduced most. The impact from bio-manipulation may

Fig. 4. Total phosphorus (TP) and chlorophyll-a (Chl-a) mean summer concentrations (mg LK1) in Aker Lake (a) and Frøyland Lake (b).

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Table 4 Theoretical residence time, average depth, lake surface area, mean total phosphorus (TP) concentration, chlorophyll-a concentration and transparency (secchi depth) for Aker Lake (1992–2000) and Frøyland Lake (1984–2000)

Aker Lake Frøyland Lake

Theoretical residence time (years)

Average depth (m)

Lake surface area (km2)

1.7 0.41

6 5.3

2.3 4.9

be one of the reasons for the poor correlation between the TP concentration and the Chl-a concentration (r2Z0.16) in these two lakes. Monitoring data for lakes over a wider range of the oligo-eutrophic scale and in average for time series data in the Eikeren watercourse, shows much better relationship between P concentration and Chl-a concentration (Fig. 5). The Eikeren watercourse is close to Aker Lake and consists of lakes polluted from agriculture and sewage. Aker Lake and Frøyland Lake fit well into this relationship (Fig. 5). 3.3. Water quality response in the lakes to catchment contributions of SS, TP and DRP Frøyland Lake is recipient for the Time Stream and Aker Lake is recipient for the Grimestad Stream. The two subcatchments constitute only 12 and 2% of the catchments of Aker Lake and Frøyland Lake, respectively. This implies that what happens in these two streams can only partly explain what happens in the lakes. However, since the Grimestad subcatchment is one of the main contributing areas to the Aker Lake and the Time subcatchment is representative for the Frøyland Lake catchment, a certain correlation can be expected. These subcatchments will at least reflect the time dynamics of the transfer to the recipient, since runoff is by far the most important factor. The correlation coefficients (r2) are presented in Fig. 6 for monthly transfer of TP from the catchments. For the Time Stream it was shown that TP concentrations in November and December gave the best relationship (r2Z0.59) with the TP concentration of the Frøyland Lake the following summer. Also TP concentration in January in the Time Stream showed some influence on the TP concentration of the Frøyland Lake. Correspondingly, for the Grimestad Stream, the best correlation (r2Z0.56) was found for

Concentration (mg LK1) TP

Chlorophyll-a

57 35

28 21

Secchi depth (m)

1.4 2.1

TP concentrations in March and April regarding lake TP concentrations the following summer (Fig. 6). Concentrations of TP in the Grimestad Stream in May also seem to be relatively important, while the summer concentrations in the Grimestad Stream were not correlated to the TP concentration of the lake. This is partly due to the low runoff during summer. The best correlation (r2Z0.75) was found for TP loads from the Grimestad catchment in March and April and the TP concentration of Aker Lake during summer. TP loads in March–April constitute 22% of the mean annual losses. The TP loads from the Grimestad catchment were much higher (3.5 times) during the winter period than in summer, and since theoretical residence time is 1.7 years in the lake (Table 3), the winter period may have a more significant influence on the lake’s water quality than the summer period. DRP from the Grimestad subcatchment showed the same correlation as TP with concentration of TP in the lake. The correlation (r2Z0.46) between TP concentration in the Time Stream, and the TP concentration

Fig. 5. Relationship between total phosphorus (TP) and chlorophylla (Chl-a) concentrations (mg LK1) for ten monitored lakes in the Eikeren watercourse, situated close to Aker Lake. Aker Lake and Frøyland Lake are also shown (adapted from Berge (1999)).

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Fig. 6. Correlation coefficients for the relationship between total phosphorus (TP) concentrations in the Frøyland Lake and the monthly TP concentrations of the Time Stream and for the TP concentration of the Aker Lake and the monthly TP concentration and load of the Grimestad Stream (a). Concentrations (b) and loads (c) of TP in March–April for the Grimestad catchment in relation to TP concentrations in the Aker Lake. Concentrations (d) of TP in November–December in the Time Stream in relation to TP concentrations in the Frøyland Lake.

in the Frøyland Lake indicates that there is a covariation between the TP concentration in the Time Stream and the rest of the catchment, i.e. the rest of the catchment behaves like the Time catchment. The relationship between lake TP concentrations and transfer of P from agricultural areas in different seasons may be used to evaluate the effect of measures to reduce TP loading. If TP losses in March–April explain most of the variation in TP concentration in the lake, as in the Grimestad/Aker Lake study, measures which reduce TP losses in March–April may need to be focused upon. In the case of Frøyland Lake, measures focusing on reduced concentration in November–December may be the most effective. Acceptable trophic status of lakes was evaluated by Berge (1987), who defined acceptable TP concentration ([P]l) in shallow (1.5–15 m deep) Norwegian lakes based on mean depth: ½Pl Z K8:68 lnðmean depth in mÞ C 31:13

(1)

For Aker Lake this gives an acceptable TP concentration of 16 mg LK1. The measured concentration in Aker Lake is 57 mg LK1 in the summer season, which indicates that the lake is heavily overloaded. Berge (1987) developed a modified Vollenweider model for Norwegian shallow lakes. TP ¼ 2:293½PlTw0:16 Q

(2)

where: TP is total annual load in kg [P]l is average annual concentration of TP in the lake (mg LK1) Tw is theoretical residence time Q is annual discharge in m3. Using this model for Aker Lake estimates the loading of TP to the open waters at 1237 kg yrK1. This figure also includes the contribution from internal loading. It is shown that in periods of high

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pH during summer with intensive algal growth, significant amounts of phosphate can leach from the sediments. This means that the model of Berge (1987) overestimates the actual external P loading in eutrophic lakes, whereas it estimates the acceptable P-load rather correctly. Contributions from the Grimestad catchment are 2.7 kg haK1 yrK1 (Z497 kg yrK1). Thus, TP losses from the rest of the catchment, plus any internal contribution from sediments within the lake itself are 740 kg yrK1 according to these calculations. Using acceptable concentration (16 mg LK1) in the above Eq. (2) gives an acceptable TP load of 507 kg yrK1. Hence, the modelled TP load (1237 kg haK1 yrK1) is approximately twice as high as the acceptable load. Berge (1988) found that internal loading in Aker Lake was relatively restricted, even though it was not estimated very accurately. If one estimates the internal loading at approximately 200–300 kg TP yrK1, it can be calculated that the necessary reduction of external P loading in Aker Lake is about 500 kg TP yrK1. This reduction in the external loading will ‘turn off’ the internal loading. Frøyland Lake is shallow, and because of a high pH, it releases more P from its sediments than the Aker Lake (Sanni, 1987). In addition, the lake is located in a windy area where resuspension of sediments may contribute to the release of Molversmyr (2002) estimated that the internal P loading in Frøyland Lake is approximately 2200 kg P yrK1, of which most takes place during summer due to high pH. Using the Berge (1987) model here also gives an acceptable P concentration of 16 mg P LK1, and an acceptable P loading of 2484 kg P yrK1. A mean TP concentration of 35 mg LK1 throughout the monitoring period gives an estimated TP load to the free waters of Frøyland Lake of about 5400 kg TP yrK1 (which also includes internal loading, see above). Subtracting the internal loading gives an external loading of 3200 kg TP yrK1. Subtracting the acceptable loading gives a need for reduced loading to the Frøyland Lake of approximately 700 kg TP yrK1, which represents 1.2 kg haK1 for the agricultural area. In the catchments of both of the studied lakes, constructed wetlands were introduced during the monitoring period as a measure to reduce sediment and TP loading to the lakes. In Frøyland Lake, 20 constructed wetlands were build from 1992 to 2000.

The effect of 11 constructed wetlands, which were build in 1995–1997 in the catchment of Aker Lake, was evaluated by Bach et al. (2003). They estimated the reduction in TP loads by constructed wetlands in the catchment of Aker Lake to be 314 kg yrK1, which is equivalent to 500 g haK1 yrK1 agricultural land. For comparison, the TP loads from the Grimestad catchment are 2800 g haK1 yrK1. The correlation between the contributions of P from the Grimestad catchment and the water quality of Aker Lake was nearly significant despite the influence of constructed wetlands and the fact that Grimestad constitutes only 13% of the Aker Lake catchment. The effect of constructed wetlands was not included in the contributions from the subcatchment and hence is an additional cause of variation in the relationship between TP concentrations in streams and lakes.

4. Conclusions Improved quality of surface water is the main goal of many measures implemented in management of agricultural production. Two systems comprising the contributions of TP from agricultural areas and the resulting effect on eutrophication of lakes have been presented. One system was influenced by a high erosion rate and P transfer, while the other system was influenced by low erosion with a high P content in the eroded material. Calculations showed that reductions in TP loading of 500 and 700 kg TP yrK1 were necessary to obtain acceptable trophic status in the Aker Lake and the Frøyland Lake, respectively. The two subcatchments (Grimestad and Time) constitute only 12 and 2% of the lakes’ catchments (Aker Lake and Frøyland Lake, respectively), and thus the relationship between input of TP from agriculture and concentration of TP in the lakes is not straightforward. Additionally, in both studied systems the normally found good correlation between P load and P concentration in lakes was confused by (1) internal P loading, which takes place during the summer months due to high pH values in epilimnic waters; (2) bio-manipulation measures, aimed at reducing the amount of algae through increasing the population of zoo-plankton, in the same period as this study was performed. The latter has obviously affected the TP/Chl-a relationship in the two lakes,

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and obscured the picture of the causal relationships between measures and effects. Despite these complicating factors, the TP loads from the Grimestad catchment (the main contributing area) in March and April explained about 75% of the variation in the summer TP concentration in the surface water recipient (Aker Lake). Correspondingly, the TP concentration in November and December in the Time Stream was found to be well correlated (r2Z0.59) to the TP concentration in the Frøyland Lake. Accordingly, for the studied systems the P transfer outside the growing season seems to be most important for the TP concentration of the lakes. This conclusion may have implications for the measures implemented to reduce TP concentration in these lakes.

Acknowledgements We thank the Norwegian Crop Research Institute (Særheim) and the County Governor in Vestfold for providing data from the monitoring stations, and Karl Kerner (Agro Lingua) for careful revision of the language. In addition, we gratefully acknowledge financial support from SLF (Norwegian Agricultural Authority) and SFT (Norwegian Pollution Control Authority) to the Agricultural Environmental Monitoring Programme.

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