Phosphorus input by nordic geese to the eutrophic Lake Arendsee ...

Phosphorus input by nordic geese

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Fundamental and Applied Limnology Archiv für Hydrobiologie Vol. 172/2: 111–119, July 2008 © E. Schweizerbart’sche Verlagsbuchhandlung 2008

Phosphorus input by nordic geese to the eutrophic Lake Arendsee, Germany H. Rönicke1,*, R. Doerffer3, H. Siewers3, O. Büttner1, K.-E. Lindenschmidt1, P. Herzsprung1, M. Beyer1 and H. Rupp2 Helmholtz Centre for Environmental Research – UFZ, Department of Lake Research Magdeburg With 2 figures and 6 tables Abstract: Phosphorus import by nordic geese (Anser fabalis and Anser albifrons) was investigated in Lake Arend-

see, located in the Saxony-Anhalt region, Altmark, Germany during the period 1996 to 1997. Phosphorus contained in geese excrement on the ice was measured in the winters 1996 and 1997. In February 1996 (after 9 days of frozen lake surface) two excrement fields amounted to 80 ha and 30 ha in area and in January 1997, 10 days after ice closure, the excrement field was 106 ha large. The weight of excrement was estimated to be 148 to 266 g m–2 fresh weight (mean 201 g m–2) in 1996 and 83 g m–2 to 408 g m–2 (mean of 243 g m–2) in 1997. The average of phosphorus content was 8.5 mg g–1 dry weight in 1997 and 9.2 mg g–1 in 1996. Based on these values the phosphorus input attributed to nordic geese was calculated. Our results demonstrated a phosphorus import in 1996 after 9 days of frozen lake surface of 251 kg and in 1997 after 10 days of freezing of 173 kg. During 100 days of wintering, the nordic geese on Lake Arendsee produced a phosphorus load of 2.8 t in 1996 and 1.7 t in 1997. Compared with the annual phosphorus import from different sources, the contribution by nordic geese was 88 % in 1996 and 92 % in 1997. Its yearly phosphorus load during the winter months appears as a significant eutrophication factor for the trophic level of Lake Arendsee. However, the annual external load is approximately 10 % of the phosphorus poolsize in the lake water, and even less when considering the amount lodged in the bottom sediments. Key words: phosphorus input, nordic geese, nutrient loading by waterfowl, lake eutrophication.

Introduction Birds are an integral part of most freshwater ecosystems but their role in the trophic dynamics of these water bodies has often been neglected. Since the 1980s, both ornithologists and limnologists became increasingly aware of the contribution of aquatic birds to nutrient levels (Kerekes 1994). The process of lake eutrophication caused by human activities such as the emmission of domestic or industrial waste water, agricultural activities, intensive 1

fish production, and recreation use is well known. It has also been shown that a high density of waterfowl may affect the nutrient status of lakes by the release of excrement into the water (Manny et al. 1975, Dobrowolski et. al. 1976, Kalbe 1982, Portnoy 1990, Marion et al. 1994, Gere & Andrikovics 1994, Scherer et al. 1995, Moore et al. 1998, Post et al. 1998, Kitchell et al. 1999, Mukherjee & Borad 2001, Olson et al. 2005). For the past 10 years, a large number of nordic geese (Anser fabalis and Anser albifrons) have been observed during the winter months on Lake Arendsee.

Authors’ addresses: Helmholtz Centre for Environmental Research – UFZ, Department of Lake Research Magdeburg,

Brückstr. 3a, 39144 Magdeburg, Germany. Helmholtz Centre for Environmental Research – UFZ, Department of Soil Research Falkenberg, Germany. 3 GKSS – Research Centre Geesthacht, Max-Planck Strasse, 21502 Geesthacht, Germany. * Author for correspondence; e-mail: [email protected] 2

DOI: 10.1127/1863-9135/2008/0172-0111

1863-9135/08/0172-0111 $ 2.25 © 2008 E. Schweizerbart’sche Verlagsbuchhandlung, D-70176 Stuttgart

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The population of over-wintering geese, which was observed every year, ranged between 5,000 and 70,000 according to surveys by the Environment Board and ornithological working groups. The geese feed on the agricultural fields in the surrounding area and spend the night on the water surface. We estimated the contamination on the ice from the excrement of the geese in the winters of 1996 and 1997, two exceptional years during which the lake froze for a period. The purpose of this paper is to calculate the annual phosphorus input by the nordic geese and compare it to the other phosphorus sources to the lake. Our investigations provide a new method for the calculation of phosphorus import by waterfowl in lakes. Usually, such estimations were made on the basis of the number of birds and their daily production of faeces. In general, the estimation of nutrient input

by waterfowl on the basis of the bird number is difficult. This is mainly due to methodological difficulties in geese counting. As a consequence, the exact geese density is often unknown. With our new method, based on analysed geese faeces, we are able to measure directly the phosphorus input into the waterbody of the lake. We show that this method for the determination of P-import by nordic geese provides a practical technique if the number of birds is unknown.

Study area Lake Arendsee is a dimictic, highly eutrophic lake in the northern part of Germany (52° 54′ N, 11° 30′ E) in a predominantly agricultural area (Fig. 1). The lake has increased in both size and depth over time with the

Fig. 1. Map of Germany showing the location

of lake Arendsee in the state of Saxony-Anhalt (light grey) and locations of major cities (dark grey).

Phosphorus input by nordic geese Table 1. Morphometric parameters for Lake Arendsee (from Rönicke 1986).

Lake parameter Location Area Maximum depth Mean depth Volume Catchment area Mean turn over time

Value 52° 54′ N, 11° 30′ E 514 ha 48.7 m 28.6 m 147 × 106 m3 29.8 km2 114 yr

latest expansion occurring in 1685 (Halbfass 1896). The basic data on Lake Arendsee are given in Table 1 (Rönicke 1986 ). In addition, this bathing lake is the most important recreational site in the region of Altmark. As late as the turn of the last century, Lake Arendsee, with exceptionally clear water and phytoplankton dominated by Desmidiaceae, was on a par with the Alpine lakes (Zacharias 1899). An undiminished high level of phosphorus and thus, year-round development of bloom-forming colonial Cyanoprocaryotes (Aphanizomenon, Anabaena and Anabaenopsis species) have been observed for the past 25 years. Investigations on the N2-fixing cyanobacterial blooms were carried out by Rönicke (1986), the paleolimnology was studied by Findlay et al. (1998) and Scharf (1998) as well as the nutrient cycle and phosphorus retention in the lake sediment by Hupfer & Steinberg (1997) and Schauser et al. (2003). For several decades before 1972, all domestic and industrial wastewater from the town of Arendsee, with a population of 4000 residents, was discharged untreated into the lake (Rönicke et al. 1995a). The lake has been eutrophic during the past 50 years. In the seventies, phosphorus concentrations ranged between 160 and 170 µg L–1, and the deepest water was anoxic from July to November. The lake is situated in a region where 60 % of the land is used for agriculture. From 1900 to 1949, the average loading by inorganic N and P from agricultural fertilizers was 10–20 kg ha–1 yr–1. Due to intensified agriculture, loading rates increased from 1950 to 1990 averaged 120 kg ha–1 yr–1 N and 80–150 kg ha–1 yr–1 P (Findlay et al. 1998). Lake Arendsee is used by 350,000 to 400,000 people during the summer as a recreation site (Rönicke et al. 1995b) and has a commercial vendace (Coregonus albula) fishery (Schultz 1992).

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Methods Calculation of the ice area with geese excrement In 1996, the ice area which was contaminated with geese droppings was estimated with a laser distance scanner. We measured the distance from the edge of the contaminated ice area to several land points and using means of the distances and their angles, we calculated the area where excrement was deposited (Fig. 2). In 1997 we used an aerial survey of geese excrement patches on ice cover. Aerial photographs were taken with a handheld Canon EOS 5 camera from a Cessna light aircraft. The zoom lense was set to 28 mm focal length. The aircraft was banked to about 60 degrees at various positions over the lake to obtain a mosaic of nearly vertical images. A true color Ektachrome 64 positive film was used. To obtain different scales for navigation of the images, as well as a higher resolution of different patches, the flight altitude was set between 200 and 600 m. The slides were scanned with a Linotype flatbed scanner in transparent mode with a resolution of 1200 dpi. Ground points for navigation were taken from a map with a scale of 1:10,000 and from ground control points in form of white crosses which were positioned on the ice sheet using a triangulation technique. The digitized images were then referenced using a 2D polynomial rubber sheet stretching technique, using a program written in IDL. The polynomial coefficients were determined from the coordinates of corresponding points of the map, ground control points and pixel coordinates. The images were contrast enhanced and, based on the discolouration of the ice, the shape of the excrement patches was determined interactively by delineation using the mouse pointer. The size of a pixel in square meters was determined from the resulting scale after referencing the images. All pixels with excrement coverage were then counted and multiplied by the size of a pixel to determine the overall area covered with geese excrement (Fig. 2).

Estimation of the excrement concentration on the ice field We collected the entire geese excrements from 1 m2 plots at different locations over the polluted ice area. During the two years of investigation, we collected 5 samples from two excrement patches in 1996, and 40 samples from one excrement patch in 1997. Total phosphorus in geese excrements was measured according to the „Deutsche Einheitsverfahren“ (DIN 38414). Solid phase phosphorus was determined in the ignition residue. The oxidation was completed by addition of ammonium nitrate solution to the crucible before heating in a muffle furnace. 0.1 to 0.3 g of the annealed sediment was mixed with 25 ml 1 N HCl and 15 minutes heated. After cooling, the suspension was filled into a volumetric flask and deionised water was filled up to the mark. From this an aliquot was filled into another volumetric flask. After addition of p-nitrophenol the solution was titrated with NaOH to yellow. The solution was discoloured by addition of sulphuric acid and KMnO4 solution was added and deionised water was filled up to the mark. The phosphorus concentration was determined photometrically by addition of ascorbic acid and ammonium molybdate at 880 nm.

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Fig. 2. Maps of lake Arendsee in February 1996 and January 1997 showing the areas where the ice was covered with geese excrement. The triangles represent marked points on the ice for identifying position in the aerial photographs.

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Estimation of total phosphorus in the water Total phosphorus in the water was measured according to the ISO 6878 (2004). The sample was heated after addition of sulphuric acid and potassium peroxodisulfate for oxidation of phosphorus containing organic material and hydrolysis of inorganic polyphosphates. An aliquot of the digested solution was filled into a volumetric flask. After addition of ascorbic acid and ammonium molybdate, phosphorus concentration was determined photometrically at 880 nm.

Estimation of the atmospheric deposition The atmospheric deposition of total phosphorus (TP) was collected with a bulk deposition sampler (Eigenbrodt; height = 1.5 m, funnel diameter = 0.25 m). The samples of the atmospheric deposition collected by the Eigenbrodt-sampler were taken daily. The incremental samples were stored at 4°C in the fridge and merged to a monthly mixed sample. Subsequently, the concentration of TP atmospherically deposited was determined by means of monthly mixed samples. A comparison between the concentrations of anions and cations measured by means of daily samples and monthly mixed samples accomplished on the basis of long term measurements did not result in significant differences (Schmidl 1999). Hence, the determination of the atmospheric deposition by means of monthly mixed samples was statistically verified. The concentration of TP in the sample was measured according to DIN 38405 Section 11. Potassiumperoxodisulfat was used to digest the sample. The TP measurement was carried out on the non-filtrated sample by using a Merck SQ 118 photometer. The TP loads were calculated by multiplying the amount of rainfall (measured daily in 1 m height above the surface by Hellmann rain gauges) and measured TP concentrations.

Results Geese excrements and phosphorus load Fig. 2 illustrates the areas covered with geese excrements and the location of sampling points on the fro-

zen lake in 1996 and 1997. Although different methods were used for the calculation of the polluted excrement fields, the results were similar in the two years of investigation. In February 1996 (after 9 days of ice cover), the two excrement fields were 80 ha and 30 ha and in January 1997 (after 10 days of ice cover) the excrement field was 106 ha large. The geese excrements collected from the sampling area ranged from approximately 148 to 266 g m–2 wet weight, with a mean of 201 g m–2, in 1996, and 83 g m–2 to 408 g m–2,with a mean of 243 g m–2, in 1997 (Table 2). The mean value of water content of the geese excrement was 88 % in 1996, and 92% in 1997 (Table 2). The mean value of phosphorus content ranged from 8.5 mg g–1 dry weight in 1997 to 9.2 mg g–1 in 1996 (Table 2). On the basis of these chemical parameters, the concentrations of geese excrements and the areas of polluted ice, the phosphorus load caused by nordic geese was calculated (Table 3). These results shows that the phosphorus input in 1996 after 9 days on frozen lake surface was 251 kg, and in 1997 after 10 days of the freezing was 173 kg. The daily phosphorus load was, thus, 27.8 kg in 1996 and 17.3 kg in 1997. After 100 days of the habitation of wintering nordic geese on Lake Arendsee, the phosphorus load may be estimated to 2.78 tons in 1996 and 1.73 tons in 1997. Annual phosphorus import from other sources Lake Arendsee has three inflows (Fig. 2) which are characterised by a low water flow during the course of a year (Table 4). On the basis of runoff and mean phosphorus concentration, the annual phosphorus load was calculated. In 1996, the inflows transported

Table 2. Parameters of geese excrement on ice-covered Lake Arendsee in 1996 and 1997. date

n

surface loading min – max water content min – max loss on ignition min – max P concentration min – max on polluted ice average ± SD average ± SD average ± SD average ± SD (%) (%) (mg g–1dry weight) (g m–2 FW)

6. Feb. 1996

5

201 ± 46

148–266

15. Jan. 1997 40 10

243 ± 88

83–408

88 ±1.2

86 – 90

76 ±1.9

72–79

9.2 ±0.5

8.6–9.5

92 ±1.2

91 – 94

84 ±6.2

68–90

8.5 ±1.7

6.7–12.2

Table 3. Phosphorus import from nordic geese excrement to Lake Arendsee in 1996 and 1997. Year Area of polluted ice (ha) 1996 1997

110 106

Time of sampling Area-specific loading Mean P content Area-specific Total P-import Daily input Calculated total-P after freeze-up on ice of excrement P-import on contaminated of Phosphorus input during winter (d) (g DW m–2) mg g–1 DW (kg ha–1) ice area (kg) (kg P day–1) (100 days) (t) 9 10

24.76 19.17

9.2 8.5

2.28 1.63

250.6 172.7

27.8 17.3

2.78 1.73

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Table 4. Annual phosphorus input from the inflows to Lake Arendsee in 1996 and 1997.

mean P concentration P annual import (mg m–3) ± SD into the lake (kg)

year

inflow

n

mean water inflow (dm3 sec–1) ± SD

1996

Drainage Fauler Seegraben Werftgraben * ∑

23 23 15

1.57 ± 1.300 17.91 ± 9.900 21.83 ± 12.30

27 ± 1 47 ± 3 54 ± 3

1.34 26.66 24.65 52.65

1997

Drainage Fauler Seegraben Werftgraben ** ∑

20 20 11

2.3 ± 1.000 10.9 ± 7.000 30.36 ± 31.6

21 ± 1 28 ± 1 44 ± 2

1.52 9.60 24.47 35.59

* the inflow was dry in January and from August to October ** the inflow was dry in January and from August to November

Table 5. Annual phosporus import by different sources to Lake Arendsee 1996 and 1997.

Source

1996 (kg P yr–1)

1997 (kg P yr–1)

Inflows Atmospheric deposition Swimmers Nordic geese

53 293

36 86

38 2780

33 1730

∑

3164

1885

Source this study Lysimeter station, Falkenberg Schulze 1981 this study

52.7 kg, and in 1997, 35.6 kg phosphorus into the lake. The atmospheric deposition, which was measured at nearby Station Falkenberg, was different in both years of investigation (Table 5). In 1996, the phosphorus import of 293 kg was three times higher than in 1997 (86 kg). Schulze (1981) estimated a daily nutrient import per swimmer of 0.094 g TP and 3.12 g TN. On the basis of the number of swimmers during the season (statistics kindly granted by the camping ground Arendsee) in 1996 of 400,000 and in 1997 of 350,000, the phosphorus load was calculated as 38 kg in 1996 and 33 kg in 1991 (Table 5). Phosphorus content in the lake Since the beginning of the 1970s, the phosphorus concentrations in Lake Arendsee have not changed significantly (Rönicke et al. 1998). In April, before the water body was stratified, the mean spring concentrations of total phosphorus ranged from 0.154 mg L–1 (1995) to 0.172 mg L–1 (2001/2002) during the past ten years. Based on the mean pelagic phosphorus concentration in spring, the lake contains approximately 23 t of phosphorus.

Discussion The role of waterbirds in nutrient cycles in lakes is rarely quantified, or assumed to be unimportant. Nevertheless, when bird populations are large in relation to the size of a waterbody, a substantial fraction of the nutrient pool may cycle through birds (Scherer et al. 1995). Table 6 shows some estimates of phosphorus import by waterfowl to lakes and reservoirs. Lake Arendsee is characterized by a very narrow area of reed banks and a limited density of submersed macrophytes. This results in a small population of waterfowl (several duck species, swans, coots, marsh warblers), which can use Lake Arendsee in spring and summer for breeding and resting. Contrary to this situation, the large number of overwintering nordic geese represents the biggest percentage of waterfowl at the lake during winter time. Although the large number of wintering nordic geese at Lake Arendsee has been a well known natural phenomenon for many decades, there are no exact counts available of their density. For the region Drömling and Altmark, Benecke (1996) reported 500–20,000 nordic geese in the years 1991 to 1995. In the winter season 1991/92 40,000 to 50,000 wintering nordic geese were documented by Tappenbeck & Raschewski (1993). For the region Altmark 40,000 nordic geese for the years 1989 to 1993 and 20,000 in 1994–1995 were reported by Rutschke & Naacke (1995). The number of nordic geese for the state of Saxony-Anhalt ranged from 29,452 (January 1997) to 77,152 (November 1997), documented by Naacke (1998/1999). All results show the wide variation in the estimated geese density, caused by the methological difficulty of counting geese. Therefore, our investigations of phosphorus input by nordic geese were based


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Table 6. Contribution by waterfowl to phosphorus inputs to selected lakes and reservoirs.

Location

Kelbra reservoir (Germany) Arendsee (Germany)

Year

1969–1976

1992

Species

P-import kg day–1

P-import kg year–1

Percentage of total P-input %

Mallard, Pochard, Black-headed Gull, Coot

0.02–0.88

117

0.7

Bean-Goose, White-fronted Goose

2.24

269

Source

Ziemann 1986

Tappenbeck & Raschewski 1993

Wintergreen Lake (USA)

1969–1972

Canada Goose

0.24

88

70

Manny et al. 1994

Grand-Lieu (France)

1981–1982 1990–1991

Heron, Cormoran, Duck, Gull, Starling

5.5 6.9

2000 2530

2.4 6.6

Marion et al. 1994

Green Lake (USA)

1992–1994

American Coot, Mallard, Gadwall, Gull

160

25–34

Scherer et al. 1995 Olson et al. 2005

Middle Creek-reservoir (USA)

2001

Greater snow Goose

1.1

850

85–93

Arendsee (Germany)

1996 1997

Bean-Goose, White-fronted Goose

27.8 17.3

2780 1730

88 92

not on the surveyed geese number, but on the direct measurement of geese excrement. Investigations on the nutrient input by wintering nordic geese in Lake Arendsee were carried out earlier by Tappenbeck & Raschewski (1993). On the basis of 15,000 geese, they reported 257 t of geese excrement and 269 kg phosphorus for a season of 120 days. Compared to the calculated P-import by nordic geese according to Tappenbeck & Raschewski (1993) our results were considerably higher. These authors calculated their P-import on the basis of 15,000 geese and their measured P- concentration in geese excrements. Therefore, the values of P-import rates by nordic geese during the winter time reported by Tappenbeck & Raschewski were considerably lower than in our calculations. We estimated a striking difference in P-import during our investigation time in 1996 and 1997. The reason for this difference was the smaller ice area contaminated in 1997 if compared to year 1996 as well as the smaller P-content of geese faeces in 1997. Geese are known to feed more actively when they first arrive on winter grounds. Later, they have a reduced feeding activity. For example Post et al. (1998) found that the majority of nutrient loading occurred when geese first arrived and that loading decreased over the course of the season. We have not found a similar behaviour at lake Arendsee. The nordic geese left the ice area of lake Arendsee in the early morning, used the surrounding agricultural fields for feeding and

this study

came back in the evening. This chronological order in their behaviour was equal over the whole period of investigation. We are rather confident, that our estimated phosphorus load over the winter season on the basis of the first 9 to 10 days of goose activity does not result in an overestimation. The nordic geese arrived on Lake Arendsee at the beginning of November and left the lake towards the end of February. Therefore, we calculated a residence period of 100 days. Tappenbeck & Raschewski (1993) used in their calculation of P-import a time scale of 120 days for geese presence. However, our observation shows that 100 days are a more realistic period for geese staying over winter on the Lake Arendsee. Rutschke (1987) reported the nordic geese migrated in November to the middle of Europe and left their overwintering places in February. According to Rutschke & Schiele (1978/1979) and Rutschke (1987), 10,000 geese led to an input of 2.2 kg per day into Lake Gülper See (Brandenburg State). The measured phosphorus content of the geese excrement (8.5 to 9.2 mg g–1 DW) corresponded with the results from Rutschke (1987). He reported a mean phosphorus content of 1 %. Smith & Johnson (1995) reported fresh seabird guano to contain 1.5 % phosphorus. The results of Tappenbeck & Raschewski (1993) show far lower TP values, ranging from 4.6 to 4.7 mg g–1 dry weight. According to Gwiazda (1996), one gram of the faeces of black-headed gull (Larus melanocephalus)

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and of mallard (Anas platyrhynchos) contains 78.6 mg P and 8.5 mg P, respectively. The loading rates caused by a dense aggregation of waterfowl (over 40,000 Lesser Snow Geese, Chen caerulescens caerulescens, and Ross Geese, Chen rossii) that winter annually at Bosque del Apache National Wildlife Refuge, New Mexico, peaked in late November at more than 30 kg phosphorus per day (Post et al. 1998). Marion et al. (1994) reported the importance of avian-contributed nitrogen and phosphorus to Lake Grand-Lieu, France where 2,000 and 2,500 kg total P were deposited over the time periods, 1981–82 and 1990–91 (Table 6). Waterfowl (Cormorants, Phalacrocorax carbo) inhabiting the area of Kis-Balaton, excreted 3.1 t of P in 1983 (Gere & Andrikovics 1992). On the basis of quantity of consumed food and of the excrement, the Little Cormorants (Phalacrocorax niger) released 3.9 t, Egrets and Herons together up to 3.98 t of P into the Bhanderai and Pandloli reservoir complex in Matar tahsil of Kheda district, Gujarat, India (Mukherjee & Borad 2001). The nutrient contribution by geese (over 6,000 migrant Canada geese, Branta canadensis, each year) to Wintergreen Lake amounts to 0.39 g P m–2 of lake surface per year (Manny et al. 1975). Compared with the annual phosphorus imports by different sources (Table 5), the contribution of the phosphorus load by nordic geese was 88 % in 1996 and 92 % in 1997 into Lake Arendsee. For Green Lake, a productive lake in urban Seattle USA, the total phosphorus in bird excrement constituted 27 % of the total phosphorus loading to the lake from all sources in 1992, 25 % in 1993 and 34 % in 1994 (Scherer et al. 1995). Kitchell et al. (1999) documented that geese increased the nutrient loading rates in some wetland ponds by up to 75 % of total phosphorus loads. Snow Geese (Chen caerulescens) contributed 85–93 % of phosphorus loaded to the Middle Creek Reservoir (Olson et al. 2005). Beyond this, there are results from other authors which documented that the role of birds in total P input is relatively small in comparison with very high inputs from human sewage, agriculture runoff and other sources. Gwiazda (1996) reported that the aviancontributed phosphorus input to a mesotrophic reservoir represented less than 1 % of inorganic P and N. Ziemann (1986) reported that only 0.7 % of total phosphorus import into the reservoir Kelbra (State Thuringia) was attributed to waterfowl. Our calculated phosphorus import rates by nordic geese to Lake Arendsee documented the significance of the wintering nordic geese. The pelagial of Lake Arendsee is characterized by a high level of phospho-

rus and a low level of nitrogen. It is a typical N limited aquatic system. Every year nitrogen fixing cyanobacteria (Anabaena, Aphanizomenon, Anabaenopsis) appeared during the summer time. Wintering geese particularly lead to high loading rates as a consequence of their colonial roosting (Post et al. 1998) and their communal behaviour. It should be pointed out, that geese feed on fields of winter corn and digest and produce faeces on lakes where they are protected from predators. It must be emphasized that only the external loading of phosphorus was considered in this study for which geese play a very significant part. The total external loading was calculated to be 2.5 t per year which agrees with the calculation by Schauser et al. (2003) (based on a per annum phosphorus balance at steady state, with the assumption that the phosphorus internal loading from the bottom sediments is removed from the water column by sedimentation). Ninety percent of this external loading is attributed to the import by geese. Nevertheless, the annual input represents only approximately 10 % of the total phosphorus present in the fully mixed water column during early spring (mean of 23 t). Hence, a scheme for lake restoration should give priority to the removal of the large amount of phosphorus already present in the lake. Acknowledgements We thank F. Frimel and W. Elsner for their support in helping us to obtain and analyse the samples. The study was supported by the Federal Ministry for Education and Research (BMBF, FV 02-WA 9545/3) of the Federal Republic of Germany. We acknowledge the assistance of all of the staff of Inland Water Research in Magdeburg.

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Submitted: 19 June 2006; accepted: 7 January 2008.

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