Spatial and seasonal variation of phosphorus fractions in bottom. sediments of the hypertrophic Swarzędzkie Lake (W Poland)

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1 Limnological Review 5 (2005) Spatial and seasonal variation of phosphorus fractions in bottom sediments of the hypertrophic Swarzędzkie Lake (W Poland) Adam Mickiewicz University, Department of Water Protection, Drzymały 24, Poznań Abstract: Total phosphorus concentration and contributions of its individual fractions were assessed in bottom sediments of the Swarzędzkie Lake, which is hypertrophic due to high external and internal nutrient loads. The major contributor to total P was the Res-P fraction (mean 45.14% and mgp gdw -1 ), which includes insoluble compounds, almost unavailable to living organisms. Its concentrations reached up to mgp gdw -1 near the mouth of the stream Mielcuch. Soluble fractions (NH 4 Cl-P, Fe-P and partially NaOH-P) accounted for only 27.6% of total P concentration in the sediments (0.511 mgp gdw -1 ). This is favourable for water quality because the dominating insoluble fraction is not bioavailable and is durably accumulated in bottom sediments. Key words: bottom sediments, lake, phosphorus, phosphorus fractions. Introduction Bottom sediments participate in phosphorus circulation in aquatic ecosystems by way of accumulation, transformation, and release of its compounds (Wiśniewski, Rzepecki, 1996). There are many clas-sifications of the sedimented phosphorus compo-unds because of their ability to return to circulation in lake ecosystem, the physicochemical conditions of this process, and the analytic procedure (Wiś-niewski, 1994). The sediment fractioning method enables separation of several groups of phosphorus compounds, differing in solubility and hence also in bioavailability. Many researchers suggest that the most bioavailable form of phosphorus is loosely adsorbed on the surface of sediment particles (NH 4 Cl -P fraction). The iron-bound fraction (Fe-P) is also very mobile. Another fraction includes phosphorus bound with aluminium and contained in organic matter (NaOH-P). The remaining fractions are phosphorus compounds with calcium (HCl-P) and other, almost insoluble inorganic and organic compounds (Res-P). An analysis of contributions of individual fractions to total P can provide information about durability of its accumulation in sediments, and about possibilities of its release into the water column (Kentzer, 2001). This study aimed to analyse the seasonal and spatial variation in contributions of individual fractions to total P concentration in bottom sediments of the Swarzędzkie Lake. Study area The Swarzędzkie Lake (Fig. 1) is a natural, throughflow water body of medium size, located in the northwest part of the town of Swarzędz, near its border with the city of Poznań in western Poland. Its total surface area is 93.7 ha, mean depth 2.6 m, max. depth 7.2 m (Szyper et al., 1992; Gołdyn, Grabia, 1998). The lake is elongated, narrowing from half its

2 124 length towards the outlet. The wider, northeastern part is much deeper, as it includes the deepest place, accumulated in its bottom sediments but also to external loads. Several types of bottom sediments were distinguished in the lake: (1) algal gyttja in the deepest place; (2) black sapropel with chemical odour around the mouth of the Mielcuch; (3) coarse detritus gyttja with numerous plant remains in the shal-low part of the lake; and (4) sandy bottom near the pier. Material and methods Fig. 1. Distribution of sediment sampling stations (modified from Kowalczewska-Madura, 2004): 1 11 sampling stations while the narrower southwestern part is only about 2 m deep. The lake is supply by the river Cybina, the stream Mielcuch, and the temporary watercourse Zielinka. The majority (75.5%) of the catchment area is covered by farmland (Szyper et al., 1994; Gołdyn, Grabia, 1998). Till the year 1991, domestic sewage from Swarzędz was discharged directly into the lake. The lake is now hypertrophic, due to the large loads of nutrients Samples of sediments for assessment of total P and its fractions were collected in the summer (July) and autumn (October) of The samples were taken with the use of Kajak s tube sampler from 11 stations in summer and 7 stations in autumn (Fig. 1). For the analysis, the surface layer (10 cm thick) of sediments was used. Phosphorus fractions were separated according to the fractioning protocol proposed by Psenner et al. (1988). In a volume of 1 cm 3 of wet sediment, we assayed: loosely bound phosphorus (NH 4 Cl-P) extraction with 1 M NH 4 Cl for 2 h; phosphorus bound with iron (Fe-P) extraction with a mixture (1:1) of 0.11 M NaHCO 3 and 0.11 M Na 2 S 2 O 4 for 2 h; phosphorus bound with aluminium and organic matter (NaOH-P) extraction with 1.0 M NaOH for 18 h; phosphorus bound with calcium (HCl-P) extraction with 0.5 M HCl for 18 h; and the residue (Res-P), which was the difference between total P concentration and the sum of the first four fractions. After each stage of extraction, the sample was centrifuged and phosphorus concentration in the supernatant was measured by the molybdate method with ascorbic acid as a reducer. Results We compared total P concentration and contributions of its five fractions (differing in solubility and bioavailability) in bottom sediments of the Swa-rzędzkie Lake in summer and autumn. The major contributor was the Res-P fraction, characterized by the lowest solubility and bioavailability. Its mean concentration in July was

3 Spatial and seasonal variation of phosphorus fractions in bottom sediments of the mgp gdw -1, which accounted for 39.08% (Fig. 2, Fig. 3). In autumn, its contribution increased to 54.66%, and its mean concentration to mgp gdw -1 (Tab. 1). In both seasons the highest values were recorded at station 8, near the mouth of the stream Mielcuch (2.737 mgp gdw -1 in July and mgp gdw -1 in October), and the lowest in sandy deposits at station 10 (0.059 mgp gdw -1 and mgp gdw -1 respectively) (Fig. 3, Fig. 5). 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% Fig. 2. Contributions of individual fractions to total phosphorus in bottom sediments of the Swarzędzkie Lake in the summer of mgp gdw Fig. 3. Concentrations of individual fractions of total phosphorus in the summer of 2003 The HCl-P fraction ranked second. Its contribution was higher in summer (mean mgp gdw -1 ) than in autumn (mean mgp gdw -1 ) (Tab. 1). In July, the highest contribution was recorded at station 5 (65.3%), and in autumn at station 10 (41.8%) (Fig. 2, Fig. 4). Fractions with the highest bioavailability (NH 4 Cl-P, Fe-P and partially NaOH-P) accounted jointly for 27.6% of total P on average. In both seasons the contribution of loosely bound phosphorus, i.e. the NH 4 Cl-P fraction, was the lowest and reached about 6%. Its mean concentration in summer (0.107 mgp gdw -1 ) was higher than in autumn (0.086 mgp gdw -1 ). Similarly, Fe-P concentration was higher in summer than in autumn, and its contribution was 9.53% on average (Tab. 1). In July its maximum concentration was recorded at station 8 (0.504 mgp gdw -1 ), whereas in autumn at station 10 (0.168 mgp gdw -1 ) (Fig. 3, Fig. 5). The NaOH-P fraction, i.e. phosphorus bound with aluminium and organic matter, had the highest contribution in summer on average 13.21% and 0.37 mgp

4 126 gdw -1 (Tab. 1). Its concentration was the highest at station 8, reaching up to mgp gdw -1 (Fig. 3). In autumn, concentration of this fraction ranged from mgp gdw -1 at station 2 to mgp gdw -1 at station 7 (Fig. 5). 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% Fig. 4. Contributions of individual fractions to total phosphorus in bottom sediments of the Swarzędzkie Lake in the autumn of mgp gdw Fig. 5. Concentrations of individual fractions of total phosphorus in the autumn of 2003 The mean contribution of these three fractions jointly amounted to 29.5% in summer and 24.8% in autumn. Their mean concentration was mgp gdw -1 in summer and mgp gdw -1 in autumn (Tab. 1). Thus only less than 1/3 of total P was bioavailable and over 2/3 were almost unavailable to living organisms, immobilized in the sediments. Table 1. Mean concentrations (mgp gdw -1 ) (A) and contributions (%) (B) of individual fractions within total phosphorus in the summer and autumn of 2003 NH 4Cl-P Fe-P NaOH-P HCl-P Res-P A B A B A B A B A B Summer Autumn The comparison of contributions and concentrations recorded in summer and autumn shows that only those of Res-P increased, while those of the other four fraction decreased. This suggests that in fact only Res-P is unavailable to living organisms,

5 Spatial and seasonal variation of phosphorus fractions in bottom sediments of the while the other fractions are quite mobile, but this hypothesis still needs to be confirmed. Discussion In the Swarzędzkie Lake, the major contributor to total P was the Res-P fraction (mean 45.14% and mgp gdw -1 ), which is almost unavailable to living organisms. Its concentrations were up to mgp gdw -1. This fraction dominated also in other lakes, like Lake Lyng (Denmark), where it reached 6.19 mgp gdw -1 in the surface layer of sediments (Søndergaard et al., 2000). In the shallow eutrophic Lake Arreskor (Denmark) it contributed 43.7% in the littoral zone and 45.1% in the profundal zone (Andersen, Ring, 1999). Second highest contribution was recorded for the HCl-P fraction (phosphorus bound with calcium), due to the high ph of water and sediments. At ph values ranging from 7.5 to 8.5, phosphate precipitate as Ca 3 (PO 4 ) 2. This compound in time can be transformed into even less soluble forms, such as fluoroapatite (Kentzer 2001). That author suggested that an increase in the HCl-P fraction can be treated as an indicator of lake eutrophication. He found a significant correlation between concentrations of phosphorus in water column and this fraction in bot-tom sediments. This results from the increase in wa-ter ph that usually accompanies the increase in tro-phic level. Consequently, insoluble compounds are formed, such as CaHPO 4 or Ca 3 (PO 4 ) 2. Thus, at high water ph, phosphorus in the HCl-P fraction in the lake is not Another bioavailable. positive aspect is that the mean contribution of the three bioavailable fractions jointly (NH 4 Cl-P, Fe-P and partially NaOH-P) to total P in the Swarzędzkie Lake is only 27.6% (0.511 mgp gdw -1 ). This attests to the small possibilities to phos-phorus mobilization from its bottom sediments. Mean contributions of individual fractions were: 6.55% (0.099 mgp gdw -1 ) in the loosely bound phosphorus NH 4 Cl-P fraction; 9.26% (0.156 mgp gdw -1 ) in the iron-bound phosphorus Fe-P fraction; and 11.83% (0.275 mgp gdw -1 ) in the NaOH-P fraction, bound with aluminium and organic matter. Similarly, in the Lake GościąŜ (Kentzer, 2001), low contributions of the two most mobile fractions (NH 4 Cl-P and Fe-P) were recorded: about 4.5% in NH 4 Cl-P and about 5% in Fe-P. In the eutrophic Lake Kujno, the sum of easily exchangeable fractions accounted for 8 55% of total P concentration in sediments, and in the Lake Gant for 3 16% (Wiśniewski, Rzepecki, 1996). The contribution of the NH 4 Cl-P fraction in the Lake Jabel (Germany) varied from 9.5 to 18.3% (Kleeberg et al., 2000), so it was similar to that recorded in the Swarzędzkie Lake. These data suggest that the contribution of those fractions to the processes of phosphorus deposition and mobilization is small. In the eutrophic Lake Kortowskie, the concentration of loosely bound phosphorus was much smaller -only mgp gdw -1, iron-bound mgp gdw -1, aluminium-bound mgp gdw -1, and calcium-bound 1.89 do 2.34 mgp gdw -1 (Tandyrak, Brzozowska, 2003). The mechanism of mobilization and deposition of phosphorus in sediments is often linked with the iron-phosphorus cycle. However, its importance decreases with increasing trophic level. In lakes with a higher trophic level, oxygen deficits are often observed in the bottom layer of water. This does not favour accumulation of iron-bound phosphorus Fe-P. The low concentration of this fraction in the Swa-rzędzkie Lake probably resulted from this process. The binding of phosphorus into complexes with hu-mic acids and iron which is important in humic la-kes appeared to be insignificant there. The low concentrations of the NaOH-P fraction were due to the low organic matter content of the bottom sediments in the lake, which was on average 17.76% (Kowalczewska-Madura, 2004). This did not favour accumulation of this phosphorus fraction. The concentration of this fraction in the Swa-rzędzkie Lake was generally low (mean mgp gdw -1 ), but it was highly variable and in summer, especially at station 8, it was relatively high. Its con-tribution was proportional to phosphorus concentration in sediments, which is confirmed by a significant correlation between the two parameters. It was the dominating fraction in Lake Gant (30 50%) (Wiśniewski, Rzepecki, 1996) and in Lake Jabel, where it reached up to 72% Although (Kleeberg bioavailable et al., 2000). phosphorus accounts for only 27% of total P concentration in sediments, it exerts a strong influence on the trophic status of the study lake. The internal loading from bottom sediments is similarly high as the external loading of the lake (Kowalczewska-Madura, Gołdyn 2005). Conclusions

6 128 The almost insoluble Res-P fraction was the major contributor to total P in bottom sediments of the study lake. Such a situation, observed also in other water bodies, is favourable for water quality because it limits the influence of bottom sediments as a source of internal phosphorus loading. This fraction, because of its extremely low solubility, is not bioavailable and is permanently accumulated in bottom se-diments. References Andersen F. Ø., Ring P., 1999, Comparison of phosphorus release from littoral and profundal sediments in a shallow, eutrophic lake, Hydrobiologia 408/409, Gołdyn R., Grabia J., 1998, Program ochrony wód rzeki Cybiny, Urząd Miasta Poznania, Wydział Ochrony Środowiska, Poznań. Kentzer A., 2001, Fosfor i jego biologicznie dostępne frakcje w osadach jezior róŝnej trofii, Uniwersytet Mikołaja Kopernika, Toruń. Kleeberg A., Nixdorf B., Mathes J., 2000, Lake Jabel restoration project: Phosphorus status and possibilities and limitations of diversion of its nutrient-rich main inflow, Lake & Reservoirs: Research and Management, 5, Kowalczewska-Madura K., 2004, Materia organiczna w osadach dennych Jeziora Swarzędzkiego, In: A. T. Jankowski, M. Rzętała (eds.) Jeziora i sztuczne zbiorniki wodne funkcjonowanie, rewitalizacja i ochrona, Uniwersytet Śląski, Polskie Towarzystwo Limnologiczne, Polskie Towarzystwo Geograficzne Oddział Katowicki, Sosnowiec, Kowalczewska-Madura K., Gołdyn R., 2005, Phosphorus internal loading of the Swarzędzkie Lake, SEFS, Abstract Book, (in print). Psenner R., Boström B., Dinka M., Pettersson K., Pucsko R., Sager M., 1988, Fractionation of phosphorus in suspended matter and sediment, Arch. Hydrobiol. Beih. Ergebn. Limnol. 30, Søondergaard M., Jeppensen E., Jensen J. P., 2000, Hypolimnetic nitrate treatment to reduce internal phosphorus loading in a stratified lake, Lake and Reservoir Management 16 (3): Szyper H., Gołdyn R., Romanowicz W., 1994, Lake Swarzędzkie and its influence upon the water quality of the river Cybina. PTPN, Pr. Kom. Biol. 74, Szyper H., Gołdyn R., Romanowicz W., Kraska M., 1992, Raport o stanie czystości Jeziora Swarzędzkiego i moŝliwościach jego rekultywacji, UAM Poznań. Tandyrak R., Brzozowska R., 2003, Fosfor w osadach dennych jeziora rekultywowanego metodą usuwania wód hypolimnionu. In: Materiały VII Ogólnopolskiej Konferencji Limnologicznej nt. Naturalne i antropogeniczne przemiany jezior, Kielce. Wiśniewski R. J., 1994, Fosfor w zbiornikach zaporowych zasilanie, kumulacja, wymiana między osadami dennymi a wodą, In: Zalewski M., (ed.) Zintegrowana strategia ochrony i zagospodarowania ekosystemów wodnych, Biblioteka Monitoringu Środowiska, WIOŚ, Łódź. Wiśniewski R. J., Rzepescki M., 1996, Osady denne stref przejściowych rzeka-jezioro i jezioro-rzeka w systemie rzeczno-jeziornym Krutyni (Pojezierze Mazurskie), Rola w krąŝeniu fosforu, In: Funkcjonowanie systemów rzeczno jeziornych w krajobrazie pojeziernym: rzeka Krutynia (Pojezierze Mazurskie), Zeszyty Naukowe Komitetu Człowiek i Środowisko 13, Streszczenie Płytkie przepływowe Jezioro Swarzędzkie (Wielkopolska) naleŝy do jezior silnie przeŝyźnionych hypertroficznych. Ma ono kształt wydłuŝony i podzielone jest na dwie części: szerszą (północno-wschodnią) gdzie znajduje się głęboczek oraz węŝszą (południowo-zachodnią) o głębokości około 2 metry. Zawartość poszczególnych frakcji fosforu w 10 centymetrowej warstwie osadów dennych oznaczano wg schematu zaproponowanego przez Psennera i in. (1988). Badania prowadzono na 11 wytypowanych stanowiskach w lipcu i 7 w październiku 2003 roku (ryc. 1). Wykazały one, iŝ największy udział w ogólnej puli fosforu posiadała frakcja Res-P (średnio 45,14%) charakteryzująca fosfor praktycznie niedostępny biologicznie. Jej koncentracje dochodziły do 2,747 mgp/g s.m. na stanowisku 8 (ryc.5). Fosfor występujący w połączeniach z wapniem (HCl-P), był drugą frakcją pod względem zarówno udziału procentowego jak i zawartości w osadach dennych. Jej koncentracje wahały się w zakresie od 0,091 (stanowisko 1) do 0,981 mgp/g s.m. (stanowisko 5) (ryc. 3 i 5). Kolejne trzy frakcje (NH 4 Cl-P, Fe-P, NaOH-P) mniej lub bardziej dostępne biologicznie, miały niewielki udział w całkowitej puli fosforu w osadach dennych tego jeziora (ryc.2 i 4). StęŜenia frakcji NH 4 Cl-P dochodziły do 0,194 mgp/g s.m. (na stanowisku 3), frakcji Fe-P do 0,504 mgp/g s.m. (stanowisko 8) a frakcji NaOH-P do 2,043 mgp/g s.m. (stanowisko 8). Porównanie zarówno udziału procentowego, jak i zawartości poszczególnych frakcji w obu okresach badawczych wykazało, iŝ w przypadku czterech frakcji (NH 4 Cl-P, Fe- P, NaOH-P i HCl-P) wyŝsze wartości wystąpiły w lecie a tylko frakcja Res-P wyŝsze koncentracje wykazywała jesienią (tab. 1).

7 Limnological Review 5 (2005) The study was supported by the Polish Committee for Scientific Research (grant no. 3PO4FO1724)