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Speciation of phosphorus in the sediments of Lake Bini (Ngaoundere-Cameroon) a
b
Sieliechi Joseph Marie , Dangwang Dikdim Jean-Marie & Noumi Guy Bertrand
b
a
Department of Applied Chemistry, National Higher School of Agro-Industrial Science, University of Ngaoundere, Ngaoundere, P.O. Box 455 Cameroon b
Department of Chemistry, Faculty of Science, University of Ngaoundere, Ngaoundere, P.O. Box 454 Cameroon Published online: 27 Feb 2014.
To cite this article: Sieliechi Joseph Marie, Dangwang Dikdim Jean-Marie & Noumi Guy Bertrand (2014) Speciation of phosphorus in the sediments of Lake Bini (Ngaoundere-Cameroon), Environmental Technology, 35:14, 1831-1839, DOI: 10.1080/09593330.2014.884171 To link to this article: http://dx.doi.org/10.1080/09593330.2014.884171
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Environmental Technology, 2014 Vol. 35, No. 14, 1831–1839, http://dx.doi.org/10.1080/09593330.2014.884171
Speciation of phosphorus in the sediments of Lake Bini (Ngaoundere-Cameroon) Sieliechi Joseph Mariea∗ , Dangwang Dikdim Jean-Marieb and Noumi Guy Bertrandb a Department
of Applied Chemistry, National Higher School of Agro-Industrial Science, University of Ngaoundere, Ngaoundere, P.O. Box 455 Cameroon; b Department of Chemistry, Faculty of Science, University of Ngaoundere, Ngaoundere, P.O. Box 454 Cameroon
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(Received 11 October 2013; accepted 11 January 2014 ) In this study, spatial and seasonal variations of phosphorus fractions in Lake Bini sediments were evaluated using a sequential extraction method. The sampling of water and sediments (surface and coring) was carried out at seven sites around the lake during the dry season and the rainy season. The results showed that phosphorus is mainly in the inorganic form (L-P+Ca–P+Fe-P) in the sediments whatever the season may be. The rank order of phosphorus extracts obtained was Fe-P>Ca–P>OM-P>L–P>Res-P. The maximum values of phosphorus (sum of each fraction) were obtained in the rainy season at the sites D6 (298.12 ± 12.37 μg P/g) and D4 (244.93 ± 11.06 μg P/g) located beside water source 2 and farmland 2, respectively. The average values of the phosphorus content vary from 05.29 ± 1.05 μg P/g to 102.58 ± 4.62 μg P/g for the upper layer (0–5 cm depth); 04.67 ± 0.66 μg P/g to 70.06 ± 2.82 μg P/g for the medium layer (5–10 cm depth) and finally 04.63 ± 0.98 μg P/g to 55.24 ± 5.17 μg P/g for the deep layer (10–15 cm depth). The results of principal component analysis showed that processes which enhance Ca–P and Fe–P accumulation are probably related to the same factor and the origin of P depends on the source of pollution. The nature of the season plays a significant role in the geochemical composition of the sediments in phosphorus and on the eutrophication level of Lake Bini. Keywords: Lake Bini; speciation of phosphorus; eutrophication; PCA; pollution
1. Introduction Phosphorus (P) is often accepted as the most critical limiting nutrient for primary lake productivity,[1–3] and its excess supply can lead to eutrophication.[4] Despite efforts to control the phosphorus load by reducing external inputs,[5] the water quality remains a problem in some lakes due to the internal P-loading.[6] Indeed, P can accumulate in sediments and, under some environmental conditions, can be released from sediment depths as low as 20 cm.[7] In this context, many studies in the past years have been made to understand the factors that affect the P release from the sediments.[8] Temperature, redox reactions, pH, dissolved oxygen concentration, nitrate, sulphates and bacterial activity are pointed as the major factors controlling P release from sediments.[4] Exchange of P in sediment–water interface constitutes a key point in the phosphorus cycle which is a sedimentary process.[2] Phosphorus (P) is present in sediment in several chemical forms depending on the sediment composition, the sedimentation rate and the physico-chemical conditions.[9] Thus, the role of the P in lake sediment in promoting lake eutrophication can be more efficiently evaluated on the basis of the contents of different P fractions, instead of total P content.[1–8] Many investigations have performed studies on P forms to allow a more precise description of the
∗ Corresponding
author. Email: [email protected]
© 2014 Taylor & Francis
potentials for P release from the sediment and to predict its future influence on lake systems. According to Yalçin et al.,[9] it is important to know which part of the stock of P can be mobilized because P remobilization from the sediments is probably controlled by its speciation. Moreover, it is well known that the bioavailability of P is strongly linked to its speciation.[8] Since then, several sequential extraction methods have been developed to distinguish between the different P fractions in the sediments.[10,11] These sequential extraction schemes made possible, in view of the potential P-bioavailability, to characterize the diverse forms in which P is distributed in sediments, classified as water-soluble P, readily desorbable P, algal available P or ecologically important P.[2] The lake Bini is located in North-West of the town of Ngaoundere (Cameroon). For several years, it has constituted a significant source of income and a means of survival for the population who practise there fishing, agriculture and the breeding at the banks. Consequently, the increased use of fertilizers, pesticides, liquid manures and animal manure for cultivation unfortunately is drained to the lake by scrubbing and streaming of the grounds. The significant invasion of this lake by watery plants and algae, the reduction in fish and its accelerated contraction for a few years have shown that it is undergoing a considerable eutrophication.
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The main objective of this work was to determine seasonal variation of the amount and forms of P in the sediments of Bini Lake by using the sequential extraction method, to evaluate their possible contributions to the P-loadings of this Lake system.
2. Experimental section 2.1. Description of the study area The lake Bini (Figure 1) is located at 15 km in the NorthWest of the town of Ngaoundere in the Adamawa region whose soil is of ferrallitic nature. It is located at 1070 m of altitude and between 1330 24.28 and 1328 40.14 of Eastern longitude and between 725 23.21 and 725 38.70 of Northern latitude. It is a crater lake with an average depth estimated at 5 m with per place maximum depths higher than 4 m in the East and the centre and the depths of 1 m in the West. Its average surface is approximately 4.5 km2 . This one is fed by various small rivers which are for the majority seasonal workers; we have the Baladji River, the Mayanga River and the Soukande River but the principal source being the Bini River. Sample site selection criteria include the presence of signs of eutrophication and the anthropic activities which are practised around the lake. We have as follows: site D1 corresponding to the exit of the lake, sites D2 and D4 located beside farmland 1 and farmland 2, respectively; site D3 corresponding to the medium, finally
Table 1. Variation of relative humidity, evaporation and precipitation during the year 2012. Month January February March April May June July August September October November December
Evaporation (mm)
Precipitation (mm)
23.6 41 40.6 72.2 79.8 81.7 83.4 82 80.6 72 63.4 56.7
203 154 283 206.2 136.2 102 108.2 206.2 103.3 134 165 164.9
0 0 0 184.8 223 169.7 231.1 240.3 249.5 125 0 0
the sites D5, D6 and D7 located beside water source 1, water source 2 and water source 3, respectively. Table 1 shows the variation of relative humidity, evaporation and precipitation of the study area. 2.2. Sampling and pre-treatment Sampling campaigns were conducted for this study during the dry season (December 2011 to March 2012) and the rainy season (April to December 2012). In the dry season, we collected 1 sample per month of water and sediment during 5 months at 7 sampling points (35 samples of water and 35 samples of sediments). Also, in the rainy season we collected 1 sample per month of water and sediment during 8 months at 7 sampling points (56 samples of water and 56 samples of sediments). Water samples were collected from the seven sites of Bini Lake. Then, collected samples were kept in 1.5 L polyethylene plastic bottles that were cleaned and rinsed many times with distilled water. All water samples were stored in an insulated cooler containing ice and delivered on the same day to a laboratory and all samples were kept at 4◦ C until processing and analysis by Rodier et al.[12] Sediment samples were collected by using a plastic pipe, 30 mm in diameter and 15 cm length. Sediment samples were taken to the laboratory, and separated into three sections and each section was 5 cm: upper layer (A: 0–5 cm), medium layer (B: 5–10 cm) and deep layer (C: 10–15 cm). These samples were air dried, homogenized and passed through a 0.25 mm sieve and stored. 2.3.
Figure 1. Map of the lake Bini showing sampling locations of the seven sites.
Relative humidity (%)
Physico-chemical analysis of water and the sediment The parameters of characterization of water such as the temperature, electric conductivity (E.C.) and the pH were measured in situ using a portable multimeter (HANNA HI8033). Dissolved oxygen was determined by the traditional method of Winkler. Turbidity was determined by
Environmental Technology the nephelometric method and the suspended matter by filtration.[12] Nitrates, phosphates, ammoniums and sulphates were determined by spectrophotometry (Rayleigh VIS-723N) according to Rodier et al.’s [12] method. The pH and electric conductivity of sediment were measured by using a portable multimeter (HANNA HI8033). Organic carbon (OC) was determined after treatment of the sample with K2 Cr2 O7 /H2 SO4 according to the Walkey– Black method. The total concentration of metals (Cu, Zn, Ni, Pb, Hg and Mn) was determined by inductively coupled plasma-mass spectrometry (Perkin Elmer ELAN-6000DRC II) with appropriate standards (BCR320, IAIE433) according to García-Delgado et al.’s [13] method. These analyses were carried out only on the upper layer of the sediment.
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2.4.
concentrations in the supernatant solutions were determined by the conventional molybdate colorimetry method.[15] 2.5. Statistical analysis Data obtained were statistically analysed by Statgraphic Centurion XV software program using the analysis of variance (ANOVA). Principal component analysis (PCA) was realized using the XLSTAT 2007 software. 3. Results and discussion 3.1. Physico-chemical characterization of water Table 2 summarizes the physico-chemical characteristics of Lake Bini. The lake shows an average temperature of 28.3 ± 0.3◦ C in the dry season and 23.2 ± 0.1◦ C in the rainy season with a pH ranging from 6.83 ± 0.01 to 7.65 ± 0.07 over the two seasons. The electrical conductivities are low and vary between 9.7 ± 0.5 and 26.6 ± 0.4 μS/cm. This lake has very low oxygen with average dissolved oxygen which is equal to 2.17 ± 0.17 mg O2 /L and 4.58 ± 0.11 mg O2 /L for the dry season and the rainy season, respectively. The suspended matters are higher in the rainy season compared with the dry season with corresponding averages of 62.19 and 33.27 mg/L, respectively. There is an increase in turbidity in the rainy season, the values range from 4.55 ± 0.21 NTU to 107.3 ± 1.84 NTU for the two seasons. The increase in temperature in the dry season induces the evaporation of water and results in an increasing in the concentration of dissolved ions. The concentrations of these dissolved ions reach their maximum value and their average concentrations are 10.93 ± 0.84, 0.79 ± 0.07, 0.56 ± 0.10 and 1.38 ± 0.09 mg/L for sulphates, nitrates, ammoniums and phosphates, respectively. On the other hand in the rainy season, dilution by rain water tends to decrease their concentrations which are equal in average 4.50 ± 0.87, 0.77 ± 0.07, 0.09 ± 0.02 and 0.84 ± 0.09 mg/L for sulphates, nitrates, ammoniums and phosphates, respectively. These dissolved ions are present at higher concentrations in Lake Bini and they might contribute to improve the internal P-loading. Indeed, phosphates’ concentration in the water
Speciation of phosphorus in the sediment
The speciation of phosphorus in the sediments was carried out by sequential extraction according to the protocol of Moreau [14] with a slight modification. This extraction scheme allows the discrimination of loosely sorbed phosphorus (L–P), inorganic phosphorus associated with aluminium (Al–P), inorganic phosphorus bonded to carbonates (Ca–P), inorganic phosphorus associated with Fe (Fe–P), phosphorus bonded to organic matter (OM-P) and residual phosphorus (Res-P). Each sample was extracted in duplicate and all the data were expressed as the average: 300 mg of dried sediment was treated with the reactant for a given time. An extraction sequence is as follows: [14] the desorption of loosely sorbed phosphorus was done by agitation in 30 mL of distillated water for 2 h, the oxidation of organic matter was done by agitation in 10 mL of H2 O2 for 24 h, the inorganic Ca–P was extracted by agitation with 30 mL H2 SO4 (0.5 N) for 12 h, the inorganic phosphorus associated with aluminium was extracted with NH4 F(0.3 N) for 6 h, the inorganic phosphorus associated with iron was extracted with 30 mL of NAOH (1 N) for 6 h and Res-P was extracted with 1 mL of H2 SO4 (0.5 N) and K2 S2 O8 for 1 h. After each extraction step, the samples were centrifuged for 15 min at 6000 rpm and the supernatants were filtered through a 0.22 mm membrane. Phosphate Table 2.
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Physico-chemical characteristics of Lake Bini water. Dry season
Parameters T (◦ C) pH E.C. (μS/cm) DO (mg O2 /L) Turbidity (NTU) MIS (mg/L) Sulphates (mg/L) Nitrates (mg/L) Ammoniums (mg/L) Phosphates (mg/L)
Rainy season
Min
Max
Mean
Min
Max
Mean
27.4 ± 0.1 6.83 ± 0.01 19.1 ± 0.7 0.63 ± 0.19 4.55 ± 0.21 20.00 7.50 ± 1.18 0.35 ± 0.05 0.25 ± 0.09 0.63 ± 0.03
29.4 ± 0.4 6.95 ± 0.00 49.7 ± 0.4 3.39 ± 0.08 107.30 ± 1.84 60.00 15.50 ± 0.71 1.13 ± 0.10 0.99 ± 0.17 1.90 ± 0.10
28.3 ± 0.3 6.88 ± 0.01 26.4 ± 0.1 2.17 ± 0.17 24.46 ± 0.79 33.27 10.93 ± 0.84 0.79 ± 0.07 0.56 ± 0.10 1.38 ± 0.09
22.6 ± 0.2 6.90 ± 0.03 09.7 ± 0.5 2.71 ± 0.12 13.2 ± 0.50 43.33 01.83 ± 0.00 0.09 ± 0.02 0.02 ± 0.01 0.29 ± 0.02
23.5 ± 0.1 7.65 ± 0.07 38.8 ± 1.7 9.30 ± 0.04 54.10 ± 0.57 72.66 08.83 ± 1.18 1.14 ± 0.12 0.21 ± 0.00 1.16 ± 0.05
23.2 ± 0.1 7.14 ± 0.04 16.2 ± 0.3 4.58 ± 0.11 23.36 ± 0.70 62.19 4.50 ± 0.87 0.77 ± 0.07 0.09 ± 0.02 0.84 ± 0.09
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Characterization of the sediment.
Parameters
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pH E.C. (μS/cm) OC (%) Cu (ppm) Ni (ppm) Pb (ppm) Hg (ppm) Mn (ppm) Zn (ppm)
Min 5.34 ± 0.00 5.90 ± 0.07 1.03 ± 0.24 6.55 ± 0.14 17.44 ± 0.94 7.59 ± 0.32 0.24 ± 0.03 69.47 ± 4.55 8.90 ± 0.47
Max 6.58 ± 0.01 20.80 ± 0.71 26.14 ± 0.75 18.97 ± 0.77 43.40 ± 1.09 10.99 ± 1.47 0.49 ± 0.03 290.76 ± 9.26 25.45 ± 3.49
column is linked to recycling and releases of P from the sediment and nitrates, sulphates are the major factors controlling this P release.[4] Nitrate, for example, may prevent or enhance P mobilization by maintaining a high redox potential or by stimulating bacteria capable of producing ‘iron reductase’.[16] 3.2. Characterization of the sediment The characteristics of the surface sediment (upper layer) of Lake Bini are given in Table 3. This table shows that the pH is acid due to ferrallitic properties of the soil with the highest values obtained in the rainy season. The electro conductivities are low like in water. The OC is in the range of 1.03–26.14%. The sediments were rich in manganese and to a lesser extent in mercury. The concentrations of nickel, copper, zinc and lead were much lower than manganese. Highest concentrations of these metals were obtained at the downstream (site D1) and upstream (site D5, D6 and D7) of lake while sites D2 and D4 located beside farmlands show moderate concentration. These metals originate by anthropic activities (mainly agriculture) which are practised around the lake. Consequently, they impair water quality for drinking, agriculture or other purposes. Moreover, excess heavy metals can lead to a potential contamination for a large variety of flora and fauna, including humans through food chains. 3.3. Phosphorous forms in the sediment 3.3.1. Labile phosphorus Figure 2(a) and 2(b) presents the variation of labile phosphorus (L-P) with depth in the dry season and the rainy season, respectively. It shows that L-P content is much higher in the dry season compared with the rainy season. This result can be explained by the phenomenon of dilution of orthophosphate contained in the water column during the rainy season (mean of precipitation in the rainy season was 174 mm with relative humidity of 41%). In contrast, the increased evaporation of water due to higher temperature in the dry season (27.4–29.4◦ C with mean equal to 28.3◦ C) (mean was 201.2 mm in the dry season compared with 118 mm in the rainy season) leads to an increase in these
ions’ concentration which then migrate into the sediment. This part follows the seasonal variations of phosphate ions in the water because it is located in the pore water and thus in direct contact with the water column. This can be illustrated by the maximum values obtained at the upper layer (A: 0–5 cm) in the dry season at site D6 (71.42 ± 3.16 μg P/g) that result in high values of orthophosphate in the water of this site at this time. In most of these sites, this increase in the dry season can also be explained by the low oxygen conditions that could cause the release of phosphorus bound to other forms in the pore water. This also justifies the evolution of this fraction which decreases with depth. In addition, this decrease in the L-P content with depth might be explained by the decrease in pH with depth, which enhances desorption of P.[17] Figure 3 shows that it represents 17% of P in the dry season and 3% in the rainy season. L-P represents a loosely sorbed P in the sediment and it is the best parameter for assessment of P-bioavailability.[5] It is a mineral fraction, dissolved in pore water in the form of orthophosphate (PO3− 4 ), directly assimilated by algae and therefore it strongly contributes to eutrophication.[9] 3.3.2. Phosphorous bound to organic matter Figure 2(c) and 2(d) presents the variation of phosphorus bound to organic matter (OM-P) with depth in the dry season and the rainy season, respectively. An increase in this fraction in the rainy season compared with the dry season shown in Figure 2(c) and 2(d) can be explained by the increase in bacterial activity with temperature.[18] Indeed, in aerobic conditions, the importance of bacterial activity is responsible for the release of phosphorus. This release can be explained by the oxidation of organic forms of phosphorus or the dissolution of the calcium fraction.[19] In addition, the increase in this fraction in the rainy season can be explained by exogenous inputs of OM by runoff due to the amount of precipitation (minimum of 125 mm and maximum of 249.5 mm) which lead to highest values of OC in the sediment obtained during the rainy season. This observation can be illustrated by the high values obtained at sites D7 (88.15 ± 2.63 μg P/g), D2 (81.46 ± 1.58 μg P/g) and D4 (63.42 ± 1.32 μg P/g) which then result in high levels of OM in water during the rainy season in each of these sites. The OM-P fraction content is similar whatever the season may be and ANOVA did not show a significant difference (p < .05) between the different layers for most sites in accordance with the observations of Yalçin et al.[9] The reduction in the OM-P content with depth could be explained in part by the mineralization of OM deposited on the surface of sediments by the water column and second by forming complexes with iron which adsorb phosphorus.[19,20] The relative contributions of OM-P were 15% in the dry season and 19% in the rainy season in our case (Figure 3). Organically bound phosphorus occurs in more or less labile forms or in
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Figure 2. (a). L-P (OM-P) content variation with depth in the dry season ((0–5) cm, (5–10) cm and (10–15) cm). (b) L-P (OM-P) content variation with depth in the rainy season ((0–5) cm, (5–10) cm and (10–15) cm). (c) OM-P content variation with depth in the dry season ((0–5) cm, (5–10) cm and (10–15) cm). (d) OM-P content variation with depth in the rainy season ((0–5) cm, (5–10) cm and (10–15) cm). (e) Ca–P content variation with depth in the dry season ((0–5) cm, (5–10) cm and (10–15) cm). (f) Ca–P content variation with depth in the rainy season ((0–5) cm, (5–10) cm and (10–15) cm). (g) Fe–P content variation with depth in the dry season ((0–5) cm, (5–10) cm and (10–15) cm). (h) Fe–P content variation with depth in the rainy season ((0–5) cm, (5–10) cm and (10–15) cm). (i) Res-P content variation with depth in the dry season ((0–5) cm, (5–10) cm and (10–15) cm). (j) Res-P content variation with depth in the rainy season ((0–5) cm, (5–10) cm and (10–15) cm).
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a refractory form that is not released during mineralization and which constitutes a fraction permanently buried in the sediment.[21] Nevertheless, Rydin [6] showed that about 60% of the organic phase can be mobilized and therefore contribute to eutrophication. 3.3.3. Phosphorus bound to carbonate Figure 2(e) and 2(f) presents the variation of phosphorus bound to carbonate (Ca-P) with depth in the dry season and the rainy season, respectively. The results show that this fraction is higher in the rainy season compared with the dry season and ANOVA between both seasons shows a significant difference (p < .05) for layers A and B but not for layer C. These seasonal variations in levels of Ca-P can probably be explained by changes in the pH of the sediment. The pH increase during the rainy season (6.31 by mean) compared with the dry season (5.98 by mean) definitely helps to increase the precipitation of Ca-P in the sediment surface, according to the observations of Dittrich et al.,[21] then increasing in Ca-P levels to the sediment surface (layer A: 0–5 cm) was obtained. In our case, this is illustrated by the highest value obtained at site D6 (111.03 ± 2.36 μg P/g) which results in a higher pH value (6.58 ± 0.01 in the rainy season) than the other site. From a hydrodynamic point of view, Lake Bini could undergo during the rainy season the influence of different water sources that provide calcium in the water column especially site D6. The decrease in Ca-P content with depth might be explained by the decrease in pH with depth, which enhances P desorption. [17] The fraction bound to calcium Ca–P is an important way of storage of phosphorus in sediments.[22] In our case, this fraction contributes 27% and 34% to sedimentary phosphorus in the dry season and the rainy season, respectively (Figure 3). The Ca–P represents the P fraction which is sensitive to low pH and was assumed to mainly consist of apatite P (natural and detritus), including P bound to carbonates and traces of hydrolyzable organic P.[1] This P fraction was deemed as a relatively stable fraction of inorganic P in the sediments.[1–8,17] This fraction was long considered little mobilized, but it can lead to phosphorus release due to a decrease in pH.[16,23] 3.3.4. Phosphorus bound to iron The variation of phosphorus bound to iron (Fe–P) with depth in the dry season is presented in Figure 2(g) and this variation in the rainy season is presented in Figure 2(h). The analysis of the Fe–P content shows that except the layer A, a significant difference (p < .05) between the two seasons for layers B and C was obtained. The increase in the Fe–P fraction content in the rainy season compared with the dry season can be explained by the higher temperature in the dry season (mean equal to 28.3◦ C with mean of evaporation of 201.5 mm and relative humidity of 77%) compared with the rainy season (mean equal to 23.2◦ C
Figure 3. Relative contribution of P-fraction to the sum of phosphorus.
with mean of evaporation of 118 mm and relative humidity of 41%) which leads to the degradation of organic matter by bacteria. This causes a decrease in the redox potential of sediments and therefore induces the reduction of the Fe–P fraction which then releases the phosphorus and iron Fe (III).[16] In the same way, this increase in Fe–P concentrations can be explained by high levels of OC in sediments during the rainy season. Indeed, organic matter and humic substances particularly can form complexes with iron ‘organic matter-iron oxides-phosphate’ and then they adsorb phosphorus.[19] The highest value obtained at the site D6 (183.00 ± 1.58 μg P/g) results in a large value of OC in sediments (in the rainy season at this station). The evolution of the Fe–P fraction with depth varies according to the sampling sites and the seasons. This difference can be explained by climatic conditions and pollution sources that could influence the composition of each sampling station. Generally, there is a decrease with depth, which could be explained primarily by the ability of phosphorus to be complexed by iron oxides (Fe (OOH)) in sediments and secondly by the anoxic conditions which increase with depth.[23] The Fe–P fraction is easily mobilizable and taking into account its great bioavailability, it is responsible for an increase in eutrophication.[5,9] The percentages of Fe–P contributing to sedimentary P are 42% and 37% for the dry and rainy seasons, respectively (Figure 3). It was reported that in heavily polluted lakes, the rank order of Fe+Al-P>Ca-P was found [24] while it was the opposite order of Fe-P+Al-P>Ca-P in mesotrophic lake.[1] This fraction is characterized by seasonal fluctuations and plays an important role in the exchange of phosphorus in the sediment–water interface.[16] The Fe–P fraction has also been suggested to be used for estimation of short- and long-term available P in the sediment, and is a measure
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of algal available phosphorus.[3,5] In this context, we can say that, Lake Bini has a high potential availability of P for algae, which is undesirable for the eutrophication control.[8]
is therefore considered unavailable to algae.[25] However, Golterman et al. [22] showed that this fraction can be mineralized when bacterial activity is intense and in anoxic conditions.
3.3.5. Residual phosphorus Figure 2(i) and 2(j) presents the variation of Res-P with depth in the dry season and the rainy season, respectively. ANOVA of Res-P contents shows that there is no significant difference (p < .05) from one season to another for all layers. We can also observe an increase in the Res-P content in the rainy season in most of the site that varies with depth. However, whatever the season may be, ANOVA did not show significant difference (p < .05) between the different layers of the sampling site. This fraction represents the smallest among phosphorus in our case, which is consistent with the results of Taoufik et al.[25] The percentages of Res-P contributing to P are 4% and 2% for the dry and rainy seasons, respectively (Figure 3). De Groot and Golterman [26] showed that this fraction is mainly composed of phytates. Res-P contains refractory organic phosphorus and inert inorganic phosphorus.[6] This fraction is recognized as very difficult to mobilize sediment as its extraction requires a very aggressive treatment and
3.4. Statistical analysis (PCA) PCA has been used widely to identify the factors influencing the variance of P-fractions in lake sediments.[6] In order to highlight the relation between the different P-forms (variables) and the sampling site during each season, a PCA was performed and the interpretations were focused on the two first principal components (axes) which explain most of the total variance. The results of this analysis are represented in Table 4. The first two factors accounted for 70.15% of the variance of data in the dry season (Table 4) and the plot of PCA is given in Figure 4(a). The first component (F1) accounts for 41.29% of the total variance while the second component (F2) represents 28.86%. Based on the PCA results (Table 4 and Figure 4(b)), two groups could be identified. The first group (I) is constituted by sites D2 and D4 that consist of organic P compound which correlated negatively with F1 and positively with F2. The second group (II) is constituted
Figure 4. (a). Correlation circle (F1 vs. F2) during the dry season. (b) Distribution of sampling sites and P-forms in the dry season. (c) Correlation circle (F1 vs. F2) during the rainy season. (d) Distribution of sampling sites and P-fractions in the rainy season.
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S.J. Marie et al. Table 4. Correlation between P-forms (variables) and axes shown by PCA. Dry season P-forms
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L-P OM-P Ca–P Fe–P Res-P Variance (%)
Rainy season
F1
F2
F1
F2
0974 −0342 0086 0712 −0695 41.29
0178 0851 −0120 −0474 −0669 28.86
−0829 −0601 0922 0902 −0526 59.77
−0160 0704 −0178 0120 −0659 20.03
by sites D1 and D3 which consist of inorganic P (L-P, Ca–P and Fe–P) which correlated positively with F1 axis. The first two factors accounted for 79.81% of the variance of data in the rainy season (Table 4) and the plot of PCA is given in Figure 4(c). The first component (F1) accounts for 59.77% of the variance and the second component (F2) represents 20.03%. Based on the PCA results (Table 4 and Figure 4(d)), three groups could be identified. The first group (I) is constituted by site D4 which consists of inorganic P (Ca–P and Fe–P) which contributed mainly to axis F1. The second group (II) is constituted by sites D1 and D3 which consist of Residual P which correlated negatively with F1 and F2. The third group constituted of sites D2 and D5 which consist of both organic P and inorganic P (L-P) which correlated negatively with F1. This analysis indicated that processes which enhance Ca–P and Fe–P accumulation in the sediments are probably related to the same factor because a strong correlation between them was observed. This analysis shows also that the origin of P depends on the source of pollution.
4. Conclusion This study of phosphorus (P) speciation in the sediment of Bini Lake showed that sediment constitutes a significant storage of this nutrient. For both seasons, the rank order of the different P extracts for this lake was Fe–P>Ca– P>OM-P>L-P>Res-P with the prevalence of inorganic forms. According to Lijklema et al.,[24] this lake can be considered heavily polluted. The composition of sediment during each season might lead to significant seasonal variations of these P forms. These seasonal variations in the capacity of the sediment to retain P and its dependency on the eutrophication level indicate that the background mechanisms are strongly linked to temperature. In most of the cases, the P content is similar between the different layers whatever the season may be and the higher values were obtained at the upper sediment layers. Overall, the sequential extraction method used in this study contributed to a better understanding of the geochemical cycle of phosphorus and it indicated that phosphorus (P) is stored in the Lake Bini sediments in relatively labile and bioavailable fractions, which suggest that some P loading
from the sediment into the water column is ultimately possible, depending on the local environmental conditions mainly the nature of the season.
References [1] Kaiserli A, Voutsa D, Samara C. Phosphorus fractionation in lake sediments-Lakes Volvi and Koronia. N Greece Chemosphere. 2002;46:1147–1155. [2] Haijun W, Hongzhu W. Mitigation of lake eutrophication: loosen nitrogen control and focus on phosphorus abatement. Prog Nat Sci. 2009;19:1445–1451. [3] Dorich R, Nelson D, Sommers L. Estimating phosphorus in suspended sediments by chemical extraction. J Environ Qual. 1985;14:400–405. [4] Kim LH, Choi E, Stenstrom MK. Sediment characteristics, phosphorus types and phosphorus release rates between river and lake sediments. Chemosphere. 2003;50:53–61. [5] Zhou Q, Gibson CE, Zhu Y. Evaluation of phosphorus bioavailability in sediments of three contrasting lakes in china and UK. Chemosphere. 2001;42:221–225. [6] Rydin E. Potentially mobile phosphorus in Lake Erken sediment. Water Res. 2000;34:2037–2042. [7] Søndergaard M, Jensen JP, Jeppesen E. Role of sediment and internal loading of phosphorus in shallow lakes. Hydrobiologia. 2003;506–509:135–145. [8] Ribeiro DC, Martins G, Nogueira R, Cruz JV, Brito AG. Phosphorus fractionation in lake volcanic sediments (Azores-Portugal). Chemosphere. 2008;70:1256–1263. [9] Yalçin S, ¸ Demirak A, Keskin F. Phosphorus fractions and its potential release in the sediments of Koycegiz Lake. Turkey Lakes ReservPonds. 2012;6:139–153. [10] Williams JDH, Jaquet JM, Thomas RL. Forms of phosphorus in the surfacial sediments of Lake Erie. J Fish Res Board Canada. 1976;33:413–429. [11] Hieltjes AHM, Lijklema L. Fractionation of inorganic phosphates in calcareous sediments. J Environ Qual. 1980;9: 405–407. [12] Rodier J, Geoffray C, Kovacsik J, Laporte M, Plissier J, Scheidhauer J, Verneaux J. Vial l’analyse de l’eau: eaux naturelles eaux résiduaires eaux de la mer. Chimie Physicochimie Bactériologie Biologie, 6e édition, Dunod, Paris; 1978. [13] García-Delgado C, Cala V, Eymar E. Influence of chemical and mineralogical properties of organic amendments on the selection of an adequate analytical procedure for trace elements determination. Talanta. 2012;88:375–384. [14] Moreau S. Investigation of Vilaine watershed [PhD dissertation]. Rennes I University; 1997. [15] Murphy J, Riley J. A modified single solution method for the determination of phosphates in natural water. Anal Chim Acta. 1962;27:31–36. [16] Boström B, Andersen JM, Fleischer S, Jansson M. Exchange of phosphorus across the sediment-water interface. Hydrobiologia. 1988;170:229–244. [17] Gonsiorczyk T, Casper P, Koschel R. Phosphorus binding forms in the sediment of an oligotrophic and an eutrophic hard water lake of the Baltic district (Germany). Water Sci Technol. 1998;37:51–58. [18] Boers P, Van Hese O. Phosphorus release from the peaty sediments of the Loosdrecht lakes (The Nertherlands). Water Res. 1988;22:355–363. [19] Taoufik M, Dafir JE. Comportement du phosphore dans le sédiment des barrages de la partie aval du bassin versant d’Oum Rabiaa (Maroc). Revue Sci l’Eau. 2002;15:235–249.
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[20] Golterman HL. Sediment as a source of phosphate for the algal growth. In: Golterman HL, editor. Interactions between sediments and freshwater. Hague: W Junk Publisher; 1977. p. 286–293. [21] Dittrich M, Casper P, Koschel R. Changes in the pore water chemistry of profundal sediments in response to artificial hypolimnetic calcite precipitation. Arch Hydrobiol Spec Issues Advanc Limnol. 2000;55:421–432. [22] Golterman HL, Paing J, Serrano L, Gomez E. Presence of and phosphate release from polyphosphates or phytate phosphate in lake sediments. Hydrobiologia. 1998;364:99–104. [23] Gomez E, Durillon C, Rofes G, Picot B. Phosphate adsorption and release from sediments of brackish lagoons:
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pH, O2 and loading influence. Water Res. 1999;33:2437– 2447. [24] Lijklema L, Koelmans AA, Portielje R. Water quality impacts of sediment pollution and the role of early diagenesis. Water Sci Technol. 1993;28:1–12. [25] Taoufik M, Kemmou S, Idrissi LL, Dafir JE. Comparison of two methods for the fractionation of phosphorus from the sediments of lower Oum Rabiaa basin (Morocco). Water Qual Res J Canada. 2004;39:50–56. [26] De Groot CJ, Golterman HL. On the presence of organic in some Camargue sediment: evidence for the importance of phytate. Hydrobiologia. 1993;252: 117–126.
Environmental Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tent20
Speciation of phosphorus in the sediments of Lake Bini (Ngaoundere-Cameroon) a
b
Sieliechi Joseph Marie , Dangwang Dikdim Jean-Marie & Noumi Guy Bertrand
b
a
Department of Applied Chemistry, National Higher School of Agro-Industrial Science, University of Ngaoundere, Ngaoundere, P.O. Box 455 Cameroon b
Department of Chemistry, Faculty of Science, University of Ngaoundere, Ngaoundere, P.O. Box 454 Cameroon Published online: 27 Feb 2014.
To cite this article: Sieliechi Joseph Marie, Dangwang Dikdim Jean-Marie & Noumi Guy Bertrand (2014) Speciation of phosphorus in the sediments of Lake Bini (Ngaoundere-Cameroon), Environmental Technology, 35:14, 1831-1839, DOI: 10.1080/09593330.2014.884171 To link to this article: http://dx.doi.org/10.1080/09593330.2014.884171
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Environmental Technology, 2014 Vol. 35, No. 14, 1831–1839, http://dx.doi.org/10.1080/09593330.2014.884171
Speciation of phosphorus in the sediments of Lake Bini (Ngaoundere-Cameroon) Sieliechi Joseph Mariea∗ , Dangwang Dikdim Jean-Marieb and Noumi Guy Bertrandb a Department
of Applied Chemistry, National Higher School of Agro-Industrial Science, University of Ngaoundere, Ngaoundere, P.O. Box 455 Cameroon; b Department of Chemistry, Faculty of Science, University of Ngaoundere, Ngaoundere, P.O. Box 454 Cameroon
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(Received 11 October 2013; accepted 11 January 2014 ) In this study, spatial and seasonal variations of phosphorus fractions in Lake Bini sediments were evaluated using a sequential extraction method. The sampling of water and sediments (surface and coring) was carried out at seven sites around the lake during the dry season and the rainy season. The results showed that phosphorus is mainly in the inorganic form (L-P+Ca–P+Fe-P) in the sediments whatever the season may be. The rank order of phosphorus extracts obtained was Fe-P>Ca–P>OM-P>L–P>Res-P. The maximum values of phosphorus (sum of each fraction) were obtained in the rainy season at the sites D6 (298.12 ± 12.37 μg P/g) and D4 (244.93 ± 11.06 μg P/g) located beside water source 2 and farmland 2, respectively. The average values of the phosphorus content vary from 05.29 ± 1.05 μg P/g to 102.58 ± 4.62 μg P/g for the upper layer (0–5 cm depth); 04.67 ± 0.66 μg P/g to 70.06 ± 2.82 μg P/g for the medium layer (5–10 cm depth) and finally 04.63 ± 0.98 μg P/g to 55.24 ± 5.17 μg P/g for the deep layer (10–15 cm depth). The results of principal component analysis showed that processes which enhance Ca–P and Fe–P accumulation are probably related to the same factor and the origin of P depends on the source of pollution. The nature of the season plays a significant role in the geochemical composition of the sediments in phosphorus and on the eutrophication level of Lake Bini. Keywords: Lake Bini; speciation of phosphorus; eutrophication; PCA; pollution
1. Introduction Phosphorus (P) is often accepted as the most critical limiting nutrient for primary lake productivity,[1–3] and its excess supply can lead to eutrophication.[4] Despite efforts to control the phosphorus load by reducing external inputs,[5] the water quality remains a problem in some lakes due to the internal P-loading.[6] Indeed, P can accumulate in sediments and, under some environmental conditions, can be released from sediment depths as low as 20 cm.[7] In this context, many studies in the past years have been made to understand the factors that affect the P release from the sediments.[8] Temperature, redox reactions, pH, dissolved oxygen concentration, nitrate, sulphates and bacterial activity are pointed as the major factors controlling P release from sediments.[4] Exchange of P in sediment–water interface constitutes a key point in the phosphorus cycle which is a sedimentary process.[2] Phosphorus (P) is present in sediment in several chemical forms depending on the sediment composition, the sedimentation rate and the physico-chemical conditions.[9] Thus, the role of the P in lake sediment in promoting lake eutrophication can be more efficiently evaluated on the basis of the contents of different P fractions, instead of total P content.[1–8] Many investigations have performed studies on P forms to allow a more precise description of the
∗ Corresponding
author. Email: [email protected]
© 2014 Taylor & Francis
potentials for P release from the sediment and to predict its future influence on lake systems. According to Yalçin et al.,[9] it is important to know which part of the stock of P can be mobilized because P remobilization from the sediments is probably controlled by its speciation. Moreover, it is well known that the bioavailability of P is strongly linked to its speciation.[8] Since then, several sequential extraction methods have been developed to distinguish between the different P fractions in the sediments.[10,11] These sequential extraction schemes made possible, in view of the potential P-bioavailability, to characterize the diverse forms in which P is distributed in sediments, classified as water-soluble P, readily desorbable P, algal available P or ecologically important P.[2] The lake Bini is located in North-West of the town of Ngaoundere (Cameroon). For several years, it has constituted a significant source of income and a means of survival for the population who practise there fishing, agriculture and the breeding at the banks. Consequently, the increased use of fertilizers, pesticides, liquid manures and animal manure for cultivation unfortunately is drained to the lake by scrubbing and streaming of the grounds. The significant invasion of this lake by watery plants and algae, the reduction in fish and its accelerated contraction for a few years have shown that it is undergoing a considerable eutrophication.
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The main objective of this work was to determine seasonal variation of the amount and forms of P in the sediments of Bini Lake by using the sequential extraction method, to evaluate their possible contributions to the P-loadings of this Lake system.
2. Experimental section 2.1. Description of the study area The lake Bini (Figure 1) is located at 15 km in the NorthWest of the town of Ngaoundere in the Adamawa region whose soil is of ferrallitic nature. It is located at 1070 m of altitude and between 1330 24.28 and 1328 40.14 of Eastern longitude and between 725 23.21 and 725 38.70 of Northern latitude. It is a crater lake with an average depth estimated at 5 m with per place maximum depths higher than 4 m in the East and the centre and the depths of 1 m in the West. Its average surface is approximately 4.5 km2 . This one is fed by various small rivers which are for the majority seasonal workers; we have the Baladji River, the Mayanga River and the Soukande River but the principal source being the Bini River. Sample site selection criteria include the presence of signs of eutrophication and the anthropic activities which are practised around the lake. We have as follows: site D1 corresponding to the exit of the lake, sites D2 and D4 located beside farmland 1 and farmland 2, respectively; site D3 corresponding to the medium, finally
Table 1. Variation of relative humidity, evaporation and precipitation during the year 2012. Month January February March April May June July August September October November December
Evaporation (mm)
Precipitation (mm)
23.6 41 40.6 72.2 79.8 81.7 83.4 82 80.6 72 63.4 56.7
203 154 283 206.2 136.2 102 108.2 206.2 103.3 134 165 164.9
0 0 0 184.8 223 169.7 231.1 240.3 249.5 125 0 0
the sites D5, D6 and D7 located beside water source 1, water source 2 and water source 3, respectively. Table 1 shows the variation of relative humidity, evaporation and precipitation of the study area. 2.2. Sampling and pre-treatment Sampling campaigns were conducted for this study during the dry season (December 2011 to March 2012) and the rainy season (April to December 2012). In the dry season, we collected 1 sample per month of water and sediment during 5 months at 7 sampling points (35 samples of water and 35 samples of sediments). Also, in the rainy season we collected 1 sample per month of water and sediment during 8 months at 7 sampling points (56 samples of water and 56 samples of sediments). Water samples were collected from the seven sites of Bini Lake. Then, collected samples were kept in 1.5 L polyethylene plastic bottles that were cleaned and rinsed many times with distilled water. All water samples were stored in an insulated cooler containing ice and delivered on the same day to a laboratory and all samples were kept at 4◦ C until processing and analysis by Rodier et al.[12] Sediment samples were collected by using a plastic pipe, 30 mm in diameter and 15 cm length. Sediment samples were taken to the laboratory, and separated into three sections and each section was 5 cm: upper layer (A: 0–5 cm), medium layer (B: 5–10 cm) and deep layer (C: 10–15 cm). These samples were air dried, homogenized and passed through a 0.25 mm sieve and stored. 2.3.
Figure 1. Map of the lake Bini showing sampling locations of the seven sites.
Relative humidity (%)
Physico-chemical analysis of water and the sediment The parameters of characterization of water such as the temperature, electric conductivity (E.C.) and the pH were measured in situ using a portable multimeter (HANNA HI8033). Dissolved oxygen was determined by the traditional method of Winkler. Turbidity was determined by
Environmental Technology the nephelometric method and the suspended matter by filtration.[12] Nitrates, phosphates, ammoniums and sulphates were determined by spectrophotometry (Rayleigh VIS-723N) according to Rodier et al.’s [12] method. The pH and electric conductivity of sediment were measured by using a portable multimeter (HANNA HI8033). Organic carbon (OC) was determined after treatment of the sample with K2 Cr2 O7 /H2 SO4 according to the Walkey– Black method. The total concentration of metals (Cu, Zn, Ni, Pb, Hg and Mn) was determined by inductively coupled plasma-mass spectrometry (Perkin Elmer ELAN-6000DRC II) with appropriate standards (BCR320, IAIE433) according to García-Delgado et al.’s [13] method. These analyses were carried out only on the upper layer of the sediment.
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2.4.
concentrations in the supernatant solutions were determined by the conventional molybdate colorimetry method.[15] 2.5. Statistical analysis Data obtained were statistically analysed by Statgraphic Centurion XV software program using the analysis of variance (ANOVA). Principal component analysis (PCA) was realized using the XLSTAT 2007 software. 3. Results and discussion 3.1. Physico-chemical characterization of water Table 2 summarizes the physico-chemical characteristics of Lake Bini. The lake shows an average temperature of 28.3 ± 0.3◦ C in the dry season and 23.2 ± 0.1◦ C in the rainy season with a pH ranging from 6.83 ± 0.01 to 7.65 ± 0.07 over the two seasons. The electrical conductivities are low and vary between 9.7 ± 0.5 and 26.6 ± 0.4 μS/cm. This lake has very low oxygen with average dissolved oxygen which is equal to 2.17 ± 0.17 mg O2 /L and 4.58 ± 0.11 mg O2 /L for the dry season and the rainy season, respectively. The suspended matters are higher in the rainy season compared with the dry season with corresponding averages of 62.19 and 33.27 mg/L, respectively. There is an increase in turbidity in the rainy season, the values range from 4.55 ± 0.21 NTU to 107.3 ± 1.84 NTU for the two seasons. The increase in temperature in the dry season induces the evaporation of water and results in an increasing in the concentration of dissolved ions. The concentrations of these dissolved ions reach their maximum value and their average concentrations are 10.93 ± 0.84, 0.79 ± 0.07, 0.56 ± 0.10 and 1.38 ± 0.09 mg/L for sulphates, nitrates, ammoniums and phosphates, respectively. On the other hand in the rainy season, dilution by rain water tends to decrease their concentrations which are equal in average 4.50 ± 0.87, 0.77 ± 0.07, 0.09 ± 0.02 and 0.84 ± 0.09 mg/L for sulphates, nitrates, ammoniums and phosphates, respectively. These dissolved ions are present at higher concentrations in Lake Bini and they might contribute to improve the internal P-loading. Indeed, phosphates’ concentration in the water
Speciation of phosphorus in the sediment
The speciation of phosphorus in the sediments was carried out by sequential extraction according to the protocol of Moreau [14] with a slight modification. This extraction scheme allows the discrimination of loosely sorbed phosphorus (L–P), inorganic phosphorus associated with aluminium (Al–P), inorganic phosphorus bonded to carbonates (Ca–P), inorganic phosphorus associated with Fe (Fe–P), phosphorus bonded to organic matter (OM-P) and residual phosphorus (Res-P). Each sample was extracted in duplicate and all the data were expressed as the average: 300 mg of dried sediment was treated with the reactant for a given time. An extraction sequence is as follows: [14] the desorption of loosely sorbed phosphorus was done by agitation in 30 mL of distillated water for 2 h, the oxidation of organic matter was done by agitation in 10 mL of H2 O2 for 24 h, the inorganic Ca–P was extracted by agitation with 30 mL H2 SO4 (0.5 N) for 12 h, the inorganic phosphorus associated with aluminium was extracted with NH4 F(0.3 N) for 6 h, the inorganic phosphorus associated with iron was extracted with 30 mL of NAOH (1 N) for 6 h and Res-P was extracted with 1 mL of H2 SO4 (0.5 N) and K2 S2 O8 for 1 h. After each extraction step, the samples were centrifuged for 15 min at 6000 rpm and the supernatants were filtered through a 0.22 mm membrane. Phosphate Table 2.
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Physico-chemical characteristics of Lake Bini water. Dry season
Parameters T (◦ C) pH E.C. (μS/cm) DO (mg O2 /L) Turbidity (NTU) MIS (mg/L) Sulphates (mg/L) Nitrates (mg/L) Ammoniums (mg/L) Phosphates (mg/L)
Rainy season
Min
Max
Mean
Min
Max
Mean
27.4 ± 0.1 6.83 ± 0.01 19.1 ± 0.7 0.63 ± 0.19 4.55 ± 0.21 20.00 7.50 ± 1.18 0.35 ± 0.05 0.25 ± 0.09 0.63 ± 0.03
29.4 ± 0.4 6.95 ± 0.00 49.7 ± 0.4 3.39 ± 0.08 107.30 ± 1.84 60.00 15.50 ± 0.71 1.13 ± 0.10 0.99 ± 0.17 1.90 ± 0.10
28.3 ± 0.3 6.88 ± 0.01 26.4 ± 0.1 2.17 ± 0.17 24.46 ± 0.79 33.27 10.93 ± 0.84 0.79 ± 0.07 0.56 ± 0.10 1.38 ± 0.09
22.6 ± 0.2 6.90 ± 0.03 09.7 ± 0.5 2.71 ± 0.12 13.2 ± 0.50 43.33 01.83 ± 0.00 0.09 ± 0.02 0.02 ± 0.01 0.29 ± 0.02
23.5 ± 0.1 7.65 ± 0.07 38.8 ± 1.7 9.30 ± 0.04 54.10 ± 0.57 72.66 08.83 ± 1.18 1.14 ± 0.12 0.21 ± 0.00 1.16 ± 0.05
23.2 ± 0.1 7.14 ± 0.04 16.2 ± 0.3 4.58 ± 0.11 23.36 ± 0.70 62.19 4.50 ± 0.87 0.77 ± 0.07 0.09 ± 0.02 0.84 ± 0.09
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Characterization of the sediment.
Parameters
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pH E.C. (μS/cm) OC (%) Cu (ppm) Ni (ppm) Pb (ppm) Hg (ppm) Mn (ppm) Zn (ppm)
Min 5.34 ± 0.00 5.90 ± 0.07 1.03 ± 0.24 6.55 ± 0.14 17.44 ± 0.94 7.59 ± 0.32 0.24 ± 0.03 69.47 ± 4.55 8.90 ± 0.47
Max 6.58 ± 0.01 20.80 ± 0.71 26.14 ± 0.75 18.97 ± 0.77 43.40 ± 1.09 10.99 ± 1.47 0.49 ± 0.03 290.76 ± 9.26 25.45 ± 3.49
column is linked to recycling and releases of P from the sediment and nitrates, sulphates are the major factors controlling this P release.[4] Nitrate, for example, may prevent or enhance P mobilization by maintaining a high redox potential or by stimulating bacteria capable of producing ‘iron reductase’.[16] 3.2. Characterization of the sediment The characteristics of the surface sediment (upper layer) of Lake Bini are given in Table 3. This table shows that the pH is acid due to ferrallitic properties of the soil with the highest values obtained in the rainy season. The electro conductivities are low like in water. The OC is in the range of 1.03–26.14%. The sediments were rich in manganese and to a lesser extent in mercury. The concentrations of nickel, copper, zinc and lead were much lower than manganese. Highest concentrations of these metals were obtained at the downstream (site D1) and upstream (site D5, D6 and D7) of lake while sites D2 and D4 located beside farmlands show moderate concentration. These metals originate by anthropic activities (mainly agriculture) which are practised around the lake. Consequently, they impair water quality for drinking, agriculture or other purposes. Moreover, excess heavy metals can lead to a potential contamination for a large variety of flora and fauna, including humans through food chains. 3.3. Phosphorous forms in the sediment 3.3.1. Labile phosphorus Figure 2(a) and 2(b) presents the variation of labile phosphorus (L-P) with depth in the dry season and the rainy season, respectively. It shows that L-P content is much higher in the dry season compared with the rainy season. This result can be explained by the phenomenon of dilution of orthophosphate contained in the water column during the rainy season (mean of precipitation in the rainy season was 174 mm with relative humidity of 41%). In contrast, the increased evaporation of water due to higher temperature in the dry season (27.4–29.4◦ C with mean equal to 28.3◦ C) (mean was 201.2 mm in the dry season compared with 118 mm in the rainy season) leads to an increase in these
ions’ concentration which then migrate into the sediment. This part follows the seasonal variations of phosphate ions in the water because it is located in the pore water and thus in direct contact with the water column. This can be illustrated by the maximum values obtained at the upper layer (A: 0–5 cm) in the dry season at site D6 (71.42 ± 3.16 μg P/g) that result in high values of orthophosphate in the water of this site at this time. In most of these sites, this increase in the dry season can also be explained by the low oxygen conditions that could cause the release of phosphorus bound to other forms in the pore water. This also justifies the evolution of this fraction which decreases with depth. In addition, this decrease in the L-P content with depth might be explained by the decrease in pH with depth, which enhances desorption of P.[17] Figure 3 shows that it represents 17% of P in the dry season and 3% in the rainy season. L-P represents a loosely sorbed P in the sediment and it is the best parameter for assessment of P-bioavailability.[5] It is a mineral fraction, dissolved in pore water in the form of orthophosphate (PO3− 4 ), directly assimilated by algae and therefore it strongly contributes to eutrophication.[9] 3.3.2. Phosphorous bound to organic matter Figure 2(c) and 2(d) presents the variation of phosphorus bound to organic matter (OM-P) with depth in the dry season and the rainy season, respectively. An increase in this fraction in the rainy season compared with the dry season shown in Figure 2(c) and 2(d) can be explained by the increase in bacterial activity with temperature.[18] Indeed, in aerobic conditions, the importance of bacterial activity is responsible for the release of phosphorus. This release can be explained by the oxidation of organic forms of phosphorus or the dissolution of the calcium fraction.[19] In addition, the increase in this fraction in the rainy season can be explained by exogenous inputs of OM by runoff due to the amount of precipitation (minimum of 125 mm and maximum of 249.5 mm) which lead to highest values of OC in the sediment obtained during the rainy season. This observation can be illustrated by the high values obtained at sites D7 (88.15 ± 2.63 μg P/g), D2 (81.46 ± 1.58 μg P/g) and D4 (63.42 ± 1.32 μg P/g) which then result in high levels of OM in water during the rainy season in each of these sites. The OM-P fraction content is similar whatever the season may be and ANOVA did not show a significant difference (p < .05) between the different layers for most sites in accordance with the observations of Yalçin et al.[9] The reduction in the OM-P content with depth could be explained in part by the mineralization of OM deposited on the surface of sediments by the water column and second by forming complexes with iron which adsorb phosphorus.[19,20] The relative contributions of OM-P were 15% in the dry season and 19% in the rainy season in our case (Figure 3). Organically bound phosphorus occurs in more or less labile forms or in
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Environmental Technology
Figure 2. (a). L-P (OM-P) content variation with depth in the dry season ((0–5) cm, (5–10) cm and (10–15) cm). (b) L-P (OM-P) content variation with depth in the rainy season ((0–5) cm, (5–10) cm and (10–15) cm). (c) OM-P content variation with depth in the dry season ((0–5) cm, (5–10) cm and (10–15) cm). (d) OM-P content variation with depth in the rainy season ((0–5) cm, (5–10) cm and (10–15) cm). (e) Ca–P content variation with depth in the dry season ((0–5) cm, (5–10) cm and (10–15) cm). (f) Ca–P content variation with depth in the rainy season ((0–5) cm, (5–10) cm and (10–15) cm). (g) Fe–P content variation with depth in the dry season ((0–5) cm, (5–10) cm and (10–15) cm). (h) Fe–P content variation with depth in the rainy season ((0–5) cm, (5–10) cm and (10–15) cm). (i) Res-P content variation with depth in the dry season ((0–5) cm, (5–10) cm and (10–15) cm). (j) Res-P content variation with depth in the rainy season ((0–5) cm, (5–10) cm and (10–15) cm).
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a refractory form that is not released during mineralization and which constitutes a fraction permanently buried in the sediment.[21] Nevertheless, Rydin [6] showed that about 60% of the organic phase can be mobilized and therefore contribute to eutrophication. 3.3.3. Phosphorus bound to carbonate Figure 2(e) and 2(f) presents the variation of phosphorus bound to carbonate (Ca-P) with depth in the dry season and the rainy season, respectively. The results show that this fraction is higher in the rainy season compared with the dry season and ANOVA between both seasons shows a significant difference (p < .05) for layers A and B but not for layer C. These seasonal variations in levels of Ca-P can probably be explained by changes in the pH of the sediment. The pH increase during the rainy season (6.31 by mean) compared with the dry season (5.98 by mean) definitely helps to increase the precipitation of Ca-P in the sediment surface, according to the observations of Dittrich et al.,[21] then increasing in Ca-P levels to the sediment surface (layer A: 0–5 cm) was obtained. In our case, this is illustrated by the highest value obtained at site D6 (111.03 ± 2.36 μg P/g) which results in a higher pH value (6.58 ± 0.01 in the rainy season) than the other site. From a hydrodynamic point of view, Lake Bini could undergo during the rainy season the influence of different water sources that provide calcium in the water column especially site D6. The decrease in Ca-P content with depth might be explained by the decrease in pH with depth, which enhances P desorption. [17] The fraction bound to calcium Ca–P is an important way of storage of phosphorus in sediments.[22] In our case, this fraction contributes 27% and 34% to sedimentary phosphorus in the dry season and the rainy season, respectively (Figure 3). The Ca–P represents the P fraction which is sensitive to low pH and was assumed to mainly consist of apatite P (natural and detritus), including P bound to carbonates and traces of hydrolyzable organic P.[1] This P fraction was deemed as a relatively stable fraction of inorganic P in the sediments.[1–8,17] This fraction was long considered little mobilized, but it can lead to phosphorus release due to a decrease in pH.[16,23] 3.3.4. Phosphorus bound to iron The variation of phosphorus bound to iron (Fe–P) with depth in the dry season is presented in Figure 2(g) and this variation in the rainy season is presented in Figure 2(h). The analysis of the Fe–P content shows that except the layer A, a significant difference (p < .05) between the two seasons for layers B and C was obtained. The increase in the Fe–P fraction content in the rainy season compared with the dry season can be explained by the higher temperature in the dry season (mean equal to 28.3◦ C with mean of evaporation of 201.5 mm and relative humidity of 77%) compared with the rainy season (mean equal to 23.2◦ C
Figure 3. Relative contribution of P-fraction to the sum of phosphorus.
with mean of evaporation of 118 mm and relative humidity of 41%) which leads to the degradation of organic matter by bacteria. This causes a decrease in the redox potential of sediments and therefore induces the reduction of the Fe–P fraction which then releases the phosphorus and iron Fe (III).[16] In the same way, this increase in Fe–P concentrations can be explained by high levels of OC in sediments during the rainy season. Indeed, organic matter and humic substances particularly can form complexes with iron ‘organic matter-iron oxides-phosphate’ and then they adsorb phosphorus.[19] The highest value obtained at the site D6 (183.00 ± 1.58 μg P/g) results in a large value of OC in sediments (in the rainy season at this station). The evolution of the Fe–P fraction with depth varies according to the sampling sites and the seasons. This difference can be explained by climatic conditions and pollution sources that could influence the composition of each sampling station. Generally, there is a decrease with depth, which could be explained primarily by the ability of phosphorus to be complexed by iron oxides (Fe (OOH)) in sediments and secondly by the anoxic conditions which increase with depth.[23] The Fe–P fraction is easily mobilizable and taking into account its great bioavailability, it is responsible for an increase in eutrophication.[5,9] The percentages of Fe–P contributing to sedimentary P are 42% and 37% for the dry and rainy seasons, respectively (Figure 3). It was reported that in heavily polluted lakes, the rank order of Fe+Al-P>Ca-P was found [24] while it was the opposite order of Fe-P+Al-P>Ca-P in mesotrophic lake.[1] This fraction is characterized by seasonal fluctuations and plays an important role in the exchange of phosphorus in the sediment–water interface.[16] The Fe–P fraction has also been suggested to be used for estimation of short- and long-term available P in the sediment, and is a measure
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of algal available phosphorus.[3,5] In this context, we can say that, Lake Bini has a high potential availability of P for algae, which is undesirable for the eutrophication control.[8]
is therefore considered unavailable to algae.[25] However, Golterman et al. [22] showed that this fraction can be mineralized when bacterial activity is intense and in anoxic conditions.
3.3.5. Residual phosphorus Figure 2(i) and 2(j) presents the variation of Res-P with depth in the dry season and the rainy season, respectively. ANOVA of Res-P contents shows that there is no significant difference (p < .05) from one season to another for all layers. We can also observe an increase in the Res-P content in the rainy season in most of the site that varies with depth. However, whatever the season may be, ANOVA did not show significant difference (p < .05) between the different layers of the sampling site. This fraction represents the smallest among phosphorus in our case, which is consistent with the results of Taoufik et al.[25] The percentages of Res-P contributing to P are 4% and 2% for the dry and rainy seasons, respectively (Figure 3). De Groot and Golterman [26] showed that this fraction is mainly composed of phytates. Res-P contains refractory organic phosphorus and inert inorganic phosphorus.[6] This fraction is recognized as very difficult to mobilize sediment as its extraction requires a very aggressive treatment and
3.4. Statistical analysis (PCA) PCA has been used widely to identify the factors influencing the variance of P-fractions in lake sediments.[6] In order to highlight the relation between the different P-forms (variables) and the sampling site during each season, a PCA was performed and the interpretations were focused on the two first principal components (axes) which explain most of the total variance. The results of this analysis are represented in Table 4. The first two factors accounted for 70.15% of the variance of data in the dry season (Table 4) and the plot of PCA is given in Figure 4(a). The first component (F1) accounts for 41.29% of the total variance while the second component (F2) represents 28.86%. Based on the PCA results (Table 4 and Figure 4(b)), two groups could be identified. The first group (I) is constituted by sites D2 and D4 that consist of organic P compound which correlated negatively with F1 and positively with F2. The second group (II) is constituted
Figure 4. (a). Correlation circle (F1 vs. F2) during the dry season. (b) Distribution of sampling sites and P-forms in the dry season. (c) Correlation circle (F1 vs. F2) during the rainy season. (d) Distribution of sampling sites and P-fractions in the rainy season.
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S.J. Marie et al. Table 4. Correlation between P-forms (variables) and axes shown by PCA. Dry season P-forms
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L-P OM-P Ca–P Fe–P Res-P Variance (%)
Rainy season
F1
F2
F1
F2
0974 −0342 0086 0712 −0695 41.29
0178 0851 −0120 −0474 −0669 28.86
−0829 −0601 0922 0902 −0526 59.77
−0160 0704 −0178 0120 −0659 20.03
by sites D1 and D3 which consist of inorganic P (L-P, Ca–P and Fe–P) which correlated positively with F1 axis. The first two factors accounted for 79.81% of the variance of data in the rainy season (Table 4) and the plot of PCA is given in Figure 4(c). The first component (F1) accounts for 59.77% of the variance and the second component (F2) represents 20.03%. Based on the PCA results (Table 4 and Figure 4(d)), three groups could be identified. The first group (I) is constituted by site D4 which consists of inorganic P (Ca–P and Fe–P) which contributed mainly to axis F1. The second group (II) is constituted by sites D1 and D3 which consist of Residual P which correlated negatively with F1 and F2. The third group constituted of sites D2 and D5 which consist of both organic P and inorganic P (L-P) which correlated negatively with F1. This analysis indicated that processes which enhance Ca–P and Fe–P accumulation in the sediments are probably related to the same factor because a strong correlation between them was observed. This analysis shows also that the origin of P depends on the source of pollution.
4. Conclusion This study of phosphorus (P) speciation in the sediment of Bini Lake showed that sediment constitutes a significant storage of this nutrient. For both seasons, the rank order of the different P extracts for this lake was Fe–P>Ca– P>OM-P>L-P>Res-P with the prevalence of inorganic forms. According to Lijklema et al.,[24] this lake can be considered heavily polluted. The composition of sediment during each season might lead to significant seasonal variations of these P forms. These seasonal variations in the capacity of the sediment to retain P and its dependency on the eutrophication level indicate that the background mechanisms are strongly linked to temperature. In most of the cases, the P content is similar between the different layers whatever the season may be and the higher values were obtained at the upper sediment layers. Overall, the sequential extraction method used in this study contributed to a better understanding of the geochemical cycle of phosphorus and it indicated that phosphorus (P) is stored in the Lake Bini sediments in relatively labile and bioavailable fractions, which suggest that some P loading
from the sediment into the water column is ultimately possible, depending on the local environmental conditions mainly the nature of the season.
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