Environ Monit Assess DOI 10.1007/s10661-014-3836-0
Phosphorus speciation in the marine sediment of Kalpakkam coast, southeast coast of India S. N. Bramha & A. K. Mohanty & R. K. Padhi & S. N. Panigrahi & K. K. Satpathy
Received: 14 January 2014 / Accepted: 14 May 2014 # Springer International Publishing Switzerland 2014
Abstract A study was carried out at Kalpakkam coast to find out the distribution of various fractions of phosphorus (P) in the marine sediment during pre-northeast monsoon period. Samples were collected from ten locations covering ~80 km2 of the inner-shelf region. Sedimentary parameters such as sand, silt, clay, and organic carbon percentage were analyzed in order to find out their relation with various P fractions. The sediment was found to be predominantly sandy in nature with low silt and clay content. Among all the fractions (loosely bound (LoP), calcium bound (CaP), iron bound (FeP), aluminum bound (AlP), and organic (OP)), CaP fraction constituted the largest portion (68.7 %) followed by organic fraction (16.3 %). The bioavailable P fractions ranged from 5 to 44 % of the total P (TP) content. Relatively high LoP content was observed at the offshore locations with comparatively high mud percentage as compared with the near-shore locations. As FeP and AlP concentrations were directly proportional to the amount of fine-grain sediment, the low levels of these fractions found in this coastal area were therefore attributed to the sandy nature of the sediments. The order of abundance of the major forms of P in the surface sediments of Kalpakkam coast was as follows: CaP>OP>LoP>AlP>FeP.
S. N. Bramha : A. K. Mohanty : R. K. Padhi : S. N. Panigrahi : K. K. Satpathy (*) Environment and Safety Division, Indira Gandhi Centre for Atomic Research, Kalpakkam, Tamil Nadu, India e-mail: [email protected]
Keywords Sequential extraction . Phosphorus fractions . Coastal sediment . Bay of Bengal
Introduction Sediment plays an important role in the overall nutrient dynamics of the coastal marine ecosystem. It has been observed that the sediment of the coastal system can act as an internal source of nutrients for the overlying water column. Among other nutrients, phosphorus (P) has been recognized as the most essential and critical nutrient in the terrestrial and coastal aquatic environments (Harrison et al. 1990; Bauerfeind et al. 1990) and is thought to control marine primary productivity over a geological time scales (Ruttenberg 2004). It is an essential constituent of tissues and cells and required for the formation of nucleic acids and energy-carrying molecules such as adenosine triphosphate (ATP). The biogeochemical cycle of the P in the marine sediment is mainly the quantity of inorganic and organic P and the P dynamics are controlled by a combination of physical, chemical, and biological properties and processes (Frossard et al. 2000; Reddy et al. 2005). By contrast, the other important micronutrients, nitrogen and sulfur, are predominately present in organic forms in sediments, and their turnover is therefore primarily controlled by biological and biochemical processes (Golterman 2004; Schimel and Bennett 2004; Bünemann and Condron 2007). Phosphates, the most abundant form of the P in the environment, are readily available in detergents,
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fertilizers, and pesticides. Consequently, monitoring of P content in the water and sediment is essential to control and avoid eutrophication of the coastal aquatic environment. Its current abundance in the earth crust is approximately 0.12 %; however, almost all P on the earth crust found in the form of minerals including apatite (chloro and floro), vivianite, wavellite, and phosphorite. Apatite {Ca (PO4)3[F, OH, or Cl]}, the largest reservoir of phosphate on Earth, is relatively insoluble in water. Most of the P in the current natural environment is present in particulate form and is not biologically available, thereby limiting primary production. The sorption of phosphate on sediments has major influences on transport, degradation, and ultimate fate of P in marine ecosystems. P is found in the nature in various forms which includes: (1) mineral forms; (2) organic forms such as phospholipids, nucleic acids, proteins, polysaccarides, nucleotides cofactors, and phosphonates; (3) dissolved in organic forms such as pentavalent, trivalent, or univalent dissolve species; (4) gaseous form in the III oxidation state; and (5) particulate or colloidal forms (Aydin et al. 2009; Frankowski et al. 2002). P in marine sediments primarily consists of inorganic forms such as: loosely bounded P (LoP), aluminum Al-bounded P (AlP), iron (Fe)-bounded P (FeP), calcium (Ca)-bounded P (CaP), and organic P (OP). These fractions of P shows a wide range of variations, which are controlled by a number of factors such as rate of sedimentation, sediment type, amount and type of organic matter, intensity of mineralization of organic matters in the sediment and water column, redox conditions in the sediments and water depth. Al, Fe, and Ca contents in the sediments are also known to influence the P fractions (Sundareshwar and Morris 1999; Jennifer et al. 2004,). Different form of P in the sediments can provide valuable information on the origin of the P, the degree of pollution from anthropogenic activities, the bioavailability, and also the burial and digenesis of P in sediments (Andrieux and Aminot 1997; Jensen et al. 1988; Schenau and De Lange 2001). A considerable amount of work on physicochemical characteristics of Kalpakkam coastal waters have been carried out (Satpathy et al. 2010, 2011). However, observations on sediment nutrients and other physicochemical characteristics are lacking. Moreover, information about concentration and variability of different P fractions in coastal sediments, particularly from this region is not available. Thus, the present study was designed with the following objectives: (1) to investigate the distribution
pattern of major P species in the surface sediments, (2) to find out the bioavailable fraction of P and (3) contribution of organic P (OP) to the total P (TP) content.
Materials and methods Study area Kalpakkam (12° 33′ N Lat. and 80° 11′ E Long.) is situated about 80 km south of Chennai city (Fig. 1). At present, a nuclear power plant (Madras Atomic Power Station (MAPS)) and a desalination plant are located near the coast. MAPS uses seawater at a rate of 35 m3s−1 for condenser-cooling purpose. After extracting the heat, the heated seawater is released into the sea. Two backwaters, namely the Edaiyur and the Sadras backwater systems are important features of this coast. These backwaters are connected to the Buckingham canal, which runs parallel to the coast. Buckingham canal receives the industrial effluents as well as domestic sewage from the human settlements in its vicinity. During the period of northeast (NE) monsoon and seldom during southwest (SW) monsoon, these two backwaters discharge considerable amount of freshwater to the coastal milieu for a period of two to three months. The Edaiyur backwater mouth remains open throughout the year due to dredging activities. The Sadras backwater receives the domestic discharge of the Kalpakkam Township, whereas, the Edaiyur backwater receives the agricultural waste from the nearby cultivable lands. This part (Tamil Nadu) of the peninsular India receives bulk of its rainfall (~65 %) from NE monsoon. The average rainfall at Kalpakkam is about 1,250 mm. With the stoppage of monsoon, a sand bar is formed between the Sadras backwater and sea due to the littoral drift, which is a prominent phenomenon in the east coast of India, resulting in a situation where in the inflow of low saline water from the backwaters to sea is stopped. The township has a population of about 50,000. Two villages inhabited by fishermen are located adjoining both sides of the township having sizable population. Mamallapuram, one of the UNESCO world heritage sites, is situated at the north of Kalpakkam coast. Palar River, which gets opened to the coast only during the NE monsoon period, is located at the southern end of this coast. According to climatology of this area the whole year has been divided into three seasons viz: (i) Post-monsoon/Summer (February–May), (ii) pre-
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monsoon or SW monsoon (June–September), and (iii) Monsoon or NE monsoon (October–January) (Nair and Ganapathy 1983; Satapathy 1996). Methodology Surface sediment samples (0–5 cm) were collected at ten locations from five transects perpendicular to Palar River (P1 and P2), Sadras backwater (S1 and S2), MAPS Jetty (J1 and J2), Edaiyur (E1 and E2) backwater, and Mamallapuram (M1 and M2) (Fig. 1). The two sampling stations at each transect were located at a distance of 0.5 and 5 km from the shore. Sediment samples were collected during September 2012 by using a Van Veen grab sampler. Samples were preserved at 4 °C in sealed plastic bags prior to processing for further analysis. For the determination of sand, silt, and clay, 20 g of sediment samples were washed with 30 % H2O2 and 1 N HCl to remove organic matter and carbonate, respectively (Van Andel and Postma 1954). Then the samples were washed in a 230 ASTM sieve (mesh size, 63 μm) repeatedly till all the silt and clay were removed. The washings were collected in a 1,000 ml measuring cylinder, which were further categorized as silt and clay by pipette analysis method (Carver 1971; Krumbain and Petti John 1938). The material retained in the sieve was dried and weighed for sand fraction. For analysis of P, samples were freeze dried and powdered by a mixture mill (Retsch make). Trace metal- or analytical-grade chemicals (Fisher Scientific) were used for preparation of reagents. Milli-Q (Millipore) water was used for all the analytical procedures. Working standard solutions were prepared from NIST standard solution (1,000 mg/l) of phosphate (Merck-Certipur). Certified reference material (CRM) sediment, CRM 7001 from the CZECH Metrology Institute, Czech Republic, was used to find out the analytical and instrument accuracy of the methods. The recovery was found to be 95 % of the certified value. P fractions in the sediments have been characterized by their differential solubility in various chemical extractants. The sequential P extraction method used in the present study determined P distribution in five different phases of sedimentary P (Aydin et al. 2009; Frankowski et al. 2002). Extraction of the sediment samples was carried out sequentially with 1.0 M NH4Cl, 1.0 M NH4F, 0.1 M NaOH, and 1.0 M H2SO4 for LoP, AlP, FeP, and CaP, respectively. TP in the sediment was determined by igniting the sediment at
550±10 °C in a muffle furnace followed by extraction of P with 1 N H2SO4 for 16 h (Wildung and Shmidt 1973). The total inorganic P (IP) is the sum of all inorganic-bound P. Organic P was calculated as the difference between TP and IP (Sanyal and De Dutta 1991). The phosphate in the extracts was estimated with the phosphmolybadate blue method (Grasshoff et al. 1999). Organic carbon was analyzed by using Shimadzu TOC analyzer (model, TOC VCP-H-SSM-500). Correlation analysis was carried out using STATISTICA (ver. 7) from Statsoft. In order to find out the distribution pattern of P, contour maps were plotted using Surfer (ver. 8.02) from Golden software.
Results Sedimentary parameters Results (range, average±standard deviation) showed that sand, silt, and clay content varied from 74.4 to 93.3 % (average, 83.2±6.4), 0.9 to 17.7 % (average, 8.9±4.9), and 2.6 to 12.9 % (average, 8.2±3.7), respectively (Fig. 2). The spatial distribution of sand, silt and clay showed that sand percentage gradually increased from station P1 to M1 (Figs. 3 and 4); whereas, it did not show any decreasing or increasing trend from P2 to M2. Similarly, a reverse trend as that of sand was observed for silt and clay which gradually decreased from P1 to M1 (Fig. 3). Relatively high mud (silt+clay) percentage was observed at Palar transect. The sand percentage was found to be higher at near-shore stations (0.5 km) than that of the offshore stations (5 km). The organic carbon content in the surface sediment ranged from 0.1 to 0.9 % with an average of 0.3±0.3 %. Though it did not show any particular spatial trend, relatively high values were observed at Palar and Mamallapuram transects (Fig. 5). OC content showed a weak positive correlation with clay content (Table 1) indicating the role of fine grained sediment in regulating OC fraction of sediment. Spatial distribution of different forms of P The concentrations of different forms of P and their spatial distribution in the surface sediments of Kalpakkam coast are depicted in Figs. 6 and 7, respectively. The highest and the lowest value of LoP were recorded at M2 (108.7 mg/kg) and E2 (27.5 mg/kg), respectively. Its average contribution to TP was 7.5 % at
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35 10.5
Chennai
30
Kalpakkam
10
Pondicherry
25
INDIA 9.5
Tamil Nadu
20
Madurai
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15 10 70
75
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12.52
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M1 M2
Kunnathur
E1
IGCAR
E2 MAPS Vengapakkam
BHAVINI
J1
Sadras back water
S1 S2
Palar River
12.48
12.49
12.5
J2
P1
80.15 80.16 80.17 80.18 80.19 Fig. 1 Study area showing the sampling locations
P2
80.2
80.21 80.22 80.23 80.24 80.25
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Mean Mean±SD Min-Max
80
Percentage (%)
60
40
20
0
Sand
Silt
Clay
OC
Fig. 2 Box and whisker diagram showing the percentage contribution of sand, silt, clay, and organic carbon (OC) content of surface sediments of Kalpakkam coast
63.6±27.2 mg/kg. Except Edaiyur transect, the LoP values increased towards off shore region. AlP ranged from 21.9 to 84.8 mg/kg with an average of 39.6± 17.5 mg/kg during the study. It contributed 5.3 % of TP content. The highest and the lowest values of AlP were observed at P2 and S1, respectively. FeP contents varied between 40.5 and 306 mg/kg with an average of
18.7±11.7 mg/kg. The spatial variation of FeP increased toward offshore except at Sadras and MAPS Jetty, which showed reverse trends. FeP constituted 2.1 % of the TP content. CaP was found to be the dominant form of fraction of P in this region, with most of the value exceeding 70 % (average, 68.7 %) of TP. CaP varied between 154.6 and 1,331.8 mg/kg with an average of
100%
80%
60%
Clay Silt
40%
Sand
20%
0% P1
S1
J1
E1
M1
P2
S2
J2
E2
M2
Fig. 3 Percentage contribution of sand, silt and clay at different sampling locations (P Palar, S Sadras, J MAPS Jetty, E Edaiyur, M Mamallapuram)
Environ Monit Assess Mamallapuram
Mamallapuram
B 12.53
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MAPS Vengapakkam BHAVINI
Palar River
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Palar River
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Sadras back water
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Vengapakkam BHAVINI
Kunnathur
Sadras back water
IGCAR
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Palar River
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Sadras back water
12.52
IGCAR
12.51
Kunnathur
12.48
12.49
12.5
12.51
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A
80.15
Fig. 4 Spatial distribution of sand (a), silt (b), clay (c), and organic carbon (d) in the surface sediments of Kalpakkam coast
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Organic carbon (%) 1.00 0.80 0.60 0.5 km 0.40
5 km
0.20 0.00 P
S
J
E
M
Fig. 5 Variation in organic carbon content (%) at different sampling transects (P Palar, S Sadras, J MAPS Jetty, E Edaiyur, M Mamallapuram)
737.3 ± 463.1 mg/kg. The highest and the lowest values were observed at M2 and E2, respectively. The spatial distribution of CaP fraction showed decreasing trend towards offshore except in the Mamallapuram transect. The IP content was found to be the dominant portion as compared with the OP content at this location. It varied from 216.2 to 1,508 mg/kg (average, 59.2±488.6 mg/kg) and contributed 83.7 % of the TP. The spatial variation showed same as CaP. OP contents (average, 16.3 %)
varied from 6.1 to 337 mg/kg (138.6±110.4 mg/kg). The spatial variation showed increasing trend toward offshore except in the Edaiyur transect. The TP contents ranged from 268.5 to 1,839.0 mg/kg with an average of 997.8±535.4 mg/kg. The highest and the lowest values of TP were observed at M2 and E2, respectively. The order of abundance of the major forms of P in the marine surface sediments of Kalpakkam was as follows: CaP>OP>LoP>AlP>FeP (Fig. 8).
Table 1 Correlation matrix exhibiting associations between different fractions of phosphorus and other sedimentary parameters in coastal sediments off Kalpakkam Variables
LoP
AlP
FeP
CaP
IP
OP
TP
Sand
Silt
Clay
LoP
1.000
AlP
0.599***
1.000
FeP
0.433
0.615***
CaP
0.586***
0.092
0.528
IP
0.643**
0.171
0.570***
0.996*
1.000
OP
0.773*
0.802*
0.391
0.264
0.331
1.000
TP
0.746**
0.322
0.601***
0.964*
0.981*
0.508
Sand
−0.265
−0.304
−0.815*
−0.362
−0.388
−0.137
−0.382
1.000
Silt
0.458
0.647**
0.923*
0.361
0.413
0.383
0.456
−0.827*
1.000
Clay
−0.149
−0.286
0.238
0.145
0.124
−0.263
0.059
−0.680**
0.152
1.000
OC
0.457
0.890*
0.412
−0.030
0.038
0.724*
0.184
−0.058
0.523
0.572***
OC
1.000 1.000
1.000
1.000
*Values are with a significance level of 0.01; **values are with a significance level of 0.05; ***values are with a significance level of 0.1
Environ Monit Assess 2000 1800
Mean Mean±SD Min-Max
1600
Concentration (mg/kg)
1400 1200 1000 800 600 400 200 0 LoP
AlP
FeP
CaP
IP
OP
TP
Fig. 6 Box and whisker diagram showing the variations in loosely bound P (LoP), calcium-bound P (CaP), iron-bound P (FeP), aluminumbound P (AlP), inorganic P (IP), organic P (OP), and total P (TP) content in surface sediments of Kalpakkam coast
Discussion Coastal sediment at Kalpakkam was found to be sand dominated with relatively low silt and clay content corroborating earlier observations from this locality (Satpathy et al. 2012). Absence of perennial riverine system, which generally brings silt and clay into the coastal waters, could be attributed to the above observation. Similarly, relatively high sand contents observed at near-shore locations as compared with that of the offshore locations could be attributed to the continuous wave action and churning of the shallow water near the coast, which prohibits the settlement of fine particles. The range of OC content observed at this location was comparable to the values reported from Krishna, Godavari Basin, and Tuticorin shelf region of the Gulf of Mannar (Mazumdar et al. 2007; Sundararajan and Srinivasalu 2010). Marginally higher OC content observed at Mamallapuram and Palar River transects could be ascribed to the anthropogenic input due to tourist activities and riverine input during the NE monsoon period, respectively. P concentration of the sea water is regulated by P release from the sediments which is further dependent on the contents of different P fractions. However, all of the P fractions are not released from the sediments to be easily available for the biota (Aydin et al. 2009). In the present study, CaP emerged as the dominant fraction of
P present at this location, which has also been reported for other coastal waters (Paludan and Morris 1999; Andrieux-Loyer and Aminot 2001). Generally, CaP is a highly stable mineral in alkaline environment (Diaz et al. 2006). In coastal sediments, P associated with Ca is present in solids of various types and origins. Though there could be two groups of Ca-associated P such as: (i) the detrital fluoroapatite of igneous and metamorphic origin and (ii) CaP in biogenic skeletal debris and carbonate fluoroapatite precipitations (Ruttenberg 1992; Berner et al. 1993), the CaP here represented all forms. Authigenic and biogenic CaP in sediments have been shown to represent a sink for reactive P (Ruttenberg 1992; Berner et al. 1993). Those forms like detrital are almost insoluble under the physicochemical conditions of marine waters and remains stable under both oxidizing and reducing conditions and cannot be released again from the sediment to the overlying water (Williams et al. 1980; Gonsiorczyk et al. 1998). Thus, P concentration in this extraction step will not be bioavailable so easily, being considered permanent burial (Jin et al. 2006). Therefore, knowledge of the level of CaP concentration is useful to evaluate the proportion of bioavailable P in coastal areas. It is well known that calcite is produced at high salinities through precipitation reactions and biological activity, both in estuarine and marine environments, which forms an adsorption substrate for dissolved phosphate (Coelho et al. 2004).
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80.2
12.53 12.52 12.51
80.25
80.15
80.25
Palar River
80.25
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IGCAR
12.5
BHAVINI
Palar River
80.15
Kunnathur
80.2
80.25
IGCAR
MAPS Vengapakkam
12.48
12.49
Sadras back water
12.49
80.2
80.2
Mamallapuram
MAPS Vengapakkam
12.48
Palar River
12.48
Sadras back water
12.5
BHAVINI
80.15
G
Kunnathur
MAPS Vengapakkam
Sadras back water
12.5
80.2
12.53
IGCAR
12.51
Kunnathur
Vengapakkam BHAVINI
12.48
Palar River
80.15
IGCAR
MAPS
12.49
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80.25
12.52
12.53 12.52 12.51 12.5
BHAVINI
Kunnathur
Mamallapuram
F
E
12.49
Vengapakkam
12.49
Palar River
80.15
Mamallapuram
IGCAR
MAPS
12.5
Vengapakkam BHAVINI
Kunnathur
BHAVINI
Palar River
80.25
12.52
MAPS
Sadras back water
80.2
IGCAR
12.51
Kunnathur
12.48
12.49
Palar River
80.15
D
12.53
12.53 12.52 12.51
BHAVINI
12.5
Vengapakkam
Mamallapuram
Mamallapuram
C
Sadras back water
MAPS
12.48
12.52 12.51 12.5 12.49
IGCAR
Kunnathur
12.48
Mamallapuram
B
Sadras back water
12.53
Mamallapuram
A
80.15
80.2
80.25
Fig. 7 Spatial distribution of a loosely bound P (LoP), b calcium-bound P (CaP), c iron-bound P (FeP), d aluminum-bound P (AlP), e inorganic P (IP), f organic P (OP), and g total P (TP) in the surface sediments of Kalpakkam coast
This could be a plausible reason behind the observed dominance of CaP in the study area, as the coastal water at this location is sparsely influenced by freshwater input throughout the year. Moreover, several authors have reported that the dominance of CaP could also be due to the transformation of OP into authigenic fluorapatite during microbial decomposition (Anshumali and Ramanathan 2007; Katsaounos et al. 2007; Hou et al. 2009). Similarly, sedimentary organic matter decomposed in situ may combine with CaCO3 from calcareous phytoplankton to form CaP (Frankowski et al. 2002), which could also contribute to its dominance in coastal areas. P bound to Fe and Al through chemisorption (Guo et al. 2000) acts as indicator of algal available P and can be used for the estimation of available P in the sediments (Kaiserli et al. 2002; Wang et al. 2006). These P fractions could be released for the growth of phytoplankton
when anoxic conditions prevail at sediment–water interface (Zhou et al. 2001). However, according to Hedley et al. (1982), this fraction is less plant available and usually associated with humic compounds and amorphous and crystalline Al and Fe oxides. Some studies have indicated that Fe/Al–P was the major sink for the available P and was in equilibrium with some other fractions (Rose et al. 2010). Moreover, P fractions in surface sediments were in a dynamic equilibrium with overlying water (Penn et al. 1995), which can exchange P with each other under certain environmental conditions (Rydin 2000). In general, dominance of FeP and AlP has been reported from areas of high freshwater influence (Coelho et al. 2004) as P gets adsorbed to Fe and Al oxides present in the freshwater (Zwolsman 1994). Thus, low FeP and AlP contents observed could be attributed to the above reason. Furthermore, as a general rule, concentrations and proportions of FeP
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100%
80% OP 60%
CaP FeP
40%
AlP LoP
20%
0% P1
S1
J1
E1
M1
P2
S2
J2
E2
M2
Fig. 8 Percentage contribution of loosely bound P (LoP), calcium-bound P (CaP), iron-bound P (FeP), aluminum-bound P (AlP), inorganic P (IP), organic P (OP), and total P (TP) in surface
sediments at different locations of Kalpakkam coast (P Palar, S Sadras, J MAPS Jetty, E Edaiyur, M Mamallapuram)
and AlP will be higher in the finer sediment (Andrieux and Aminot 1997). Stone and English (1993) in a specific study of Fe/Al–P concentrations in different grainsize fractions have observed an inverse relationship between FeP and AlP concentrations and particle size. The present stud also corroborated with the observations of Stone and English (1993) as FeP and AlP showed positive correlation (r=0.647 and r=0.923, respectively) with silt content. Moreover, FeP showed a strong negative correlation (r=−0.815) with sand. Thus, the low levels of Fe/Al–P found in this coastal area were therefore attributed to the sandy nature of the sediments. Concentration of loosely adsorbed P (also called water soluble, labile, exchangeable+carbonate-associated, and hydrolyzed P) directly determines the bioavailability of P (Zhou et al. 2001). It gives an estimate of the immediately available P for consumption. Contribution of LoP was the third highest in the present investigation. LoP showed positive correlations (p≥0.005) with OP, IP, and TP, which showed that it maintained a constant proportion with all these fractions. Some of the studies have reported increased LoP with increase in fine grain particles (Andrieux and Aminot 1997). Though there was no significant correlation between LoP and mud fractions (silt+clay) in the present instance, a relatively high LoP content was observed toward offshore locations (average LoP content, 73.9 mg/kg) as compared with the near shore locations (average LoP content, 54.3 mg/kg). The above observation could be attributed
to the fact that average mud percentage of the offshore and near shore locations was 18.5 and 15.8, which indicated a general increase in LoP content with increase in mud content. OP in marine sediment includes apatite bounded phosphate and biochemical components such as nucleic acid, lipid, and sugar that are bound to P. (De Groot 1990). According to Berner (1980), OP has been considered as a source of dissolved phosphate present in interstitial water in sediment due to bacterial regeneration. Increase in organic matter with that of fine sediment fractions leading to increase in OP has been reported from some areas (Salomons and Gerritse 1981). In the present investigation, relatively high OP contents observed at offshore locations could be attributed to the comparatively high mud fraction and OC content found at those locations. A strong positive correlation (r=0.724; p≥0.001) found between OP and OC further supported the above observations. Contribution of OP to TP was the second highest in the present study, which suggested that mineralization of the phosphate-containing organic matter at the sediment surface (Lee et al. 1977) is the predominant process at this location. Similar phenomenon has also been reported from the coastal waters of South India (Nair et al. 1993). Even though, TP is not representative of the reactive fraction of P in sediments, it is usually the only component that can be compared with
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existing data based on lesser defined geochemical determinations. TP showed positive correlation with almost all the inorganic fractions (LoP, r= 0.964; FeP, r= 0.601; CaP, r=0.964; and IP, r= 0.981), whereas it did not yield any significant correlation with the organic fraction. TP as well as the OP content observed in the present study were comparable to that of the other marine environments of India and elsewhere (Renjith et al. 2011; Aydin et al. 2009). Considering the important role of P in marine productivity and its existence in various fractions (soluble and nonsoluble), the notion of bioavailability must be taken into account. Bioavailable P in sediments corresponds to the amount that can be released easily, for algal growth (Sonzogni et al. 1982). Though bioavailability can be assessed by bioassays (De Jonge et al. 1993), knowledge of the various forms of P is useful in determining the upper limit of the potentially bioavailable P. CaP, which remains insoluble in marine conditions and FeP and AlP, which cannot be released unless anoxic environment prevails, practically do not contribute to the bioavailable fraction. The present study area being an open shallow coastal environment that supports turbulence and mixing, the CaP and FeP and AlP fractions cannot be bioavailable under normal circumstances. However, LoP can progressively be released when the phosphate concentration in the water column is lower than that in the pore water. Similarly, OP can be progressively bioavailable due to remineralization. Therefore, initially only LoP and OP should be considered as potentially bioavailable fractions (Andrieux and Aminot 1997), which contributed from 5 to 44 % (average, 23.8 %) of the TP during the present study.
Conclusions This paper presents baseline report on P speciation in the coastal sediments of Kalpakkam. The coastal sediment at Kalpakkam was found to be sand dominated. The CaP fraction comprised the largest percent (68.7 %) to the total sedimentary P followed by the organic fraction (16.3 %). Bioavailable P fractions ranged from 5 to 44 % of the TP content. Relatively high LoP content was observed at the offshore locations with comparatively high mud percentage as compared with the nearshore locations. As FeP and AlP concentrations are directly proportional to the amount of fine-grain
sediment, the low levels of Fe/Al–P found in this coastal area were therefore attributed to the sandy nature of the sediments. The order of abundance of the major forms of P in the marine surface sediments of Kalpakkam was as follows: CaP>OP>LoP>AlP>FeP. The present observation encourages future intensive research focusing to elucidate the role of different seasons in speciation of P in Kalpakkam coastal sediment.
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Phosphorus speciation in the marine sediment of Kalpakkam coast, southeast coast of India S. N. Bramha & A. K. Mohanty & R. K. Padhi & S. N. Panigrahi & K. K. Satpathy
Received: 14 January 2014 / Accepted: 14 May 2014 # Springer International Publishing Switzerland 2014
Abstract A study was carried out at Kalpakkam coast to find out the distribution of various fractions of phosphorus (P) in the marine sediment during pre-northeast monsoon period. Samples were collected from ten locations covering ~80 km2 of the inner-shelf region. Sedimentary parameters such as sand, silt, clay, and organic carbon percentage were analyzed in order to find out their relation with various P fractions. The sediment was found to be predominantly sandy in nature with low silt and clay content. Among all the fractions (loosely bound (LoP), calcium bound (CaP), iron bound (FeP), aluminum bound (AlP), and organic (OP)), CaP fraction constituted the largest portion (68.7 %) followed by organic fraction (16.3 %). The bioavailable P fractions ranged from 5 to 44 % of the total P (TP) content. Relatively high LoP content was observed at the offshore locations with comparatively high mud percentage as compared with the near-shore locations. As FeP and AlP concentrations were directly proportional to the amount of fine-grain sediment, the low levels of these fractions found in this coastal area were therefore attributed to the sandy nature of the sediments. The order of abundance of the major forms of P in the surface sediments of Kalpakkam coast was as follows: CaP>OP>LoP>AlP>FeP.
S. N. Bramha : A. K. Mohanty : R. K. Padhi : S. N. Panigrahi : K. K. Satpathy (*) Environment and Safety Division, Indira Gandhi Centre for Atomic Research, Kalpakkam, Tamil Nadu, India e-mail: [email protected]
Keywords Sequential extraction . Phosphorus fractions . Coastal sediment . Bay of Bengal
Introduction Sediment plays an important role in the overall nutrient dynamics of the coastal marine ecosystem. It has been observed that the sediment of the coastal system can act as an internal source of nutrients for the overlying water column. Among other nutrients, phosphorus (P) has been recognized as the most essential and critical nutrient in the terrestrial and coastal aquatic environments (Harrison et al. 1990; Bauerfeind et al. 1990) and is thought to control marine primary productivity over a geological time scales (Ruttenberg 2004). It is an essential constituent of tissues and cells and required for the formation of nucleic acids and energy-carrying molecules such as adenosine triphosphate (ATP). The biogeochemical cycle of the P in the marine sediment is mainly the quantity of inorganic and organic P and the P dynamics are controlled by a combination of physical, chemical, and biological properties and processes (Frossard et al. 2000; Reddy et al. 2005). By contrast, the other important micronutrients, nitrogen and sulfur, are predominately present in organic forms in sediments, and their turnover is therefore primarily controlled by biological and biochemical processes (Golterman 2004; Schimel and Bennett 2004; Bünemann and Condron 2007). Phosphates, the most abundant form of the P in the environment, are readily available in detergents,
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fertilizers, and pesticides. Consequently, monitoring of P content in the water and sediment is essential to control and avoid eutrophication of the coastal aquatic environment. Its current abundance in the earth crust is approximately 0.12 %; however, almost all P on the earth crust found in the form of minerals including apatite (chloro and floro), vivianite, wavellite, and phosphorite. Apatite {Ca (PO4)3[F, OH, or Cl]}, the largest reservoir of phosphate on Earth, is relatively insoluble in water. Most of the P in the current natural environment is present in particulate form and is not biologically available, thereby limiting primary production. The sorption of phosphate on sediments has major influences on transport, degradation, and ultimate fate of P in marine ecosystems. P is found in the nature in various forms which includes: (1) mineral forms; (2) organic forms such as phospholipids, nucleic acids, proteins, polysaccarides, nucleotides cofactors, and phosphonates; (3) dissolved in organic forms such as pentavalent, trivalent, or univalent dissolve species; (4) gaseous form in the III oxidation state; and (5) particulate or colloidal forms (Aydin et al. 2009; Frankowski et al. 2002). P in marine sediments primarily consists of inorganic forms such as: loosely bounded P (LoP), aluminum Al-bounded P (AlP), iron (Fe)-bounded P (FeP), calcium (Ca)-bounded P (CaP), and organic P (OP). These fractions of P shows a wide range of variations, which are controlled by a number of factors such as rate of sedimentation, sediment type, amount and type of organic matter, intensity of mineralization of organic matters in the sediment and water column, redox conditions in the sediments and water depth. Al, Fe, and Ca contents in the sediments are also known to influence the P fractions (Sundareshwar and Morris 1999; Jennifer et al. 2004,). Different form of P in the sediments can provide valuable information on the origin of the P, the degree of pollution from anthropogenic activities, the bioavailability, and also the burial and digenesis of P in sediments (Andrieux and Aminot 1997; Jensen et al. 1988; Schenau and De Lange 2001). A considerable amount of work on physicochemical characteristics of Kalpakkam coastal waters have been carried out (Satpathy et al. 2010, 2011). However, observations on sediment nutrients and other physicochemical characteristics are lacking. Moreover, information about concentration and variability of different P fractions in coastal sediments, particularly from this region is not available. Thus, the present study was designed with the following objectives: (1) to investigate the distribution
pattern of major P species in the surface sediments, (2) to find out the bioavailable fraction of P and (3) contribution of organic P (OP) to the total P (TP) content.
Materials and methods Study area Kalpakkam (12° 33′ N Lat. and 80° 11′ E Long.) is situated about 80 km south of Chennai city (Fig. 1). At present, a nuclear power plant (Madras Atomic Power Station (MAPS)) and a desalination plant are located near the coast. MAPS uses seawater at a rate of 35 m3s−1 for condenser-cooling purpose. After extracting the heat, the heated seawater is released into the sea. Two backwaters, namely the Edaiyur and the Sadras backwater systems are important features of this coast. These backwaters are connected to the Buckingham canal, which runs parallel to the coast. Buckingham canal receives the industrial effluents as well as domestic sewage from the human settlements in its vicinity. During the period of northeast (NE) monsoon and seldom during southwest (SW) monsoon, these two backwaters discharge considerable amount of freshwater to the coastal milieu for a period of two to three months. The Edaiyur backwater mouth remains open throughout the year due to dredging activities. The Sadras backwater receives the domestic discharge of the Kalpakkam Township, whereas, the Edaiyur backwater receives the agricultural waste from the nearby cultivable lands. This part (Tamil Nadu) of the peninsular India receives bulk of its rainfall (~65 %) from NE monsoon. The average rainfall at Kalpakkam is about 1,250 mm. With the stoppage of monsoon, a sand bar is formed between the Sadras backwater and sea due to the littoral drift, which is a prominent phenomenon in the east coast of India, resulting in a situation where in the inflow of low saline water from the backwaters to sea is stopped. The township has a population of about 50,000. Two villages inhabited by fishermen are located adjoining both sides of the township having sizable population. Mamallapuram, one of the UNESCO world heritage sites, is situated at the north of Kalpakkam coast. Palar River, which gets opened to the coast only during the NE monsoon period, is located at the southern end of this coast. According to climatology of this area the whole year has been divided into three seasons viz: (i) Post-monsoon/Summer (February–May), (ii) pre-
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monsoon or SW monsoon (June–September), and (iii) Monsoon or NE monsoon (October–January) (Nair and Ganapathy 1983; Satapathy 1996). Methodology Surface sediment samples (0–5 cm) were collected at ten locations from five transects perpendicular to Palar River (P1 and P2), Sadras backwater (S1 and S2), MAPS Jetty (J1 and J2), Edaiyur (E1 and E2) backwater, and Mamallapuram (M1 and M2) (Fig. 1). The two sampling stations at each transect were located at a distance of 0.5 and 5 km from the shore. Sediment samples were collected during September 2012 by using a Van Veen grab sampler. Samples were preserved at 4 °C in sealed plastic bags prior to processing for further analysis. For the determination of sand, silt, and clay, 20 g of sediment samples were washed with 30 % H2O2 and 1 N HCl to remove organic matter and carbonate, respectively (Van Andel and Postma 1954). Then the samples were washed in a 230 ASTM sieve (mesh size, 63 μm) repeatedly till all the silt and clay were removed. The washings were collected in a 1,000 ml measuring cylinder, which were further categorized as silt and clay by pipette analysis method (Carver 1971; Krumbain and Petti John 1938). The material retained in the sieve was dried and weighed for sand fraction. For analysis of P, samples were freeze dried and powdered by a mixture mill (Retsch make). Trace metal- or analytical-grade chemicals (Fisher Scientific) were used for preparation of reagents. Milli-Q (Millipore) water was used for all the analytical procedures. Working standard solutions were prepared from NIST standard solution (1,000 mg/l) of phosphate (Merck-Certipur). Certified reference material (CRM) sediment, CRM 7001 from the CZECH Metrology Institute, Czech Republic, was used to find out the analytical and instrument accuracy of the methods. The recovery was found to be 95 % of the certified value. P fractions in the sediments have been characterized by their differential solubility in various chemical extractants. The sequential P extraction method used in the present study determined P distribution in five different phases of sedimentary P (Aydin et al. 2009; Frankowski et al. 2002). Extraction of the sediment samples was carried out sequentially with 1.0 M NH4Cl, 1.0 M NH4F, 0.1 M NaOH, and 1.0 M H2SO4 for LoP, AlP, FeP, and CaP, respectively. TP in the sediment was determined by igniting the sediment at
550±10 °C in a muffle furnace followed by extraction of P with 1 N H2SO4 for 16 h (Wildung and Shmidt 1973). The total inorganic P (IP) is the sum of all inorganic-bound P. Organic P was calculated as the difference between TP and IP (Sanyal and De Dutta 1991). The phosphate in the extracts was estimated with the phosphmolybadate blue method (Grasshoff et al. 1999). Organic carbon was analyzed by using Shimadzu TOC analyzer (model, TOC VCP-H-SSM-500). Correlation analysis was carried out using STATISTICA (ver. 7) from Statsoft. In order to find out the distribution pattern of P, contour maps were plotted using Surfer (ver. 8.02) from Golden software.
Results Sedimentary parameters Results (range, average±standard deviation) showed that sand, silt, and clay content varied from 74.4 to 93.3 % (average, 83.2±6.4), 0.9 to 17.7 % (average, 8.9±4.9), and 2.6 to 12.9 % (average, 8.2±3.7), respectively (Fig. 2). The spatial distribution of sand, silt and clay showed that sand percentage gradually increased from station P1 to M1 (Figs. 3 and 4); whereas, it did not show any decreasing or increasing trend from P2 to M2. Similarly, a reverse trend as that of sand was observed for silt and clay which gradually decreased from P1 to M1 (Fig. 3). Relatively high mud (silt+clay) percentage was observed at Palar transect. The sand percentage was found to be higher at near-shore stations (0.5 km) than that of the offshore stations (5 km). The organic carbon content in the surface sediment ranged from 0.1 to 0.9 % with an average of 0.3±0.3 %. Though it did not show any particular spatial trend, relatively high values were observed at Palar and Mamallapuram transects (Fig. 5). OC content showed a weak positive correlation with clay content (Table 1) indicating the role of fine grained sediment in regulating OC fraction of sediment. Spatial distribution of different forms of P The concentrations of different forms of P and their spatial distribution in the surface sediments of Kalpakkam coast are depicted in Figs. 6 and 7, respectively. The highest and the lowest value of LoP were recorded at M2 (108.7 mg/kg) and E2 (27.5 mg/kg), respectively. Its average contribution to TP was 7.5 % at
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63.6±27.2 mg/kg. Except Edaiyur transect, the LoP values increased towards off shore region. AlP ranged from 21.9 to 84.8 mg/kg with an average of 39.6± 17.5 mg/kg during the study. It contributed 5.3 % of TP content. The highest and the lowest values of AlP were observed at P2 and S1, respectively. FeP contents varied between 40.5 and 306 mg/kg with an average of
18.7±11.7 mg/kg. The spatial variation of FeP increased toward offshore except at Sadras and MAPS Jetty, which showed reverse trends. FeP constituted 2.1 % of the TP content. CaP was found to be the dominant form of fraction of P in this region, with most of the value exceeding 70 % (average, 68.7 %) of TP. CaP varied between 154.6 and 1,331.8 mg/kg with an average of
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Fig. 4 Spatial distribution of sand (a), silt (b), clay (c), and organic carbon (d) in the surface sediments of Kalpakkam coast
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Organic carbon (%) 1.00 0.80 0.60 0.5 km 0.40
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M
Fig. 5 Variation in organic carbon content (%) at different sampling transects (P Palar, S Sadras, J MAPS Jetty, E Edaiyur, M Mamallapuram)
737.3 ± 463.1 mg/kg. The highest and the lowest values were observed at M2 and E2, respectively. The spatial distribution of CaP fraction showed decreasing trend towards offshore except in the Mamallapuram transect. The IP content was found to be the dominant portion as compared with the OP content at this location. It varied from 216.2 to 1,508 mg/kg (average, 59.2±488.6 mg/kg) and contributed 83.7 % of the TP. The spatial variation showed same as CaP. OP contents (average, 16.3 %)
varied from 6.1 to 337 mg/kg (138.6±110.4 mg/kg). The spatial variation showed increasing trend toward offshore except in the Edaiyur transect. The TP contents ranged from 268.5 to 1,839.0 mg/kg with an average of 997.8±535.4 mg/kg. The highest and the lowest values of TP were observed at M2 and E2, respectively. The order of abundance of the major forms of P in the marine surface sediments of Kalpakkam was as follows: CaP>OP>LoP>AlP>FeP (Fig. 8).
Table 1 Correlation matrix exhibiting associations between different fractions of phosphorus and other sedimentary parameters in coastal sediments off Kalpakkam Variables
LoP
AlP
FeP
CaP
IP
OP
TP
Sand
Silt
Clay
LoP
1.000
AlP
0.599***
1.000
FeP
0.433
0.615***
CaP
0.586***
0.092
0.528
IP
0.643**
0.171
0.570***
0.996*
1.000
OP
0.773*
0.802*
0.391
0.264
0.331
1.000
TP
0.746**
0.322
0.601***
0.964*
0.981*
0.508
Sand
−0.265
−0.304
−0.815*
−0.362
−0.388
−0.137
−0.382
1.000
Silt
0.458
0.647**
0.923*
0.361
0.413
0.383
0.456
−0.827*
1.000
Clay
−0.149
−0.286
0.238
0.145
0.124
−0.263
0.059
−0.680**
0.152
1.000
OC
0.457
0.890*
0.412
−0.030
0.038
0.724*
0.184
−0.058
0.523
0.572***
OC
1.000 1.000
1.000
1.000
*Values are with a significance level of 0.01; **values are with a significance level of 0.05; ***values are with a significance level of 0.1
Environ Monit Assess 2000 1800
Mean Mean±SD Min-Max
1600
Concentration (mg/kg)
1400 1200 1000 800 600 400 200 0 LoP
AlP
FeP
CaP
IP
OP
TP
Fig. 6 Box and whisker diagram showing the variations in loosely bound P (LoP), calcium-bound P (CaP), iron-bound P (FeP), aluminumbound P (AlP), inorganic P (IP), organic P (OP), and total P (TP) content in surface sediments of Kalpakkam coast
Discussion Coastal sediment at Kalpakkam was found to be sand dominated with relatively low silt and clay content corroborating earlier observations from this locality (Satpathy et al. 2012). Absence of perennial riverine system, which generally brings silt and clay into the coastal waters, could be attributed to the above observation. Similarly, relatively high sand contents observed at near-shore locations as compared with that of the offshore locations could be attributed to the continuous wave action and churning of the shallow water near the coast, which prohibits the settlement of fine particles. The range of OC content observed at this location was comparable to the values reported from Krishna, Godavari Basin, and Tuticorin shelf region of the Gulf of Mannar (Mazumdar et al. 2007; Sundararajan and Srinivasalu 2010). Marginally higher OC content observed at Mamallapuram and Palar River transects could be ascribed to the anthropogenic input due to tourist activities and riverine input during the NE monsoon period, respectively. P concentration of the sea water is regulated by P release from the sediments which is further dependent on the contents of different P fractions. However, all of the P fractions are not released from the sediments to be easily available for the biota (Aydin et al. 2009). In the present study, CaP emerged as the dominant fraction of
P present at this location, which has also been reported for other coastal waters (Paludan and Morris 1999; Andrieux-Loyer and Aminot 2001). Generally, CaP is a highly stable mineral in alkaline environment (Diaz et al. 2006). In coastal sediments, P associated with Ca is present in solids of various types and origins. Though there could be two groups of Ca-associated P such as: (i) the detrital fluoroapatite of igneous and metamorphic origin and (ii) CaP in biogenic skeletal debris and carbonate fluoroapatite precipitations (Ruttenberg 1992; Berner et al. 1993), the CaP here represented all forms. Authigenic and biogenic CaP in sediments have been shown to represent a sink for reactive P (Ruttenberg 1992; Berner et al. 1993). Those forms like detrital are almost insoluble under the physicochemical conditions of marine waters and remains stable under both oxidizing and reducing conditions and cannot be released again from the sediment to the overlying water (Williams et al. 1980; Gonsiorczyk et al. 1998). Thus, P concentration in this extraction step will not be bioavailable so easily, being considered permanent burial (Jin et al. 2006). Therefore, knowledge of the level of CaP concentration is useful to evaluate the proportion of bioavailable P in coastal areas. It is well known that calcite is produced at high salinities through precipitation reactions and biological activity, both in estuarine and marine environments, which forms an adsorption substrate for dissolved phosphate (Coelho et al. 2004).
Environ Monit Assess
80.2
12.53 12.52 12.51
80.25
80.15
80.25
Palar River
80.25
12.53 12.52 12.51
IGCAR
12.5
BHAVINI
Palar River
80.15
Kunnathur
80.2
80.25
IGCAR
MAPS Vengapakkam
12.48
12.49
Sadras back water
12.49
80.2
80.2
Mamallapuram
MAPS Vengapakkam
12.48
Palar River
12.48
Sadras back water
12.5
BHAVINI
80.15
G
Kunnathur
MAPS Vengapakkam
Sadras back water
12.5
80.2
12.53
IGCAR
12.51
Kunnathur
Vengapakkam BHAVINI
12.48
Palar River
80.15
IGCAR
MAPS
12.49
Sadras back water
80.25
12.52
12.53 12.52 12.51 12.5
BHAVINI
Kunnathur
Mamallapuram
F
E
12.49
Vengapakkam
12.49
Palar River
80.15
Mamallapuram
IGCAR
MAPS
12.5
Vengapakkam BHAVINI
Kunnathur
BHAVINI
Palar River
80.25
12.52
MAPS
Sadras back water
80.2
IGCAR
12.51
Kunnathur
12.48
12.49
Palar River
80.15
D
12.53
12.53 12.52 12.51
BHAVINI
12.5
Vengapakkam
Mamallapuram
Mamallapuram
C
Sadras back water
MAPS
12.48
12.52 12.51 12.5 12.49
IGCAR
Kunnathur
12.48
Mamallapuram
B
Sadras back water
12.53
Mamallapuram
A
80.15
80.2
80.25
Fig. 7 Spatial distribution of a loosely bound P (LoP), b calcium-bound P (CaP), c iron-bound P (FeP), d aluminum-bound P (AlP), e inorganic P (IP), f organic P (OP), and g total P (TP) in the surface sediments of Kalpakkam coast
This could be a plausible reason behind the observed dominance of CaP in the study area, as the coastal water at this location is sparsely influenced by freshwater input throughout the year. Moreover, several authors have reported that the dominance of CaP could also be due to the transformation of OP into authigenic fluorapatite during microbial decomposition (Anshumali and Ramanathan 2007; Katsaounos et al. 2007; Hou et al. 2009). Similarly, sedimentary organic matter decomposed in situ may combine with CaCO3 from calcareous phytoplankton to form CaP (Frankowski et al. 2002), which could also contribute to its dominance in coastal areas. P bound to Fe and Al through chemisorption (Guo et al. 2000) acts as indicator of algal available P and can be used for the estimation of available P in the sediments (Kaiserli et al. 2002; Wang et al. 2006). These P fractions could be released for the growth of phytoplankton
when anoxic conditions prevail at sediment–water interface (Zhou et al. 2001). However, according to Hedley et al. (1982), this fraction is less plant available and usually associated with humic compounds and amorphous and crystalline Al and Fe oxides. Some studies have indicated that Fe/Al–P was the major sink for the available P and was in equilibrium with some other fractions (Rose et al. 2010). Moreover, P fractions in surface sediments were in a dynamic equilibrium with overlying water (Penn et al. 1995), which can exchange P with each other under certain environmental conditions (Rydin 2000). In general, dominance of FeP and AlP has been reported from areas of high freshwater influence (Coelho et al. 2004) as P gets adsorbed to Fe and Al oxides present in the freshwater (Zwolsman 1994). Thus, low FeP and AlP contents observed could be attributed to the above reason. Furthermore, as a general rule, concentrations and proportions of FeP
Environ Monit Assess
100%
80% OP 60%
CaP FeP
40%
AlP LoP
20%
0% P1
S1
J1
E1
M1
P2
S2
J2
E2
M2
Fig. 8 Percentage contribution of loosely bound P (LoP), calcium-bound P (CaP), iron-bound P (FeP), aluminum-bound P (AlP), inorganic P (IP), organic P (OP), and total P (TP) in surface
sediments at different locations of Kalpakkam coast (P Palar, S Sadras, J MAPS Jetty, E Edaiyur, M Mamallapuram)
and AlP will be higher in the finer sediment (Andrieux and Aminot 1997). Stone and English (1993) in a specific study of Fe/Al–P concentrations in different grainsize fractions have observed an inverse relationship between FeP and AlP concentrations and particle size. The present stud also corroborated with the observations of Stone and English (1993) as FeP and AlP showed positive correlation (r=0.647 and r=0.923, respectively) with silt content. Moreover, FeP showed a strong negative correlation (r=−0.815) with sand. Thus, the low levels of Fe/Al–P found in this coastal area were therefore attributed to the sandy nature of the sediments. Concentration of loosely adsorbed P (also called water soluble, labile, exchangeable+carbonate-associated, and hydrolyzed P) directly determines the bioavailability of P (Zhou et al. 2001). It gives an estimate of the immediately available P for consumption. Contribution of LoP was the third highest in the present investigation. LoP showed positive correlations (p≥0.005) with OP, IP, and TP, which showed that it maintained a constant proportion with all these fractions. Some of the studies have reported increased LoP with increase in fine grain particles (Andrieux and Aminot 1997). Though there was no significant correlation between LoP and mud fractions (silt+clay) in the present instance, a relatively high LoP content was observed toward offshore locations (average LoP content, 73.9 mg/kg) as compared with the near shore locations (average LoP content, 54.3 mg/kg). The above observation could be attributed
to the fact that average mud percentage of the offshore and near shore locations was 18.5 and 15.8, which indicated a general increase in LoP content with increase in mud content. OP in marine sediment includes apatite bounded phosphate and biochemical components such as nucleic acid, lipid, and sugar that are bound to P. (De Groot 1990). According to Berner (1980), OP has been considered as a source of dissolved phosphate present in interstitial water in sediment due to bacterial regeneration. Increase in organic matter with that of fine sediment fractions leading to increase in OP has been reported from some areas (Salomons and Gerritse 1981). In the present investigation, relatively high OP contents observed at offshore locations could be attributed to the comparatively high mud fraction and OC content found at those locations. A strong positive correlation (r=0.724; p≥0.001) found between OP and OC further supported the above observations. Contribution of OP to TP was the second highest in the present study, which suggested that mineralization of the phosphate-containing organic matter at the sediment surface (Lee et al. 1977) is the predominant process at this location. Similar phenomenon has also been reported from the coastal waters of South India (Nair et al. 1993). Even though, TP is not representative of the reactive fraction of P in sediments, it is usually the only component that can be compared with
Environ Monit Assess
existing data based on lesser defined geochemical determinations. TP showed positive correlation with almost all the inorganic fractions (LoP, r= 0.964; FeP, r= 0.601; CaP, r=0.964; and IP, r= 0.981), whereas it did not yield any significant correlation with the organic fraction. TP as well as the OP content observed in the present study were comparable to that of the other marine environments of India and elsewhere (Renjith et al. 2011; Aydin et al. 2009). Considering the important role of P in marine productivity and its existence in various fractions (soluble and nonsoluble), the notion of bioavailability must be taken into account. Bioavailable P in sediments corresponds to the amount that can be released easily, for algal growth (Sonzogni et al. 1982). Though bioavailability can be assessed by bioassays (De Jonge et al. 1993), knowledge of the various forms of P is useful in determining the upper limit of the potentially bioavailable P. CaP, which remains insoluble in marine conditions and FeP and AlP, which cannot be released unless anoxic environment prevails, practically do not contribute to the bioavailable fraction. The present study area being an open shallow coastal environment that supports turbulence and mixing, the CaP and FeP and AlP fractions cannot be bioavailable under normal circumstances. However, LoP can progressively be released when the phosphate concentration in the water column is lower than that in the pore water. Similarly, OP can be progressively bioavailable due to remineralization. Therefore, initially only LoP and OP should be considered as potentially bioavailable fractions (Andrieux and Aminot 1997), which contributed from 5 to 44 % (average, 23.8 %) of the TP during the present study.
Conclusions This paper presents baseline report on P speciation in the coastal sediments of Kalpakkam. The coastal sediment at Kalpakkam was found to be sand dominated. The CaP fraction comprised the largest percent (68.7 %) to the total sedimentary P followed by the organic fraction (16.3 %). Bioavailable P fractions ranged from 5 to 44 % of the TP content. Relatively high LoP content was observed at the offshore locations with comparatively high mud percentage as compared with the nearshore locations. As FeP and AlP concentrations are directly proportional to the amount of fine-grain
sediment, the low levels of Fe/Al–P found in this coastal area were therefore attributed to the sandy nature of the sediments. The order of abundance of the major forms of P in the marine surface sediments of Kalpakkam was as follows: CaP>OP>LoP>AlP>FeP. The present observation encourages future intensive research focusing to elucidate the role of different seasons in speciation of P in Kalpakkam coastal sediment.
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