Environmental Earth Sciences

, Volume 62, Issue 6, pp 1209–1218

Phosphorus fractions in the surface sediments of three mangrove systems of southwest coast of India

Authors

    • Department of Chemical Oceanography, School of Marine SciencesCochin University of Science and Technology
  • C. S. Ratheesh Kumar
    • Department of Chemical Oceanography, School of Marine SciencesCochin University of Science and Technology
  • K. R. Renjith
    • Department of Chemical Oceanography, School of Marine SciencesCochin University of Science and Technology
  • T. R. Gireesh Kumar
    • Department of Chemical Oceanography, School of Marine SciencesCochin University of Science and Technology
  • N. Chandramohanakumar
    • Department of Chemical Oceanography, School of Marine SciencesCochin University of Science and Technology
Original Article

DOI: 10.1007/s12665-010-0609-0

Cite this article as:
Joseph, M.M., Ratheesh Kumar, C.S., Renjith, K.R. et al. Environ Earth Sci (2011) 62: 1209. doi:10.1007/s12665-010-0609-0

Abstract

The phosphorus fractions in three tropical mangrove systems of Cochin region were analysed by sequential extraction method. Iron-bound phosphorus was the major fraction in the first two stations, while station 3 was exclusively dominated by calcium-bound phosphorus. Compared to other stations, about tenfold increase in total phosphorus content was observed at station 3. This station is a congregation of communally breeding birds, and there is accumulation of bird guano. Mineralogical analysis showed the presence of monetite, a thermodynamically metastable calcium phosphate mineral, in this unique system. The excreta and carcass of the birds in this sanctuary seems to be the reason for the formation of monetite, which is favoured by periodic fluctuations in redox potential. The high mass percentages of calcium and phosphorus by XRF and SEM–EDS analysis confirm the existence of calcium phosphate mineral at station 3. First two stations did not show any noticeable difference in phosphorus fractions and inorganic fractions constituted to about 65% of total phosphorus. But at station 3, inorganic fractions were about 92%. Low C:P ratios and low organic phosphorus content indicated active mineralisation of phosphorus at station 3. Bioavailable fractions of phosphorus at stations 1 and 2 were about 75%, whereas 98% of the total phosphorus was bioavailable at station 3. Since the bulk of the total phosphorus is bioavailable, these mangrove sediments have the potential to act as source of phosphorus to the overlying waters.

Keywords

Mangrove sedimentsPhosphorus fractionsSequential extractionGeochemistryGuanoMonetite

Introduction

Mangroves are highly productive wetland ecosystems, which play an important role in the biogeochemical cycles of the coastal environment (Jennerjahn and Ittekkot 2002; Feller et al. 2003). The biogeochemistry of mangroves is the least understood one because of their sediment complexity due to the tidal influx of allochthonous organic matter and also the input of local vegetation. Phosphorus is an essential element that limits marine primary productivity, by which it is intimately involved in marine biogeochemical cycles (Howarth et al. 1995; Tyrell 1999). Phosphorus cycle in tropical mangroves is multifarious because of the periodic flooding of sediment with both fresh and saline waters (Salcedo and Medeiros 1995). Phosphorus retention and release in mangrove sediments depends on several factors including pH, redox potential, tidal inundation, the nature of phosphorus compounds supplied to the sediment–water interface, sedimentation rate, bioturbation, diagenetic processes, etc. (Silva and Sampaio 1998; Ingall and Van Cappellen 1990; Ruttenberg and Berner 1993; Schenau and de Lange 2001).

Phosphorus fractionation in sediments can provide valuable information on the origin of phosphorus, the degree of pollution from anthropogenic activities, the bioavailability and also the burial and diagenesis of phosphorus in sediments (Andrieux and Aminot 1997; Jensen et al. 1998; Schenau and De Lange 2001). There is a conspicuous lack of information about concentrations and variability of different phosphorus fractions in intertidal sediments. The phosphorus fractions in three typical mangrove systems of Cochin region were analysed to quantify different forms of phosphorus in these systems and to find the processes leading to the fractional distribution of phosphorus.

Materials and methods

Study area

Three mangrove systems in the northern arm of Cochin estuary, southwest coast of India (09°50′N, 76°45′E), which is a Ramsar Site (No. 1214), were chosen for the present study (Fig. 1). Station 1, Puthuvyppu, is a mangrove nursery maintained by the fisheries research unit of Kerala Agricultural University, located about 100 m away from the estuarine front. It is free from sewage inputs. The dominant mangrove flora found here are Avicennia officinalis and Bruggeria gymnorhiza (Sebastian and Chacko 2006) (Fig. 2). Station 2, Murikkumpadam, is a densely populated fishermen settlement. The dominant species of this system are Acanthus ilicifolis, Rhizophora apiculata, Rhizophora mucronata, Excoeacaria agallocha, and two mangrove associates Clerodendronica and Acrostica (Sebastian and Chacko 2006). The discharge of sewage and disposal of garbage and solid waste are the main source of pollution here. These two stations form part of the island called Vypin, which is one of the most densely populated coastal zones. Station 3, Mangalavanam, is a patchy mangrove area (2.74 ha) in the heart of Cochin City. This habitat consists primarily of Avicennia officinalis with occasional patches of Acanthus ilicifolius and Rhizophora mucronata sp. (Subramaniyan 2000). This mangrove forest (Fig. 2) is the home of many exotic and rare varieties of migratory birds. Forty-one species of birds were recorded from Mangalavanam representing 12 orders and 24 families, and the most common bird species found here are little cormorant (Phalacrocroax niger) and night heron (Nychcorax nychcorax) (Jayson 2001). But the urban developmental pressure has been affecting negatively this sanctuary. The heavy vehicular traffic, siltation and waste deposition in the area and piling up of non-biodegradable waste in the water body of Mangalavanam are some of the visible signs of distress regarding the sanctuary. This is an almost closed system with a single narrow canal linking the estuary, which is the only source for tidal propagation. During low tide, the system is completely drained of water. There are very few studies on the phosphorus geochemistry in these systems.
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Fig. 1

Study area showing the mangrove stations

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Fig. 2

Mangrove vegetation in the Cochin estuary

These mangroves experience tidal inundation from the adjacent micro-tidal Cochin estuarine system. Tides at Cochin are of a mixed semi-diurnal type, with the maximum spring tide range of about 1 m (Srinivas 1999). Cochin estuary is under the profound influence of monsoon, which contributes to about 71% of the annual rainfall (Jayaprakash 2002) and accordingly three seasons are prevailing: monsoon (June–September), post-monsoon (October–January) and pre-monsoon (February–May).

Sampling and analytical methodology

Samples of water and sediments were taken from these three locations during December 2005, April 2006 and July 2006. Surface water samples were collected during high tide using a decontaminated plastic bucket. Surface sediment samples were taken from the study areas using a decontaminated plastic spoon. To a get a true representation of the system, sediment samples were collected from three different parts of each system and pooled together for analysis. Samples were transported to lab on ice and stored in a deep freezer till analysis. All the analyses were carried out in triplicates, and the average is reported.

General hydrographical parameters and nutrients of the water samples were analysed using standard methods (Grasshoff et al. 1983). Textural analysis of the sediment was done based on Stoke’s law (Krumbein and Petti John 1938). Sediment samples were air dried and finely powdered using agate mortar for further analyses. Powder X-Ray diffraction analysis was carried out to find the mineralogy of the sediments (Moore and Reynolds 1997). Total carbon, nitrogen and sulphur were determined using a Vario EL III CHNS Analyser. Sediment organic carbon was estimated by the procedure of El Wakeel and Riley modified by Gaudette and Flight (1974). Representative samples were analysed using X-ray fluorescence (XRF) for finding the major elements. Major elemental composition of the sediment in station 3 also was analysed using SEM–EDS. Thermo gravimetric analysis (TGA) was carried out to find out the loss of ignition. Iron content of the sediment was estimated using Flame AAS (Perkin Elmer 3110) after digestion using di-acid mixture (1:5 HClO4:HNO3). Accuracy of the analytical procedure was checked using standard reference material BCSS-1, which showed a recovery of 93%.

The sequential extraction scheme by Golterman (1996) using chelating agents was employed for estimating different phosphorus fractions. Compared with other methods, the chelating agents allow a specific extraction of inorganic phosphorus with less destruction of organic phosphorus (Golterman 1996). Iron-bound phosphorus (Fe-IP) was extracted with buffered Ca-EDTA/dithionite and calcium-bound fraction (Ca-IP) subsequently with Na-EDTA. In the next step, acid soluble organic phosphorus (ASOP) was extracted with H2SO4 and then alkali soluble organic phosphorus (Alk-OP) with 2 M NaOH at 90°C for 2 h. Residual organic phosphorus (ROP) was measured after 1 h K2S2O8 digestion in an acid medium. All the extractions were carried out under mild continuous shaking, and the results are expressed on the dry weight basis.

Results

Southwest monsoon showed profound influence on the study region, creating seasonal variations in the hydrographical parameters (Table 1). Salinity varied widely in the study region, and near marine character was seen during pre-monsoon season and fresh water character during monsoon season. pH varied from 6.6 to 7.6, and alkalinity varied from 68 to 216 mg CaCO3/l. Dissolved oxygen varied from hypoxic to saturated conditions (1.4–10.2 mg O2/l). Inorganic phosphate were higher during the pre-monsoon season and varied between 5.3 and 49.7 μmol/l, while nitrite and nitrate varied from 0.4 to 2.1 μmol/l and from 1.4 to 8.1 μmol/l, respectively. Silicate ranged from 3.6 to 63.0 μmol/l. Station 3 was less alkaline, and showed lower silicate and higher nitrate concentration. These variations could be due to its environmental setting as this bird sanctuary experiences limited water exchange with the estuary because of its almost closed in nature. The first two stations are closer to the estuarine front compared to the third one, especially the second station lies closer to the bar mouth.
Table 1

Seasonal variations of hydrographical parameters in the study region

Parameters

Station 1

Station 2

Station 3

Post

Pre

Mon

Post

Pre

Mon

Post

Pre

Mon

pH

7.4

6.6

7.1

7.6

7.1

7.0

7.2

7.5

Salinity (psu)

29.2

34.0

1.6

28.5

34.0

1.3

13.9

33.8

Alkalinity (mgCaCO3/l)

144

164

132

132

216

132

68

92

DO (mgO2/l)

1.4

6.4

3.0

2.9

10.2

4.2

4.2

4.2

Nitrite (μmol/l)

0.4

1.2

1.4

1.2

1.2

1.4

2.1

1.5

Nitrate(μmol/l)

2.7

2.2

2.4

4.4

1.4

1.6

8.1

4.4

Phosphate (μmol/l)

16.4

49.7

14.8

5.3

28.5

16.2

7.5

18.4

Silicate (μmol/l)

61.2

50.0

63.0

23.0

20.2

43.4

4.0

3.6

Texture analysis revealed that silt was the major fraction in these mangrove sediments (Table 2). XRD analysis showed that perovskite and sodalite were the dominant minerals in the stations 1 and 2, respectively. Presence of monetite, a rare calcium phosphate mineral, was observed at station 3 (Fig. 3). XRF analysis revealed very high calcium and phosphorus at station 3, when compared to the other two stations (Table 3). The SEM–EDS analysis of station 3 sediment also showed high mass percentages of calcium (9.79) and phosphorous (5.33). TGA analysis showed a moisture content of 7%, and the weight loss after 900°C was about 25%. The sediment pH ranged from 5.9 to 7.1 in the study region. Eh values showed that the sediments were generally anoxic, and station 3 is found to be highly reducing. The mangrove sediments were rich in total carbon (2.91–7.64%), and organic carbon constituted 75–93% of total carbon. The nitrogen and the sulphur content ranged from 0.27 to 0.66% and from 0.22 to 1.96%, respectively. Fe in the sediments varied from 4.26 to 5.83% in the study region.
Table 2

General sedimentary characteristics of the study region (±SD)

Parameters

Station 1

Station 2

Station 3

Post

Pre

Mon

Post

Pre

Mon

Post

Pre

Mon

Sand (%)

8.87 ± 2.9

6.73 ± 1.9

36.60 ± 4.8

0.94 ± 0. 1

4.34 ± 1.1

2.32 ± 0.9

10.46 ± 1.9

23.59 ± 2.6

Silt (%)

58.92 ± 4.2

71.79 ± 6.8

28.51 ± 3. 0

59.38 ± 6.9

59.97 ± 2.1

63.78 ± 6.7

70.47 ± 5.7

55.56 ± 3.1

Clay (%)

32.20 ± 2.3

21.45 ± 2.2

34.89 ± 2.7

39.68 ± 2.8

35.69 ± 2.7

33.90 ± 2.8

19.07 ± 2.2

20.97 ± 3.1

pH

7.0

6.2

5.9

7.1

6.5

6.5

6.6

6.7

Eh

−10

−98

−16

−53

12

−41

−398

−337

Total carbon (%)

4.72 ± 0.04

3.73 ± 0.11

6.75 ± 0.07

6.25 ± 0.11

2.91 ± 0.06

3.31 ± 0.05

7.64 ± 0.09

5.52 ± 0.09

Organic carbon (%)

3.9 ± 0.03

2.8 ± 0.01

6.3 ± 0.13

4.9 ± 0.05

2.2 ± 0.02

2.5 ± 0.03

6.7 ± 0.13

5.0 ± 0.06

Total nitrogen (%)

0.32 ± 0.01

0.34 ± 0. 07

0.50 ± 0.01

0.46 ± 0.01

0.27 ± 0.05

0.29 ± 0.01

0.66 ± 0.01

0.47 ± 0.01

Total sulphur (%)

0.62 ± 0.10

0.32 ± 0.01

0.25 ± 0.01

0.63 ± 0.01

0.35 ± 0.01

0.22 ± 0.00

1.14 ± 0.02

1.96 ± 0.02

Total iron (%)

4.56 ± 0.11

5.83 ± 0.21

4.26 ± 0.12

5.71 ± 0.17

5.76 ± 0.34

5.74 ± 0.29

5.74 ± 0.29

5.62 ± 0.36

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Fig. 3

XRD spectra of the mangrove sediments

Table 3

Major elemental composition and TGA results (weight %) of the mangrove sediments showing manifold increase in calcium and phosphorus content at station 3

Compound

Station 1

Station 2

Station 3

SiO2

40.030

39.570

35.060

TiO2

0.748

0.820

0.552

Al2O3

16.63

17.94

13.14

MnO

0.033

0.040

0.034

Fe2O3

7.99

8.96

4.86

CaO

1.434

0.807

7.278

MgO

2.901

2.799

1.947

Na2O

3.250

2.325

2.302

K2O

1.547

1.316

0.885

P2O5

0.630

0.622

6.760

SO3

0.096

0.158

1.25

Cr2O3

0.034

0.033

0.02

CuO

0.009

0.007

0.01

NiO

0.010

0.010

0.000

Rb2O

0.006

0.006

0.003

SrO

0.029

0.016

0.054

ZnO

0.000

0.012

0.044

ZrO2

0.013

0.010

0.012

Loss of ignition at 110°C

7.40

7.67

7.34

Loss of ignition at 900°C

24.61

24.63

26.66

Fractionation of phosphorus in mangrove sediments (Table 4) revealed significant spatial variations for different phosphorous fractions among the three systems under study. Fe-IP varied from 825 to 2,080 μg/g among the mangrove systems. Fe-IP was the major fraction in the first two stations accounting for 38.2 and 37.57% of the total phosphorus, respectively. Ca-IP ranged from 505 to 24,764 μg/g. Station 3 was exclusively dominated by Ca-IP contributing to about 87% of total phosphorous. ASOP varied from 201 to 1,555 μg/g, and higher values were observed at station 3. Alk-OP ranged from 428 to 738 μg/g. This fraction was very low at station 3 (2.2%), while it showed similar distribution pattern in other two stations. R-OP ranged from 48 to 92 μg/g. R-OP was the smallest fraction (0.2–3.8% of total phosphorus). Total phosphorus, calculated as the sum of all fractions, varied between 2,226 and 28,665 μg/g. Compared to other two stations, station 3 showed about tenfold increase in total phosphorus content.
Table 4

Different fractions of phosphorus in the study region (μg/g ± SD)

Parameters

Station 1

Station 2

Station 3

Post

Pre

Mon

Post

Pre

Mon

Post

Pre

Mon

Fe-IP

909 ± 24

856 ± 22

825 ± 19

982 ± 32

1,142 ± 34

979 ± 26

2,080 ± 31

1,129 ± 36

Ca-IP

532 ± 17

727 ± 21

541 ± 13

803 ± 17

1,019 ± 26

505 ± 28

24,287 ± 189

24,764 ± 171

ASOP

236 ± 5

201 ± 3

289 ± 7

318 ± 6

322 ± 8

274 ± 10

1,499 ± 27

1,555 ± 16

Alkali-OP

506 ± 12

452 ± 13

501 ± 21

738 ± 21

428 ± 11

539 ± 17

735 ± 18

504 ± 10

R-OP

66 ± 1.9

72 ± 1.5

70 ± 2.1

92 ± 2.7

76 ± 1.2

90 ± 2.1

64 ± 1.6

48 ± 1.4

Total-P

2,249 ± 56

2,308 ± 58

2,226 ± 54

2,933 ± 71

2,987 ± 73

2,387 ± 59

28,665 ± 209

28,000 ± 192

First two stations did not show any noticeable difference in phosphorus fractions and inorganic fractions constituted to about 65% of total phosphorus. But at station 3, inorganic fractions were about 92%.

Analysis of variance (ANOVA) showed that there are no significant seasonal variations for any of the phosphorus fractions in the study region. But considerable spatial variations were observed for Ca-IP and ASOP (p ≪ 0.001), which were significantly higher at station 3. Fe-IP did not show any significant variation between stations, while ROP was significantly (p = 0.01) lower at station 3. Correlation analysis (Table 5) showed that Ca-IP had highly significant positive correlation with total sulphur and highly significant negative correlation with Eh. ASOP also showed highly significant positive correlation with total sulphur and highly significant negative correlation with Eh. pH did not show correlations with any of the parameters. Fe-IP had no correlation with Fe, while it showed highly significant positive correlations with other two bioavailable fractions. Ca-IP showed highly significant positive correlation with ASOP and significant negative correlation with clay.
Table 5

Pearson correlation between the sedimentary parameters in the study region

 

Sand

Silt

Clay

pH

Eh

Total carbon

Organic carbon

Total nitrogen

Total sulphur

Fe

Fe-IP

Ca-IP

Acid-OP

Alkali-OP

R-OP

Sand

1.00

              

Silt

−0.86

1.00

             

Clay

−0.16

−0.32

1.00

            

pH

−0.59

0.39

0.17

1.00

           

Eh

−0.18

−0.17

0.81

−0.10

1.00

          

Total carbon

0.48

−0.32

−0.22

0.06

−0.55

1.00

         

Organic carbon

0.61

−0.43

−0.25

−0.05

−0.56

0.99

1.00

        

Total nitrogen

0.43

−0.17

−0.43

−0.03

−0.74

0.95

0.94

1.00

       

Total sulphur

0.26

−0.10

−0.57

0.35

−0.83

0.42

0.45

0.51

1.00

      

Fe

−0.66

0.71

−0.30

0.24

−0.36

−0.30

−0.37

−0.06

0.21

1.00

     

Fe-IP

−0.11

0.39

−0.54

0.16

−0.77

0.50

0.47

0.68

0.44

0.37

1.00

    

Ca-IP

0.26

0.03

−0.74

0.13

−0.97

0.52

0.55

0.70

0.90

0.28

0.74

1.00

   

Acid-OP

0.27

0.00

−0.69

0.14

−0.96

0.53

0.56

0.70

0.90

0.28

0.74

1.00

1.00

  

Alkali-OP

−0.23

0.24

−0.01

0.50

−0.44

0.71

0.60

0.70

0.24

0.22

0.58

0.35

0.36

1.00

 

R-OP

−0.59

0.28

0.70

0.13

0.62

−0.29

−0.39

−0.36

−0.74

0.21

−0.28

−0.71

−0.69

0.26

1.00

Discussion

Inorganic phosphate in surface waters was higher during pre-monsoon season for all stations. These values were comparatively higher than those reported for Cochin estuary and also a reversal of the general seasonal trend (Menon et al. 2000). The C:P ratios (organic carbon: total phosphorus, w/w) of these mangrove sediments (1.8–28.6) were lower than Redfield ratio (Hecky et al.1993), indicating that the organic matter is enriched with phosphorus and tends to accumulate. Sediment phosphorus is particularly important in shallow systems as various forms of bioavailable phosphorus in the upper sediments can be a major source of phosphorus to the water column biota via numerous physically, chemically and biologically mediated processes (Reddy and D’Angelo 1994; Kufel et al. 1997). Hence, these mangrove sediments may act as a source of phosphate in the water column.

Fractionation analysis of phosphorus in the mangrove sediments showed that station 3 is abnormally enriched with Ca-IP. XRD analysis of the sediments showed the presence of monetite, an anhydrous calcium phosphate mineral (CaHPO4) at station 3. The high mass percentages of calcium and phosphorus by XRF and SEM–EDS analysis confirm the existence of calcium phosphate mineral at station 3.

This station is a congregation of communally breeding birds. The bird excreta and their remains are not effectively flushed away by tides as this mangrove system is linked to the adjacent micro-tidal estuary by a narrow canal, the only source for tidal propagation. This results in the accumulation of bird guano and carcass of the birds, rich source of phosphorus.

Monetite formation is interpreted as being the result of reaction between guano and clay mineral or carbonate rocks (Onac and Veres 2003). Eh analysis revealed that this unique system is highly reducing during tidal influx. There are also reported evidences for high anoxic condition of this unique ecosystem. Authigenic filamented pyrites have been reported at Mangalavanam (Rosily 2002), the presence of which can be taken for the highly anoxic conditions (Jeng and Huh 2001). 5α-cholestan-3β-ol, which is reported in anaerobic sediments (Volkman et al. 1998), is also found in Mangalavanam (Narayanan 2006). During low tide, when the sediments get exposed to the atmosphere for considerably longer periods, there may be chances for an oxygenated microenvironment. These periodic fluctuations in redox potential create a metastable condition, which might favour the formation of monetite. Drier and acidic conditions of the sediment favour the formation of monetite (Onac and Veres 2003). The presence of monetite indicates the complexity of the system as it is a thermodynamically metastable calcium phosphate mineral (Effler 1987). The presence of monetite validates the dominance of Ca-IP at station 3.

Calcium-bound phosphorus dominates in mangrove sediment (Silva and Mozeto 1997), due to its stability under the redox (Eh) variations observed in the mangroves (Nriagu 1976; Silva et al. 1998). Highly significant negative correlation of Ca-IP with Eh and its highly significant positive correlation with sulphur, the redox indicator, also suggested that there is a preferential accumulation of Ca-IP when the system is highly reducing. According to Silva and Sampaio (1998), acidic pH in sediments provides high stability to Ca-IP, which also explains the high values of this phosphorus fraction at station 3.

The increase in Ca-IP during pre-monsoon at all the stations might be due to the increase in salinity. Similar trend is reported in marine sediments presumably by the accumulation of calcium under high salinity, which favours apatite formation (Ryden et al. 1997). Silva and Mozeto (1997) also suggested that phosphorus combined with calcium under high salinity acts as a principal mechanism for its retention.

The Fe-IP was the major fraction at stations 1 and 2. Generally release of this phosphate fraction from the sediment is controlled by sulphate reduction (Caraco et al. 1989) and is considered more bioavailable under the redox (Eh) variations observed in the mangroves sediments (Silva and Mozeto 1997; Caraco et al. 1989). Sulphide produced from sulphate respiration may reduce the iron-oxides, and thus, promote a release of Fe-IP (Jensen et al. 1995; Howarth et al. 1995). However, sulphate reduction is generally of less importance in intertidal zones because of periodic aeration in the environment (Kristensen et al. 1992; Alongi 1998; Holmer et al. 1999) causing the formation of Fe(OH)3 (Crosby et al. 1984). Furthermore, the mangrove trees are able to excrete oxygen through their root system, producing an oxygenated microenvironment (Silva et al. 1991) capable of trapping phosphorus as FePO4 through the formation of Fe(OH)3.

The involvement of iron in the dynamic equilibrium between the sediment and water as explained above has led to the suggestion that an iron-dependent threshold exists for the sediment’s ability to bind phosphorus. Jensen et al. (1992) showed that the retention capacity was high as long as the Fe:P ratio exceeds 15 (by weight), while Caraco et al. (1993) suggested that this ratio should be above 10 to regulate phosphorus release. Fe:P ratios of the sediments were higher than 15 in the study region, except at station 3. This justifies higher concentration of Fe-IP at stations 1 and 2. The Fe:P ratio was very low at station 3, resulting in lower percentage of Fe-IP in the system. Periodic fluctuations in the redox potential of this system might also result in lower Fe-IP content at station 3.

The ASOP includes apatite-bound phosphate and biochemical components such as nucleic acids, lipids and sugars which bound to phosphate (De Groot 1990). ASOP had highly significant negative correlation with Eh and highly significant positive correlation with total sulphur. This indicates that highly reducing environment is favourable for ASOP similar to Ca-IP. Alk-OP generally constitutes humic phosphate and phytate phosphate (Golterman et al. 1998). Phytate or phytic acid (inositol hexaphosphate) is an organic phosphate that is widely spread in plants (Hess 1975), soils (Stevenson 1982) and aquatic sediments (De Groot and Golterman 1993). Phytate is relatively stable as it can be strongly adsorbed onto iron hydroxide and multivalent cations (De Groot and Golterman 1993). Irrespective of the higher content of total organic matter, the labile fraction was very low in these mangrove sediments, signalling to the dead organic matter accumulation (Joseph et al. 2008). This might result in the higher concentration of Alk-OP, which is non-bioavailable. As a result of the diagenetic reorganisation of phosphorus within sediments, organic phosphorus concentrations usually decrease with time as it is ultimately transformed to authigenic phosphorus during diagenesis (Ruttenberg and Berner 1993; Anderson et al. 2001).

Iron- and calcium-bound inorganic fractions and acid soluble organic fractions of phosphorus are generally considered to be bioavailable (Diaz-Espejo et al.1999). But Fe-IP is more important than Ca-IP in terms of potential availability of phosphorus under the redox (Eh) variations observed in the mangroves sediments (Silva and Mozeto 1997; Caraco et al. 1989). Bioavailable fractions of phosphorus at stations 1 and 2 were about 75%, whereas 98% of the total phosphorus was bioavailable at station 3. Since the bulk of the total phosphorus is bioavailable, these mangrove sediments have the potential to act as source of phosphorus to the overlying waters.

Organic phosphorus in the study region ranged from 8 (station 3) to 38.6% (station 1) of total phosphorus. Generally organic-bound phosphorus accounted for 6–19% of total in coastal sediments (Hirata 1985). The high percentage organic-bound phosphorus at stations 1 and 2 indicated that the mineralisation of phosphorus is less, where as at station 3, active mineralisation is taking place. C:P ratio at station 3 was very low, and it is reported that mineralisation of organic phosphorus and C:P ratio are inversely related (Reddy and Delaune 2008). Moreover, organic phosphorus mineralisation is high under anaerobic condition than under aerobic conditions (Bridgham et al. 1998). Very high reduction potential at this station results in higher mineralisation and subsequently lower concentration of organic phosphorus.

The variations in the phosphorus content in the three systems could also be favoured by the difference in local vegetation (Alongi 1989). Organic matter associated with Avicennia sediments, because of the presence of more degradable organic matter, can sustain a higher rate of microbial activity than Rhizophora and, as a consequence, better recycling of nutrient elements (Lacerda et al. 1995). In Rhizophora sediments, either microbial conversion is negligible or the organic components are more refractory (Alongi 1989). Station 3 is dominated by Avicennia, and the high amount of inorganic phosphorus in this system could be deduced to be a signal of higher levels of diagenetic activity.

Conclusion

Phosphorus fractionation analysis of three tropical mangrove systems in the Cochin region showed that these systems seem to accumulate phosphorus, primarily as Ca-IP and Fe-IP fractions. Bulk of the total phosphorus is bioavailable, suggesting that these mangrove sediments have the potential to act as source of phosphorus to the overlying waters. The first two stations behave identically, and Fe-IP was the major fraction in these stations. Station 3 is unique because of the accumulation of bird guano, and it resulted in about tenfold increase in the total phosphorus content. This station is exclusively dominated by Ca-IP, and a rare mineral, monetite, was also detected in the system. The unique geochemical characteristics found at this station warrants further investigation.

Acknowledgments

The authors gratefully acknowledge the facilities and the support provided by the Director, School of Marine Sciences and the Dean, Faculty of Marine Sciences, Cochin University of Science and Technology. We extend our sincere thanks to the anonymous reviewer for giving valuable comments to improve the manuscript.

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