Mangrove forests occupy several million hectares of coastal area worldwide and support an ecosystem, comprised of plants, animals and microorganisms, having a dynamic environment. The total area covered by mangroves in India is estimated to be 6,740 km², which accounts for about 7% of the world’s total mangrove vegetation. Mangrove ecosystem is known to be very rich due to high amount of dissolved and particulate organic matter which acts as a homeland for diverse microbes. Sediments are originated basically from soil erosion, decomposition of plants and animals within the estuary itself. A lot of previous reports are available over the last few decades on the biogeochemical role of estuaries and mangrove ecosystems [1,2], but there is little information which links the role of these ecosystems in enhancing production functions of the adjacent agro ecosystems through nutrient export. Many reports are available on the physico-chemical features of Indian estuaries [3-11]. Mangroves ecosystem of Odisha is a very less explored ecosystem. Odisha is located between 17°49’ N and 22°34’ N latitudes and between 81°27’ E and 87°29’ E longitudes. Out of 52,472 sq.km of forested area mangrove forests constitute 243 sq. km. Odisha consists of mangroves in five different areas such as Bhitarakanika, Dhamra, Mahanadi, Devi and Budhabalanga. In the present study, the physico-chemical parameters such as pH, conductivity, organic carbon, salinity, alkalinity and total phosphorus were analysed from sediments of five different sampling sites of Odisha.

Study area and description of sampling sites

Samples have been collected from five different mangrove areas of Odisha (Figure 1) and the sampling sites were described as follows.

Figure 1: Map showing distribution of mangrove ecosystems of Odisha.

Bhitarakanika (SITE-1): Bhitarkanika is the second largest mangrove ecosystem of India situated at Long. 20°30’ N and Lat. 86°45’ E. The mangroves cover an area of 650 sq. km. The sediment samples were collected from five different localities such as Kantaikhai (BKS- 1), Khola (BKS-2), Dangamal (BKS-3), Mahisamunda (BKS-4) and Dangmal (BKS-5).

Dhamra (SITE-2): Dhamra mangrove ecosystem is present in the bank of river Dhamra situated at Long. 20°47’ N and Lat. 86°56’ E. A small patch of mangroves present in the port site has been identified for protection by the Dhamra port. Sediment samples were collected from Kalanali jora site (DHS-1), Chandinipal (DHS-2), Dhamra MAPOT (DHS-3), Jyotsnamayee (DHS-4), and Kalanali mangrove site (DHS-5).

Mahanadi (SITE-3): Mahanadi mangrove is located in the combined delta of the rivers Mahanadi and situated at Long. 20°17’ N and Lat. 86°42’ E. The Mahanadi delta occupied an area of 9,000 sq.km. Sediment samples were collected from Kansardi (MHS-1), MHS-2, MHS-3, Ghangholia jora site (MHS-4) and MHS-5.

Devi Mouth (SITE-4): The Devi mouth mangrove is located in the district of Jagatsinghpur (9° km) situated at Long. 19° 58’ N and Lat. 86° 22’ E. Sediment samples were collected from the mangrove areas of Machamachikuda (DVS-1), Bandar (DVS-2), Nadiakhia (DVS-3), Nentai (DVS-4) and Kiakhala (DVS-5).

Budhabalanga (SITE-5): The Budhabalanga estuarine mangrove situated at Long. 21° 28’ N to Lat. 87° °5’ E. Sediment samples have been collected from Balaramgadi (BDS-1), Haldigudi (BDS-2), Nandachaha (BDS-3), Sanipala (BDS-4) and Budhabalanga River (BDS-5).

Collection and processing of sediment

Sediment samples were collected with an air dried Beckman’s grab in the month of October from all the sampling areas. The sediment collected were aseptically transferred to the labeled polythene bags and kept in an ice-chest box before transferring to the laboratory.

Samples were collected from five different locations of each sampling sites. The collected sediments were air dried at room temperature in the laboratory for 7-10 days for evaporation of moisture content. The dried samples were further crushed to fine texture in a ceramic mortar, re-packaged in labeled polythene bags and stored in the laboratory for sediment characterization study.

Physico-chemical characterization of sediment

Before analysis, the air-dried followed by oven-dried sediment samples were sieved using 2.0 mm mesh sized sieve and 0.5 mm sieve for other analyses. For all the tests, the sediment was dissolved in required amount of milliQ water and filtered. The filtrate was used for further tests. pH and conductivity of the sediment samples were measured by using a digital pH and conductivity meter. Salinity, alkalinity and total phosphorous were analyzed following the methodology of APHA [12]. The oven dried sediment samples were powdered with grinder (Tetsch, model, RM 100). Then required amount of grounded sample was taken for total organic carbon analysis following Walkely and Black method [13].

pH

A little variation in pH was found among different stations. It was found to be minimum i.e., 6.91 ± 0.042 and maximum i.e., 7.7 ± 0.06 at Bhitarakanika. In sediments of Dhamra, pH varied from 6.23 ± 0.13 to 7.75 ± 0.16. In Mahanadi, it varied from 6.27 ± 0.04 to 8.3 ± 0.03 whereas in Devi and Budhabalanga it ranged from 4.32 ± 0.12 to 7.54 ± 0.21 and 8.22 ± 0.09 to 8.77 ± 0.10 (Figure 2).

Figure 2: pH variation between five sampling sites of mangrove estuary.

Conductivity

There was a little variation in conductivity of sediments. The conductivity ranged from 2087 ± 167 to 2199 ± 189 μS/cm at Bhitarakanika whereas it was 1782 ± 167 to 2236 ± 167 μS/cm, 2095 ± 235 to 2564 ± 256 μS/cm, 1327 ± 113 and 1675 ± 138 μS/cm, 2312 ± 266 and 2457 ± 245 μS/cm at Dhamra, Mahanadi, Devi and Budhabalanga mangrove sediments respectively (Figure 3).

Figure 3: Conductivity variation between five sampling sites of mangrove estuary.

Salinity

The salinity varied a lot among all the sites. In Bhitarakanika, it ranged from 0.86 ± 0.067 PSU to 4.63 ± 0.16 PSU. It was maximum i.e., 3.73 ± 0.198 PSU and minimum i.e., 0.75 ± 0.084 PSU at Dhamra. In the sediments of Mahanadi delta, salinity varied from 0.78 ± 0.087 to 3.18 ± 0.13 PSU whereas in mangrove sediments of Devi and Budhabalnga estuary, it was between 1.15 ± 0.12 PSU to 3.84 ± 0.21 PSU and 0.39 ± 0.05 PSU to 0.67 ± 0.072 PSU respectively. It was highest in Bhitarakanika and lowest at Budhabalnga (Figure 4).

Figure 4: Salinity variation between five sampling sites of mangrove estuary.

Organic Carbon (OC)

The sediment organic carbon in different sampling sites varied greatly. It was between 51.86 ± 0.11 to 36.42 ± 0.14 mg/g; 13.88 ± 0.05 to 24.68 ± 0.21 mg/g; 14.1 ± 0.09 mg/g to 44 ± 0.14 mg/g; 10.98 ± 0.055 to 48.48 ± 0.05 mg/g and 6.7 ± 0.12 to 21.3 ± 0.24 mg/g in mangal sediments of Bhitarakanika, Dhamra, Mahanadi, Devi and Budhabalanga respectively (Figure 5).

Figure 5: OC variation between five sampling sites of mangrove estuary.

Alkalinity

In mangrove sediments, the alkalinity varied a lot. In Bhitarakanika it varied from 4.7 ± 0.18 to 7.64 ± 0.16, whereas in other sites such as Dhamra, Mahanadi, Devi and Budhabalanga it varied from 6.78 ± 0.13 to 7.85 ± 0.11; 5.85 ± 0.12 to 7.74 ± 0.12; 4.7 ± 0.18 to 7.64 ± 0.16; 7.75 ± 0.09 to 8.75 ± 0.25 respectively. The Budhabalanga sediments are more alkaline than other mangrove sediments (Figure 6).

Figure 6: Alkalinity variation between five sampling sites of mangrove estuary.

Total Phosphorus (TP)

TP was found to be maximum at Budhabalanga mangrove sediments. It was minimum i.e., 58.94 ± 0.63 mg/ml at BKS-3 whereas least 42.52 ± 1.13 mg/ml at BKS-4 was observed in Bhitarakanika. In Dhamra, it was 18.9 ± 0.652 to 44.88 ± 0.91 mg/ml at DHS-1 and DHS- 5. Total phosphorus content was from 22.12 ± 0.78 to 32.96 ± 1.98 mg/ ml in MHS-2 and MHS-5; 14.23 ± 0.61 to 25.5 ± 1.11 mg/ml at DVS-1 and DVS-5 and 6.68 ± 1.56 to 14.34 ± 1.57 mg/ml at BDS-5 and BDS-4 (Figure 7).

Figure 7: Total Phosphorus analysis between five sampling sites of mangrove estuary.

In sediment analysis, six parameters such as pH, conductivity, and salinity, OC (Organic carbon), Alkalinity and TP (Total Phosphorus) were studied. A significant variation was observed between all the data’s between the groups and within the groups by ANOVA tests. From the F-test, all the parameters were found to have significant differences (p<0.05) among each other in all the five sampling sites. The F-values were 8.781, 36.586, 3.327, 6.986, 6.296, 1.264 for pH, conductivity, salinity, OC, alkalinity and TP respectively where p<0.05 (Table 1). While studying the correlation study between all the parameters taken in the present study in Bhitarakanika, pH was positively related to conductivity (r=0.336), salinity (r=0.970, p<0.01), alkalinity (r=0.653, p<0.01) and conductivity (r=0.447, p<0.05) whereas negatively related to OC and TP. Conductivity was positively correlated with maximum parameters studied while a positive significant correlation was observed with salinity (r=0.447, p<0.05) and alkalinity (r=0.588, p<0.01). Salinity was found to possess negative relation with maximum parameters studied (Table 2). The statistical analysis for correlation study between all the physico-chemical parameters of Dhamra indicates that pH has positive relation with most of the parameters while significantly related to salinity (r=0.772, p<0.01) (Table 3). The results for correlation analysis among all the physico-chemical parameters of Mahanadi mangrove sediments, showed that pH has positive significant relation (p<0.05) with conductivity and alkalinity (r=0.479, 0.454) whereas positively related to all the parameters studied. OC has negative correlation with alkalinity and TP (Table 4). While studying the correlation in Devi mangrove sediments, pH was positively significantly related to most of the parameters such as conductivity (r=0.482, p<0.05), OC (r=0.774, p<0.01) and alkalinity (r=0.670, p<0.01). Conductivity was negatively related to maximum parameters studied (Table 5). In Budhabalanga mangrove sediments, pH has negative relation with alkalinity and TP (r=-0.128, -0.387) whereas positively related to all. Conductivity has positive relation with all parameters except OC and TP (r=-0.042, -0.535). Salinity has negative relation with OC whereas positive relation was found with other parameters studied (Table 6).

Table 1: Analysis of Variance (ANOVA) between the physico-chemical parameters and microbial diversity (actinobacterial and fungal) recorded in mangrove sampling sites of Odisha.

Table 2: Correlation coefficient between physico-chemical parameters in sampling sites of Bhitarakanika.

Table 3: Correlation coefficient between physico-chemical parameters in sampling sites of Dhamra.

Table 4: Correlation coefficient between physico-chemical parameters in sampling sites of Mahanadi.

Table 5: Correlation coefficient between physico-chemical parameters in sampling sites of Devi.

Table 6: Correlation coefficient between physico-chemical parameters in sampling sites of Budhabalanga.

Different physico-chemical parameters were investigated such as pH, conductivity, salinity, OC, alkalinity and TP of the sediments. Soil texture of the different locations of the mangroves of Odisha was also studied. Analysis of Variance (ANOVA) worked out for different physico-chemical parameters are given in Table 1. Results of correlation coefficient for different parameters recorded at five different sites are given in Tables 2-6. Sediment composition studies have indicated the predominance of clay as the major component followed by sand and silt in all the stations.

pH is controlled by both biogenic and abiogenic reactions. In the present study, the sediment was acidic or near neutral or slightly alkaline in all the stations. The acidic pH may have resulted from Humic Acid (HA) formed from decaying organic matter (leaves), which is consistent with the report of the Niger Delta swamp environment [14] where observed pH values in the acidic domain were attributed to the presence of humic acid. In the present study, among the five sites studied, pH of Budhabalanga was maximum (8.45 ± 0.09) is alkaline in comparison to Devi having minimum (5.79 ± 0.14) is acidic in nature which is consistent with the range (4.5 to 9) specified by Costerton et al. [15] for microbial activity to take place whereas in contrast more uptake of CO₂ by the photosynthetic organisms could have increased the pH level. Similar range of pH has also been reported by earlier workers [16-18]. A significant correlation obtained between for pH with salinity, alkalinity and conductivity (Tables 2-6) has revealed the influence of salinity over pH. Alkalinity was more i.e., 8.17 ± 0.156 at Budhabalanga and less i.e., 5.95 ± 0.21 at Devi, which indicates that alkalinity is directly related to pH. Alkalinity changes in salinity might affect microorganisms in two different ways. They adapt to the changed salinity or they are replaced by microorganisms that are adapted to the changed conditions [19]. The salinity is the chloride content of sediments, which are known to be controlled largely by the frequency and duration of tidal submersion, subaerial evaporation and leaching. The salinity was maximum i.e., 4.63 ± 0.16 PSU and minimum i.e., 0.39 ± 0.05 PSU at Bhitarakanika and Budhabalnaga respectively (Figure 3). The characteristics of sediments further revealed that the chloride conc. of sediment depends on the salinity of overlying water. Similar observations were made by Sanders et al. [20], Reddy [21]. The conductivity is directly related to salinity, i.e., where the salt conc. is more, the conductivity will be more. Organic carbon represents the organic matter in the sediments and this is of potential significance for mangrove productivity. In the present study, maximum conc. of OC i.e., 51.86 ± 0.11 mg/g was recorded at Bhitarakanika whereas minimum amount i.e., 6.7 ± 0.12 mg/g was recorded at Budhabalanga sediments of Odisha. The wide range of fluctuations was attributed to the variation of temp. In the sediment, nature of vegetation, rate of accumulation of dead and decayed foliage’s, topography, soil texture and depth of water. Similar observation was made by Kumar [22,23] in mangrove biotope of Cochin estuary [24]. Phosphorus is efficiently adsorbed by the fine sediments of muddy areas rather than the coarse grained sediments. This is probably the reason for mangroves growing luxuriantly in muddy environments. Whatever Phosphorus may be present gets bound with calcium, and is effectively held within the sediments [25]. Many organisms utilize both organic and inorganic forms of Phosphorus; however inorganic phosphorus seems to be more appreciated by plants than organic phosphorus [26]. There is a little detectable mobilization rate of TP from the mangrove sediment to the overlaying water through the mineralization process by microbial activity or due to degradation of organic matter [27-29]. Higher conc. of phosphate i.e., 50.7 ± 0.05 mg/L was noticed and it was less 10.62 ± 0.61 mg/ml at Budhabalanga (Table 7) [30].

Table 7: Variation of some physicochemical parameters (mean ± S.E.) at different stations in mangrove sediment of Odisha.

Conclusively, in this study the sediment physico-chemical parameters such as pH, conductivity, Organic Carbon content, salinity, alkalinity and total Phosphorus content from the mangrove sediments of Odisha were assessed. Out of the five mangrove study areas of Odisha, the mangrove sediments of Bhitarakanika is highly nutrient rich ecosystem due to the amount of organic carbon content whereas the Budhabalanga sediments has less organic carbon content. The sediments of Bhitarakanika were having slightly neutral pH which indicates that the better growth and survival of highly diverse organisms. However, free exchange of mangrove water (along with nutrient) with the adjacent coastal area must be related to the hydrodynamic nature of the mangrove-fringed tidal channel. Such characteristics, export eutrophic and nutrient rich mangrove water seaward, thus enhances coastal productivity. The present study will highlight on the physico-chemical constituents of sediments from mangrove environment which will highlight on the nutrient recycling in this particular area and the present baseline information in sediments would form a useful tool for further ecological assessment and monitoring of the coastal ecosystems of Odisha.

The authors are thankful to Prof. B.K Mishra, Director, Institute of Minerals and Materials Technology, Bhubaneswar for providing necessary facilities to carry out the research work.