Keywords

1 Introduction

Mangrove forests are one of the world’s most productive tropical ecosystems confined to the coastal deltaic zones of tropics and subtropics, predominantly between 25° N and 25° S with an estimated total coverage of 1.7 to 2.0 × 105 km2 (Borges et al. 2003). Mangroves are of much significance as they act as a barricade against cyclones, protect coastal erosion and provide habitat for a number of commercially important aquatic organisms (Kathiresan and Bingham 2001).

Pollution of the natural environment by trace metals is a worldwide problem because trace metals have toxic effects on living organisms when they exceed a certain concentration (Chakraborty et al. 2009; Dias and Nayak 2016). Trace metal toxicity holds greater significance as they tend to accumulate in the sediments as well as biota through various physical, chemical and biological processes (Leivouri 1998; Jonathan et al. 2010; Kumar and Ramanathan 2015). Increasing urbanization and industrialization has increased the release of higher domestic sewage, industrial effluents, trace metals and other toxic pollutants to coastal areas (Agoramoorthy and Hsu 2005; Ranjan et al. 2008). Mangrove plants have a special feature to store trace metals without being damaged even at higher concentration than terrestrial flora (MacFarlane et al. 2007). This capacity increases with the increasing age of mangroves (Panda et al. 2013). The present chapter examines the status of major estuarine mangrove complexes (Fig. 9.1) across the Indian coast with reference to occurrence of trace metals in the sediments.

Fig. 9.1
figure 1

Distribution of mangroves in India (Adopted from Kumar 2000)

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1.1 Distribution of Major Indian Mangrove

According to the Forest Survey of India (FSI) report in 2015, the total mangrove cover in the country stands at 4628 km2. The major Indian mangroves are described in brief as below.

Sundarbans (West Bengal)

The Indian Sundarbans at the apex of the Bay of Bengal (between 21°40′ N and 22°40′ N latitude and 88°03′ E and 89°07′ E longitude) is located towards the northeast coast of India. It is a tide-dominated estuarine wetland at the lower deltaic plains of the Ganges-Brahmaputra Rivers. This wetland is a complex network of tidal creeks, surrounding hundreds of tidal islands exposed to different elevations at high and low semidiurnal tides (Sarkar et al. 2008; Jonathan et al. 2010; Banerjee et al. 2012; Kumar and Ramanathan 2015).

Bhitarkanika (Odisha)

Bhitarkanika is located between 20°04′–20°08′ N latitudes and 86°45′–87°50′ E longitudes in the Kendrapara district of Odisha. It is the second largest mangrove ecosystem in India comprising mangrove forests, rivers, creeks, estuaries, backwater, accreted land and mud flats. Bhitarkanika mangroves, spread over an area of 672 km2, were declared as a wildlife sanctuary in 1975. It is a tide-dominated mangrove with areas of high tidal range of semidiurnal nature with mean tide level of 1.5–3.4 m (Sarangi et al. 2002).

Coringa and Gaderu (Andhra Pradesh)

Coringa and Gaderu mangrove ecosystems are located to the north of Gautami-Godavari River (82° 15′ and 82° 22′ E latitude; 16° 43′ and 17° 00′ N longitude) with an area of 132 km2. The Gautami-Godavari River opens into Kakinada bay via the main distributaries Gaderu (length 11 km) and Coringa (length 26 km). The area between Gautami-Godavari and Kakinada bay has dense vegetation of mangrove forests and mudflats belonging to Coringa wildlife sanctuary (Ray et al. 2006).

Pichavaram (Tamil Nadu)

Pichavaram mangrove forest is located between Vellar and Coleroon estuaries at 11° 02′ N latitude and 79° 47′ E longitude. The area has 51 islets, separated by intricate waterways that connect the Vellar and Coleroon estuaries. The southern part close to the Coleroon estuary is mainly mangrove vegetation, while the northern part close to the Vellar estuary is dominated by mudflats. The Vellar estuary opens into the Bay of Bengal at Parangipettai and is connected with the Coleroon River, a distributary of the River Cauvery. The Pichavaram mangrove is influenced by mixing of three types of waters: (1) neritic water from the adjacent Bay of Bengal, (2) brackish water from the Vellar and Coleroon estuaries and (3) fresh water from an irrigation channel (Khan Sahib Canal) and also from the main channel of the Coleroon River. The mangrove covers an area of about 1100 ha, of which 50% is forest, 40% waterways and the remaining sandflats and mudflats (Ramanathan et al. 1999; Ranjan et al. 2013).

Kumarakom Mangrove (Kerala)

Kumarakom mangrove is located between latitude 09°28′ and 10°10′ N and longitude 76°13′ E. Kumarakom mangrove and mangrove-associated forests are seen along the southeastern border of the Vembanad estuary. It was one of the best mangrove ecosystems of Kerala before commissioning of Thannirmukkam barrier constructed on Vembanad estuary to prevent saltwater intrusion during droughts. Now the mangrove ecosystem has severe regeneration problems owing to the drastic changes in the physico-chemical conditions caused by the Thannirmukkam barrier and the intensive shell mining taking place in the areas close to the Kumarakom mangroves (Badarudeen et al. 1996).

Goa Mangrove (Goa)

Goa mangrove and associated swamps can be observed along most of the water bodies within the estuarine reaches. It is located on central western coast of India between 14°53′–15°48′ N latitudes and 73°40′–74°20′ E longitudes. The most prominent and extensive backwaters with mangroves are located along the east of the capital city Panaji. The total area covered by the estuaries in Goa including the major Mandovi-Zuari estuarine complex is approximately 120 km2 (Attri and Kerkar 2011).

Mumbai (Maharashtra)

The city of Mumbai lies between 18°55′ N and 19°20′ N latitudes and 72°45′ E and 73°00′ E longitudes. This city has at its northern end the Ulhas River, Thane Creek on the southeastern end and the Arabian Sea on the west. Extensive mudflats and mangroves are present along both estuarine banks. The creek is fringed with mangroves along both banks, along with heavy industrialization and urbanization (Fernandes et al. 2012).

Gulf of Kachchh (Gujarat)

The Gulf of Kachchh (GoK) is situated in Saurashtra in the western state of Gujarat. The GoK, occupying an area of 7300 km2, is biologically one of the most productive and diversified habitats along the western coast of India. The northern shore with numerous shoals and creeks also sustains large stretches of mangroves. The majority of the mangrove cover of Gujarat area is confined to the gulf. Of the 991 km2 of mangrove cover in the state, those along the gulf contribute 96% of the total. Due to high salinity, grazing and cutting pressure, Kachchh mangroves have stunted growth and are only 1–2 m tall, whereas at untouched patches mangroves gain a height up to 5 m (Ramaswamy et al. 2007; Chakraborty et al. 2014).

Andaman and Nicobar (Andaman and Nicobar Islands, ANI)

Andaman and Nicobar archipelago, consisting of over 572 islands and islets, lie in the lap of Bay of Bengal (between latitude 6°45′–13°41′ N and longitude 92°12′–93°57′ E) to the eastern side of India. The islands, about 1200 km east from the mainland of India, have a total land area of about 8249 km2 covered by luxuriant tropical rain forest. Fringing reef and barrier reef dominate the Andaman Islands forming a natural barrier to the wave energy from the Bay of Bengal (Ramesh et al. 2006). Coral reefs form the dominant ecosystem creating ground for sea grasses and mangroves in the lagoons and creeks protected by the reef (Nobi et al. 2010).

2 Analytical Methods

2.1 Contamination Factor (CF)

The CF is the ratio obtained by dividing the mean concentration of each metal in the soil (Cn) by the baseline or background value (concentration in unpolluted soil, Håkanson et al. 1980)

$$ CF={C}_n/{B}_n $$
(9.1)

C n in the above formula is the concentration of the examined element ‘n’ in the surface sediments, and B n is the geochemical background concentration of the metal ‘n’. B n usually refers to world’s average shale or upper continental crust composition as a reference value. CF is categories into four categories, as follows: CF <1 as low level, CF between 1 and 3 as moderate level, CF 3–6 as considerable, and CF >6 as very high level of contamination.

2.2 Pollution Load Index (PLI)

The PLI is calculated as the nth root of the product of the contamination factor (CF) of all trace metals. The equation for PLI developed by Tomlinson et al. (1980) is given below:

$$ PLI=\sqrt[ n]{\left( CF1\times CF2\times CF3\times \dots \times CF n\right)} $$
(9.2)

In the above equation, CF denotes the contamination factors of different metals in the samples. The PLI >1 indicates pollution, whereas <1 indicates no pollution.

2.3 Enrichment Factor (EF)

The EF is a geochemical index based on the assumption that, under the natural sedimentation conditions, there is a linear relationship between a reference element and other elements. Elements that are most often used as reference are Al and Fe (Mucha et al. 2003; Esen et al. 2010; Kumar et al. 2016). The EF is defined as given below (Ergin et al. 1991):

$$ EF={\left( M\ /\ Fe\right)}_{\mathrm{sample}}/{\left(\mathrm{M}/ Fe\right)}_{\mathrm{background}} $$
(9.3)

where (Me/Fe)sample is the metal to Fe ratio in the sample of interest; (Me/Fe)background is the geochemical background value of metal to Fe ratio. The value of EF denotes the following category of enrichment of trace metals:

  • EF < 2 – deficient to minimal enrichment

  • EF = 2–5 – moderate enrichment

  • EF = 5–20 – significant enrichment

  • EF = 20–40 – very high enrichment

  • EF >40 – extremely high enrichment

2.4 Geo-Accumulation Index (I-geo)

The index I-geo is widely used as a measure of sediment contamination. It denotes contamination by comparing current metal contents with preindustrial levels. The content accepted as background is multiplied by the constant 1.5 in order to take into account natural fluctuations of a given substance in the environment as well as very small anthropogenic influences. The I-geo as per Muller (1981) is described by the following equation:

$$ \mathrm{I}- geo={ \log}_2\left({\mathrm{C}}_n/1.5{\mathrm{B}}_n\right) $$
(9.4)

The interpretation of the results is as follows:

  • I-geo ≤0 – practically uncontaminated

  • 0< I-geo <1 – uncontaminated to moderately contaminated

  • 1< I-geo < 2 – moderately contaminated

  • 2< I-geo <3 – moderately to heavily contaminated

  • 3< I-geo < 4 – heavily contaminated

  • 4< I-geo <5 – heavily to very heavily contaminated

  • I-geo ≥5 – very heavily contaminated

2.5 Potential Ecological Risk Index (Eif)

The Eif is used to express quantitatively the potential ecological risk of a given contaminant suggested by Håkanson (1980). It is calculated by the following equation:

$$ Eif= CF\times Tif $$
(9.5)

where Tif is the toxic response factor for a given substance [toxic response index for different metals are Hg = 40, Cd = 30, As = 10, Cu = Pb = Ni = 5, Cr = 2 and Zn = 1 given by Håkanson (1980)], and CF is the contamination factor. The following terminologies are used to describe the risk factor:

  • Eif <40 – low potential ecological risk

  • 40≤ Eif < 80 – moderate potential ecological risk

  • 80≤ Eif <160 – considerable potential ecological risk

  • 160≤ Eif <320 – high potential ecological risk

  • Eif ≥320 – very high ecological risk

2.6 Potential Toxicity Response Index (RI)

The RI was originally introduced by Håkanson (1980) to assess the degree of heavy metal pollution in soil, according to the toxicity of metals and the response of the environment. The calculation of RI is as given below:

$$ RI=\varSigma \mathrm{Eif} $$
(9.6)

There are four categories of RI: RI <150 low-grade, 150≤ RI <300 moderate, 300≤ RI <600 severe and 600≤ RI serious risk.

3 Scenario of Metal Pollution in Selected Indian Mangrove Ecosystem

Metal distributions are very uneven within mangrove forests; it varies with depth, distance from the coast, anthropogenic activities, types of vegetation and hydrology (Kehrig et al. 2003; Marchand et al. 2006; Chatterjee et al. 2009). The sources are usually difficult to identify as some have natural sources and others have anthropogenic, as point and non-point, sources. The concentration of Al, As, Cd, Cr, Cu, Co, Fe, Mn, Ni, Pb and Zn in mangrove sediments from different studies, published so far, for Indian mangrove is shown in Table 9.1.

Table 9.1 Trace metal concentration in different mangrove ecosystems of India
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The Sundarbans and Pichavaram have higher concentration of Al, As, Cd, Co, Cr, Fe and Mn (Table 9.1). In the tsunamigenic sediments in Pichavaram mangroves, Ranjan et al. (2008) reported higher concentration of almost all trace metals. Coringa-Gaderu, GoK and Manakudy estuary have higher concentration of Zn, Ni and Fe, whereas Coringa and Kumarakom have higher concentration of Cd.

3.1 Contamination Factor (CF)

An overall synthesis of reported CF in sediments from various mangrove habitats in India is presented in Table 9.2. Almost all studies of Indian mangrove ecosystem show most of the trace metals contamination in the range from ‘low’ to ‘moderate’. Few exceptions were ‘Fe’ in Sundarbans, Pichavaram, Muthupet, Sharavati and Goa and ‘Cd’ in Coringa-Gaderu, Pichavaram, Vellar and GoK. The CF for Co was high in Sharavati estuary and Goa (Feranandes and Nayak 2016; Dias and Nayak 2016).

Table 9.2 Contamination factor (CF), enrichment factor (EF), pollution load index (PLI), potential toxicity response index (RI) with respect to metals in major Indian mangrove ecosystems
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3.2 Enrichment Factor (EF)

The scenario of trace metal enrichment in various mangrove ecosystems of India is presented in the Table 9.2. In Sundarbans mangrove ecosystem, trace metal enrichment level is deficient to minimal, except for Cd and Ni, in most of the studies (Chowdhury et al. 2015). Enrichment factors for most of the trace metals in the mangroves of Muthupet, Vellar, Sharavati, Kumarakom, Goa, Coringa and Gaderu, Gulf of Kachchh and Vellar were deficient to minimal. Pichavaram also seemed to have somewhat similar enrichment pattern of trace metals except Cd. The mangrove sediment in Vellar had high enrichment of Cd and that in Pichavaram (Tsunamigenic sediment, Ranjan et al. 2008), Coringa-Gaderu and Gulf of Kachchh showed extremely high enrichment of Cd. This extremely high enrichment of Cd is due to the geogenic origin and deposition of marine sediments during the tsunami episode of 2004. This can be attributed to the fact that marine sediments from deep oceans are rich in Cd, which might have been trapped in mangrove sediments after the tsunami.

Extremely high enrichment of Pb in Bhitarkanika mangrove ecosystem indicates that Pb is precipitated at around the redox boundaries (Lee and Cundy 2001); the downward flux of Pb is bound to biogenic particles (Lambert et al. 1991) which increase its concentration. Enrichment of Cd in this estuarine environment can be linked to both natural and non-point anthropogenic sources. The abnormally high enrichment in tsunamigenic sediment can be attributed to various factors (Purvaja and Ramesh 2000; Subramanian 2004), viz. anthropogenic wastes from nearby soils brought back by tsunami waves and discharge from nearby industries (aquaculture effluents, domestic sewage). Factors like enhanced organic matter content, flocculation due to varying salinity regimes and transportation of sea sediments to the coastal zone contribute significantly towards the enrichment of heavy metals in sediments.

3.3 Pollution Load Index (PLI)

The pollution load of Indian mangrove ecosystem is presented in Table 9.2. With only few exceptions, almost all the mangrove ecosystems seemed to be polluted. Pollution of Pichavaram and Goa mangrove can be attributed to fishing, aquaculture, agricultural runoff, shipping, industries, tourism and boating. It was interesting to note that while most of the studies in Sundarbans showed the area to be highly polluted w.r.t. PLI, the recent findings by Chowdhury et al. (2015) were different.

3.4 Potential Toxicity Risk Index (RI)

Potential toxicity risk index (RI), presented in Table 9.2, reveals that the RI was in ‘low-grade’ category in all the studies done so far in Kumarakom, Muthupet, Goa and Bhitarkanika mangroves. Similar was the case with Sundarbans except for the study done by Banerjee et al. (2012). In Pichavaram, Coringa-Gaderu, Vellar and Gulf of Kachchh, the RI fell in ‘serious’ category.

3.5 Geo-Accumulation Index (I-geo)

Geo-accumulation index (I-geo) of various heavy metals in different mangrove ecosystems of India is presented in Table 9.3. Of all the mangroves in India, maximum investigations were undertaken in the Sundarbans. These investigations suggest different accumulation status of heavy metals in the mangrove sediments, ranging from ‘practically uncontaminated’ (Zn) to ‘moderate to strong accumulation’ (Fe and Mn). The category of geo-accumulation of metals in Pichavaram mangrove sediments differed from that reported in Sundarbans (practically uncontaminated includes Al, Mn, Pb and Zn; contaminated includes Cu; strongly accumulated includes Cd). However, tsunamigenic sediments showed different patterns of metal accumulation.

Table 9.3 Geo-accumulation index (I-geo) and potential ecological risk index (with respect to metals) in major Indian mangrove ecosystems
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In the mangroves of Coringa-Gaderu (Table 9.3), the I-geo for Fe, Cr, and Ni were in ‘practically uncontaminated’ category, and Cu, Co, Mn and Pb in ‘uncontaminated to moderate’ category. In Kumarakom mangrove sediments, I-geo values for Cr, Cu, Mn and Zn were in ‘practically uncontaminated’ category, and Fe was found to be ‘moderately accumulated in the sediments’. Most of the heavy metals examined in sediments of GoK and Bhitarkanika mangrove systems were found in “practically uncontaminated”’ category. Similarly, Al, Cu and Zn were in ‘practically uncontaminated’ category in Muthupet and Goa mangroves.

3.6 Potential Ecological Risk Index (Eif)

An overall picture of potential ecological risk is presented in Table 9.3 for the Indian mangrove ecosystems. In all the Indian mangrove ecosystems examined, Al, As, Cr, Cd, Ni, Pb and Zn seemed to have ‘low potential ecological risk’, whereas Cd had ‘high potential ecological risk’ (in Pichavaram, Coringa-Gaderu, GoK and Vellar).

4 Processes Governing the Distribution of Trace Metals

Other than the inputs from the terrestrial runoff and marine inputs (Peters et al. 1997), processes like redox potential, organic matter content, texture of the sediment, flocculation-deflocculation, etc., control the distribution and availability of the trace metals in the mangrove ecosystems. The anaerobic sediments of mangrove with high sulphur content favour trace metal retention, especially Fe and Mn, by combining with sulphides (Silva et al. 1990). Further, the Fe-Mn complex encourages retention of other metals due to the geochemical affinity with other trace metals. Hence, it is also referred as sink of trace metals (Förstner and Wittman 1983). Changing redox potential (oxidation) also causes metal desorption from the sediment to the water column making it mobile and bioavailable (Silva et al. 1990; Marchand et al. 2006). Sediment organic matter acts as a metal carrier (Ray et al. 2006), changing salinity regimes encourages flocculation (Eckert and Slokowitz 1976) and fine particles enhance metal accumulation by precipitation of metals as hydroxide coating over them (Förstner and Wittman 1983; Salomons and Förstner 1984).

The high Cd concentration in surface sediments of Pichavaram mangrove was attributed to the combined effect of both non-point anthropogenic inputs (fertilizers brought in by Vellar, Uppanar and Coleroon river) as well as natural sources, along with inputs of Cd-rich sediment from deep ocean carried by the tsunami (Li et al. 2006; Seralathan et al. 2006). Enrichment of Pb in Pichavaram mangrove indicates that the metal is precipitated around the redox boundaries (Lee and Cundy 2001), and the downward flux of Pb is bound to biogenic particles (Lambert et al. 1991) which increases its concentration. Aquaculture effluents, agricultural runoff and domestic sewage also play a role in Pb enrichment in the area (Purvaja and Ramesh 2000; Subramanian 2004). A higher value of Pb at Manakudy estuary mangrove (161.254 μg/g) were due to the local redox condition that allowed Pb to coprecipitate with Mn during Mn-oxide formation in the superficial segment (Lee and Cundy 2001). Early diagenesis of organic matter in sediments may also cause trace metal redistribution (Prajith et al. 2016).

5 Concerns

Accumulations of trace metals have been already reported in many Asian mangroves and associated flora and fauna (Sandilyan and Kathiresan 2014). Trace metals’ presence has been reported in more than 30 mangrove species and their various tissues (Lewis et al. 2011). Trace metals have been found in Avicennia and Barringtonia species of Kerala mangroves showing variable metal accumulation (Thomas and Fernandez 1997). Similarly, trace metals have been reported from stems and leaves of Bhitarkanika mangroves with stem showing higher accumulation (Panda et al. 2013). Bioaccumulation of trace metals in leaves of mangroves from Pichavaram revealed higher concentration of Pb than the normal range of contamination factor reported for plants (Agoramoorthy et al. 2008). Mangroves support various species like fishes and crabs with 60% of commercial fishes in Fiji and India (Lewis et al. 2011). Thus, it may pose threat for many fishes and crustaceans as roots of mangroves have higher accumulation capacity (Sandilyan and Kathiresan 2014). Other bottom dwelling faunal species like crabs, shrimps and filter feeders also face the risk of metal accumulation (Kaviraj and Guhathakurta 2004). Trace metals from mangrove environment can make up their way to human beings directly or indirectly via herbal medicine, honey, seafood and dairy products (Sandilyan and Kathiresan 2014). Recently, Pb and Cr concentration in important edible species of bivalves in the Vellar estuary were reported to be higher than the permitted WHO/EPA limits (Ponnusamy et al. 2014).

Mangrove ecosystems act as a trap for contaminants transported to the coastal ecosystems (Sandilyan and Kathiresan 2014), adversely affecting various species surviving there and ultimately affecting the food chain. Metal stress can weaken the mangrove vivacity (vigour) making it prone to pathological attacks (Awal 2014) and also affecting the general biodiversity in those habitats.

6 Summary and Conclusion

The increase of trace metals in the mangrove sediment may be attributed to many factors like the abundance of fine particles having a greater surface area, which increases precipitation of metals as hydroxide coating (mainly Fe and Mn) over them. Factors like high organic matter content, flocculation due to varying salinity and transportation of deep shore sediments to the coastal zone also contribute significantly towards enrichment of heavy metals in sediments. The high values of metals like Fe and Mn in the mangrove sediment could be attributed to the precipitation of the respective metal sulphide compounds in anaerobic sediments. Fe-Mn also shows very close distribution pattern of enrichment in surface/subsurface layers of sediment cores which might be due to the early diagenetic processes as well as the strong association to the geochemical matrix between the two elements. The precipitated Fe in the form of oxyhydroxides has the affinity to scavenge other metals such as Cu and Pb, as they pass through the water en route to the sediment. Many trace metals like Cd, Cu, Pb and Zn have a wide range of sources, mainly anthropogenic, like untreated waste discharge from industries, agricultural runoff, sewage effluent and other surface runoffs. Tsunamigenic sediments show higher contamination factor, enrichment factor and potential ecological risk index than other mangrove sediments, which might be due to higher waste discharge at that time along with tsunami-driven sediments derived from the deep ocean. Thus, it shows that mangrove ecosystem act as the sink for trace metals; many mangrove species also act as a bioaccumulator of trace metals that after dying and degeneration enhances the trace metal concentration in mangrove ecosystem thus increasing its vulnerability.