Trace Metals in a Tropical Mangrove Wetland

The Indian Sundarban wetland and its adjacent Hooghly (Ganges) River Estuary are situated in the low-lying, meso-macrotidal, humid, and tropical belt, influenced severely by semidiurnal tides of high amplitude. This globally significant estuarine deltaic complex deserves special attention as they support to sustain rich and diverse flora and faunal communities. However, this productive but vulnerable wetland suffers from environmental degradation and becomes susceptible to chemical pollutants including trace metals which have changed the estuarine geochemistry by complex processes. The 24 studied sampling sites are representative of the variable environmental and energy regimes that cover a wide range of substrate behavior, wave–tide climate, geomorphologic–hydrodynamic regimes, and distances from the sea (Bay of Bengal). They have diverse human interference with a variable degree of exposure to heavy metal contamination. The chapter has been dealt with details of selection of the study sites, sampling strategy, instrumental techniques in environmental chemical analyses, and application of statistical methods for interpretation of data. In addition, the diagnostic of sediment contamination and ecological risks using several important indexes along with consensus sediment quality guidelines (SQGs) are also presented along with practical examples and their solutions.

The chapter presents a comprehensive account regarding concentration, distribution, and possible pollution sources of selected trace elements in the surface sediment samples (top 0–10 cm; <63 μm in grain size) collected from the intertidal regions of the meso- macrotidal estuarine complex covering Sundarban mangrove wetland (SWM) and Hooghly River Estuary (HRE), eastern part of India. The total concentrations of the elements (expressed in μg g⁻¹ dry weight) showed a wide range of variations as follows: Al 7563.61–64,012.77, Fe 8326.24–44,653.68, Mn 143.49–728.08, V 13.25–93.43, Cr 11.58–82.75, Co 3.22–17.97, Ni 7.80–52.81, Cu 5.82–53.72, Zn 19.70–118.95, Pb 2.90–24.06, Cd 0.01–0.45, and As 1.85–10.66. Their distribution was likely to be determined by sediment transport pathways, hydrodynamic conditions, and input from point and nonpoint sources. The concentration of all elements (except Cd) was found to be maximum at the sampling site Frazergunj; hence, it can be identified as a “hot spot.” Assessment using consensus-based sediment quality guidelines (SQGs) indicates that the concentrations of Ni have exceeded both ERM and PEL values which might result in frequent harmful effects on sediment-dwelling organisms. At the sampling site Budge Budge in HRE, both geoaccumulation index (I _geo) and enrichment factor (EF_Cd = 2.68) values revealed significant pollution by Cd which might have been derived from anthropogenic sources. However, the mean ecological risk index (RI = 31.39 ± 12.21) indicated low ecological risk for this estuarine environment. Pearson correlation showed significant positive correlation between elements and also good correlation of elements with finer fractions in sediments.

The chapter examines a detailed account of the geochemical assessment of ten trace metals (TMs) in total and acid-leachable (ALTM) fractions of the surface sediments (grain size < 63 μm; top 0–10 cm) from Sundarban coastal regions and adjacent Hooghly River Estuary (HRE), India. Total TM concentration exhibited a wide range of variations in the following descending order (mean values expressed in mg kg⁻¹ dry weight): Fe (32343.40 ± 6726.21) > Mn (549.51 ± 121.85) > Zn (87.63 ± 15.86) > Cr (64.83 ± 12.81) > Ni (55.67 ± 8.66) > Cu (49.13 ± 20.24) > Co (41.48 ± 9.03) > As (36.65 ± 11.74) > Pb (24.78 ± 2.64) > Cd (4.63 ± 1.65). An overall similar trend was also discernible for leachable fractions, except two hazardous trace metals, Pb and As. The mean bioavailability for the TMs revealed that Pb was the most mobile (27.10%), followed by Cu, Zn, and As (13–17%) and Cd, Ni, Co, and Cr (4–7%), mainly concerned with multitude of human activities (industrial effluents, discharge of sewage, agricultural runoff, etc.). Maximum concentration of majority of the TMs was evident at the lower stretch of the estuary. The geoaccumulation index (I_geo) and contamination factor (CF) values affirm the intensity of severe pollution by Cd and As. The ecological risk associated with the TMs in sediment was considered to be occasionally observed for As, Cd, Cu, and Ni (effects range low (ERL)–effects range medium (ERM)) and frequently observed for Ni (greater than ERM) based on assessment using consensus-based sediment quality guidelines (SQGs). A similar pattern of correlations was observed for TMs and ALTMs as evident from Pearson correlation matrix, whereas ALTMs exhibited significant association with silt. The study provides a useful tool for sustainable management in this productive estuarine complex.

The use of bioindicator organisms for the biomonitoring of heavy metal toxicity and their ecological effects draws special attention. Many marine invertebrate species fulfill the following criteria: sensitivity to a wide range of inorganic contaminants (especially to heavy metals), cost-effectiveness for repeatable tests, and readily interpretable biological consequences of pollution. Among the potential marine invertebrates used as bioindicators, littoral benthic species form key components to address the metal bioavailability. Polychaetes (Annelida) are the dominant component of soft-bottom macroinvertebrate communities, forming a potential macrozoobenthos group. The chapter has been designed to investigate the trace metal bioaccumulation strategies in diverse macrozoobenthos groups (polychaetous annelids and bivalve and gastropod mollusks) along with the host sediments from the intertidal regions of Indian Sundarban mangrove wetland. The suitability of using these organisms in biomonitoring of heavy metals has been elaborated with the present research findings. Both species-dependent variability and temporal variations were pronounced. A high degree of organ specificity was evident in the bivalves where gill and mantle exhibited higher metal accumulation due to ion exchange property of the mucous layer covering these organs while shells represent very poor accumulation. The organisms are differentially selective for a range of metals, and these variations might be influenced by a number of intrinsic and extrinsic factors. Moreover, the concentration of heavy metals in the tissues of marine invertebrates depends on the accumulation strategy adopted by each species for each metal. The enrichment of Mn (maximum 730 mg kg⁻¹) and Zn (maximum 320 mg kg⁻¹) was recorded for all the polychaetes followed by Cr, Cu, As, Co, Ni, and Pb, showing sharp variations between the species. However, concentrations of Sn and Hg were very low with small variations between the species. An overall high bioconcentration factor (BCF) was recorded in the capitellid worm Mastobranchus indicus. The results of the investigation indicate metal bioaccumulation in these macrozoobenthos group has the potential for use in any future regulatory framework monitoring and eventually controlling ambient metal pollution levels.

The sequential extraction process is a unique tool in assessing the specific geochemical forms and binding states in the sediments and their ecological risk on the biotic species and thus confirming the environmental behavior of metals (mobility, pathways, and bioavailability). Different sequential extraction schemes have been used for the geochemical fractionation study of metals in sediments. Owing to the need for standardization and subsequent validation of extraction schemes for sediment analysis, the European Community Bureau of Reference (BCR) introduced in 1993 a new three-step sequential extraction. This BCR sequential extraction method analyzes different fractions of metals in the soil: acid extractable (water soluble, exchangeable, and bound to carbonates), reducible (bound to Fe and Mn oxides), oxidizable (bound to sulfides and organic matter), and residual. Water-soluble and exchangeable forms are considered readily mobile and available to biota, while metals that are incorporated into the crystalline lattices of clays appear relatively inactive. BCR procedure was followed in the present study to determine the concentrations of 11 trace metals (Al, As, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, Zn) in different geochemical phases of sediments collected along the Hugli (Ganges) River Estuary and adjacent Sundarban mangrove wetland, India. This is considered as the most conventional method as it is simple, is applicable to all kinds of sediments, is low cost, and has easily understandable and comparable results. The chapter presents detailed information with respect to the origin, mobilization and transport, biological availability, and potential toxicity to various species.

Assessment of sediment quality is of prime importance for proper management of marine environment as required by the Water Framework Directive (2000/60/EC) (WFD) and Marine Strategy Framework Directive (2008/56/EC) (MSFD) that considered sediment as the crucial component for sound management of water bodies. Organotin compounds (tributyltin (TBT), dibutyltin (DBT), monobutyltin (MBT), and triphenyltin (TPhT)) are widely used as active ingredients in marine antifouling coating to resist the settlement of biofouling agents. The chapter explores the quantification and speciation of butyltin contamination levels in surficial intertidal sediments of Indian Sundarban mangrove wetland as well as adjacent Hooghly (Ganges) estuary. Concentration of TBT, DBT, and MBT exhibited a wide range of variations for 16 studied stations of diverse environmental stresses. The correlation coefficient values showed positive significant values between the compounds as follows: MBT and DBT (r = 0.62; p = 0.01) and DBT and TBT (r = 0.54; p = 0.03). The degradation of TBT assessed by BT degradation index (BDI) revealed recent input of BTs at eight studied stations and very recent degradation or absolutely no degradation at four other stations. Based on Australian sediment quality guidelines (SQGs), it is revealed that TBT concentrations in majority of study sites are high and to be considered as potential threats for sustenance of benthic organisms.

The case study highlights the concentration of total arsenic and individual arsenic species in four soft-bottom benthic marine polychaetous annelids of diverse feeding guilds from the intertidal regions of Indian Sundarban wetland. An additional six sites were also considered exclusively for surface sediments as this would provide important information regarding cycling of arsenic in this estuarine system. Polychaetes (Perinereis cultrifera, Ganganereis sootai, Lumbrinereis notocirrata, and Dendronereis arborifera) and the host sediments were collected adopting a standard protocol and subsequently measured for their T_As arsenic content using inductively coupled plasma mass spectrometry (ICP-MS). Concentration of arsenic (As) in polychaete body tissues exhibited wide range of variations, suggesting species-specific characteristics and inherent peculiarities in arsenic metabolism. Arsenic was generally present in polychaetes as arsenate (As V, ranges from 0.16 to 0.50 mg kg⁻¹) or arsenite (As III) (from 0.10 to 0.41 mg kg⁻¹) (30–53% as inorganic As) and dimethylarsinic acid (DMAV < 1–25%). Arsenobetaine (AB < 16%) and PO₄-arsenoriboside (8–48%) were also detected as minor constituents, while monomethylarsonic acid (MAV) was not detected in the polychaetes. The maximum T_As (14.7 mg kg⁻¹ dry wt) was recorded in the polychaete D. arborifera inhabited in the vicinity of a sewage outfall in which the majority of As was present as an uncharacterized compound (10.3 mg kg⁻¹ dry wt) eluted prior to AB. For host sediments, total As ranged from 2.5 to 10.4 mg kg⁻¹. The results support the importance of speciation analysis of arsenic, because of the ubiquitous occurrence of this metalloid in the environment, and its variable toxicity depending on the chemical form, present.

Mangroves are unique halophytic biogenic community in intertidal zones grown in tropical and subtropical latitudes and frequently act as a metal biofilter in estuarine environments. These specialized evergreen plants grow in physiologically dry soil, possessing highly specialized features that make them adaptive to the stressed coastal estuarine regions, viz., pneumatophores, prop roots, salt-excreting leaves, viviparous mode of germination, etc. Mangrove habitats favor accumulation of fine-sized sediment particles and organic detritus, resulting in the entrapment of both inorganic and organic pollutants. Therefore trace metal concentration in mangrove plants along with the host sediments (rhizosediments) would be considered as a potential tool on evaluating the status of metal contamination. The present chapter highlights the potential role of mangroves in sequestering trace metals from rhizosediments considering three representatives of the genus Avicennia, namely, A. officinalis (Linnaeus) 1753, A. marina (Forssk.) Vierh. 1907, and A. alba (Blume) 1826, provided with salt-excreting glands belonging to the family Avicenniaceae in Sundarban wetland. These dominated facultative mangrove trees have specific adaptive mechanisms with a physiological, morphological, and anatomical features to alleviate toxicity of metals. However, the mechanisms of internal and outer metal detoxification of these plants are still only partially understood.