Mangrove ecosystem is a very unique ecosystem in the Earth, which is under threat due to habitat loss, aquaculture expansion, overharvesting and increase of pollution load. In this review paper, world-wide status of mangrove habitat loss, role of mangrove to act as a sink of pollutants and carbon capture (carbon sequestration), accumulation and biomagnifications of heavy metals is discussed. Emphasis has been given to understand the effect of heavy metals, organic and inorganic pollutants on the mangroves and the natural ability of this ecosystem to tolerate the pollution load. Lastly the guidelines of mangrove research for the developing countries are also suggested.
Mangroves are coastal forests found in sheltered estuaries and along river banks and lagoons in the tropics and subtropics. The term “mangrove” describes both the ecosystem and the plant families that have developed specialized adaptations to live in this tidal environment [
Mangrove restoration work has been carried out in the Tutuila Island, American Samoa [
Basically Mangroves are woody halophytic plants, which exist in the conditions of high salinity, extreme tides (
tress are all indicative of the evolutionary selection to persist in muddy, brackish coastal environment on physiologically dry soil.
Mangroves are salt tolerant species and can take up water despite of high osmotic potential of soil water and even if the salt is absorbed, it is excreted through the salt glands in the leaves.
To mark out the general mangrove community along with other flora and fauna, the term “mangal” was proposed [
The exact number of species is still under discussion and ranges from 50 to 70 according to different classifications [
Mangrove ecosystems can be used as indicators of coastal change or sea-level rise. These ecosystems are so specialized that any minor variation in their hydrological or tidal regimes causes noticeable mortality [
Unfortunately this unique ecosystem is itself being destructively harmed by the progress of civilization. Recent assessments on extent of mangroves worldwide suggests that between 1990 and 2010 there is a reduction of 3% of mangroves cover throughout the world and reasons are primarily land conversions for coastal development, rice production and aqua cultural projects [
About 25 countries of Asia have mangrove ecosystem, with climatic variation, ranging from arid (Arabian Peninsula) to sub-tropical (China, Japan) to humid tropical (South East Asia) [
water input from rivers makes Bangladesh, India, Malaysia, Thailand and Indonesia a favorable place for growth of well structured mangroves, where the trees grow to a height of 30 - 50 m.
In Asia, Sundarbans, is the world’s largest contiguous mangrove patch covering an area of 10,000 km2 and is the part of the progradation delta of Ganga-Brahmaputra-Meghna river systems that comprises of an area of 80,000 km2 [13-15] and recognized internationally as the UNESCO (United Nations Educational, Scientific and Cultural Organization) World-Heritage site, The transboundary forest of Sundarbans is spread over two countries, of which 60% is in Bangladesh and 40% in India. This mangrove ecosystem is affected by numerous cyclonic storms [
Like all other green species, Mangrove has got definite role against the pollution. It has natural ability to act as a sink of anthropogenic and industrial pollutants. Mangrove ecosystems are specific in numerous aspects (e.g. carbon and nutrients cycles, sediment characteristics, tidal conditions) which are expected to affect the speciation, and therefore the bioavailability of contaminants [
Rise in industrialization and uncontrolled anthropogenic pressure on virgin mangrove patches has been increase in recent years, however, mangroves ecosystem adapted themselves by acting as natural pollution sink. Mangrove soils/sediments are usually fine-grained, water-logged and receive allochthonous organic matter from terrigenous origins [
Salt marshes or mangroves are characterized by highly anoxic reducing soil, with high decomposer activity [
Mangrove ecosystem retains toxic metals and stops it from infiltrating into the marine ecosystems. Different mangrove forest areas across the world have varying level of pollution load. A correlation is observed between total organic carbon (TOC) and heavy metal concentration [
Marine, estuarine organisms can bioaccumulate trace metals and pollutants and it is expressed by biota-sediment accumulation factor (BSAF) which is actually a ratio of concentration of pollutants in the tissue and concentration of the same pollutant in the sediment. Recent research [
Environmental degradation due to impact of nutrient and heavy metal pollutants, can give raise to asphyxiated swamp, where Dissolved Oxygen (DO) falls. There is a substantial amount of litter, vegetation present in the mangrove ecosystem for decomposition by microbial action and through detritus food chain. But lack of oxygen is eventually gives rise to the dead zone [
whereCF = contamination factor;
n = number of metal;
CSample = metal concentration of sediment and;
CBackground = mean metal concentration from healthy mangrove swamp.
PLI is indicative of number of times the contamination of metal in sediment exceeds that in natural unpolluted environment.
The mean concentration of metals (mg/kg. dw) namely Zn, Cu, Ni, Pb, Cr and V in sediment at asphyxiated and healthy mangrove ecosystems of Qua Iboe vary from 36.3 - 179.4, 29.2 - 43.2, 3.6 - 37.4, 39.6 - 93.8, 0.15 - 0.53 and 2.9 - 9.3, where the former have higher metal accumulation potential [
Several studies reported the accumulation of non-nutrients metal in mangrove sediment and bioaccumulation to aerial tissues. Mangrove ecosystem is used as an effective pollution sink, where the pollutants from different industrial and anthropogenic activities are diverted into the mangrove ecosystem. Paper and petroleum effluents are also one of the major sources of pollution in mangrove ecosystem. The, toxicity studies for mangrove plants have focused on the effects of trace metals (Cu, Cd, Hg, Mn, Pb and Zn), oil residues, some herbicides and raw wastewater. Under controlled conditions, the effect of trace pollutants on mangrove plants were studied in detail and it reveals that photosynthesis, growth, and biomass was reduced due to their effect and it finally increases mortality [
The contaminant accumulation in sediments and bioaccumulation pathway on mangrove ecosystem is presented in
The metal concentration in sediment often differs geographically for the same trace metal. Literatures indicated that, out of all trace metals Mn accumulation was reported highest (sometime Fe) and least for Cd. The general sequence is Mn (=Fe) > Zn > Cr > Pb > Cu > Cd. Concentration of metals (µg/gm dry wt) in mangrove tissues is reported as Mn (4.5 - 2472) > Zn (0.7 - 1988) > Pb (0.02 - 225) > Cu (0.5 - 207) > Cd (0.01 - 3.1). Different species have shown different degree of metal accumulation potential.
Metal concentration is usually higher in mangrove roots than aerial parts. BCF (Bioconcentration Factor) are usually low in mangrove tissues other than roots, thus mangrove tissues are not generally considered as effective indicator of pollution. Out of 60 mangrove species, 33 species are used for toxicity test [
There Cu and Pb were found to be accumulated in higher concentration in root tissues than sediment concentrations, whereas in leaf tissue Cu, Zn was found more than 10% of that in the root [
There is a trend of change in mangrove biodiversity in different parts of the globe. Most of the investigation is revolving around the bioaccumulation potential of different mangroves, which reveals that Avicinnea sp is one of the most tolerant species in respect to heavy metals, amongst mangroves. In Indian scenario there is a clear increase of A. marina in different mangrove patches. Thus one can derive at this point that pollution factor can also be a potential reason for their dominance. So more pollutants would mean proliferation of only pollution tolerant mangroves to flourish and ecosensitive species would be replaced, and henceforth would result in deterioration of mangrove biodiversity.
Literature reviews stated that trace metals, Polycyclic Aromatic Hydrocarbons (PAHs), Persistent Organic Pollutants (POPs), Pharmaceuticals and Personal Care Products (PPCPs) and Endocrine Disrupters Compounds (EDCs) have been detected in various mangrove compartments (water, sediments and biota) [
Carbon emission is the major cause for climate change and global warming. There is a 36% raise in global CO2 emission from 1992 to 2008 and it shows a steady mean increase from 357 ppmv (parts per million by volume) in 1992 to 389 ppmv in 2011 [
It will be a mere repetition if we again elaborate or point out the multilayered function of this unparallel ecosystem. Therefore protection and conservation of mangrove ecosystem in the developing countries should be given priority. Along with the above factors, chemical pollution, particularly accumulation and bio-transformation of toxic metals could be a significant factor for reduction of mangrove biodiversity. Mangrove sediment is soft and muddy, extended up to a depth of 3 - 4 m, rich in organic carbon, and acted as a “carbon reservoir”. The assessment of distribution of carbon profile (both spatial and temporal) is essential to determine the C-sequestration potential of mangrove ecosystem for developing countries.
The authors are grateful to Ms. Namrata Dey Roy, (Lecturer Susil Kar College, University of Calcutta) for helping in editing the manuscript.