International Journal of Marine Science, 2016, Vol.6, No.9, 1-20
3
culprits in this area are Hg, Cd, Cr, Pb, As, Zn, and Cu. Out of these Hg and Cd snatches more attention due to
their well-known toxic effects and biomagnification efficiency. Industrial discharge, agriculture run off,
combustion, urban discharge, mining etc are the sources by which the heavy metals reach the marine environment
(Bilandzic
et al
., 2011).
The major toxicity effects of heavy metals are following
- The blocking of essential functional groups of the biomolecules like proteins and enzymes
- The displacement of a metal ion from a biomolecule
- The inhibition of function of biomolecules by the modification of the structure
The heavy metals do not decompose naturally and in aquatic environment some of them get converted to more
toxic forms thereby posing a real threat to marine organisms and human health. The best example is the
conversion of mercury into methyl mercury (MHg). The concern about the metal transfer in the marine food web
and its ecotoxicological studies stem from the Minamata incident which occurred in the 1950’s in Japan (Wang,
2002). Mercury is a global pollutant and it is highly persistent in the environment (Lacerda and Fitzgerald, 2001).
Due to the very low solubility product of its compounds the major portion of mercury that reaches the coastal sea
gets precipitated and accumulated in the sediments (Spada
et al.
, 2012). Here it can stay for a longer period
undergoing many transformations. The conversion of mercury to its organic form - methyl mercury under
favourable conditions is more dangerous (Ullrich
et al.
, 2001). The monomethyl form of mercury is the major part
of mercury in fishes and shell fishes. Hence its consumption on regular basis may serious threat to humans (Giani
et al.
, 2012, Agah
et al
., 2007). Cadmium accumulates mainly in the kidneys and liver of marine organisms
(Ozden
et al
., 2010). The levels of mercury and cadmium in the fishes and mussels from various sites are shown in
the Table 1.
The accumulation of metals varies according to the chemical form. Claisse
et al
. (2001) found that mussels
accumulated higher Hg and MHg concentrations in their soft tissues than oysters, but they have less MHg than
fish and hence apparently present a smaller risk to human consumers. Mussels generally accumulate more metals
than any other organism. A review on metal accumulation in Mediterranean mussel Mytilus galloprovincialis,
revealed that the concentrations of toxic metals were in the following order: As>Pb>Cd>Hg (Stankovic and Jovic,
2012).
3.2 Plastic Pollution
Plastics constitute the most significant part of marine litter deposits and all rubbish floating in the oceans. Marine
litter consists of items that have been made or used by people and deliberately discarded into the sea or rivers or
on beaches; brought indirectly to the sea with rivers, sewage, storm water or winds; accidentally lost, including
material lost at sea in bad weather (fishing gear, cargo); or deliberately left by people on beaches and shores
(UNEP, 2003). Monitoring the extent of plastic pollution in the marine environment at a global scale is
complicated due to the large spatial and temporal heterogeneity in the amounts of plastic debris and also due to
our limited understanding of the pathways followed by plastic debris and its long-term fate (Ryan
et al
., 2009).
Plastics are dumped in huge volumes in well-used beaches, lakes, navigation channels and other forms of water
masses. Most plastics are less dense than water, and it enable them to float and readily be transported for long
distances from source areas. Floating plastic debris have become a global problem now because they are carried
across ocean basins, contaminating even the most remote islands and polar regions (Barnes,
et al
., 2009). The UN
Environment Programme estimated that in 2006 that every square mile of ocean contains 46,000 pieces of floating
plastic (UNEP, 2006).
Plastics do not degrade easily and thus poses a real threat to the marine world (Laist, 1997). Most plastics break
down slowly through a combination of photo degradation, oxidation and mechanical abrasion (Andrady, 2003).
Except for expanded polystyrene, plastics take much longer time to degrade in water than they do on land, mainly
