Metal Transport and Its Impact on Coastal Ecosystem

Abstract

Heavy metal pollution has risen as an alarming threat in the aquatic systems, including coastal ecosystems comprising of mangroves, salt marshes, wetlands, bays, and estuaries. Heavy metals that pollute the coastal ecosystems mainly consist of metals like arsenic (As), lead (Pb), cadmium (Cd), chromium (Cr), zinc (Zn), copper (Cu), nickel (Ni), and manganese (Mn). The transport of heavy metals in coastal ecosystems occurs through various natural as well as anthropogenic sources. The natural sources comprise natural leaching of bedrocks, transportation from land, and input from freshwater systems, while the anthropogenic sources include mining, smelting, and industrial effluent, followed by agricultural and domestic runoff. Rapid economic growth has further accelerated the transport of heavy metals in coastal ecosystems. The pollution caused by heavy metals not only is restricted to the water but also affects the sediments and biological systems. The heavy metals are not degraded naturally as organic matter and are frequently returned to the system through physicochemical and biological processes, posing risk to the health of humans and the ecosystem. The heavy metals have a strong affinity for particle surfaces; hence the majority is deposited in the sediments because of processes like adsorption and coprecipitation. The mobility, speciation, and bioavailability of these heavy metals are dependent on physical and chemical properties such as pH, redox potential, organic content, and salinity, rendering them as potential pollutants. The slack water conditions in coastal areas encourage heavy metal accumulation; high levels of organic, clay, and sulfide content enhance the adsorption of these metals, while the high rate of sedimentation enhances permanent deposition of locally formed metal sulfides and refractory metal-organic complexes. Such transformations pose a great threat as due to bioavailability they enter the food chain and biological systems causing adverse effects on biological and ecosystem health. Several research works present the health impacts and ecological effects caused due to the contamination of heavy metals by assessment of enrichment factor, ecological risk index, geo-accumulation index, and pollution load index. However, owing to deteriorating water quality, more extensive studies are required. Therefore, it is extremely important to look into the sources, processes, fate, and consequences of the heavy metals in coastal ecosystems, and design appropriate management policies to save the ecosystem from being further polluted through the contamination of heavy metals.