Heavy Metals in the Marine Environment—An Overview

Abstract

Heavy metals are the natural constituents of the Earth’s crust, found in very low concentrations; however, human activities such as industrial waste discharges, agricultural practices, coastal construction and dredging have inevitably increased the metal concentrations in the marine environment. Heavy metals are classified as both essential and non-essential elements. Essential heavy metals have known biological roles and are only toxic above threshold concentrations whilst non-essential heavy metals lack any known biological role in marine invertebrates and exhibit high degree of toxicity if allowed to accumulate at metabolically active sites. Heavy metals are non-biodegradable, persistent and toxic to the environment, thus causing serious eco-toxicological problems. Heavy metals tend to bioaccumulate and the extent of their bioaccumulation is dependent on the total amount, the bioavailability of each metal in the environmental medium and the route of uptake, storage and excretion mechanisms. Metal speciation influences metal bioavailability and toxicity to biota, its transportation and mobilization, and its interaction with the environment. The actual metal speciation is influenced by factors such as pH, the types and concentrations of inorganic ligands and organic ligands and colloidal species present.