**2. Metals pollution**

Unlike other pollutants, which may be visibly buildup in the environment, trace metals in the environment may accumulate unnoticed to toxic levels. These metals pollutants in the aquatic environment can come from natural or anthropogenic sources. Metals are serious pollutant in our natural environment due to their toxicity, persistence, and bioaccumulation problems

**Figure 3.** Bioaccumulation process of metals concentration in fish. Picture adapted from http://www.hydro-industries. co.uk/case-studies.htm?id=10 [2].

(**Figure 3**). Some are highly toxic and persistent, and have a strong tendency to become concentrated in marine food webs. Excess of these metal levels in aquatic environment may pose a health risk to humans and to the environment.

Organisms require certain trace amounts of some metals, including cobalt, copper, iron, manganese, and zinc in their growth process. Excessive levels of essential metals in the environment, however, can be detrimental to the organism itself. Besides that, nonessential metals of particular concern to surface water systems are cadmium, chromium, mercury, lead, and arsenic, and these metals have no biological function. Metals pollution in aquatic environment can be categorized into four major groups [3] according to their pollution potential:

