Effects on Aquatic Organisms - Water Pollution
Toxic water pollutants such as metals, pesticides, insecticides and chemicals affect aquatic species directly whereas the non-toxic organic load may eliminate some aquatic species indirectly by reducing the DO of water. The sensitive species such as Trout, and Bass that require high level of oxygen are eliminated first. With further decrease in DO, leeches, catfish and carp get killed and only the resistant species, for example, eel and hardy worms survive. Trout and salmon are excellent indicators of pollution. The aquatic flora of lakes and ponds is also affected by slight variations in sustaining elements, thus affecting the whole ecological system.
Generally plants and animals have the ability to wash out many poisons out of their system but certain chemicals such as pesticides, insecticides, methyl mercury tend to accumulate in the body. For example, DDT, an organochloride, does not dissolve in water but dissolves in fats, therefore, it accumulates in the organism for a long period. This is called bioaccumulation. These poisons then move in other organisms through food chain. For example, 0.02 ppm concentration of DDT in a lake entering into a bird through plankton (5 ppm), fish (400-300 ppm) may go up to 2000 ppm. Similarly, spray of DDT on soil (9.9 ppm) reaches 445 ppm in a Robin through plant (44 ppm). The increase in concentration of a toxin in food chain at higher level is called biomagnifications. Low levels of DO in water body expedite bioaccumulation because the rate of breathing of fish increases with decrease in DO. Hence it is forced to take in large amounts of water which may be polluted with toxic metals and chemicals. So their intake in fish would increase many fold than in the water body.