Many aquatic animals, frogs in particular, are EXCELLENT indicators, because amphibians have permeable skin. That means they get their water through osmosis; they soak it up through their skin. This is interesting, because they also soak up any pollutants (dioxins, PCB's) that happen to be in the water. read more
Pollutants may not have a particularly large effect on the aquatic world because the volume of water diffuses the pollutant to lower the concentration so that there is little effect except far up the food chain. However, the more pollution there is, the further down the food chain the animals will be dying off. read more