Sometimes that reaction is positive, and sometimes it is negative. For example, right after World War II, a pesticide called DDT was used a great deal. As a result, many birds starting having trouble reproducing, and several bird species were in danger of extinction. Introducing DDT into the environment had an unexpected, negative result. If a chemical is in lake water, it may have an effect on frogs.
Frogs have soft, thin skin that easily absorbs any chemical in the water.
Also, when frogs reproduce, they must lay their eggs in the water. These eggs don't have a shell. They are coated with something more similar to jello. Any chemicals in the water will be absorbed into this jello covering and affect the frog that develops. Because frogs are so vulnerable, it is easy to see how a chemical in lake water might effect them.