The level of pollution in the food chain, based on estimates of DDT concentration in aquatic environment increases in the following sequence:

Explanation

This phenomenon is known as biomagnification. It's the increasing concentration of a substance, such as a toxic chemical, in the tissues of organisms at successively higher levels of the food chain.  

1) Water plants absorb DDT from the water.
2) Snails consume water plants and accumulate DDT.
3) Fish eat snails and concentrate DDT further.
4) Birds prey on fish, accumulating the highest concentration of DDT.

This process is particularly harmful to top predators, as they can suffer from reproductive problems, weakened immune systems, and even death due to high levels of toxins.