How is energy lost from from an ecosystem?

Energy that is not used in an ecosystem is eventually lost as heat. Energy and nutrients are passed around through the food chain, when one organism eats another organism. ... In each case, energy is passed on from one trophic level to the next trophic level and each time some energy is lost as heat into the environment.

well, that's easy; the answer's c, "it reduces contamination of the water and substrate of the estuary."