Why does studying pollutants in dolphins give a better representation of pollution impacts on an ecosystem than studying marsh grasses?

Organisms low on the organic phenomenon like plants absorb chemicals like pollutants that are in their surroundings, whether or not within the soil, water, or air. Once a fish comes by and chow that plant, he’s intake the pollutants within the plant too. 
Dolphins are thought-about animal sentinels for environmental health owing to their long lives, native residences, and role as apex predators.