Final answer:
DDT, a pesticide used to regulate mosquito breeding in a lake, affects the trophic levels in the associated food chain. It bioaccumulates in organisms, particularly birds at the top of the food chain, leading to thin eggshells and population decline. The ban on DDT led to population recoveries.
Step-by-step explanation:
DDT, a pesticide used to regulate mosquito breeding in a lake, can affect the trophic levels in the associated food chain. DDT is a persistent and toxic substance that bioaccumulates in organisms at each trophic level, leading to higher concentrations in organisms higher up the food chain. When birds at the top of the food chain, such as eagles and pelicans, consume prey that has accumulated DDT, the chemical affects their ability to produce eggs with normal shell thickness, resulting in a decline in population size. The ban on DDT in the United States in 1972 allowed affected bird populations to recover.