My guess is that fat-soluble toxins that may occur in freshwater ecosystems would most affect apex species in these ecosystems. This is because fat-soluble toxins are stored in the fat reserves of animals, and then bioaccumulate up the food chain. So in this case, most of the toxin is not secreted, but stored within the food chain, and bioaccumulates, with the concentration becoming higher the further up the food chain.
The answer is written on the pic I sent you with this
