Biomagnification
Persistent Organic Pollutants
Some pollutants, such as DDT (a pesticide) and PCBs (carcinogens used in plastic manufacturing), last a remarkably long time prior to breaking down. These are known as persistent organic pollutants, or POPs. DDT has a half-life of 30 years and PCBs have a half-life of 10 years, meaning they can still be present in the environment decades after they were last used. POPs are synthetic, carbon-based molecules that are fat-soluble, resulting in them building-up in organisms' fatty tissues and not readily leaving the body through things like urine as they do not dissolve easily in water.
As they do not break down easily, they can affect environments far from their original source. They can travel over long distances, being carried by wind and water, prior to being deposited in a new environment. Many modern synthetic compounds are now designed to break down much sooner, in an attempt to avoid the issues associated with POPs.
While not all endocrine disruptors are POPs, many of them are, lengthening their potential negative effects on the ecosystem.
Endocrine Disruptors
Some POPs are what is known as an endocrine disruptor - chemicals that disrupt the endocrine system of organisms (probably not surprising given the name). These molecules are similar in structure to hormones, allowing them to bind to hormone receptors, either blocking hormones from binding when they should or activating a pathway that shouldn't be activated at the time. This can lead to birth defects, developmental disorders, and imbalances in the sex of various species but particularly aquatic species.
Bioaccumulation and Biomagnification
Fat-soluble molecules can build up within the cells of organisms; this is known as bioaccumulation. As organisms that were bioaccumulating certain compounds are eaten, these molecules can amplify in concentration with each level of a food chain, which is known as biomagnification.
 The molecule is taken in by producers, which are then eaten by primary consumers. The consumers take in the pollutants that had built up in the producers, and what they have is then taken in by the secondary consumers and so on. Due to the 10% rule, organisms at higher trophic levels need to eat more and more to receive enough energy. Large predators that are high on the food chain have the highest levels of the chemical within them.
You may be familiar with the concept of biomagnification without even knowing it. Have you ever heard that you shouldn't eat certain types of fish every day? That's because of this exact thing. Mercury, when it enters aquatic environments, is converted into methylmercury by bacteria. It builds up in the food chain and becomes more prevalent in animals higher up the food chain. As mercury is a neurotoxin, it is highly toxic and can disrupt a great many physiological functions.
DDT is a pesticide and endocrine disruptor that was commonly used throughout the world for agricultural purposes. As it gets into aquatic ecosystems, it is taken in by plankton and then biomagnified at higher trophic levels. Top predators, such as raptors like the bald eagle, experienced significant negative impacts, including thinned eggshells and developmental deformities. This led to bald eagles becoming endangered and the eventual passing of the Endangered Species Act in the United States, with the banning of DDT being a result.