Final answer:
Aquatic life dies from ingesting toxic substances that biomagnify, including PCBs and heavy metals. These toxins can accumulate through the food chain, affecting high trophic level species and resulting in significant environmental concerns like dead zones.
Step-by-step explanation:
Thousands of fish, birds, and other aquatic life die each year from ingesting toxic substances that biomagnify through the food chain. Substances such as polychlorinated biphenyls (PCBs), heavy metals including mercury, lead, and cadmium, as well as other contaminants like polycyclic aromatic hydrocarbon (PAH), and trichloroethylene (TCE) can accumulate in aquatic ecosystems, resulting in high concentrations in predatory fish species and birds that consume these fish.
The process of biomagnification, illustrated by studies such as those by the National Oceanic and Atmospheric Administration (NOAA), shows that predators like the walleye can accumulate more than four times the amount of PCBs compared to the primary producers (phytoplankton). This demonstrates how even low environmental concentrations of these toxins can lead to high levels in apex consumers and beyond. Excessive nutrients, like phosphorus and nitrogen from fertilizer runoff, can cause algae overgrowth, depleting oxygen and creating dead zones, further impacting fish and aquatic life.