Final answer:
The concept of eutrophication, sparked by excessive nutrients from runoff, leads to overgrowth of algae and depletion of dissolved oxygen, causing dead zones and impacting water quality. Cultural eutrophication and dead zones, like that in the Gulf of Mexico, are notable consequences of such pollution.
Step-by-step explanation:
The question regarding toxicants degrading water quality, particularly focusing on nutrients such as nitrogen and phosphorus, relates to the concept of eutrophication. Eutrophication is a process whereby excessive nutrients, largely nitrogen (N) and phosphorus (P), from sources like fertilizers and sewage runoff, lead to overgrowth of algae and microorganisms.
This excessive growth depletes dissolved oxygen in the water, causing hypoxic conditions and potentially leading to the death of aquatic organisms, establishing what are known as dead zones. Cultural eutrophication is a significant environmental issue due to the challenge of regulating the nonpoint source pollution responsible for high N and P levels. Notably, about 50% of lakes in North America, Europe, and Asia are adversely influenced by cultural eutrophication, and the Gulf of Mexico has a renowned dead zone tied to these nutrients.
Oxygen-demanding waste, such as decaying organic matter, contributes to high biochemical oxygen demand (BOD), further decreasing water quality and impacting the living conditions for aquatic life like clean-water fish and shellfish. While natural ecosystems may maintain low BOD and high dissolved oxygen levels, urban and agricultural runoff significantly alters this balance, leading to environmental distress.