Name two human activities that increase nutrient levels in streams and lakes.

• A. Urban forestry and dam building
• B. Agricultural fertilizer runoff and sewage/discharge
• C. Oil spills and plastic waste
• D. Bike lanes and sidewalk cleaning

Answer: (B)

Excess nutrients entering streams and lakes from human activities cause eutrophication, leading to algal blooms that can block light and disrupt aquatic ecosystems. The two activities that most directly raise nutrient levels are agricultural fertilizer runoff and sewage or wastewater discharge. Fertilizers are rich in nitrates and phosphates; when fields are irrigated or rain washes over them, these nutrients get carried into nearby waterways. Sewage and wastewater introduce high levels of nitrogen and phosphorus through human waste and detergents, especially when discharge isn’t fully treated. The resulting nutrient boost fuels algae growth; when the algae die, bacteria decompose the organic matter, using up dissolved oxygen and creating hypoxic conditions that harm fish and other aquatic life.

The other options don’t fit as clearly because they involve pollutants other than nutrients or can even influence nutrient dynamics in different ways (for example, some practices may reduce runoff or alter nutrient transport rather than directly increasing nutrient levels).