What environmental issue arises from fertilizer runoff into lakes?

Fertilizer runoff into lakes leads to high levels of nitrates and phosphorus, which are key nutrients found in many fertilizers. When these nutrients enter water bodies, they can contribute to a process called eutrophication, where nutrient overloads stimulate excessive growth of algae. This phenomenon can create a range of ecological problems, including the depletion of oxygen levels in the water as the algae die and decompose, leading to fish kills and loss of biodiversity.

In contrast, options referring to increased fish populations, decreased algae growth, or lower water temperatures do not accurately capture the consequences of fertilizer runoff. In fact, the rise in nutrient concentrations from agricultural runoff often disrupts aquatic ecosystems, making high nitrates and phosphorus levels a crucial environmental issue associated with fertilizer use.