What effect does agricultural runoff containing fertilizers most likely have on algae populations in lakes and streams?

Agricultural runoff containing fertilizers typically introduces an excess of nutrients, particularly nitrogen and phosphorus, into aquatic ecosystems. This process leads to a phenomenon known as eutrophication, where the increased nutrient levels stimulate rapid algal growth, creating algal blooms.

As algae multiply excessively, they eventually die off and decompose, a process that consumes dissolved oxygen from the water. This reduction in dissolved oxygen can be detrimental to fish, shellfish, and other aquatic organisms that rely on adequate oxygen levels for survival. Consequently, the overall health of the aquatic ecosystem is compromised as the oxygen levels drop, leading to dead zones where fish and other aerobic life can no longer thrive.

The other options do not accurately reflect the impact of fertilizer runoff on algae populations. For instance, while it might seem that nutrient-rich water could lead to increased clarity, the opposite generally occurs as algal blooms cloud the water. Similarly, agricultural runoff does not affect water level or temperature variations in a direct, predictable manner related to algae populations.