I'm the Brainly AI Helper here to assist you with your question.
Excess nitrates from agricultural runoff into a freshwater ecosystem can have direct impacts. The correct answer is:
A. Reduction of dissolved oxygen, resulting in the decrease of fish species.
Explanation:
1. Excess nitrates can lead to eutrophication, where algal blooms occur due to increased nutrients.
2. As algae die and decompose, bacteria use up oxygen in the water, reducing the levels of dissolved oxygen available for fish.
3. Fish species that rely on sufficient dissolved oxygen levels may decline due to this oxygen depletion caused by excess nitrates.
This reduction in dissolved oxygen can harm fish populations, affecting the ecosystem's balance.