How large can the annual hypoxic zone in the central basin of Lake Erie be?

• A. 1,000 square miles
• B. 2,000 square miles
• C. 3,861 square miles
• D. 4,500 square miles

Answer: (C)

The area of the central basin that lacks sufficient oxygen grows in late summer when nutrients from fertilizers fuel algae, and the decomposition of that algal biomass consumes the dissolved oxygen in bottom waters. Lake Erie’s central basin tends to stratify in warm weather, so the low-oxygen water doesn’t mix upward, allowing the hypoxic zone to expand horizontally as oxygen is depleted. Under favorable conditions, this hypoxic zone can reach a large size—the commonly cited upper extent is about 3,861 square miles—making it the largest typical annual area affected. This reflects how nutrient loading, water temperature, and mixing all interact to drive oxygen levels in the lake. The other numbers are smaller or larger than the commonly observed maximum, so 3,861 square miles best represents the upper bound scientists report for the annual extent.