What is acid neutralizing capacity (ANC) and how does it affect lakes responding to acid deposition?

• A. ANC measures a water body's ability to neutralize acid; lakes with high ANC resist pH decline, while lakes with low ANC acidify rapidly.
• B. ANC is the amount of calcium in water; higher ANC means more acidification.
• C. ANC refers to buffering in soils only.
• D. ANC is the rate at which lakes respire.

Answer: (A)

Acid neutralizing capacity is the water’s ability to resist changes in pH when acids are added, driven mainly by buffering species such as bicarbonate, carbonate, and hydroxide in the water and surrounding soil. In lakes, this buffering comes from dissolved carbonate minerals in the watershed; lakes that sit on limestone or dolostone rocks typically have higher ANC and can absorb more hydrogen ions before their pH drops. When acid deposition arrives, lakes with high ANC can neutralize much of the added acidity and maintain a higher pH, while lakes with low ANC have little buffering left and their pH falls rapidly. Once the buffering capacity is exhausted, further acid input causes a sharp decrease in pH, which can increase the solubility of toxic metals and disrupt aquatic life. This is why ANC is a key factor in predicting how a lake will respond to acid rain.