Aquatic Sensitivity to Acid Deposition

The response of aquatic environments to acid deposition depends on their sensitivity to acids and the amount of acid input they receive. Most ecosystems have natural buffering capacity, often provided by materials like limestone, which neutralize incoming acids. When sufficient buffering capacity exists, excessive acid input does not immediately change the pH of the water, though it gradually depletes its acid neutralizing capacity (ANC).

The ability of a watershed to accommodate acid deposition is largely determined by soil composition, bedrock, and vegetation, but other factors also play a role. Steep slopes, shallow soils, and frozen ground reduce the contact time between precipitation and neutralizing materials, leading to incomplete neutralization of water entering lakes and streams. Additionally, biological processes contribute to neutralization—algae produce buffering compounds during photosynthesis, while microbes in sediments help reduce sulfur compounds to less harmful forms.

ANC and pH interact in a critical way: as acid is added to a buffered system, pH remains relatively stable until ANC is nearly depleted. Once ANC approaches zero, even small amounts of acid cause drastic pH drops, leading to conditions intolerable for most aquatic life.

Lower pH levels result in toxic metals leaching from the watershed, reduced aquatic growth, and the disappearance of sensitive species, from algae to fish. In drinking water supplies, acidification can cause metals such as iron, copper, and lead to dissolve from distribution pipes, leading to unsafe water quality.