Maine

Abstract

The chemical imprint of acidic deposition is evident as increased SO₄²⁻ concentrations in virtually all Maine surface waters, but the effect on acidity status is small. Fewer than 100 nondystrophic lakes are currently estimated to be acidic (ANC ≤ 0), representing < 1.7% of the total number, and < 0.1% of the lake area. Two-thirds of the acidic lakes are smaller than 4 ha, and many are groundwater recharge seepage lakes. Low DOC (< 417 µmol L⁻¹) acidic lakes (about 60% of the total number of acidic lakes) are acidic largely because of atmospheric deposition of acids. High DOC drainage lakes (about 10% of the total) are acidic because of a combination of natural and anthropogenic acids, and high DOC seepage lakes (about 30% of the total) are acidic largely because of organic acidity. High-elevation lake systems are most chemically influenced by acidic deposition. No low DOC lakes are known to have pH < ~5.0.

Base cation release is the most important factor in the generation of ANC. Other factors, such as NO₃⁻ uptake and SO₄²⁻ retention, also contribute to ANC production. In addition to atmospheric deposition, acidifying processes include the production of organic acids and NH₄⁺ uptake. A majority of the region’s watersheds are inferred to be presently conservative with respect to SO₄²⁻ on an annual basis. Nitrate loss from watersheds occurs seasonally during snowmelt, or episodically in response to rainfall events.

Historically, nonsignificant decreases in acidity and SO₄²⁻ concentrations in deposition have occurred during the 1980s. Slight, often significant, increases in surface water ANC have occurred during the same period. Few, if any, lakes or streams are expected to become acidic in the next several decades with steady-state atmospheric deposition. A number of lakes and streams spanning the natural pH range are fishless or have depleted populations, but acidic deposition cannot be directly implicated in the status of fisheries.