Tea-Stained Lakes
Tea-Stained Lakes: The Surprising Reason Northern Waters Turn Dark Brown
Summary: If your favorite northern lake looks like a giant vat of brewed tea, you aren't looking at pollution or dirt. This color comes from Tannins, which are natural organic compounds that leach out of fallen leaves, pine needles, and peat moss in the surrounding forest. Much like a tea bag steeping in a mug, the rain washes these organic acids into the lake, staining the water. While it looks dark, "tea-stained" water is often a sign of a healthy, forested watershed.
The Science Behind It: The technical term for this "tea" is Dissolved Organic Matter (DOM), specifically humic and fulvic acids. These substances are the byproduct of the incomplete decomposition of plant material in wetlands and forest floors. Research shows that these tannins act as a natural "sunscreen" for the lake. By absorbing UV radiation, tannins protect sensitive aquatic organisms from solar damage. However, this staining also limits light penetration, which restricts the depth at which aquatic plants can grow. Scientific studies suggest that "brownification"—the increasing tea-color in northern lakes—is often linked to recovering ecosystems and changes in precipitation patterns. Because tannins are acidic, these lakes often have a lower pH, which influences which fish species, such as Walleye, will thrive.