How acid or alkaline is your lake?

A characteristic you can’t see or feel can have subtle or significant effects on life in your lake. It’s called pH, and it’s a measure of how acid or alkaline your lake’s water is.

We know that water molecules contain two atoms of hydrogen and one atom of oxygen (H₂O). However, some of those molecules actually exist as positively charged hydrogen ions (H⁺) and negatively charged hydroxide ions (OH^-).

In pure water, those ions exist in essentially equal numbers. But when chemicals are added to water, the balance can shift in one direction or the other. A solution with more hydrogen ions is acidic; a solution with more hydroxide ions is basic, or alkaline.

pH is measured on a scale from zero (extremely acidic) to 14 (extremely alkaline). Pure water, which is considered neutral, has a pH of 7. Relating this to common substances, lemon juice is a fairly strong acid (pH just over 2), while household ammonia is strongly alkaline (pH about 12).

Lake waters are not that strongly acidic or alkaline. Their pH falls generally in a range from about 6 to 8, close to neutral. Interestingly enough, natural rainwater is fairly acidic (pH about 5.6), and air pollutants like sulfur dioxide and nitrogen oxides can lower the pH significantly – causing the harmful phenomenon of acid rain.

Fortunately, most lakes contain substances that neutralize (or “buffer”) acids, thus keeping the pH stable. One of the most important of these is calcium carbonate (limestone).

How does pH affect life in your lake? That’s complicated, but it determines how well certain fish species, plants, insects and other life forms survive and reproduce. For example, at pH below 6.5, walleye spawning is inhibited, and smallmouth bass disappear below pH 5.5.

pH can also determine the extent to which certain pollutants are released into the water from sediments in the lake bottom. For example, a change in pH can cause more phosphorus to dissolve in water, making it available to feed algae. In addition, many scientists believe that higher acidity is related to the release of toxic mercury into lake water. The mercury then can accumulate in fish.

pH and its effect on lake life is a complex subject. In healthy lakes, the effects are mostly subtle – pH is just one of many qualities that make each lake unique.