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pH is a measure of how acidic a solution is. The pH scale ranges from 0 to 14. Solutions with a pH of less than 7 are acidic, and those with a pH greater than 7 are basic (or alkaline). Distilled water is neutral and has a pH of 7.
Knowledge of pH is important because most aquatic organisms are adapted to live in solutions with a pH between 5.0 and 9.0. The pH in an estuary tends to remain constant because the chemical components in seawater resist large changes to pH. Biological activity, however, may significantly alter pH in an estuary.
Through a process called photosynthesis, plants remove carbon dioxide (CO2) from the water and expel oxygen (O2). Since CO2 becomes carbonic acid when it dissolves in water, the removal of CO2 results in a higher pH, and the water becomes more alkaline, or basic. When algae naturally begin to increase in estuaries during the spring, pH levels tend to rise. An overabundance of algae (called an algal bloom) may cause pH levels in an estuary to rise significantly, and this can be lethal to aquatic animals.