The term pH measures the concentration of hydrogen ions in a solution, determining its acidity or alkalinity. For freshwater ecosystems, pH stability is paramount because it governs the solubility of nutrients and the physiological processes of aquatic life. Maintaining a stable pH level is essential for the chemical balance that sustains biological activity within a river, lake, or stream.
The pH Scale and Healthy Water Ranges
The pH scale is a logarithmic gauge running from 0 to 14, where each full number represents a tenfold change in acidity or alkalinity. A value of 7.0 is defined as chemically neutral, balancing hydrogen and hydroxide ions. Solutions below 7.0 are acidic, while those above 7.0 are classified as alkaline or basic.
For most natural freshwater systems, the acceptable range for a healthy environment falls between 6.5 and 8.5. This range supports the greatest diversity of aquatic organisms. The water’s resistance to abrupt changes in pH is known as its buffering capacity, provided primarily by dissolved compounds like bicarbonates and carbonates. These compounds absorb excess hydrogen ions to stabilize the pH when acids are introduced.
How pH Changes Affect Aquatic Ecosystems
Deviations from the optimal pH range introduce physiological stress to aquatic life, disrupting basic bodily functions. When the water becomes too acidic, the delicate gill structures of fish can be damaged, impairing their ability to take in oxygen. Prolonged exposure to low pH, particularly below 5.0, can also prevent the successful hatching of fish eggs and reduce reproductive success across many species.
A decline in pH also alters water chemistry by increasing the solubility of toxic metals naturally present in sediment. Aluminum, which is harmless in its bound state, becomes mobilized at lower pH levels and can accumulate in a fish’s gills, causing respiratory failure. Amphibians are particularly susceptible to minor pH shifts due to their permeable skin, making them sensitive indicators of water quality.
Conversely, an increase in alkalinity above pH 9.0 is detrimental to aquatic organisms. High pH increases the toxicity of un-ionized ammonia, a common waste product, posing a threat to fish health. Highly alkaline conditions can also limit the availability of certain micronutrients, such as iron and manganese, necessary for the growth of aquatic plants.
Natural and Human Factors Modifying Freshwater pH
The natural chemistry of a freshwater body is determined by the surrounding geology and soil composition. Systems flowing over limestone or other carbonate-rich bedrock tend to have a higher buffering capacity and a more alkaline pH due to dissolved minerals. Conversely, waters draining through sandy soils or granite bedrock possess fewer natural buffers, making them more vulnerable to acidification.
Organic matter, such as peat or decaying vegetation, generates organic acids that can naturally lower the water’s pH, sometimes to levels between 4.5 and 6.0. Biological processes like the respiration of aquatic organisms release carbon dioxide, which forms carbonic acid and causes daily fluctuations in pH. Photosynthesis by aquatic plants and algae consumes carbon dioxide, causing the pH to rise during the day and fall at night.
Human activities are a significant source of pH modification, often leading to rapid and detrimental changes. Burning fossil fuels releases sulfur dioxide and nitrogen oxides, which return to the earth as acid deposition (acid rain). This influx of strong acids rapidly depletes a water body’s buffering capacity, leading to widespread acidification in sensitive regions. Runoff from agricultural areas containing fertilizers and industrial wastewater discharges also introduce substances that alter the water’s pH balance.