Algae are photosynthetic organisms that live in aquatic environments, ranging from single-celled phytoplankton to larger seaweeds. They use sunlight to convert carbon dioxide and water into energy, a process that significantly impacts the water’s chemistry. The measure of acidity or alkalinity in water is known as pH, operating on a scale from 0 to 14, where 7 is neutral. A higher concentration of hydrogen ions results in a lower, more acidic pH, while a lower concentration means a higher, more alkaline pH. Actively growing algae generally cause the pH of the surrounding water to increase, making it more alkaline.
How Photosynthesis Drives pH Up
The increase in water pH during daylight hours is a direct result of algae consuming carbon dioxide ($\text{CO}_2$) for photosynthesis. When $\text{CO}_2$ dissolves into water, it forms carbonic acid ($\text{H}_2\text{CO}_3$), which is mildly acidic. The formation of this acid releases hydrogen ions ($\text{H}^+$), which lowers the pH of the water.
During periods of light, algae actively remove dissolved $\text{CO}_2$ from the water column. This consumption disrupts the natural balance of the $\text{CO}_2$/carbonic acid system. As the concentration of $\text{CO}_2$ decreases, the chemical equilibrium shifts, causing carbonic acid molecules to break down to replace the lost $\text{CO}_2$. This breakdown removes the acidic component from the water, leading to a reduction in the concentration of hydrogen ions. Reducing the hydrogen ion concentration causes the pH to rise toward the alkaline range. In systems with a dense population of algae, this process can be rapid, sometimes pushing the pH from 7.0 up to 9.0 or 10.0 within a few hours. The ability of the water to resist these changes, known as its buffering capacity, determines the magnitude of this daily pH swing.
The Nighttime Drop in Water pH
The increase in pH during the day is countered by a reversal of the chemical process when sunlight is absent. Once the sun sets, photosynthesis stops. Algae and all other organisms in the water, including bacteria and fish, continue cellular respiration throughout the night.
Respiration consumes oxygen and releases $\text{CO}_2$ back into the water. This continuous influx of $\text{CO}_2$ overnight causes the concentration of dissolved carbon dioxide to increase significantly. The newly introduced $\text{CO}_2$ reacts with water to form carbonic acid, which releases hydrogen ions. This acid production causes the water’s pH to drop, making it more acidic by the early morning hours. The extent of this nightly drop depends heavily on the density of the algae and other respiring organisms. In highly productive aquatic systems, the daily pH fluctuation can be substantial, sometimes varying by more than a full pH unit between the afternoon peak and the pre-dawn low.
Consequences of pH Fluctuation in Aquatic Systems
These daily fluctuations in water pH have practical implications for the health of aquatic life, particularly in closed systems like aquariums and ponds. Rapid and large swings in pH, often exceeding 1.0 unit, place extreme stress on fish and invertebrates. Maintaining a stable internal chemistry becomes difficult when the external water parameters are constantly changing.
Ammonia Toxicity and High pH
A concern related to high pH levels is the increased toxicity of ammonia. Ammonia is a common waste product excreted by fish and is present in the water as two forms in equilibrium: the relatively harmless ammonium ion ($\text{NH}_4^+$) and the highly toxic un-ionized ammonia ($\text{NH}_3$). As the water’s pH rises above 7.0, the equilibrium shifts, converting a much larger proportion of the less harmful ammonium into the toxic un-ionized ammonia.
If intense daytime photosynthesis pushes the pH up to 9.0, the concentration of toxic free ammonia can increase by a factor of 100 or more, rapidly reaching lethal levels. This makes the high pH caused by intense algal growth a major water quality management challenge, as it can turn a relatively safe level of ammonia into a deadly concentration for fish and other sensitive organisms.