How Does the Acidity of Water Affect Photosynthesis in Water Plants?

About Aquatic Plants

• All aquatic plants, including algae and other phytoplankton or floating plants, absorb nutrients through their cell walls from the surrounding water. Rooted plants will also absorb some nutrients through these roots also. Like other plants, they produce their own carbohydrates and proteins using the energy of the sun through the process of photosynthesis. The oxygen plants release during this process can make a substantial contribution to the amount of oxygen dissolved in the water and available to fish and other organisms.

Photosynthesis And pH

• During the day, when photosynthesis is going on, plants absorb carbon dioxide and release oxygen. At night, the process of respiration is dominant, using up oxygen and releasing carbon dioxide. This carbon dioxide acts as an acid in the water, lowering the pH temporarily. The water becomes less acid again during the day as carbon dioxide is used up.

  The amount of change in pH depends on how much dissolved calcium carbonate and magnesium carbonate is found in the water. These bond with the carbon dioxide and reduce the increase in acidity. Calcite; calcium carbonate or dolomite lime; calcium and magnesium carbonate, are often used to make ponds less susceptible to pH changes.

About Phosphorus

• Phosphorus is usually the limiting factor in plant growth for water plants, unlike land plants that are often limited by nitrogen availability. It is essential to the process of photosynthesis and helps with the formation of sugars, starches and oils. Excess phosphorus from fertilizer runoff can cause large algae blooms in ponds and lakes.

  Phosphorus becomes less available as the pH of pond water drops below a pH of 6. Increasing the alkalinity by adding lime will benefit plant growth by making more phosphorus available, thus promoting better photosynthesis.

Liming And Fertilization

• Phosphorus may be increased by applying a high phosphorus fertilizer but until the pH is raised it will not be completely available to the plants. Use dolomite lime, commonly available in nurseries, to add both calcium and magnesium. These chemicals are also essential for photosynthesis and should be added to the water. The dissolved minerals will buffer the pH, preventing large changes by absorbing the daily excess of carbon dioxide. Add one and a half times the amount you'd use on the surrounding soil but don't worry too much about an excess. Lime will not dissolve at a pH above 8.3.