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Life in Freshwater

pH in Ponds and Lakes

pH can vary enormously between ponds and lakes, due to bedrock, topography and even the species of plant present. For example on heathlands where ponds form there are likely to be Sphagnum or bog mosses present. These actively exchange ions across their cell walls. As they take in minerals they compensate by releasing hydrogen ions which increase the acidity.

The most significant aspect to pH is the amount of carbonic acid present. Carbon dioxide disolves in water to produce this and so it is a measurement of the level of C02 available for photosynthesis. The ions dissociate to yield hydrogen and hydrogen carbonate. Measurement of the pH of a pond over a 24 hour period will demonstrate this well. At dawn the water will be acidic due to the high level of carbon dioxide released through respiration by all the living organisms present. However, as the sunlight becomes available photosynthesis occurs and the level of carbon acid declines. By mid-day the water may become increasingly alkaline until the sun sets. With the reduction in light photosynthesis halts and only respiration is occuring in the organisms. pH decreases accordingly. In regions rich in calcium the hydrogen carbonate combines with the calcium to form chalky deposits on plants and species like the stoneworts thrive.

Some animals are specific to calcium-rich, alkaline water like the crayfish. Snails need calcium for their shells and so will be limited to water rich in this mineral. Plankton may also be specific to alkaline waters.

Acidification of lakes through acid rain can completely destroy the communities present.

Abiotic factors:


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