Question

During the day time when the sun is bright, the PH of the water in a small pond with plenty of submerged green plants may rise whereas at night, the PH of the water in small ponds may fall. Why?

Answer 1

During bright sunlight photosynthesis takes place in plants and use up the carbon dioxide dissolved in the water, this dissolved CO${}_2$ forms a weak carbonic acid with a PH about 5 to 6, if it is removed PH of the pond water rises. Where as during night only respiration takes place in green plants, it produces CO${}_2$ gas which dissolves in the water to produce carbonic acid. So the PH of the water in the pond may fall.