The coloured microbes provide energy for the whole system from the sunlight shining down on it. On a sunny day, bubbles arise on top of the mats of algae, whether they be blue-green, green, brown or red. Often, they break away and rise to the surface, disappearing there. On a sea-grass flat, there are even more bubbles, and sometimes they form a steady stream, mainly from the tips of the leaves. These bubbles are oxygen, and the reason they appear is that the water is already supersaturated. During the night, the bubbles cease because there is no photosynthesis, but respiration continues. The pH of the well-buffered seawater in the open ocean is slightly over 8 and is alkaline. In the estuaries it can reach 9.4 on a bright, sunny day on a sea-grass flat where photosynthesis is active; at night it sometimes falls to an acidic state with less than pH 7. The high pH is due to reduction of the carbonic acid content of the water from photosynthesis, and the low pH to reverse changes due to respiration increasing the carbonic acid. This means that the plants, especially the microscopic plants, control the chemical, and therefore the biological processes in an estuary.
KeywordsHydrogen Sulphide Green Sulphur Bacterium Smut Fungus Sewer Outfall Assimilate Carbon Dioxide
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