Lakes and rivers are polluted because the composition of their water has been changed. One object of the present invention is an approach to revert polluted lakes and rivers to their originally pristine conditions.
In mountains where the drainage basins are underlain by granites or other crystalline rocks, the lake water is nearly neutral or somewhat acidic. The pH value is less than 7, commonly ranging from 5.5 to 6.5. Diatoms are the common plankton that bloom in those lakes. In places, the lake water is even more acidic. Where the pH of lake water drops below 5 no organisms, not even diatoms, could survive. The lake water is crystal-clear, but it is also sterile.
In foothills or plains county, where the drainage basins are underlain by limestones or other sedimentary rocks, the lakes are mostly alkaline. Limestones or other carbonate rocks are present to provide carbonate ions to buffer the acidity. The pH value of Lake Zurich water, for example, ranges from about 7.4 in the spring to 8.85 in the summer. Green algae are the most common plankton in alkaline lakes.
Case history studies of Lake Zurich, of Er-Hai and Dian-Ci lakes of Yunnan, China revealed that the most significant chemical change of those polluted lakes is the increase of pH of the lake water. The maximum pH value of water is more than 8.5 in Lake Zurich an in Er-Hai and more than 9 in Dian-Ci. The increase of alkalinity favours the growth of green algae and the precipitation of lacustrine chalk.
Paleolimnological studies in Switzerland and elsewhere confirm that the kind of plankton growth in lakes depends upon the pH of lake water. In studying the varves of Lake Zurich, for example, it was noted that diatom frustules are the sediment of the spring and lacustrine chalk, precipitated by green algae, is the sediment of summer and autumn. The winter varve is commonly clay-sized detritus. The changing composition of the plankton reflects the changing pH of Lake Zurich water. After the spring overturn of lake water, the bottom water, which is only slightly alkaline, reaches the surface. A mixing gives the surface water a minimum pH value of about 7.5 in April. The spring is the season of diatom blooms in the nutrient-rich lake water. Consequently the spring varve is rich in diatom frustules. With the coming of summer, diatoms ceases to reproduce, and green algae become the predominating plankton. The pH of lake water is increased some 10-fold to 8.7 or 8.8, accompanied by the precipitation of calcite, which forms the lacustrine chalk in the summer varve. The plankton growth was inhibited during the winter, before the annual cycle of varve-sedimentation starts again with the diatom blooms of the spring.
This knowledge of plankton ecology forms the theoretical basis of the invention. It is true that the growths of diatoms and of green algae both require nutrients, but the predominating plankton may be diatoms or green algae; their growth depends upon the season and the pH of lake waters. Diatoms can reproduce only in the spring. They could grow in slightly alkaline lake waters, but they alone could survive in the acidic waters of mountain lakes. The growth of green algae is suppressed in waters where the diatoms bloom. The growth is suppressed in the spring and is inhibited in acidic waters. The green algae become, however, the predominating plankton in polluted lakes where the lake water is extremely alkaline, even in the spring when diatoms should bloom. This is the critical factor causing the pollution of lake and river waters by green algae.
Considerable efforts have been expended to suppress the growth of green algae through attempts to reduce the nutrients concentrations in lake waters. Those efforts have not been successful. Furthermore, the nutrients are not necessarily harmful, if the lake plankton are diatoms rather than green algae. Not the plankton growth, but the kind of plankton growth is the key to solving the pollution problem. Green algae are commonly not consumed by fish. Special kinds of fish introduced to Dian Ci Lake could consume, but not digest, green algae. The fish excrement consists of diminutive debris to be suspended in lake water to cause worse pollution. In natural waters where green-algae growth is suppressed, rich nutrients are consumed by diatoms. Diatoms are good feed for fish, and the diatom-fed fish excrement is in the form of faecal pellets.
Consequently, mountain lakes hosting diatoms are good breeding ground for fish, and the lake water is commonly crystal-clear because diatom frustules and faecal pellets fall rapidly to the bottom of the lake.