It is the natural ageing of a lake by the biological enrichment of its water. In a young lake, the water is cold and clear supporting little life. With time, due to introduction of nutrients such as nitrogen and phosphorus, there is growth of aquatic organisms. Slowly and steadily as the fertility of the lake increases, organic remains begin to get deposited on the lake’s bottom.
Over the centuries, as silt and organic debris pile up, the lake starts growing shallower and warmer with warm water organisms replacing those that thrive in a cold environment. Marsh plants take root in the shallows and begin to fill in the original lake basin and eventually, the lake gets converted into land. This process may take thousands of years depending on climate, lake size, etc.
(ii) Biological magnification
It refers to an increase in concentration of the toxicant at successive trophic levels. Few toxic substances present in industrial water can undergo biological magnification in aquatic food chain.
These toxic substances are not excreted or metabolised and get accumulated in organism’s body. Due to this, these substances pass to the next trophic level via food chain. This phenomenon is well known, in case of mercury and DDT pollution.
(iii) Groundwater depletion and ways for its replenishment
Waste water including sewage can be treated in an integrated manner by utilising a mixture of artificial and natural processes. One such example is the town of Areata situated along the northern coast of California. An integrated waste water treatment process was developed with the help of biologists from Humboldt State University in this town. Integrated waste water management occurs in two major steps:
Step I- In this, conventional sedimentation, filtering and chlorine treatments are given, but even after this stage, heavy metals and toxic pollutants remain in the water.
Step II- To overcome this, the biologists had taken an innovative approach and developed a series of six connected marshes grown with suitable plants, fungi, algae and bacteria. As the water flows through the marshes, the pollutants are neutralised, absorbed or assimilated, by the plants, etc. The marshes also constitute a sanctuary, with high level of biodiversity in the form of fishes, animals and birds. A citizens group called Friends of the Areata Marsh (FOAM) take care of this wonderful project.