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Society normally wishes to maintain a reasonable level of productivity in its lakes, rivers, and estuaries and this requires the presence of modest levels of nutrients. Historically, many water bodies have progressed from low productivity or oligotrophic settings to productive mesotrophic conditions to over enriched eutrophic or hypertrophic conditions. The results are often algal or cyanobacterial mats, anoxia, and fish kill leading to greatly reduced biodiversity.
The accumulation of nutrients in a water body causes ecological imbalances and results in ‘Eutrophication’. Nutrient input causes growth of algae and weeds. If the algae and weeds are not consumed by Zooplankton and Fish, they die and decompose and the nutrients remain in the waterway.
On the other hand if the algae are consumed by Zooplankton and Fish they are transported up the food chain and hence the water remains clean and well oxygenated. The dead plant matter also accumulates as sludge on the lake bed and makes it shallower. The nutrient inputs come from sewage, agricultural runoff, storm water, etc.
The dead algae are decomposed by aerobic bacteria which use up dissolved oxygen. These bacteria increase the biological oxygen demand (BOD) of the ecosystem. Biological oxygen demand is important because it affects the amount of dissolved oxygen available to all species in an aquatic ecosystem.
A higher BOD indicates a lower availability of dissolved oxygen for Zooplankton and Fish. This lower concentration of oxygen causes many fish to suffocate, and as they die, the number of oxygen-demanding decomposers increases even more.
The amount of algae in a lake depends on the amount of nutrients available for growth. Long-term management of algae involves nutrient reduction to the water body. However, long-term nutrient reduction can be expensive and takes many years to significant improve water quality.
To control nuisance algae especially Blue Green Algae (BGA) and water weeds like Water Hyacinth, Duck weed etc. there are many chemical solutions like usage of Herbicides, Peroxides and Copper based Algaecides. These are generally harmful to the environment and are short term remedies.
Biological treatments for nutrient removal are based on use of Nitrogen reducing bacteria or enzymes in Bio-remediation and use of water and terrestrial plants for Phyto-remediation. Although both these techniques are effective their main drawback is adaptation of bacteria to the environment where the treatment is carried out and also land area or reed bed area required for Phyto-remediation of large water bodies or lakes.
Phycoremediation is defined as the use of algae to remove pollutants or nutrients from the environment or to render them harmless. Algae can fix carbon-dioxide by photosynthesis and remove excess nutrients efficiently at minimal cost. In addition oxygen produced photosynthetically with algae can relieve BOD in the water. The use of micro algae for the treatment of municipal wastewater has been subject of research and development for several decades.
Oligotrophic lakes are dominated by Diatom Algae and Eutrophic lakes by Cyanobacteria. In eutrophic lakes, diatoms may dominate in winter and spring and blooms of cyano-bacteria occur only in summer or spring.
In winter and spring the water quality is generally good but deteriorates in summer and autumn due to the algal blooms. The seasonal succession of phytoplankton is depicted very well on the website Water For The World (http://waterfortheworld.net/)
Therefore the solution to prevention of eutrophication is to grow Diatoms even in summer and autumn. This is based on the principle of controlled photosynthesis proposed by Oswald in 1950s and by Officer and Ryther in 1980s.
Growing good algae to prevent growth of bad algae is a unique idea.
Diatoms are microscopic plants which use nitrates and phosphates to grow along with other nutrients like silica, iron, copper, molybdenum, etc. They use Carbon dioxide and produce Oxygen and can also accumulate heavy metals.
By triggering the growth of these algae many problems related to lake pollution can be solved. Growth of diatoms also reduces the growth of harmful algae like BGA. In nature Diatom algae which is the most dominant and varied species of algae in majority of water ways are competitors for nutrients with other algae like BGA. Growing one type of algae in open waters is complicated process but diatoms have a distinctive advantage because of their requirement for silica. By using this advantage we can trigger diatoms in open waters.
Using diatoms to control water quality is a unique idea for effective, eco-friendly and cost effective lake management. It does not need any special equipment, construction and skilled manpower. It harnesses the enormous potential of diatom algae to consume nutrients rapidly and thereby control other nuisance algae like BGA and water weeds.
Diatoms are present in all waterways and hence there would be no need to introduce Diatoms spores. This is unlike bio-remediation solutions in which bacteria is introduced into the waterways. Diatoms help not only in restoring water quality but also in improving the biodiversity of the lakes by restoring the natural food web.
Sewage, fertilizer, etc., contains more of nutrients and less of micro-nutrients. Diatom algae are the last group of algae to have evolved about 200 million years ago and they are more complex than other algae and require more micro-nutrients.
Thus diatoms cannot grow once the micro-nutrients in the water are exhausted. This typically occurs by summer and is the main reason for Cyano blooms in summer and autumn. Dosing micro-nutrients into waterways to enable diatoms to grow even in summer and autumn is a unique and new phyco-remediation solution.