Usually to globally fix the problem one would have to implement sound watershed management and pollution controls to reduce the sources of pollutant. In some occasions, local fix  can be achieved though implementing certain types of ecological restoration or other engineering methods, such as vegetation restoration, enhanced circulation/mixing, algae harvesting, or dredging etc.  However, these methods, though widely applied, are often questionable without a detailed study to determine the effectiveness of each engineering measures, and how to make the engineering measures work. For example, take dredging as example,  without a clear understanding of the relationship between the scale/location of dredging and the water quality response, as well as how long would the benefit attained by a dredging last under current loading condition, to blindly implement a dredging project might be costly and ineffective.  Same issues also apply to the application of other ecological engineering methods. Therefore, sound and science-based quantitative design method should be applied to support the decision making for implementing ecological restoration efforts for lake water quality and ecological system protection.

I agree with Rui.

The key is management of the catchment and this requires a holistic approach and close collaboration between the various regulators.

Thanks Alan. The watershed management is the ultimately solution, however, it also take very long to reach the reduction target. So in some waterbody, it is possible to combine the watershed scale approach with in-water management methods.  In one of my recent studies,  the response of floating macroalgae, DO, and nutrient to watershed load reduction, and to harvest of floating macroalgae,  dredging, and modifying circulation pattern in a lagoon are analyzed, and it was found that these different measures all have their own benefits, though  it is very important to implement them at proper magnitudes or frequency. For example, while harvesting once a year seems only has limited effect, but twice a year would provide significant benefit.  In short, to develop an effective plan for lake water quality improvement is a difficult task, but it is definitely worthwhile before spending tremendous money on some ineffective management/engineering measures

Nice Q Dr Ramachandra Mohan.. 

All above replies are great and equally important. 

Any lake across the globe is prone for pollution by nature (soil, nutrients by way of degradation of leaves, figs-phenolics). Similarly as you know anthropogenic which need not be explained.  

What can be added to maintain a lake or river or coastal water is to make the polluting bodies -both on the banks and upstream riven/streams to strictly follow/ obey waste minimization technique(s) which is very much essential before going for control or treatment. 

Second is helping the lake water to have its own healthy ecosystem. 

Third is minimize air polluting industrial contaminants. Even antarctica is polluted with pesticides and the mode of transmission is 'air' and not by ocean currents. 

Fourth is Pesticide inflow by any of the agro or other sources. 

Fifth is to go for cleaner methods of treatment (If using for drinking purpose) rest all is known to all. 

For all maintaining good DO, is verry much essential.

Also maintaining lower COD, BOD is essential. 

Nice Q.   

Lake quality are at optimum, if the ecological conditions are close to those which are regarded as natural for the respective climatic zone. Parameters for this are dissolved oxygen and oxygen consumption, redox potential, dissolved organic carbon, ammonia, nitrate, depth of light penetration (Secchi depth), reactive and total phosphorus, and dissolved metals and semi metals (e.g. As, Sb). Electrical conductivity (corrected for the reference temperature) gives a first indication about the salinity. The course of temperature in the water column tells you about mixing between different water layers. Some microbial parameters (e. coli, streptococci, b. typhimurium and others) tell you, if this water body is suitable for bathing or drinking.

Suspended matter can consist of plankton or suspended inorganics (e.g. clays) - depending on temperature, nutrient input, erosion of the bottom sediments and water flows, the geological characteristics of the environment, as well as Input of nutrients and contaminants.

Suitable figures of recommeded conditions cannot be given for an unknown water body 

• Identification and elimination of overboard discharges
• Continued support for and implementation of CSO elimination at POTWs
• Prohibition of distribution and use of Phosphorus-containing fertilizers in the watersheds of sensitive water bodies
• Prevention of the introduction of invasive species
• Enhanced riparian buffer development, maintenance and protection to reduce NPS pollution from agro/industrial areas

Борьба с загрязнением может оказаться дорогой. Поэтому надо оценить риски, зависящие от пределов устойчивости биоценоза озера и степени загрязнения воды относительно нормативов качества используемой человеком озерной воды, В результате этой оценки Вы увидите приоритеты, в соответствии с которыми и надо тратить деньги. Как составной элемент оценки загрязненности рекомендую биологическое тестирование (на токсичность) на мелких ракообразных, которые могут быть основой пищевой цепи в экосистеме озера. . 

Dear Dr 

Can anyone translation above in English 

Look for migratory birds , fish species or any fresh water creatures that inhabit the lake . Blood and lipid profile of these creatures are a good indication of any possible toxins being present in the lake . Migratory birds carry the toxin with them from many paces , and so either the lake you are monitoring is polluted , or is affected by the migratory effect of the birds . As lakes are by and large fresh water bodies , DO levels will possibly be larger than salt water bodies, and drop in DO levels will also reflect pollution . 

A major source of nutrient/pesticide imput is wet and dry deposition from the atmosphere. In the case of Lake Winnipeg we have estimated that as much as 10% of the phosphate input is from this source.

Phosphor is the most important lake pollutant.

I don't agree that lakes generally are getting more polluted. Many developed countries have implemented advanced wastewater treatment and experience slow recovery of eutrophic lakes.

Lake pollution is generally caused by various sources. Export of nutrients through drainage water from agricultural lands is one of the major ways to create eutrophic condition which is the major driver for hypoxic zone. To reduce the nutrient export in the lakes, drained water can be treated for nutrients by various management practices such as denitrifying woodchip bioreactors, constructed wetlands, and controlled drainage.

The guidelines reffered in CPCB for various parameters are to be followed and remedials as well as preventive measure are essential

At first you should classify the kind and possible source of contamination: nutrients, soluble metal salts, hygienic parameters, degradable or non- degradable organic substances (e.g. hormones, pharmaceutics, detergents), radioactive substances. Your question is too general to be answered.

The most General test seems to be the Daphnia test; they have to be replaced regularly, to have them all at the same Age.

In particular, I can do multi-element Analysis; this Shows you the Level of Phosphate, sulfate and metals, as well as the characteristics of the water. At the sampling site, you have to note oxygen, pH, conductivity, and temperature, secchi depth, algal growth, colour. You should also have a Screening Parameter for halogenated organic compounds, and hydrocarbons.

At first you should classify the kind and possible source of contamination: nutrients, soluble metal salts, hygienic parameters, degradable or non- degradable organic substances (e.g. hormones, pharmaceutics, detergents), radioactive substances. Your question is too general to be answered.

The most General test seems to be the Daphnia test; they have to be replaced regularly, to have them all at the same Age.

In particular, I can do multi-element Analysis; this Shows you the Level of Phosphate, sulfate and metals, as well as the characteristics of the water. At the sampling site, you have to note oxygen, pH, conductivity, and temperature, secchi depth, algal growth, colour. You should also have a Screening Parameter for halogenated organic compounds, and hydrocarbons.