Apart from the huge cost of dredging a lake there are other disadvantages. The disruption to the surrounding areas can be a problem, but the cost to wildlife is incalculable because, when you throw away the bad stuff, you are also throwing away beneficial life from within your lake. This has a devastating effect upon the natural balance of the lake causing different problems which take advantage of the unstable conditions.
Another consideration is what to do with the sludge that is dredged from the bottom of the lake? Some farmers are able to spread it, because it is highly nutritious, but it can also be highly toxic so care has to be taken with its disposal.
If you use the kinder, Aquaplancton method, not only is the cost to you less, but also the life within your lake is preserved and the environment is protected.