Simply that, if you start cutting the trees surrounding your lake, you eliminate the natural mechanisms that keep water and soil together around the lake. The result, is that your lake, through erosion and leaching, gets tons and tons of nutrients that it certainly does not need. Within a few years, it's paralyzed by aquatic vegetation that should not have grown there. All this because around your lake, people wanted to build a little city in the country. It's the same with septic sewage systems.
Sewage reaches the lake without having been purified by the soil.
It carries nutrients that helps algae grow. Everybody knows this phenomenon. The lake becomes, in my humble opinion, rather disgusting in some areas. The lake tries its best to digest the stuff, but it's provided with too much and is unable to assimilate it all. An ecosystem To sum it up, a lake is a living thing that nature has quietly formed over hundreds and even thousands of years.