why freshwater do not contain algae?

As alga need nitrate, which is a nutrient for growth of plants,they will only present in water where has nitrate.
In freshwater, there are not much bacteria. Therefore less nitrate is formed when break down of organic substance by bacteria. Less nitrate, less alga, till all dispear.
Hence, algae may not present in freshwater.

definitely freshwater has algae!! esp. green algae like Spirogyra. Check it out here
http://en.wikipedia.org/wiki/Green_alga
http://en.wikipedia.org/wiki/Spirogyra

2006-12-12 14:08:25 補充：
In my personal experience, fresh water has more alga than sea water. You may find algal boom in shallow seas, but most of the ocean, alga is less concentrated than most fresh water bodies. The only lakes which has less algae is the ones high up in the mountains, away from pollution.

2006-12-12 14:08:38 補充：
Our drinking water has no alga only because all was killed in the process of water treatment. I bet the reservoir and the Tong Jiang (東江) where our water comes from has more algae than the sea water!!