Found this interesting article on BBC Earth News:
BBC - Earth News
Hennie
BBC - Earth News
In late June 2008, the waters and shores at the Qingdao venue hosting the Olympic sailing regatta experienced a massive green tide covering about 600 sq km. Lasting over two weeks, it took more than 10,000 people to clean up, removing over one million tonnes of algae from the beach and coast...
...Within 10 days, these patches had moved away from the coast and into the Yellow Sea, covering 1,200 sq km, and impacting about 40,000 sq km of ocean, making it the largest algal boom, or green tide, ever recorded.
Hennie