Look at the satelite photograph of the black sea.suggest why the algal bloom is around the edges of the sea , rather than spread all over it.

they grow only at such a depth where sunlight reaches, the maximum depth at which plants attached to the bottom can grow is 20 meters (there is also a record depth of 54 meters) Plants no longer grow deeper and all for the same reason - there is no light...

or if the question is already like this, then only because there is no light coming in!