Its linked to the likelihood of algal blooms occuring, which is in turn linked to the availability of nutrients (nitrates and phosphates), sunlight supply, temperature and whether the waterbody is moving (river, stream etc) or stationary (lake, pond etc). Though I think nutrients should be focused on more...
Algal blooms accelerate eutrophication (which by itself is a natural process) and under the aid of humans (effluent discharge, sewerage etc) its called cultural eutrophication.
Hope that helps.