Thick bluegreen algae films or layers can be bad in several ways, taste and odor in fish flesh, cause of low DO in deeper waters, unsightly, overall poor water quality, some types can be toxic to animals drinking the water.
Most of the nuisance blue green algae can get their nitrogen source from the atmosphere and not the water. Most of the nuisance bluegreen film, surface, algae types grow best in the heat of the summer and are indicative of too many dissolved nutrients in the water, especially phosphorus. Thus fertile fish ponds often have bluegreen algae blooms in the warmest summer months.
The production of all that plant mass on the surface is just a small protion or fraction of what a pond is capable of producing.
Did I answer your questions or do you have more? Try to be specific with questions.
