(1) this is a natural phenomenon:

Red tide is a natural ecological phenomenon that some tiny (2-20 microns) phytoplankton, protozoa or smaller bacteria in the ocean explode or suddenly gather under certain conditions, resulting in discoloration of water bodies.

"Water bloom" is a natural ecological phenomenon in fresh water, which is only caused by algae, such as cyanobacteria (strictly called cyanobacteria), green algae, diatoms and so on. When "water bloom" occurs, the water body is generally blue or green. These two kinds of red tides and "water blooms" in nature have been recorded in the ancient history of China. They quickly disappeared in nature, causing no harm to aquatic animals and humans.