Algae bloom is made up of microscopic planktonic suspended algae (also known as phytoplankton - approx 4 microns in diameter).
Suspended algae are plant life and just like plants they flourish during the summer months when conditions are ideal: more sunlight, warmer water temperatures and higher levels of nutrients.
Pigments (chlorophyll, fucoxanthin and carotenoids) in the algae (green water) absorb light energy and use it to convert carbon dioxide and nutrients into new cell biomass through photosynthesis.
The primary nutrients of concern are nitrogen and phosphorus. Nitrogen is a by-product of decomposed fish waste, uneaten fish food and accumulated sludge in the bottom of the pond.
The "pea soup" like floating algae bloom will eventually take over your entire pond resulting in potential fish fatalities, due to the decreased oxygen levels in the water.
At night, both algae and fish consume oxygen from and exhale carbon dioxide into the system. Algae compete with the fish for available oxygen in the water. A potentially serious impact of an algae bloom is the risk of an algae crash triggered by temperature or barometric pressure.
When an algae bloom collapses, dead algae cells settle to the bottom of the pond, adding to the decomposing sediments oxygen demand. If the crash is severe, the oxygen supply can be quickly depleted endangering the fish unless backup aeration is available.
Additionally, as the dead algae cells rupture, they release organic nitrogen and phosphorous back into the water, adding to the systems nutrient load. The biological cycle starts again with bacteria converting the organic nutrients to inorganic elements, which are then available to be recycled - and the algae bloom continues.
Algae does indeed have it's benefits: It is a valuable source of nutrients for your koi and goldfish, particularly their recently hatched fry. Unfortunately, green water, caused by floating algae bloom also prevents you from being able to see your pond fish; this may have dire consequences if there are sick or injured fish in your pond as you fail to notice them and their symptoms for some time.
Algae influences several factors in your pond system, including the levels of oxygen, pH, carbon dioxide and nutrient levels. Algae are microscopic plant life who like their larger cousins (plants) photosynthesize. During daylight hours when photosynthesis occurs floating algae produces oxygen, uses up carbon dioxide and removes nutrients.
However at night time the opposite happens and the algae bloom begins to respire. In ponds with large fish loads the concentration of carbon dioxide rises quickly and the water pH level rises. Fish not used to such sharp shifts may initially show signs of stress.
During night time hours both green water and your koi and goldfish fight for the pond's oxygen supply. Heavy algae blooms often result in fish fatalities, due to suffocation.
There are various methods of controlling the build up of algae bloom, some more permanent than others. Such methods include the use of aquatic pond plants, using shade, using pond algaecides and in my opinion the best method, the use of a UV sterilizer also known as a UV clarifier or simply as a UVC.
Maintaining crystal clear water, free from the dreaded green water algae bloom is the aim of the vast majority of us pond keepers. Whilst a pond with some suspended green algae may not necessarily be a bad thing it will detract from the idea of the fish pond i.e. keeping and being able to see your Koi, goldfish and other pond inhabitants.
Suspended single celled algae also known as floating algae or pea soup algae is a blight for many a pond keeper, particularly during the warm summer months when there is an abundance of pond nutrients and sunlight.
In my opinion a Pond UV sterilizer or UV Pond Clarifier is the best all round method of controlling algae in water. The terms UV sterilizer and UV Clarifier are used to mean the same thing on this write, although there are actually some differences.
For our purposes though please take them to mean the same thing.
Although there are other green water pond treatment methods including shading, using pond algaecide products, using aquatic plants and other chemical treatments, I would recommend UV sterilization (UV Clarification) because it will cost you less money in the long run as well as save you lots of time and effort.