Pond aeration is the process of adding oxygen into the water. While this does occur naturally both through contact with the surrounding atmosphere and photosynthesis through pond plants, there are some roadblocks to successful levels of dissolved oxygen. Low oxygen levels can lead to many undesirable effects.
Start with the Obvious: Fish
All aquatic life relies on oxygen. Koi fish are the most popular pond additions. They are part of the carp family and originated from rice paddies in China and Japan. Successful rice harvests require clean, fresh water. Koi are accustomed to similar water quality. They are also used to high levels of oxygen that the millions of plants would provide in a rice paddy.
Warm water holds less oxygen than cooler water. In the summer, many pond owners use waterfall features, pumps, and filters. These help maintain oxygen levels. The oxygen from the surface enters the water, and surface area increases as the water splashes or moves across objects like rocks. This also helps mix the water from all levels. This is important for months when the sun is warm enough to heat the top layers of the water. Without help, the warmer water does not mix with the cooler water from the depths of the pond. This leaves the top layer with all the oxygen from the atmosphere and photosynthesis. Animals that stay near the bottom regularly (crayfish, prawns) constantly are in colder and less-oxygenated water. Those that travel to all areas of the pond, such as koi, tend to stay toward the surface, which leaves them vulnerable to predators.
Danger increases when temperatures begin to drop. This causes all layers of the pond to cool and mix. This turnover is a major cause of death in ponds. Ideally, aeration would keep layers mixed evenly year-round. Unfortunately, pond owners in cold climates must shut down much of their equipment for the season. It is important that those with fish still maintain aeration through the use of heaters, bubblers, and other aerating devices.
Other Living Things
Not having fish is no excuse for a lack of aeration. Plants require oxygen, too. While plants produce oxygen, they also produce debris and excess nutrients that accumulate as sludge and cloudiness. Both beneficial bacteria and algae eat the waste products of decomposition (ammonia, nitrate, and phosphate). Beneficial bacteria are living organisms that require oxygen. Low oxygen means bacteria struggles. Unsightly algae do the job instead. Algae will rob any remaining good plants of oxygen left in the water.
Furthermore, plants use the sun to produce oxygen (photosynthesis). During the night or during inclement weather, they need an additional source of oxygen to really thrive.
Mucky surfaces, excessive algae, and murky water are not the only downfalls to low oxygen levels. Ponds without aeration are also prone to foul odors. Bacteria working to decompose plant matter and debris use oxygen (aerobic bacteria). Anaerobic cousins are less efficient. Also, they produce carbon dioxide and hydrogen sulfide as they digest their food. That gives the smell of rotten eggs! Ponds will become unpleasant to visit if left without aeration because anaerobic bacteria (stinky) or algae (unappealing) will handle the waste consumption.
The moral of the story is to make sure there is a source of oxygen in ponds. Solar aeration is also the option you can choose. For fish health, plant health, and pond enjoyment, the best advice is to make sure there is a constant source of aeration at all times of the year.