Planktonic algae, also known as blue-green algae, are free-floating, microscopic plants suspended in the top of water so they can get the sunlight necessary for photosynthesis (which is how they create energy). Algae and aquatic plants both consume carbon dioxide and give off oxygen in the process of photosynthesis. Algae compete with aquatic plants for nutrients and sunlight; when they crowd out aquatic plants, algae can explode in growth, making the water look murky and green.
Competition for Carbon Dioxide
Carbon dioxide exists in water in the form of carbonic acid, H2CO3, that lowers the pH of water. Algae and aquatic plants both take carbon dioxide from the water during the day. At night they release carbon dioxide into the water. When algae overwhelm aquatic plants, the plants do not get enough carbon dioxide.
Competition for Nitrogen
Algae also compete with aquatic plants for nitrogen. Nitrogen is produced by dying and decaying plants and by the waste of fish and animals. Manure deposited by ducks, geese and livestock (drinking from ponds) can cause nitrogen to increase in the water, causing a growth of algae at the expense of aquatic plants.
Suspended blue-green algae, also called planktonic algae, look slimy and make the water look green. Murky water caused by blue-green algae cuts the light needed by aquatic plants with roots. Cold temperatures in the winter inhibit the reproduction of planktonic algae, which is why ponds are clearest in the winter.
Bright green, slimy hair or string algae form filaments that can grow several feet long. These filamentous algae cling to the sides of pond and can smother aquatic plants. They also help reduce the amount of green algae suspended in the water, helping make it more clear. Filamentous algae grow more in the summer when the nutrient levels in the water are high.
Aquatic plants will compete better with algae if the water is rich with potassium. This can be accomplished by adding potassium-only fertilizer called potash. Potassium chloride, 0-0-60, marketed as muriate of potash, sulfate of potash 0-0-60 or magnesium sulfate of potash, 0-0-22, will all help plants compete with algae.
Adding an "algae bar" that slowly releases copper will kill algae. The drawback is that if the doses are too high, copper will kill fish.