The pond gets some oxygen from mixing of wind and wave action but usually not enough to support a fish population. Most of the oxygen available for fish to breathe comes from the photosynthesis of phytoplankton. Plankton has two jobs in the pond, it is the base of the food chain and it produces oxygen. As the sun comes up in the morning plankton begin to produce oxygen. they produce oxygen all day long (as long as the sun is hitting the water)As day turns to night the plankton begin to respire (use oxygen) When fertilizing your pond its critical not to over fertilize or you will "grow" too much plankton. the excessive amount of plankton will use up all the oxygen in the pond at night and you will wake up to dead fish in the morning.
Note: carbon dioxide has an inverse relationship, just before sunrise the CO2 level is at its highest for the day because everything is the pond has been respiring all night. as the sun comes up the phytoplankton begin to take up CO2 to produce oxygen. as the day progresses the CO2 level drops in the pond as the O2 level rises. this uptake of CO2 is a primary factor effecting pH, therefore ponds typically are more acidic just before sunrise and move towards basic pH reading in the afternoon. the pH level, oxygen level and CO2 level change all day and all night long. Just how much it changes depends on the alkalinity,hardness, water temp. etc.
Plankton is so good at producing oxygen that it can do it on a bright full moon and in northern ponds it can produce oxygen even under the ice. Thats why you hear about the guys up north sweeping or blowing snow off the pond, its so the sunlight can pass through the ice.
Aquatic plants also produce oxygen but they use it as well. its also possible to have so many plants in the pond that the oxygen is used up at night. the next time you see a thick weed bed give it a poke with a stick or paddle. the bubbles you see coming out of the mat are oxygen bubbles.
mark, your pond is full of phytoplankton and zooplankton just not in high enough densities to color the water green. you can have good fishing in an unfertilized pond, your pond just wont hold as many fish. take a bass heavy pond for example. If you begin a fertilization program and your pond is full of 12 inch stunted bass, your bass wont grow. you will just have more 12 inch stunted bass. proper balance between predator and prey are absolutely necessary to large healthy fish. if your pond is balanced it will produce large fish even without fertlizer.
