[bekko]

During the day, the single cell algae in green water consume carbon dioxide for photosynthesis, produce oxygen as a photosynthesis byproduct, while simultaneously consuming oxygen for respiration and releasing carbon dioxide through respiration. The net effect during the day is to consume carbon dioxide and produce oxygen. During the night, there is no photosynthesis going on so no carbon dioxide is consumed and no oxygen is produced, but oxygen is still being consumed through respiration and carbon dioxide is still being released through respiration. Obviously, there is a net loss of oxygen and net increase in carbon dioxide at night.

In a static mud pond, the effects of green water are oxygen, carbon dioxide and pH diurnal cycles. After spending most of the day photosynthesizing, the oxygen level is very high - saturation or above. The carbon dioxide level is very low. The removal of carbon dioxide (a weak acid) causes the pH to rise. At night when photosynthesis shuts down, oxygen is still being consumed and carbon is still being produced through respiration. So, the oxygen and pH increases all day and declines all night while the carbon dioxide declines all day and increases all night.



What usually kills fish first in a green water pond is night time oxygen depletion. This usually occurs when there are days of bright sunshine followed by a few days of heavy cloud cover. The days of bright sun allows the algae population to rise. The more algae cells there are, the more oxygen is produced during the day and the more oxygen is consumed at night. If there is a very high algae cell density and the weather turns cloudy, then the algae are not able to produce as much oxygen through photosynthesis during the day, but they still consume the same amount of oxygen through respiration. By dawn, the oxygen which was produced by photosynthesis the day before may be depleted and the fish are gasping at the surface or dead. There is another phenomenon where a heavy rain can "turn over" a static pond and cause dissolved oxygen depletion but we will leave that for another time.



In a heavily aerated green water pond, the cycles are muted and the dangers are much reduced or eliminated. Aeration allows oxygen to reach saturation during the day, but aeration can actually remove oxygen if the pond would otherwise be slightly supersaturated. Daytime aeration also replenishes some of the carbon dioxide which is being consumed through photosynthesis. At night, aeration replenishes the oxygen being depleted by respiration and drives off excess carbon dioxide being produced by respiration. There will still be oxygen and carbon dioxide cycles, but they will not be dangerous if the amount of aeration is properly sized for the pond volume and bio-load.



The primary benefit of green water is the food chain that develops because of it. The single cell algae are consumed by many micro- and macro-organisms. The micro-bugs may be consumed by crustaceans and other larger forms. Some of these larger forms are consumed by koi. In fact, carp in general are very efficient at harvesting the biomass one or two steps up the food chain from green water. This natural forage can seem almost magical as it allows fish to grow rapidly and develop a very healthy appearance. It is the perfect diet which we try to emulate with our prepared foods. The benefits of the natural diet out-weight any negative effects associated with diurnal water quality cycles.



However, this natural feeding process and the availability of koi forage is highly controlled by fish density. If there are too many fish, the population of larger forms of forage creatures never has a chance to develop properly because the grazing pressure is too high. We can use supplemental feed (out of a bag) to increase the availability of stuff to eat and allow a modest increase in fish density without sacrificing growth. However, when the line is crossed and the natural forage is being over-grazed, that growth-enhancement effect is lost and the fish derive little benefit from the green water environment. You can see the same thing happen with range-fed chickens.



This, I feel, is the basis for our preference for green water mud ponds where the fish density is very low, and clear-water concrete ponds where the fish density is very high.



-steve hopkins