[MikeM]

In another thread we were discussing algae in the water column. It was observed that we might do better for our koi if we could maintain and control a green water environment in a koi pond, and it was commented that filtration based on nitrifying bacteria rather than algae has proved far better suited to koikeeping and the artificial environment of the manmade koi pond. All of which got me thinking about pond dynamics and reading some of the literature on pond dynamics.

Mud ponds and natural ponds are very different from our wholly artificial backyard koi ponds. The natural processes at work are very complex and have to be understood as interacting with several processes occurring simultaneously. When a mud pond is first constructed there is relatively little nitrogen present. Nitrogen is added through fertilizers applied to promote microbe growth as a food for fry, through feeds given to the fish, the wastes fish produce and through the leaves and debris blown in from the surrounding area. Within the water column, harmful (to fish)ammonia and harmless ammonium will be in equilibrium according to the pH of the water. Ammonia can diffuse directly from the pond water into the atmosphere, it is directly consumed by the phytoplankton, it is directly adsorbed by clay in the pond soils, it is nitrified into nitrate by bacteria, and in the anaerobic soil layers nitrate is converted to nitrogen gas which diffuses through the water column into the atmosphere. Studies show that nitrogen leaves the pond system through denitrification and ammonia volatization, with some bound into the substrate clays.

Phosphate is similarly introduced and is rapidly consumed by phytoplankton. Pond soils adsorb phosphates, with clay soils having a great deal of adsorptive capacity. The pond soils gain an ever increasingly high phosphate content in an insoluble form. Organic matter decomposing in the substrate releases phosphate that is quickly bound into the soil. Unlike nitrogen, phosphate does not exit the pond through conversion into a gas. It builds up in the substrate. In a natural pond, rooted plants will extract phosphate from the submerged soils. In a mud pond there are few or no rooted plants and the phosphate concentrations can become quite high.

Clay soils inherently have high aluminum content. Aluminium ions are continuously creating an equilibrium in the soil between the ions adsorbed on soil particles and ions in the water. The aluminum ions will convert to aluminium hydroxide, releasing hydrogen ions, which acidify the soils. The mud pond will be limed to counter this process. The natural pond will reach its own acidic substrate balance, with plants adding different processes.

The newly dug pond has little organic content. Immediately upon being filled with water, this begins to change. Run-off entering the pond brings suspended mineral particles and organic matter. Algae die and settle onto the bottom. Decomposition and mineralization occurs. Simple carbohydrates and protein are quickly decomposed. Complex carbohydrates and cell wall material decomposes more slowly, accumulating on the bottom. Typically in about 4 or 5 years a balance of decomposition is reached. The slow rot is consuming as much of the accumulation of degradable organics as is being deposited on the pond bottom. A gradual thickening of a mineralized substrate then begins, so in the mud pond it is the general practice to drain, till and "renovate" the pond soils at least every 4 or 5 years. A natural pond does not receive such interference.

There is often a misunderstanding that the mud ponds in which koi are raised are equivalent to natural ponds. They are not. They are more natural than our concrete and liner koi ponds, but not really the same as a natural pond. Both, however, are far more complex than our simple koi pond systems. In the koi pond we eliminate the interaction of water with soil...which means we avoid our koi pond water participating in the full range of chemical processes at work. I've not even touched on the ionization of calcium, magnesium, etc., etc. We should never think that we are copying nature. We are not. We use natural processes in our biofilters to serve a goal, but we are very purposefully avoiding as much of the natural processes as we can. We cannot control those processes so well.