Although it is sometimes called an algae bloom, normally the names it is called are unprintable. For some, it seems to happen every Spring (also sometimes in the Fall). For others, it is almost a way of life.
A limited number of pond keepers have never or rarely experienced this "wonder" of nature. It is said that the Koi thrive in it, but you cannot see them to tell if they are thriving or not. You have heard many reasons why your water turns green and tried assorted mechanical wizardry and various chemical concoctions to clear it, (which may or may not have been harmful to your Koi), but it is still green.
There is a lot of "snake oil" out on the market to clear green water.
Green water is caused by an excessively large number of tiny organisms in the water. Called phytoplankton, these minute plants are part of the algae family that has thousands of distinct species found in water (and ice) throughout the world. These organisms are very small, with the most common ones found in our ponds being around 15 microns (0.0006 inches) in diameter. All pond water contains large numbers of different kinds of these plants and other microorganisms. Water that appears to be crystal clear just doesn't have as many.
Some of the statements that follow are somewhat controversial, but they are based on several years of research and experimentation dealing with the subject. From this research, I have concluded that within our biologic converters, a third group of bacteria exist. When these heterotroph bacteria consume dead algae in an aerobic environment, they release an enzyme, possibly used to help them digest the dead algae. The flow of water through the media carries surplus amounts of this enzyme back into the pond where it kills off the other algae.
This enzyme appears to be effective against many species of string algae as well as the bloom algae. It does not seem to have as much effect on the string algae which is only partially submerged or within a high flow area, i.e. in a splashing brook or around a waterfall. This may have to do with contact time requirements. The short blackish-green mat algae found on the walls of a "healthy" pond is composed primarily of dead string algae which is also believed to be a result of control by the antibiotic. Further, this mat area may also be providing a portion of the enzyme as it is being broken down by the heterotroph bacteria.
This seems to explain what we see in our ponds much better than many of the traditional myths which I believe arise from invalid extrapolations and application of true scientific findings based on studies of large lakes and oceans. Most of these findings just simply do not apply to the essentially closed environment of an established, circulating Koi pond. We will discuss only two of the myths here. For more and a detailed description of the experiments leading to these conclusions, see my article in the
Mar-Apr 1998 issue of KOI USA.: It is true that algae needs light to grow and reproduce. But what is interesting is the small amount of light that is actually required. Controlled experiments using reduction in sun light of 90% still show significant algae growth. There are many examples of ponds that are heavily shaded but quite green and just as many others with direct sun exposure that have no algae bloom problems at all. There have been positive results reported of completely covering a pond suffering from green water with an opaque plastic cover for 5-10 days. I'm not too sure what the Koi think about this but it is obviously not an acceptable permanent solution. I do recommend providing shade over a pond, but more for temperature stability than for algae control.
So, what is the solution? It seems to be simply a properly sized biologic converter and a proper flow rate of oxygenated water through it. The bio-converter must be large enough to support the heterotroph bacteria colonies which need considerably more space than just the nitrification bacterial colonies. This has led to two rules of thumb. The first is that the amount of water in the pond and filter system should be circulated through the bio-converter at least once per hour.
Second is that a flow rate of approximately 150 gallons per hour per square foot of media should be used. As an example of a 1500 gallon pond, we should be moving 1500 gallons of water through the bio-converter each hour and the bio-converter cross sectional area exposed to water flow should be 10 square feet. The thickness of the media is determined by the media selection.
Bubble bead or similar type pressurized filters do not generally have sufficient internal surface area to support the heterotroph colonies necessary for the enzyme production although they can provide the area necessary for the smaller nitrification colonies. They do an excellent job of capturing the dead algae and other solids. During the frequent backwashing processes, however, the dead algae and much of the heterotroph bacterial colonies are removed from the system giving insufficient time for the enzyme to be produced. This is why ponds using these type filters almost always require an ultraviolet system to handle the green water problem. A properly sized UV system will do a good job on eradicating the bloom algae. It will not affect the string algae, only the phytoplankton that actually pass through the unit. There are also some indications that the UV radiation may destroy or at least weaken any enzyme action.