If you remember from some previous posts, the idea behind the biofiltered pond is that when the water is pumped (slowly!) through the sand in the sand filtration area, a thin biological layer of “good” bacteria and other small organisms digest and break down the viruses, “bad” bacteria, algae and raw nutrients (carbon, nitrogen, phosphorus, potassium) in the water. The water plants planted in the sand can slowly extract these nutrients to grow, thus taking the nutrients out of the pond entirely.

The target then is to starve the water of nutrients to such an extent that the algae us unable to grow resulting in the “crystal clear water” one associates with a beautiful and healthy pond. All done without UV, chlorine or other chemical processes.

While I don’t expect to reach this ideal target any time soon due to the fact that we decided not to line the pond with cement or plastic for simplicity and cost reasons. As a result, there are probably enough nutrients in the clay soil on the edge of the pond to grow some algae for quite a while. But looking at the pond regularly, I have started to think that the water is slightly clearer than it was before.

You can never really tell about these things unless you measure them, which I will get to eventually.

But I recently noticed one indirect sign that this theoretical process is actually working. I’ve taken pictures of the water grasses and other plants that we planted in the sand in the past and from them you can see all that plant matter growing up in the water. Clearly, all of that plant matter must come from nutrients in the water. But take a closer look at this picture:

If you consider the closer bunches of plants in the water, keep in mind that they are all the same kind of plants, planted at the same time. While they have all grown and filled out, the bunch on the right has grown much more than the others. That one happened to be planted on the very edge touching the soil, so it is getting more nutrients from the soil, while the others are limited to only the nutrients in the water. Since the smaller ones on the left have all grown, at least in the beginning, there was enough nutrients in the water for growth. And up until recently, they seemed to all be growing at the same rate. But only recently have I noticed that the left ones seem to have stopped growing, or at least slowed substantially down compared to the one that has access to the soil nutrients.

This seems to indicate that the filtration is functioning, and the amount of nutrients in the water is getting less and less.