Pond Algae And Water Clarity

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The relationship between pond algae and water clarity can seem a little confusing for a pond owner. Clear water is not always good water, and one type of pond algae is the best prevention method for undesirable pond algae. There are many types of pond algae, some desirable, and some not.

If pond algae, of the pond scum or blanket weed type is forming a dense mat on your pond or small lake, there is about a 98% chance that the water beneath the mat is crystal clear. Check it out, and then come back, and I will tell you how I know this.

Back so soon?

It was clear wasn’t it? How do I know? Simple. Pond algae needs sunlight to grow. Pond algae needs soil to grow. For sunlight to reach the bottom of your pond, where pond algae gets it start in life, the water has to be clear. As the pond algae grows, some of the oxygen it produces by photosynthesis is deposited on the bottom of the mat of forming pond scum, and gives it the buoyancy to rise to the top of the pond. Submersed pond weeds, and blanket weed pond algae need clear water to produce, and in spite of all the thousands of products on the market that tout the virtues of clear water, and why their product will produce clear water, clear water is not desirable in a pond or lake where fish are producing.

Types of unclear water

Let’s make a distinction between types of unclear water. Do not confuse muddy or turbid water with the properly green colored water produced in a healthy pond or lake. Muddy water is not healthy water. In fact, water containing suspended soil particles is bad for the fish population. It is an irritant to fish, which causes them to feed less, fidget more, and often lose weight. It also prevents the natural photosynthesis process needed for the food chain. No, the water color needed for a healthy fish pond has a green tint, that will prevent sunlight from reaching more than 18 to 24 inches below the surface. There are products on the market that will color the water to prevent sunlight from reaching the bottom, but they have the same issue that makes muddy water undesirable. They suspend the food chain activity. So what is this magic color that prevents sunlight and provides the beginning of the food cycle?


Phytoplankton is a single cell algae that becomes suspended in lake and pond water, and is important for what it produces and what it prevents.

Oxygen production

Phytoplankton produces oxygen through photosynthesis which becomes available to aquatic life. Without oxygen in the water, your fish will die.

Shade and cooling

The phytoplankton type of pond algae provides shade for the water which helps to keep it cool. This is very important in the hotter parts of our country during the summer months.

Food production

  • Phytoplankton is the food source for the tiny creatures known as zoo-plankton
  • Zoo-plankton becomes food for insects
  • Insects become food for small fish
  • Small fish become food for large fish
  • Large fish become food for you.

Weed and pond algae prevention

The tinting effect of phytoplankton prevents sunlight from reaching the pond bottom, which prevents pond algae and submersed weed problems.

Producing phytoplankton

So, how can you produce this miracle pond product? First: If you have a heavy cover of lake weeds or pond algae, get rid of it.

Check the water color.

You can do this with a tin foil pie pan nailed to the end of a stick, although a more accurate measure can be attained using a “secchi disk” designed specially for this. Either way, if you can see the disk or pan at a depth greater than 18 to 24 inches, your pond is too clear to be healthy.

Test the waters pH

You can do this with a swimming pool test kit that you can purchase at most places that sell pool supplies. If the pH is between 8.6 and 6.8 you can go on to the next step. If it is lower than 6.8, the acidity is too high and will need to be changed before the next step. Agricultural limestone will correct the problem. I ton of agricultural limestone on one surface acre of water will raise the pH by 1 point on average. If you have a 1 acre pond, and the pH is 4.8, you will need at least 2 tons of limestone to bring your pond up to the minimum pH needed for healthy aquatic life. Do this before taking the next step.

Fertilize your pond

Fertilization will produce algae bloom, or phytoplankton in your pond. The right fertilizer is a fertilizer high in phosphorous and low in the other 2 elements. Phosphorous is the middle number in the 3 numbers listed on a bag of fertilizer. You need to add 40 lbs of 20% phosphorous, or 20 lbs of 40% phosphorous, or something similar in ratio per acre to initiate algae bloom. See also: How To Fertilize A Pond Or Lake

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