Murky Waters - Getting to the Bottom of Pond Clarity Issues

Murky or unclear pond water is most commonly an aesthetic issue and the causes for the water's discoloration can vary. Ponds are open, living ecosystems containing plants, fish and other aquatic organisms so it can be expected that the natural processes involved with such can cause changes in  water clarity. These changes can vary from season to season and depend on pond water depth and movement. Below are some of the most common causes of murky pond water.

Algae Blooms

The base of the pond food chain is planktonic algae. Planktonic algae are microscopic single-celled organisms living throughout the water column. This type of algae can color the water green, reddish, or brown and is sometimes mistaken for silty or muddy water. Planktonic algae can be distinguished from sediment in water by holding a clear glass of pond water up to a bright light. If most of the particles in the water move erratically, the pond is experiencing a bloom. If the particles do not move, sediment is the cause of the water discoloration. Planktonic algae provide a food source for tiny aquatic organisms called zooplankton, which in turn become food for larger macroinvertebrates and fish. In warm summer weather, this algae can multiply rapidly in blooms that seem to appear overnight, changing the appearance of the water quickly.

A similar microscopic organism that can be of special concern for ponds is cyanobacteria. Cyanobacteria are photosynthetic, single-celled bacteria that are commonly referred to as blue-green algae. Large populations of cyanobacteria are often called HABs (harmful algal blooms) because cyanobacteria can produce toxins that can be dangerous to humans or animals that use pond water. HABs can make water appear as if green paint or pea soup has been spilled on the surface. If you notice this pea soup appearance in your pond, keep people and pets out of the water until the bloom subsides to avoid exposure to potential toxins.

Muddy or Turbid Water

Muddy or turbid pond water is frequently caused by runoff from disturbed or eroded areas around the pond, or from bottom-dwelling fish, like carp, and from mammals like muskrats who burrow in pond banks. Muddy water is best addressed by eliminating its source. This might include planting grass or other native vegetation on exposed areas, putting a layer of rocks over exposed banks, or removing muskrats or bottom-dwelling fish. Muddy water is very common in new ponds and usually disappears as vegetation grows around the pond. In established ponds, muddy water can almost always be traced to a preventable source. Avoid mowing your pond right up to its edges, and maintain a riparian area around your pond with native plants, grasses, and shrubs to help intercept any sediment and runoff from surrounding land. This will also help to keep your pond water clear.  If your pond is in a pasture, fencing livestock out of the pond, or limiting their access to a small portion of the pond edge will help reduce bank erosion. 

Persistent muddy water problems that aren't being solved by reducing source inputs may be the result of suspended clay particles in the water column. Clay, due to its particle makeup, does not clump or settle out easily, leaving it suspended in water and causing a cloudy appearance. This suspended clay can be treated with additions of ground limestone, hydrated lime, gypsum, or alum, which are flocculating agents that allow the clay to clump together and settle to the pond bottom. Be sure to add the correct amount according to your pond's size and depth. Application recommendations and additional guidelines are available in the Extension publication Management of Fish Ponds In Pennsylvania.

Organic Matter Buildup

It is inevitable that ponds will accumulate organic matter from rotting plants, leaves, detritus, and other things entering the pond. This may become more of an issue if your pond is surrounded by large trees that lose their leaves in the fall. Tannins from rotting leaves and other matter can give pond water a tea-colored hue. While this coloring won't cause issues for the pond, leaves can be removed when they fall if their presence is bothersome.

Measuring Water Clarity

Sediment or turbidity in pond water can be measured using a simple device called a Secchi disk. This black-and-white patterned weighted disk is lowered into the water until it is barely visible, and the depth of the water is recorded. The higher the value, or the lower down you can see the disk in the water, the clearer the water is. This measurement can be taken from the pond bank or dock, or from a boat for a reading in the middle of a larger pond. Another tool, a transparency tube, can be used to measure water clarity by looking down through a vertical column of water to identify the depth at which a smaller version of a secchi disk at the bottom of the tube is visible.  

A Final Word

Ponds are living systems full of plants and animals. Identifying the causes of water clarity issues can help pond owners to be stewards of these ecosystems and keep them functioning as healthy pond environments. 

For more information on pond water quality: Water quality concerns for ponds