Water Turbidity

What is turbidity? Turbidity is a very general term that describes the “cloudiness” or “muddiness” of water. Turbidity can be caused by many substances, including microscopic algae (phytoplankton), bacteria, dissolved organic substances that stain water, suspended clay particles, and colloidal solids. Although turbidity can be a problem in many different types of water, turbidity caused by suspended clay tends to occur most often in soft, poorly-buffered (low alkalinity) waters. Some of the substances that cause turbidity are more desirable in fish culture or recreational farm ponds than others. In moderate amounts, phytoplankton is a desirable form of turbidity because it provides food for microscopic animals (zooplankton) and filter-feeding fish, and improves water quality by producing dissolved oxygen and removing potentially toxic compounds such as ammonia. On the other hand, turbidity caused by clay particles is generally undesirable because it keeps light from penetrating the water, and light is required for algal growth. At very high concentrations, clay particles can also clog fish gills or smother fish eggs. Turbidity also may be objectionable to pond owners from an aesthetic standpoint.

Some sources of clay turbidity are runoff from clear-cut or overgrazed watersheds, road or building construction, the activities of cattle watering in farm ponds, pond bank erosion from wave action, excessive aeration, or the feeding activities of certain bottom- dwelling fish such as common carp or buffalo. This fact sheet will discuss the control of undesirable forms of turbidity, specifically that caused by suspended clay particles.

 The effect of clay turbidity on dissolved oxygen

The dissolved oxygen in sportfish or farm ponds normally fluctuates widely during the summer. During the day, plant photosynthesis increases the oxygen concentration; during the night, plant and fish respiration reduces the oxygen concentration in the water. Clay turbidity reduces the magnitude of daily fluctuations in dissolved oxygen concentration, so that it gets neither very high nor very low. However, muddy water tends to have a lower average concentration of dissolved oxygen than water with a green phytoplankton bloom. Clay turbidity can sometimes develop quite suddenly, as when heavy storm runoff enters the pond or high winds churn the water and cause bottom soils to be resuspended. In such cases, oxygen may decline to critically low levels and make it necessary to aerate the pond.

 The effect of turbidity on off-flavor in fish

Not much algae can grow in muddy water because clay particles limit the penetration of light into water. Blue-green algae are adapted to the dimly lit waters of moderately turbid ponds. Unfortunately, some of these algae can cause off-flavor in fish, which could be reason enough to clear water of clay turbidity. Interestingly, extremely muddy ponds have few, if any, algae in the water and often less problem with offflavor than moderately muddy ponds.