Causes of Water Quality Problems: Understanding the Components That Impact a Pond's Ecosystem

Landscape & Irrigation, November 2006 | Go to article overview

Causes of Water Quality Problems: Understanding the Components That Impact a Pond's Ecosystem


From maintaining good water quality to being able to identify, diagnose and find a cure to what might be causing aquatic problems; managing the water quality of your pond or lake can be challenging. Excessive aquatic vegetation, fish kills and foul odors are all common ailments that pond owners face. And with the array of contributing factors that can create these aquatic problems, it makes finding a solution that much more difficult.

The best way to find a solution is to obtain a better understanding of the components that impact the delicate balance of a pond's ecosystem. From a lake manager's, perspective the most significant factors are: light and temperature, nutrients, and oxygen.

Light and temperature

Sunlight is of major significance to lake dynamics as it's the primary source of energy. Most of the energy that controls the metabolism of a lake comes directly from the solar energy utilized in photosynthesis. Photosynthesis will occur only in the upper layer of the pond, or euphotic zone, this is the area in the water column that sunlight is able to penetrate. Shallow bodies of water less than 9 feet in depth more commonly experience problems such as bottom--rooted weeds or benthic algae.

Thermal stratification is a term meaning temperature layering. As the sun shines on a pond, it warms the surface water. This water becomes lighter than the cooler, denser water that is trapped at the pond's bottom. As the hot summer season progresses, the difference in temperature between the warm surface water and the colder bottom water increases. As a result, the water becomes stratified or separated into layers and the top and bottom layers of the lake do not mix with each other. The area created between the warm and cold layers, called the thermocline or metalimnion, can act as a physical barrier, preventing any vertical mixing in the lake, and encourage algae growth throughout the warm surface waters.

Thermal stratification impacts the water quality in a lake primarily because of its effect on dissolved oxygen levels, the way we measure how water holds oxygen. Compared to cooler water, warm water has a diminished capacity to hold oxygen. In fact, water at 52 degrees Fahrenheit can hold over 40 percent more oxygen than water at 80 degrees Fahrenheit. As water temperature increases, the water's capacity to hold oxygen decreases.

Dissolved oxygen in a lake comes primarily from photosynthesis and wave/wind action. During stratification, bottom waters are removed from both of these sources, and an anoxic--or no oxygen condition--occurs. Aquatic organisms require oxygen to survive; in its absence, organisms must move from the anoxic area or die. Anoxic bottom waters lose most, if not all, of the zooplankton and aerobic bacteria necessary for efficient and effective digestion; while less effective, more pollutant-tolerant forms of anaerobic bacteria will develop.

The lack of dissolved oxygen sets in motion a series of chemical reactions that further reduce water quality: sulfide is converted to hydrogen sulfide, insoluble iron is converted to soluble forms, suspended solids increase and a severe decrease in the decomposition of waste materials on the pond bottom will occur.

Thermal stratification occurs in a seasonal cycle with the thermocline becoming more severe in late summer and late winter. Lakes and ponds in warm-weather regions experience a shorter annual cycle, spending more time in late summer and early fall conditions.

Shallow ponds offer the water manager an even greater challenge. Ponds less than 6 feet in depth tend to be very warm, allowing for the entire water column to be productive with weed and algae growth. These types of ponds need extra consideration when determining the correct water management solution.

Nutrients

The second essential factor in our lake management discussion is the impact of nutrients on the aquatic ecosystem. …

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