Understanding Pond Dynamics: Seasonal Changes and Factors Contributing to Poor Water Quality

Landscape & Irrigation, December 2007 | Go to article overview

Understanding Pond Dynamics: Seasonal Changes and Factors Contributing to Poor Water Quality


As the seasons change, so do pond and lake dynamics. The two climatic factors that have the greatest impact on seasonal variations in water quality are temperature and light. Biological activity in the pond is directly tied to these two factors. Because the market tends to deal with symptoms of poor water quality (algae, odors, etc.) as opposed to causes, focus on water quality management fades when the symptoms disappear.

Temperature has significant impact on both plant and animal life, simple and complex, in the aquatic ecosystem. Since few forms of animal life in the pond are warm blooded, the effects of lower temperatures are significant. Fish and other cold-blooded species slow and will become dormant when temperatures drop low enough. As temperatures drop, so do the metabolic rates of bacteria and protozoa--Mother Nature's "garbage disposals" so to speak. Digestion rates are tied to water temperature, the lower the temperature, the slower the rate of decomposition. A simple rule of thumb for bacterial decomposition rates (i.e., oxygen demand) appears in Chart 1.

To achieve similar decomposition rates, the bacteria population must be increased considerably in cold water. Plant life is also significantly affected by colder water and we see much slower growth rates in autumn and winter. The good news is colder water has the capacity to hold 40 percent more dissolved oxygen.

Light plays a significant role in the process of photosynthesis. As light decreases, so do the photosynthetic rates of plants. They are growing slower or not at all. Bottom-rooted weeds and algae miraculously seem more manageable, or disappear.

[ILLUSTRATION OMITTED]

The one constant in autumn and winter is nutrient loading in the pond or lake. Non-source pollutants and fertilizers will continue to leach or run off into the lakes. Leaves; salt from parking lots; water fowl and fish waste; dead or dying aquatic plants, including algae, sink to the bottom. The end result: as bacterial digestion rates slow and approach net zero, the nutrient levels of the pond continue to increase. This places the pond or lake out of balance, adding to the sludge bed and the "aquatic compost" pile at the pond or lake bottom, and is guaranteed to give the lake manager bigger headaches next year.

To avoid those headaches throughout the upcoming seasons, a little time invested in gaining a deeper understanding of the dynamics of water quality management will allow you to be proactive in the management process.

Water chemistry

The water chemistry of a lake offers significant clues as to why we see certain aquatic problems and what possible solutions are available to the property management. For chronic problem lakes, a basic lake water chemistry analysis is a must. Most well drillers can offer these services; you can find them by going online or by looking in the Yellow Pages under "water analysis." The tests prescribed here should cost less than $250, and will help you understand the varied elements that can contribute to the health of your pond or lake, allowing you to develop a management program (see Chart 2).

* Dissolved oxygen is the first and most important test the customer should consider. It plays the ultimate role in lowering nutrients in the lake. Primarily, oxygen converts phosphorus to an insoluble form that plants cannot use as a nutrient. Second, oxygen supports aerobic digestion. Aerobic digestion lowers the total amount of organic nutrient in the lake. Third, oxygen prevents anaerobic bacteria and the gases--methane, hydrogen sulfide and ammonia--that cause foul aquatic odors. Be sure to take the dissolved oxygen reading late at night or, for the best reading, just before dawn. This is when the lake will experience oxygen crisis. A lake needs more than 4mg/l to sustain aquatic life and healthy lake dynamics.

* BOD represents the lake's total oxygen demand. We look for less than 5mg/l as a minimum. …

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