Your Pond and the Seasons

Pond Dynamics: Understanding Autumn

adapted from an article by Charlie Barebo, CEO & International Regional Manager Otterbine-Barebo
(Otterlink Oct/Nov 2005)

As the seasons change, so do pond and lake dynamics. The two climatic factors which 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. As such, a thorough understanding of these seasonal changes will help to guide you to timely solutions.

Temperature has a 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. See the chart for a simple rule of thumb for bacterial decomposition rates (i.e. oxygen demand).

Temperature (°C)	Decomposition Rate
28 to 39	Optimum
13 to 27	5 to 15% slower
9 to 13	30% slower
5 to 9	Over 60% slower

To achieve similar decomposition rates the bacteria population must be increased considerably in cold water. Plant life as well, is significantly affected by colder water and we see much slower growth rates in the Autumn season. The good news is colder water has the capacity to hold more dissolved oxygen, in fact more than 40% more.

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

The one constant in Autumn is nutrient loading in the pond. Non-source pollutants and fertilizers will continue to leach or run off into the pond. Leaves, 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 out of balance, adding to the sludge bed and the "aquatic compost" pile at the pond bottom. It's guaranteed to give the pond owner bigger headaches next year.

* Look for upcoming articles on the other seasons.