By Dr. Josette La Hée, Senior Environmental Research Consultant
In Part 1, we learned about the causes of the odor. Low Oxygen + Plenty of Anaerobic Bacteria + Lots of Organic Matter = Hydrogen Sulfide (aka rotten egg odor). Now we can dig deeper into each component beginning with oxygen. If there is a good amount of oxygen in your pond, then the oxygen-needing aerobic bacteria will happily breakdown organic matter without releasing hydrogen sulfide. If there is very little oxygen in your pond, the non-oxygen-needing anaerobic bacteria will step in and breakdown the organic material, but as they do it, they will give off hydrogen sulfide and produce that rotten egg smell.
Temperature, water depth and water circulation all affect the oxygen levels in a pond. In general, wind blowing over the surface of the pond will circulate and mix the water in shallow ponds. This circulates oxygen throughout the whole pond. Also, oxygen producing organisms (e.g. plants or algae) in the water can supply the pond with significant amounts of oxygen. Oxygen throughout the whole pond means aerobic bacteria can effectively breakdown organic matter, without the production of hydrogen sulfide.
Deeper ponds are much different. If a pond is deep enough, it can have distinct layers of water. Temperature changes drive the division of water layers called stratification . In Florida, stratification is especially common in the warm summer months.
The top water layer has warmer water, more light, and less organic matter than the bottom layer. Although the water in the top layer is warm and holds less oxygen, the smaller amounts of organic matter mean less oxygen is being used up by bacteria working to breakdown this material. In addition, the oxygen levels in this water layer remain high because they are constantly replenished by
If oxygen levels remain high, then oxygen-needing bacteria dominate, and in the absence of non-oxygen-needing bacteria, there is no buildup of hydrogen sulfide and so there is no odor.
The bottom layer of water is cooler and darker than the surface and sits above the sediment, which is where most of the organic matter lies. Oxygen-needing bacteria in the bottom layer work to breakdown organic matter. They use up the available oxygen quickly and no more oxygen can be added to this layer because:
This is why, even though the bottom water is cooler than the top, it holds less oxygen. Once most of the oxygen becomes depleted, the non-oxygen-needing bacteria take over. As they breakdown organic material, they release hydrogen sulfide. Over time, hydrogen sulfide builds up in the lower water layer and it is trapped as long as the pond remains stagnant.
In stratified ponds, that layer of stinky water stays locked up at the bottom, becoming more saturated with hydrogen sulfide. As the warm summer gives way to cooler fall temperatures, the surface water cools, its density increases, and it starts to sink to the bottom of the pond. This mixing cycle, referred to as turnover, forces the water at the bottom of the pond up.
The mixing of the surface and lower waters causes the release of the once trapped hydrogen sulfide and results in a mass emission of very foul gas and an unpleasant October surprise. In addition to the foul smell, this mixing can also cause fish kills, algae blooms and other problems.
The best way to avoid the problems associated with seasonal turnover is to prevent your pond from stratifying in the first place by encouraging consistent water mixing all year round. While the water of shallow ponds can be mixed just by wind action, deeper ponds may need some help to get the job done. Many people use artificial methods to help with water circulation. The most common of which is aeration.
Aeration refers to the pumping of pressurized air into a pond at strategic points, which forces the water in the pond to circulate. Constant circulation prevents stratification and eliminates the buildup of hydrogen sulfide in the hypolimnion. Aeration also consistently forces oxygen-rich surface water to circulate throughout the pond, which results in an even distribution of oxygen throughout the water column. This oxygen rich environment favors the growth of aerobic bacteria and makes it possible for them to consistently breakdown organic matter without the risk of running out of oxygen. As long as circulation continues, oxygen will remain available, anaerobic bacteria will remain at bay and there will be no rotten egg odor.
Dr. La Hée conducts research for Aquatic Systems on urban aquatic ecology. The primary focus of her research is the use of sustainable management techniques for restoring impaired lakes and ponds. As Head of Limnological Research, Dr. La Hée spent three years working with biologists in our field offices. She designed and ran lake aeration studies on multiple lakes throughout Florida. Additionally, she developed the lake assessment and monitoring protocols used in our lake management programs. In addition to research, Dr. La Hée conducts teaching and training sessions for lake management groups. She has presented research at conferences including the North America Lake Management Society and the Florida Aquatic Plant Management Society.