Why Is My Lake Smelling Like Sulfur and Old Grass?
Why Is My Lake Smelling Like Sulfur and Old Grass? The Secrets of Late Summer Water
Summary:
If you have noticed that your lake or pond seems to have a distinct "stink" as August rolls around, you are certainly not alone. During the height of summer, the water that was crisp and fresh in May often transforms into something much more pungent. This shift is usually caused by a combination of high heat, heavy plant growth, and a lack of oxygen in the deepest parts of your lake. It is a natural part of the water body’s lifecycle, though it can certainly be an unpleasant surprise for homeowners.
The most common scents you will encounter are a "rotten egg" smell or a strong, earthy, fishy odor. These are usually the result of organic material like algae, weeds, and leaves breaking down at the bottom of the lake. In the late summer, this decomposition happens at a rapid pace because the water is at its warmest. When there isn't enough oxygen to go around, the bacteria doing the cleanup produce gases that eventually make their way to the surface and into your nose.
Another major contributor is the bloom of blue-green algae, which thrives in the late summer sun. These organisms can release compounds that smell like freshly cut grass or, in some cases, something much more foul. While it might be off-putting, understanding that your lake is essentially a massive, living compost pile helps explain why these odors peak just before the autumn cooling begins. It is a sign that the ecosystem is working hard to process the season's growth.
The Science Behind It:
The primary driver of late-summer odors in temperate lakes is a phenomenon known as thermal stratification. As solar radiation increases throughout the summer, the upper layer of water, or the epilimnion, becomes significantly warmer and less dense than the cold, deep water, known as the hypolimnion. This density difference creates a physical barrier called the thermocline, which prevents atmospheric oxygen from reaching the bottom. According to research from the University of Minnesota Extension, this isolation leads to an anoxic environment where dissolved oxygen is completely depleted by the decomposition of organic matter.
In the absence of oxygen, anaerobic bacteria begin to dominate the benthic zone. These microbes utilize sulfate as an electron acceptor during the degradation of organic material, a process that produces hydrogen sulfide ($H_{2}S$) gas. This gas is the culprit behind the characteristic "rotten egg" smell. Because the lake is stratified, the $H_{2}S$ remains trapped in the deep water until a storm event, significant wind, or the beginning of the fall turnover causes the layers to mix, releasing the gas into the atmosphere.
Furthermore, the late summer season often sees a peak in Cyanobacteria (blue-green algae) biomass. These organisms produce secondary metabolites such as geosmin and 2-methylisoborneol (MIB). Research published in Water Research indicates that these compounds are responsible for the "earthy" or "musty" odors often associated with source waters. These metabolites are released into the water column during the natural senescence and subsequent lysis of algal cells as blooms begin to die off due to nutrient depletion or temperature fluctuations.
Additionally, the accumulation of senescing aquatic macrophytes contributes to the odor profile. As large swaths of pondweed and other vascular plants reach the end of their life cycle, their rapid breakdown by heterotrophic bacteria releases a variety of volatile organic compounds (VOCs). This biological "oxygen demand" further stresses the lake's chemistry, leading to a cycle of nutrient release and further microbial activity. The interplay between thermal physics, anaerobic biochemistry, and algal lifecycles creates a complex olfactory environment unique to the late summer transition.
Sources / References:
- University of Minnesota Extension: https://extension.umn.edu/lakes-and-wetlands/water-quality
- North American Lake Management Society (NALMS): https://www.nalms.org/home/lake-management/