By Scout Nelson
Shallow prairie lakes across the Upper Midwest experience natural changes throughout the year. Water clarity, color, smell, and biological activity often change with the seasons. These changes are mainly linked to temperature, sunlight, nutrients, algae growth, and the breakdown of organic matter.
Understanding these patterns helps people enjoy lakes safely and supports better decisions about water quality and lake management.
Ryan Lefers, Griffith Endowed Chair in Agriculture and Water Resources, South Dakota Water Resources Institute Director and SDSU Extension Water Management Specialist, along with Sushant Mehan and Hala Qashou, studied seasonal lake changes and new ways to monitor water quality.
Their work examined how traditional water tests and modern technologies can help scientists better understand lake conditions and identify changes in aquatic systems.
How Lake Water Quality Changes Throughout the Year
Shallow prairie lakes in South Dakota, North Dakota, Minnesota, Iowa, and Nebraska often follow a similar seasonal pattern.
Spring: Nutrients and Algae Begin to Increase
During spring, melting snow and rainfall carry nutrients from nearby land into lakes. Longer daylight hours and warmer water support algae growth. Water may remain relatively clear during this period, while fish become more active.
Early Summer: Nutrients Move Through the Lake
As temperatures rise, organic matter left from winter begins to break down. This process releases nutrients, including nitrogen and phosphorus compounds, into the water. These nutrients can support further algae growth and may gradually increase water cloudiness.
Mid to Late Summer: Algae Growth and Decomposition Increase
Warm water and available nutrients create favorable conditions for algae blooms. When algae die, microorganisms break them down, releasing nutrients back into the water. This process can increase cloudiness and produce unpleasant odors. These changes are common in many shallow lakes across the region.
Fall: Water Conditions Begin to Stabilize
As temperatures decline, algae growth and decomposition often slow. Water clarity may improve, and biological activity begins to settle. The lake enters a new seasonal cycle.
Lake Mitchell Shows These Seasonal Patterns
Lake Mitchell is an artificial reservoir in Davison County, South Dakota. Built in 1922 by damming Firesteel Creek, it provides recreation, flood control, and water supply benefits. The lake covers about 280 acres and reaches a maximum depth of approximately 22 feet, although water levels vary.
Agricultural runoffs can carry nutrients and sediment into the lake. These materials may contribute to algae growth and changes in water quality.
SDSU scientists examined Lake Mitchell using traditional water tests and newer technologies. Their monitoring methods include nutrient testing, pigment measurements, oxygen measurements, electronic-nose sensors, and hyperspectral imaging.
In spring, chlorophyll-a, a pigment found in algae, can increase as sunlight and nutrients become available. During early summer, decomposing organic matter releases nutrients, while water cloudiness may rise. In July and August, algae growth, decomposition, and nutrient recycling can occur together. As fall approaches, cooler conditions often slow these processes and support improved water clarity.
New Technologies Improve Lake Monitoring
Traditional water quality monitoring often requires collecting samples and sending them to a laboratory. Results may take days, making it harder to identify changes immediately. New tools may provide faster information without requiring direct contact with lake water.
Hyperspectral Imaging
Hyperspectral cameras measure reflected light from the lake's surface. Algae, sediment, and dissolved organic matter reflect light in different ways. Scientists can study these patterns to identify algae, locate possible bloom areas, and monitor changes over time.
At Lake Mitchell, hyperspectral measurements closely match laboratory results for chlorophyll-a. This suggests that the technology may provide a useful way to estimate algae levels quickly and monitor large areas.
Electronic Nose Sensors
Electronic-nose, or e-nose, sensors detect patterns in gases released during the breakdown of algae and other organic matter. These gases may include ammonia and sulfur-related compounds.
The sensors can help scientists identify changes in decomposition, nutrient recycling, and odor conditions. They may also provide early signs that lake conditions are changing.
Together, these technologies may help lake managers respond to algae blooms faster, improve monitoring, and reduce the time and cost needed for some water quality checks. However, they complement rather than fully replace traditional sampling and laboratory tests.
Why Seasonal Water Quality Matters
Understanding seasonal lake changes helps people make informed choices about fishing, swimming, and boating. Spring and fall may offer clearer water, while summer can bring more algae and changing water conditions.
A green lake in summer may reflect common seasonal activity, but not every algae bloom is harmless. Some blooms can contain harmful toxins. People should follow local water advisories and avoid contact with water when officials warn of unsafe conditions. Pets should also be kept away from affected water.
Recognizing normal seasonal patterns also helps communities identify unusual events, such as unexpected fish deaths or sudden changes in water quality.
Simple Steps to Protect Lake Water Quality
Residents and lake users can help reduce nutrient pollution through simple actions:
- Keep grass clippings and leaves out of streets and storm drains.
- Use fertilizers only when needed and avoid applying them before rainfall.
- Pick up pet waste to prevent nutrients and bacteria from entering waterways.
- Maintain native plants and vegetation near shorelines to slow runoff and trap sediment.
- Support local lake cleanups, watershed projects, and water monitoring programs.
- Follow local water quality updates and recreation advisories.
Protecting Lakes for the Future
Seasonal changes in shallow prairie lakes are part of natural lake processes, but excess nutrients from surrounding land can make water quality problems worse. Research at Lake Mitchell helps scientists understand how algae, nutrients, and decomposition interact throughout the year.
Photo Credit: pixabay-klawson
Categories: South Dakota, General