Written collaboratively by Ryan Lefers, Sushant Mehan, and Hala Qashou.
Shallow prairie lakes across the Upper Midwest change dramatically throughout the year. Their water can shift from clear to green, from fresh‑smelling to earthy, and from great fishing conditions to slower days. These changes aren’t random — they follow a predictable seasonal cycle driven by weather, nutrients, algae, and natural biological processes.
Understanding this cycle helps lake users know what to expect, stay safe, and appreciate how lakes function. It also helps communities make better decisions about lake management and water quality.
How Shallow Prairie Lakes Change Through the Seasons
Although every lake is unique, most shallow prairie lakes in South Dakota, North Dakota, Minnesota, Iowa, and Nebraska follow a similar seasonal pattern:
Spring: Nutrients arrive and algae wake up
- Snowmelt and spring rains wash nutrients into lakes.
- As sunlight increases and water warms, algae begin to grow.
- Water is often clearer, and fish are active.
Early Summer: Nutrients move through the system
- Organic matter left over from winter starts to break down.
- This decomposition releases nutrients that fuel more algae growth.
- Water may become slightly cloudier.
Mid–Late Summer: Peak algae and peak decomposition
- Warm water and abundant nutrients create ideal conditions for algae. Blooms become more common.
- When algae die, they decompose, releasing nutrients back into the water and sometimes producing odors.
- Turbidity (cloudiness) increases.
- This pattern is extremely common in shallow lakes across the region.
Fall: Cooling, clearing, and stabilizing
- As temperatures drop, algae slow down.
- Decomposition decreases, and water clarity often improves.
- Lakes begin to “reset” for the next year.
Lake Mitchell: A clear example of these seasonal patterns
Lake Mitchell is an artificial reservoir located in Davison County, South Dakota, USA. The lake was originally constructed in 1922 by damming Firesteel Creek to provide recreation, flood control, and water supply for the surrounding community. It covers approximately 280 acres and has a maximum depth of approximately 22 ft, although water depth varies seasonally with precipitation and management practices. Lake Mitchell receives runoff from an agricultural watershed, making it sensitive to nutrient enrichment, algal blooms, sediment loading, and seasonal fluctuations in water quality. (South Dakota Game, Fish and Parks; City of Mitchell).
Lake Mitchell follows the same seasonal cycle seen in many Upper Midwest lakes — but because it receives runoff from a relatively large, agricultural watershed, the changes can be more noticeable.
To evaluate water quality trends in Lake Mitchell, SDSU scientists used both traditional water quality tests and some promising new monitoring tools, including:
- Traditional water‑quality tests (nutrients, pigments, oxygen)
- Electronic‑nose sensors (gas signals from decomposition)
- Hyperspectral cameras (light signatures from algae and organic matter)
These tools helped reveal how Lake Mitchell moves through the seasonal cycle.
Seasonal Cycle
Spring (March–April): Early algae growth
- Chlorophyll‑a (living algae) increases as sunlight and nutrients become available.
- Water is often clearer, and algae growth is moderate.
Early Summer (May–June): Nutrient mobilization
- Organic matter begins to break down, releasing nutrients like ammonia, nitrate, and phosphate.
- Turbidity increases slightly.
Mid–Late Summer (July–August): Algae and decomposition overlap
- This is the busiest time of year for lake biology.
- Algae grow rapidly.
- Dead algae decompose, releasing nutrients.
- Turbidity increases, reducing sunlight penetration.
- Ammonia and sulfur‑related signals rise.
- In August, Lake Mitchell showed strong correlations among nutrients, algae, decomposition indicators, and hyperspectral signals — a sign that many processes were happening at once.
Late Summer–Fall (August–October): Stabilization
- As temperatures cool:
- Algae slow down
- Decomposition decreases
- Nitrification (conversion of ammonia to nitrate) becomes more visible
- Water clarity often improves
- This fall recovery is typical of lakes throughout the region.
Why SDSU Scientists Are Evaluating New Tools for Understanding Lakes: Hyperspectral imaging and electronic nose sampling
Traditional water‑quality monitoring often includes collecting water samples and sending them to a lab — a process that is not instant and can often take days to get a result. New technologies are being evaluated to monitor lakes in real time without ever touching the water.
Hyperspectral Imaging
Hyperspectral cameras measure how sunlight reflects off the lake surface. Different substances — algae, sediment, dissolved organic matter — each reflect light in their own unique “signature.”
This allows scientists to:
- Detect algae from a distance
- Map bloom hotspots across the lake
- Track changes hour‑by‑hour
- Monitor water quality without boats or sampling bottles
In Lake Mitchell, hyperspectral imaging closely matched laboratory measurements of chlorophyll‑a, showing it is a reliable, fast, non‑contact tool.
Electronic‑Nose (E-Nose) Sensors
As algae and organic matter decompose, they release gases such as ammonia and sulfur compounds. E-nose sensors detect these gases and help identify:
- When decomposition is increasing
- When nutrient recycling is happening
- When odors may appear
- When water quality is shifting
These sensors provide early warnings of changes that might affect recreational choices, fish and wildlife.
Why these tools matter
Rapid, non-contact tools like hyperspectral imaging and e-nose may allow lake managers to:
- Respond faster to algae blooms
- Monitor water quality on a more continuous basis
- Reduce the cost and time of water quality sampling
- Improve public communication and safety
Lake Mitchell is one of the regional examples showing how these new technologies may transform lake monitoring in the future.
How This Knowledge Improves Your Lake Experience
Understanding seasonal lake behavior helps you:
- Plan recreation.
Spring and fall often offer clearer water and great fishing. Mid‑summer may bring more algae, which can affect decisions around swimming and boating. Knowing when algae are most active helps you make informed decisions about recreation or letting pets in the water. - Put into context what you see.
A green lake in August is normal. A sudden fish kill in cool weather is not. - Support better lake management.
When the community understands natural seasonal lake cycles, discussions about managing water quality become more productive and less reactive.
Simple Ways You Can Help Improve Water Quality
You don’t need to be a scientist to make a difference. Small actions add up:
- Keep grass clippings and leaves out of the street.
They wash into storm drains and feed algae. - Use fertilizer appropriately.
Even small amounts can wash into the lake, so only apply what you really need. Avoid applying before rain to help prevent runoff. - Pick up animal and pet waste.
It’s a major source of nutrients and bacteria in urban watersheds.
- Plant and maintain vegetation as a filter.
Especially in buffer areas where water flows through or collects. Plants, especially native plants, slow runoff and trap sediment. - Support local lake improvement projects.
Community cleanups, watershed improvements, and monitoring programs all help. - Stay informed.
Understanding seasonal lake behavior and water quality helps everyone make better decisions.
The Big Picture
Lake Mitchell’s seasonal cycle — spring nutrient movement, summer algae growth, late‑summer decomposition, and fall stabilization — is not unusual. It matches what scientists see in many lakes across the Upper Midwest.
By learning how lakes change and taking small steps to reduce nutrient runoff, we can all help protect the lakes we enjoy for fishing, boating, swimming, and community recreation.
Declaration of Generative AI and AI-Assisted Technologies in the Writing Process
During the preparation of this work, the author used Copilot to make minor editorial corrections. After using this tool/service, the authors reviewed and edited the content as needed and take full responsibility for the article’s content.
References
- Dodds, W. K., & Whiles, M. R. (2020). Freshwater ecology: Concepts and environmental applications of limnology (3rd ed.). Academic Press.
- Kalff, J. (2002). Limnology: Inland water ecosystems. Prentice Hall.
- Wetzel, R. G. (2001). Limnology: Lake and river ecosystems (3rd ed.). Academic Press.
- City of Mitchell. (n.d.). Lake Mitchell.
- South Dakota Game, Fish and Parks. (n.d.). Lake Mitchell.
