Analyzing ecological history through mud

The scientists at the St. Croix Watershed Research Station (SCWRS) help us understand our changing climate by analyzing how lakes around the world have changed over time. How do they do this? By capturing data about water chemistry and clarity – and by taking a very close look at mud.

In a recent collaboration with the Red Lake Band of Chippewa and the Minnesota Pollution Control Agency (MPCA), the SCWRS shared its expertise in paleolimnology, the study of the physical, chemical, and biological information preserved in freshwater lake sediments. Their work painted a clearer picture of a lake system’s health and history to guide lake management going forward.

Lakes’ histories, at their core
The Research Station is one of the world’s authorities in sediment coring, an important tool of paleolimnology. Dr. David Burge, a SCWRS assistant scientist, explains the process this way: “We take a big plastic tube and shove it in the lake bottom, cap it off, and pull the mud up. The newest, most recent mud is on the top and the oldest mud is on the bottom.” 

When the scientists get the core back to the lab, the analysis begins. “We carefully slice it into one- to two-centimeter sections, and then we date the intervals so we know when those sediments were deposited in the bottom of the lake,” Burge continues. “We go back in time as we move down the core, and we analyze the lake sediments for phosphorus, algae pigment, diatom microfossils, and a ton of other things.”

Diatoms are single-celled aquatic algae with cell walls made of glass that preserve well in lake sediments. “They respond to change so quickly,” Burge says. “We can see ecological change in the diatom community more readily than if we measure a suite of different chemical parameters.” 

Cores don’t always display significant changes. “In that instance, you can say that the lake is behaving as it always has,” Burge explains. “I can look at a core from some lakes in Minnesota, though, and tell you when cottage development or logging occurred in a lake’s watershed. You don’t even have to give me the dates – I can see the diatoms change in the core.”

Making sense of the Red Lake story
In 2016, the Research Station brought its expertise to one of Minnesota’s famous lake systems. 

The Upper and Lower Red Lakes make up the largest lake within Minnesota. All of Lower and a big portion of Upper Red Lake are under the jurisdiction of the Red Lake Band of Chippewa. The tribal Department of Natural Resources works in partnership with the Environmental Protection Agency and MPCA to monitor the health of the lakes. 

There are three main indicators of lake health: chlorophyll-A, a measure of algae growth; total phosphorus, a macronutrient that algae love for growing; and Secchi depth, a measurement of how clear the water is. These measurements can be influenced by a lake’s geographic location; its depth and resulting ability to rid itself of phosphorus by burying it in its sediment; and pollutants contributed by natural weathering, industry, and agriculture. Lake managers use these measures as sentinels of change in water quality and compare their lake’s values to a range of standards set by the state.

Under their own initiative and sovereignty, The Red Lake Department of Natural Resources (RLDNR) has monitored Upper and Lower Red Lakes for water quality since 1990. Where other agencies may be spread thin, the RLDNR has the unique opportunity to focus monitoring efforts on the Red Lakes. Through the monitoring and following Minnesota Pollution Control Agency framework, the RLDNR identified that the Red Lakes were exceeding the state’s nutrient standards (thought of as nutrient limits), registering relatively high for algae growth and phosphorus and low on water clarity compared to other lakes in their region. “This is puzzling because the watershed is small in terms of the lake size and has very little or no development or agriculture,” Burge says. Knowing there was no change in the 30 years of monitoring data, the Tribe sought funding and reached out to the Research Station to further investigate the Lakes' histories.

“In Minnesota, there are two things that can happen when a lake exceeds nutrient criteria,” Burge continues. “The traditional route is to pursue remediation. The other route is to look at the lake’s history and see if it requires a special set of regulations.” 

Digging into that history is what the SCWRS set out to do.

Finding answers in the sediment
Scientists from the Research Station collected and analyzed cores from the Red Lakes. What they found told them something important: the data are consistent. “The diatom communities and the geochemical properties that we measure have been stable over the last 150 to 200 years,” says Burge. “With no significant change to the ecology of the Red Lakes, this suggests that what we see today is close to what we saw before the Industrial Revolution.” 

The Red Lakes are shallower than many of the lakes in their region. Because of this, the wind stirs them up, and the phosphorus that is normally buried is resuspedend. This wind-introduced phosphorus gives algae exactly what it needs to grow. As a result, the algal growth and nutrient data of the Red Lakes more closely resemble the shallower lakes of southern Minnesota.

Using that evidence, the Research Station suggested to the lake managers that the data from recent decades would be appropriate to define a new set of site-specific standards for the Red Lakes. “Rather than being governed by their ecoregion’s values, the Red Lakes are now in the process of adopting their own special nutrient criteria,” Burge says, “That way, lake managers won’t have to flag them every year when they exceed the traditional criteria, and they have a baseline for reference.”

The takeaways
The Red Lake project is a great example of the Science Museum’s mission in action: The research is actively informing policy. The evidence collected by SCWRS scientists, which is published in a recent issue of Lake and Reservoir Management, has influenced a plan for new nutrient standards for the Red Lakes. 

Also important, though, is the relationship that this research has founded. Historically, tribes and state agencies have not had good working relationships. “This partnership has demonstrated that the parties involved all have different resources and skill sets to contribute to the success of the project,” Burge says. “It’s a good example of how everyone can work together moving forward, drawing upon historical and ancestral knowledge, documented standards, and scientific expertise to help ensure the health of our most important bodies of water.”