This is a picture of the diatom Navicula reinhardtii (originally described by Grunow) as seen through a light microscope. The scale bar is 10 micrometers or 1/100th of a millimeter. This diatom was found in a sediment core from a wetland area along the St. Croix River, near the town of Osceola Wisconsin, in a section of the core that was dated to 1820 A.D. N. reinhardtii is an example of a benthic diatom; benthic diatoms live attached to plants or rocks. Diatoms are photosynthetic, and therefore need light to survive; a water body must be clear enough for light to penetrate to the bottom in order to support a benthic diatom community. So, at times when benthic species such as N. reinhardtii are found in abundance it is an indication that the water must have been clear.

This is a picture of the diatom Stephanodiscus hantzschii (originally described by Grunow) as seen through a light microscope. The scale bar is 10 micrometers or 1/100th of a millimeter. This diatom was found in a sediment core from the north basin of Sauk Lake (Stearns and Todd Co. MN) dated to 2003 A.D. S. hantzschii is an example of a planktonic diatom, which lives suspended in the water column rather than attached to plants or rocks. Previous research has shown that small Stephanodiscus species, such as S. hantzschii, tend to dominate the diatom community composition in lakes that have high levels of phosphorus in the water. This sample was dominated by small planktonic Stephanodiscus species, indicating that in 2003 phosphorus levels were high in this lake, and that the waters were not clear, due to the large percentage of planktonic instead of benthic diatoms.

Diatoms are a common group of microscopic algae that are found in nearly every lake and stream around the world. They form a cell wall made of silica (biologically produced glass), which preserves well in lake-bottom sediments, and different species can be identified by the unique pattern of this glass cell wall. Diatoms are sensitive to changes in their aquatic environment, and research has been done to determine the environmental preferences and tolerances of different species. Therefore, researchers like myself can analyze the community composition of diatoms and use that information to infer a lake’s water chemistry, past and present.

“Sediment cores can be taken from lakes, dated using radioactive isotopes, and analyzed for diatoms to reconstruct the lake’s water chemistry over time. At the St. Croix Watershed Research Station, we use the information obtained from diatom specimens like these to understand the natural variability of these systems, as well as how lakes have changed as humans have impacted the landscape.
– Joy Ramstack, Assistant Scientist, St. Croix Watershed Research Station