Learn
about diatoms, Lake Mendota, and the ecology at play.
What are diatoms?
Diatoms are microscopic single-celled algae that live just about anywhere with enough moisture. This means they can live in damp soils or even on trees in humid places, but they especially live in open water. Many diatoms photosynthesize just like traditional plants, but not all. Some take nutrients from decaying matter at the bottoms of lakes or rivers.
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All diatoms have an outer shell made of silicon dioxide: glass. The diatoms take silica from sand and other sources, and incorporate it into their own shells to create something hard and protective while still allowing them to absorb light.
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"Diatom" means "cut in half," which is a feature that nearly all diatoms have. Their glass outer shell is made up of two valves which enclose the internal parts of the diatom, like bread holding in the fillings of a sandwich. This two-valve feature is how diatoms reproduce: over time the diatom cell will grow by thickening each valve face, until eventually the two valves split apart and each become their own cell.
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The three images to the right illustrate this process: The first image shows a diatom in "valve view," where only the top valve is visible. The second image shows the same species in "girdle view," as if rotated to the side, where both valves are seen. The third image shows that species in girdle view as it is in the process of splitting into two cells.
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Because they are made of glass, diatom valves that are processed for viewing under a microscope can chip or shatter like some of the ones above.
Valve view
Girdle view, one cell.
Girdle view, splitting.
Diatom with raphe
Diatom without raphe
Diatoms with raphe, the line-like system used to crawl around, are most likely to live on or near the water's bottom where they have a surface. Diatoms without a raphe are likely free-floating in the open water. If a lake is sampled and there are many more diatoms with raphe than those without, we might infer that there is much more nutrients at the bottom of the lake than in the open water. This means that we might expect bottom-dwelling fish like carp to eat more and do better than open-water fish like walleye.
Why do we care about diatoms?
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Diatoms and other algae are the base of the aquatic food web. They are incredibly abundant in the summer, making them a huge source of food for small bugs and zooplankton. These creatures are in turn eaten by small or young fish. If the diatoms are not eaten, they die and fall to the bottom of the water where their inner nutrients are decomposed. Those nutrients are then absorbed by rooted plants and other animals which live in the water's bottoms. Without diatoms and other algae, the entire aquatic ecosystem would collapse.
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As scientists, we care about diatoms because they inform us about what is going on in the water. Each species has a unique habitat and preferred conditions within the ecosystem. This means that if the water suddenly becomes more acidic or the nutrient levels change, the kinds and numbers of diatoms will change in response, letting us see what is happening. Diatoms have such a variety of shapes and patterns that it is easy to identify their specie and relate them to an environmental event.
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Because they make good environmental indicators, diatoms can show us what has happened to the environment way back in history. Diatom shells are made of inorganic material, so they don't decompose when they die. Instead, they fall to the bottom of the water and accumulate in layers of mud, similar to rings on a tree. By looking deep into this accumulated mud, researchers can see what species were present thousands of years ago and make hypotheses about what the other parts of the environment were like too. With this information about how the water was under different conditions, scientists can create predictions about how the water will change if the environmental conditions shift again in the future.
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What's so special about Lake Mendota?
Lake Mendota is often given the title "The most studied lake in the world." It sits right at the edge of the University of Wisconsin-Madison, one of the top research institutes in the country. Scientists have been studying Lake Mendota for well over a hundred years, and the research program that developed has had a major influence on the study of lakes all over the world.
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Because the lake has been studied for so long, there is an incredible amount of data and information known about its particular patterns, chemistry, and species. This data allows researchers to better understand how all of the parts of a lake affect one another, over the course of a day or over decades. Diatoms provide an opportunity to increase our understanding of how lakes will act and change today, back through history, and in the future.
Researchers diving into Lake Mendota to collect samples from the bottom of the lake.