Few object are more beautiful than the minute siliceous cases of the diatomaceae: were these created that they might be examined and admired under the high powers of the microscope? ~ Charles Darwin
As Darwin remarks, diatoms are beautiful. They have unique, intricate cell walls that help in their identification, because in addition to their beauty, they can tell a lot about past environmental conditions. My research uses diatoms as a proxy (a preserved item that acts as a ‘natural archive,’ capable of telling us something about climate in the past) to explore past environmental conditions in lakes. Diatoms are a type of single celled organisms called algae. These organisms are found in many wet environments including soils, but I focus on diatoms in lakes.
Diatoms are unique from other types of algae because they have siliceous or glass-like cell walls, and therefore are well preserved in lake sediments. This makes them good proxies of past climates. Additionally, diatom species, like other algae have a variety of environmental preferences. These preferences can range from mixed or stable water conditions, to high nutrient or light levels and provide the basis for climate inferences.
To use diatoms as proxies a sediment core needs to be taken. Typically, these cores are collected from the deepest spot in the lake using a corer (Figure 2). This device essentially sends a tube to the bottom of the lake, which is then forced into the lake bottom using a weight. Sediment is collected in the tube (more recent sediments at the top) and then brought to the surface. The tube is sealed to prevent sediments from falling out and the core is subsequently sectioned so only a ‘slice’ of sediment is retrieved at a time. Each of these sections will be processed later to remove organic matter and then applied to a microscope slide so the diatoms can be identified and counted. This makes it easier to identify the species of diatoms under the microscope. And accurate identification is essential when using diatoms as proxies. My research uses diatoms as a proxy for water stability in lakes.
Figure 2: Diagram of how core samples are collected. From Dixit et al. 1992.
During summer months, lakes stratify, or separate based on temperature (warmer water at the surface and cooler water at the bottom) thus creating a lake’s temperature profile. The temperature profile is split into three distinct layers: epilimnion at the surface, metalimnion in the middle of the lake, and the hypolimnion at the bottom. These layers do not mix easily, and as a result the surface water typically becomes depleted in nutrients as photosynthetic organisms (like diatoms) use these nutrients throughout the summer for photosynthesis. At the same time the bottom water becomes depleted in oxygen which fish and other aquatic organisms use during the course of the summer, signifying the importance of slight or complete mixing of a lake. In temperate lake, the stratified layers of the lake mix following ice out in the spring and just prior to ice forming in the fall. This mixing is important to bring nutrients to the surface and oxygen to the bottom of the lake, and can occur due to strong wind events.
When wind crosses a lake, its speed can increase because it can travel across the surface of the lake uninterrupted. The longest distance the wind can travel across the lake in this manner is called fetch. Larger lakes have larger fetches and may be impacted by wind more than smaller lakes. Research by Fee et al (1996) states that in lakes larger than 500 ha wind becomes the primary driver influencing mixing depth. When wind moves across a lake it can cause the surface layer of the water to mix. As the wind gets stronger, the mixing can move deeper in the lake. During strong enough wind events the layers of the lake can break down and the lake can completely mix.
Diatoms can be used to infer lake mixing. Certain species of diatoms called Aulacoseira subarctica prefer more mixed or turbulent conditions (Lund 1954, 1971) because their siliceous cell walls are thicker than some other diatoms, making them heavier. This could be detrimental to this species because light levels decrease as you move deeper in the lake. Therefore, if A. subarctica is not being mixed in the water column it can fall to a depth where there is not enough light to photosynthesize. As a result, these species are found in greater numbers when there is mixing within the water column that continually circulates the diatoms back up to the surface. Conversely, another species of diatoms called Discostella stelligera prefers a stable water column and therefore is found in high numbers when there is little to no mixing (Saros et al. 2012).
Using these environmental preferences scientists can examine the number of each species found in the core and infer what the lake conditions may have been at a given time—in this case infer if there were periods of increased mixing possibly due to the wind. Being able to use diatoms in this manner provides a great tool to understand past climates and try to predict how future climates may impact lakes.
Dixit SS, Smol JP, Kingston JC, and Charles DF. 1992. Diatoms: Powerful indicators of environmental change. Environmental Science and Technology. 26: 23-33.
Fee EJ, Hecky RE, Kasian SE, and Cruikshank D. 1996. Effects of lake size, water clarity, and climatic variability on mixing depths in Canadian Shield Lakes. Limnology and Oceanography 41:912-920.
Lund J. 1954. The Seasonal Cycle of the Plankton Diatom, Melosira Italica (EHR.) Kutz. Subsp. Subarctica O. Mull. Journal of Ecology Vol. 42 (1): 151-179
Lund J. 1971. An Artificial Alteration of the Seasonal Cycle of the Plankton Diatom Melosira Italica Subsp. Subartica in an Engish Lake. Journal of Ecology Vol. 59 (2): 521-533
Saros JE, Stone JR, Pederson GT, Slemmons KEH, Spanbauer T, Schliep A, Cahl D, Williamson CE, and Engstrom DR. 2012. Climate-induced changes in lake ecosystem structure inferred from coupled neo-and paleo-ecological approaches. Ecology 93(10):2155-2164