Diatoms and Paleolimnology
This learning module is designed to demonstrate how diatoms can be used in applied environmental studies. It can be adapted for various educational levels and does not require access to microscopes or any other laboratory equipment. The material was developed using virtual microscopy technology and a collection of high-resolution images of permanent diatom slides from the ANSP Diatom Herbarium available online thanks to the support of the National Science Foundation (project #1938128).
Paleolimnology
Paleolimnologists study environmental history of waterbodies and their watersheds by analyzing sediments accumulating on the bottoms of lakes and lagoons. In addition to investigating physical and chemical properties of sediments, paleolimnologists explore fossil remains of organisms that used to live in these waterbodies. Siliceous frustules of diatoms are often well-preserved in lake sediments and are used as valuable environmental indicators.
Lake St. Croix study
Dr. Mark Edlund and colleagues (2009) analyzed sediment cores from Lake St. Croix at the Minnesota-Wisconsin border to reconstruct history of human impact on water quality of starting from the time of European settlement to modern time. They took several cores in the lake, which is a natural impoundment of St. Croix River and estimated sediment age, accumulation rates, and physico-chemical properties. They also analyzed diatoms in two cores (1B and 6B) that span the period from the early 1800s to 1999.
Access the scanned Lake St. Croix slides at the virtual microscopy platform OMERO.
Scroll down to the slide GC68116 which represents the deepest 180-182 cm section of the core 6B. Isotope dating determined that this sediment was deposited before 1840s, the time of European settlement. Zoom in and observe relatively few diatoms among sediment particles. The majority of diatoms are benthic meaning that they live on the bottom of rivers and lakes or are attached to submerged objects, such as aquatic plants. Some examples of benthic diatoms characteristic for this section are marked on the slide scan. They include “small fragilarioids” such as Staurosira construens and Staurosirella leptostauron, monoraphid Cocconeis neodiminuta and biraphid Cavinula scutelloides.
Explore this slide moving across it, focusing on diatoms at different focal planes and zooming in and out. Some planktonic diatoms are present in the deep parts of both cores, but at very low abundance. This indicates low nutrient concentration in lake water, insufficient to support large populations of phytoplankton.
Compare the bottom sediments to those from the upper portion of the core that correspond to modern (1999) conditions. In the top section of core 6B (slide GC68094) diatoms are much more abundant and planktonic centric diatoms are dominant. The largest circular valves belong to Stephanodiscus niagarae, which is also present in older sediments, but in low numbers. Aulacoseira ambigua and Aulacoseira granulata were likewise present in deep sections of the core but became much more abundant with time. Some species, such as Cyclostephanos invisitatus were absent in older sediment, but are abundant in upper sections of both cores.
Check additional slides and try to find and identify other diatoms in these cores using Diatoms of North America taxa pages and printed floras. Common benthic taxa in Lake St. Croix include Staurosirella pinnata, Pseudostaurosira brevistriata and P. brevistriata var. inflata, Staurosira construens var. venter, Fragilaria vaucheriae, Cocconeis placentula, Amphora pediculus, and Planothidium frequentissimum. Among planktonic taxa relatively common ones are Fragilaria crotonensis, F. capucina var. mesolepta, and Stephanodiscus parvus indicative of nutrient enrichment.
The shift in diatom assemblage composition from benthic to planktonic diatoms indicates eutrophication, but when did it happen? This is important to know to understand which impacts caused water-quality degradation. You can enumerate diatoms in virtual slides and will see that for example, the proportion of planktonic species starts increasing in core 6B approximately from section 82-84 cm, which corresponds to 1920s, while in the core 1B this increase is noticeable from section 34-36 cm corresponding to 1950s. Differences between the cores reflect land use in the respective watersheds and the timing of eutrophication onset. Core 1B contains fewer diatoms than 6B, and the abundance of mineral particles in the older sediments of 1B points to severe erosion resulting from logging. In combination with sedimentation rate, algal pigment, and biogenic silica data, the diatom record suggests that during the first few decades of European settlement, the St. Croix River ecosystem remained relatively stable. Logging and even 19th-century farming had only a mild impact on water quality. It was subsequent development and urbanization that led to significant phosphorus enrichment and a shift from a benthic- to a plankton-dominated system.
Finally, compare Lake St. Croix sediment diatom samples to those collected from other lakes, for example EMAP dataset. Many EMAP lakes have remarkably different diatom assemblages from those found in Lake St. Croix, reflecting differences in lake physical and chemical characteristics. Many have much more abundant diatom assemblages, while diatoms in Lake St. Croix cores are often obscured on slide by silt particles. Nevertheless, it is remarkable that even slides that do not look entirely “clean” and perfect can yield valuable and rich scientific evidence about environment and ecosystems.