The role of geology behind diatom communities
Geological setting in Finland
The last glaciation, approximately 10 000 years ago, eroded and deposited material leaving behind a landscape full of glacial landforms in northern Europe. These landforms include thick layers of basal tills, glaciofluvial eskers, and moraines, to name a few. The retreating glacier also left behind a multitude of lakes in Finland. These lakes are often shallow and elongated in the direction of the ice flow. A majority of the lakes are located in central-eastern Finland, called the Finnish Lake District, where a relatively coarse-grained basal till covers most of the land surface. These tills deposited under actively flowing ice lobes that flushed away the finer materials. However, in the intersection of two actively flowing ice lobes, a passive interlobate area formed. This wedge-shaped area pierces central Finland in a NW-SE direction and is characterized by fine-grained basal till. The coastal areas of Finland, on the other hand, were under water after the ice margin retreated and, thus, clays deposited on the bottom of the ice lakes. Later on, land uplift related to the post-glacial rebound elevated these clayey areas above the sea level.
Most Finnish lakes are shallow and elongated in the ice flow direction of the last glaciation.
So what does this have to do with diatoms?
Traditionally, the lakes near the Finnish coast have been considered naturally more nutrient-rich than the lakes in other parts of Finland due to their clayey catchments. These areas have also been subjected to the most long-lasting and intensive farming. However, prior research has suggested that naturally nutrient-rich lakes could be more common in Finland than previously thought. This is where diatoms come into play. In my PhD research, I have utilized diatoms to study the natural nutrient conditions in a part of the fine-grained till area in central-eastern Finland and coarse-grained till areas surrounding it. I analyzed the diatom assemblages of recent sediments and sediments taken from a depth that represents time before the area was permanently populated in the 16th century from nearly 50 lakes. With a diatom-total phosphorus transfer function, developed based on the recent samples, I reconstructed the pre-disturbance total phosphorus concentrations of these central-eastern Finnish lakes.
And what did I find?
The lakes in the fine-grained till area are, indeed, naturally more eutrophic than those in the coarse-grained till area. These naturally nutrient-rich lakes have pre-disturbance diatom communities that are characterized by an abundance of planktonic Aulacoseira taxa (e.g. A. subarctica and A. ambigua) as well as small, benthic fragilarioid taxa. The lakes in the coarse-grained till area, on the other hand, have more diverse natural diatom assemblages that consist of cyclotelloid species associated with lower nutrient concentrations, Tabellaria, Asterionella, and small benthic Navicula and Achnanthes taxa in addition to Aulacoseira and benthic fragilarioids. According to the diatom-inferred reconstructions, human disturbance has caused further eutrophication particularly in the naturally eutrophic lakes, whereas the nutrient poorer lakes have changed less. There is also a clear increase in certain diatom species, such as A. ambigua and Cyclotella stelligera towards the present day. Therefore, it is clear that the diatom communities of lakes are not only sensitive to human disturbance but also to the small-scale variation in the geological aspects of their catchments.
Diatoms from lakes in the fine-grained till area (left) and the coarse-grained till area (right).
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