Diatom of the Month - April 2018: Diatoma tenuis
by Mira Tammelin*
The road meanders through snow-covered forests and between frozen lakes (Figure 1). What a perfect day in a winter wonderland! Or is it?
Figure 1. Winter wonderland in the boreal forest of Finland (Photo by Mira Tammelin).
Worrisome findings have been made recently in North America about the large-scale salinization and alkalinization of fresh surface waters not only in arid regions, but also in temperate, humid areas (Dugan et al., 2017; Kaushal et al., 2018). Kaushal et al. (2018) talked about freshwater salinization syndrome as the result of anthropogenic salt inputs (e.g. from road de-icing), accelerated weathering, and increased biological alkalinization. If left unmanaged, this syndrome may threaten the biodiversity and ecosystem services of freshwaters (Dugan et al., 2017; Castillo et al., 2018; Kaushal et al., 2018). For instance, Kaushal et al. (2005) estimated that many surface waters in the northeastern United States could become impotable and toxic to aquatic organisms during the next century, if salinization continues at its current rate. Freshwater salinization is particularly visible in densely-populated, urbanized areas, although climate and geology affect the outcome in addition to anthropogenic activities (Kaushal et al., 2017; Kaushal et al., 2018).
Similarly to North America, Fennoscandia has one of the highest lake area densities in the world (Messager et al., 2016). However, contrary to the densely populated northeastern United States, the boreal coniferous forest of Finland (Figure 2; part of the taiga biome), where we started our story, is a relatively remote and sparsely populated area. Its numerous lakes are generally shallow and diluted with low alkalinity (Simola and Arvola, 2005) and, thus, sensitive to human impact (Smol, 2016). Diatoms are excellent indicators of water quality changes in response to human activities, as demonstrated by recent increases in the abundances of small cyclotelloid and elongate pennate diatoms due to warming climate and eutrophication (Rühland et al., 2015).
Figure 2. The boreal forest zones and forested areas of Finland.
The diatom of this month, Diatoma tenuis C. Agardh (Figure 3) is an elongate pennate diatom that has linear valves with a typical length from 20 to 85 µm and width from 2.9 to 4.9 µm, capitate ends, and transapical ribs (Potapova, 2010). Its shape and the ability to form colonies provides D. tenuis a competitive advantage (i.e. reduced sinking velocity) when competing for light and nutrients (Rühland et al., 2015). D. tenuis has been linked with human-impacted conditions in boreal lakes, for example, in Finland, Russia, and Canada (Davydova et al., 1999; Pieniz et al., 2006; Räsänen et al., 2006). However, it has also been found from the high conductivity freshwaters of the Canadian High Arctic (Antoniades et al., 2004). A recent regional study in Northern Savonia, located in the northern part of the Finnish Lake District, revealed that D. tenuis occurs only in the surface sediments of a few lakes that represent the upper ends of the electrical conductivity and pH gradients in the area (Tammelin et al., 2017). In general, these Northern Savonian lakes are characterized by Aulacoseira, cyclotelloid, and small fragilarioid taxa.
Figure 3. Valve view (upper) and girdle view (lower) of Diatoma tenuis (Photo by Mira Tammelin).
Diatoma tenuis is particularly abundant in the relatively small, shallow, and naturally eutrophic Lake Kirmanjärvi (Figure 4), where it first appeared during the 1970s (Kauppila et al., 2012; Tammelin et al., 2017). Lake Kirmanjärvi has an electrical conductivity of 9.9 mS m-1 and pH 7.3. The lake is located right next to the main road in Northern Savonia, the European route E63. Correspondingly, the second highest abundances of D. tenuis in the studied North Savonian lakes were found from two lakes located similarly close to the main road (Tammelin et al., 2017). Therefore, the abundance of this month’s diatom appears to be controlled by freshwater salinization and alkalinization caused by road de-icing rather than climate warming or eutrophication. This suggests that Diatoma tenuis could be used as an indicator of the freshwater salinization syndrome in the diluted lakes of Northern Savonia and other boreal winter wonderlands.
Figure 4. Lake Kirmanjärvi in Northern Savonia, central-eastern Finland, and a microscope view of its surface sediment diatom sample where Diatoma tenuis is abundant (Photo by Mira Tammelin).
*PhD student in the Lake and marine sediment research group, Department of Geography and Geology, University of Turku, Finland
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