2020 has been exceptional due to the COVID-19 pandemic. As we all know, most meetings, conferences and workshops were either cancelled, changed to online events, or postponed to next year or the unforeseeable future. Luckily, the 6th Nordic-Baltic diatom intercalibration and harmonization workshop, arranged by Maria Kahlert from the Swedish University of Agricultural Sciences (SLU), belonged to the group “changed to online events” and was held in November 2020. Although meeting in the beautiful countryside scenery by Lake Erken not far from the Swedish city of Uppsala would have been fantastic, holding the workshop online was way better than having no workshop at all – and gives more opportunities for early-career diatomists to participate as well. 

But wait – what, in fact, is a diatom intercalibration and harmonization workshop? Maria Kahlert’s one-hour lecture on the topic is freely available here, hosted by Diatoms of North America. From a personal point of view, an intercalibration and harmonization workshop is one way to test how good you are at identifying diatom taxa. The Nordic-Baltic workshop includes pre-workshop diatom identifications to test and score one’s skills (Figure 1). It’s also a good way to find out which taxa are generally difficult to identify: there are always some taxa that are problematic to beginners as well as to more experienced people. Tips on how to identify difficult taxa are discussed during such workshops. And, at least the constantly evolving diatom taxonomy and scientific nomenclature keep the long-term hard-core diatomists entertained and awake.

Figure 1. Already on the first day of the workshop a variety of topics were discussed, many of which were linked to practical identification tips and rules 

Intercalibration and harmonization exercises are important if you work in e.g. diatom research, taxonomy or bioassessment/biomonitoring. Harmonizing the use of nomenclature, i.e. using the same taxon name for the same looking specimen across different labs, is crucial for comparisons of ecological status or biological indicator values; hence very important for water managers concerned with the applicability of international regulations (e.g. EU Water Framework Directive (WFD). Participating in such intercalibration and harmonization activities gives a diatomist a chance to understand how their taxonomic identifications compare to the ones made by other participants and the auditors (see example of comparison from Maria’s lecture, roughly at the 20 minutes mark). This year, the Nordic-Baltic workshop was attended by ~30 diatom-loving participants from countries such as Iceland, Lithuania, Germany, Estonia, the Netherlands, Finland and Sweden. The educators and auditors were Bart van de Vijver from Belgium and Amelie Jarlman from Sweden. The combined skillset of these two experts formed the target level for the participants; the diatom identification skills of each participant were compared to the auditors. In the Nordic-Baltic workshop, the Bray-Curtis similarity was used to determine how close the participants were to the two auditors: B-C value of 0.6 or above means that a participant’s identifications can be seen as replicates of the auditors’ identifications (Kelly 2001). In other words, if you get at least 0.6 from all samples, you pass. The certificate of passing the test can be a real advantage for someone who identifies diatoms for a living.

How does the process actually work? My experiences come from being a three-time participant of the Nordic-Baltic diatom intercalibration and harmonization workshops; the protocols in similar activities elsewhere may differ or not from what I describe next. People working with diatoms are invited to participate in the upcoming exercise, and the invitation can be forwarded to others to boost participation. Later the raw samples, i.e. in our Nordic-Baltic case, benthic diatom material and water, are sent to the participants, who then have to make permanent slides from the samples and identify the diatoms, using the same methods and taxonomic literature. When raw samples are included in the exercise, part of the dissimilarities among participants and auditors may arise from e.g. contamination or mistreating the samples in the laboratory. Another option is to send ready-made slides to the participants, when the dissimilarities will be more clearly related to identification skills. This year, there were three raw samples included in the Nordic-Baltic exercise. They were collected from Swedish rivers and streams varying in nutrient contents and pH. Samples taken from various environmental contexts means that participants will need to have a general knowledge of taxa well adapted to oligotrophic, mesotrophic, eutrophic, and even saprobic or otherwise contaminated waters. Following the Swedish method, the participants were also asked to mark down how many of the valves were deformed, to which degree, and whether the deformation is visible in the outline of the valve or in the inner structures (e.g. striae or raphe). Deformed valves, or teratological forms, may appear following environmental stress,  such as heavy metal contamination; this information could serve as an additional tool for biological monitoring and assessment (Falasco et al. 2009).

What did I learn during the workshop? I can proudly say that I learned that the figures below do NOT contain diatoms (Figure 2). The participants were encouraged to upload figures to the online workshop platform for the auditors and other participants to comment on them. I uploaded the figures below because I wanted to find out what the “weird sticks” I saw actually were (another participant had also seen them and called them “pointy ones”). It wasn’t long before we got an answer: probably these are distorted Heliozoan scales (see Leonov 2010)!

Figure 2. An honest question: what are these weird sticks? The agreement was that they are most likely distorted Heliozoan scales, and not diatom material

We also discussed some groups of taxa that have recently been through some changes in their taxonomy and taxonomic nomenclature (e.g. small naviculoids in Wetzel et al. 2015). Problematic taxa groups involving Fragilaria gracilis and Gomphonema exilissimum were also discussed in detail and Bart told us about upcoming changes in taxonomy. It is like hearing inside information of what is going on; getting to know updates in taxonomy and tips on how to identify difficult taxa makes you want to run to your microscope and start identifying diatoms at once. 

Have you ever wondered how to recognize if a species in your microscope view is colony-forming or not, when there are two valves on top of each other? Two valves from different cells appear less clearly when zooming, and the striae of these two valves appear “like you’ve drunk too much booze”. When the two valves belong to the same cell, they are usually more neatly positioned and it is easier to zoom and focus on the details of the two valves. Bart shared this tip to the participants, and perhaps I wasn’t the only one who memorized it due to the descriptive words he used. That’s holiday spirit alright!

On that note, 2020 comes to an end, and so this is the last DOM post for 2020. The entire DOM team would like to thank you, our readers and guest writers, for this year. We hope to continue receiving inspiring blog posts next year, especially from new authors, such as students as well as leading diatomists. We are here to help. Season’s Greetings and Happy New Year 2021!

Figure 3. Due to the pandemic-related restrictions in our laboratory, I was not able to take high-quality photos from this year’s intercalibration slides for the post. Instead, I am sharing a photo of a Christmas present I just got: they look a lot like Eunotia tetraodon (left) and Gomphonema acuminatum (right). Thank you Marja and Tapio for the great present! 

Annika Vilmi is a researcher at the Finnish Environment Institute (SYKE). Email Annika or drop a message below if you have any questions about the post.


  • Falasco, E., Bona, F., Badion, G., Hoffmann, L. & Ector, L. 2009. Diatom teratological forms and environmental alterations: a review. Hydrobiologia, 623: 1-35.
  • Kelly, M. 2001. Use of similarity measures for quality control of benthic diatom samples. Water Research, 35 (11): 2784-2788.
  • Leonov, M.M. 2010. Heliozoans (Heliozoa, Sarcodina, Protista) of Fresh and Marine Waters of the European Part of Russia: Species Composition, Morphology, and Distribution. Inland Water Biology, 3 (4): 344–355.
  • Wetzel, C.E., Ector, L., Van de Vijver, B., Compère, P. and Mann, D.G. 2015. Morphology, typification and critical analysis of some ecologically important small naviculoid species (Bacillariophyta) Fottea, Olomouc 15 (2): 203–234.