The history of the revised indicator list
Recently, the paper: ‘A revised list of diatom ecological indicator values in The Netherlands’ appeared in Ecological Indicators 172 (113219): 1-8, by Adrienne Mertens, Jako van der Wal, Geurt Verweij, Bert Pex, Arthur van Dulmen & Herman van Dam (2025) https://doi.org/10.1016/j.ecolind.2025.113219.
The appendices to this paper include 1) ecological indicator values for 2037 fresh, brackish and marine diatom taxa, 2) synonyms and taxonomic references to authoritative literature, 3) ecological data from hundreds of literature sources and own experience with indicator values for pH, salinity, nitrogen metabolism, oxygen, saprobity, trophic state, moisture and 4) four-letter Omnidia codes for electronic data processing.
The history of this article goes back to 1972 when I began studying diatoms. As an MS student, I conducted my first diatom study, in a pollution gradient in the Naardermeer marshland, the oldest nature reserve in the Netherlands. To correctly interpret the results of the analyses, I compiled the ecological data from the literature into a card index, which was later greatly expanded and a copy of which can be consulted here. Shortly after completing my studies, I was asked to investigate the influence of a landfill on the water quality of the Ankeveense Plassen (a valuable broad area), the results of which I published in De Levende Natuur in 1975. To properly convey the results to the area managers (not diatom specialists), I gave here for the first time a classification of the indicator numbers for pH, salinity, organically bound nitrogen and oxygen, mainly based on the works of Hustedt and Cholnoky. I assigned these numbers to the 39 most common species from the area.
At that time, several MS students studied the diatoms from the fresh and brackish waters around Amsterdam asked me for additions to the 1975 list. Then at the Dutch Research Institute for Nature Management, I had the opportunity, in between different projects, to make a checklist of the Dutch diatoms from fresh and brackish waters, including indication numbers for saprobity, trophic state and moisture in addition to pH, salinity, organically bound nitrogen and oxygen. The indicator values were estimated from my experience and taken from hundreds of publications in Western European languages and Russian. Starting in 1982, Adrienne Mertens started to assist me in this process, and Jos Sinkeldam also provided technical assistance. In 1994, this list of 948 taxa was published in a special issue of the Netherlands Journal of Aquatic Ecology commemorating Albert van der Werff, the author of the ‘Diatom flora of the Netherlands’. Within a few years, the list was being used and widely cited by researchers worldwide.
Adrienne and I went to work for AquaSense, a hydrobiological consulting firm, in 1994, with Adrienne continuing to analyse diatom samples from various water types across the country. I was responsible for acquiring, managing and reporting hydrobiological research and monitoring projects. Gradually, Adrienne gained in-depth knowledge of the taxonomy and ecology of diatom taxa in the Netherlands, which we applied in many projects we conducted in commission of water and nature managers and engineering firms. AquaSense was acquired by Grontmij in 1999, and I left this large company in 2006 to continue as an independent consultant, and Adrienne followed a few years later. Invariably, we continued to work together.
Over time, the water board laboratories and other consulting firms appointed diatomists, such as Jako van der Wal, Geurt Verweij, Bert Pex and Arthur van Dulmen. They, too, obtained more and more knowledge about the ecology of Dutch diatoms. In 2023, the Dutch Foundation for Applied Water Research (STOWA) asked us to create a revised list of indicator values for monitoring purposes within the scope of the European Water Framework Directive. Adrienne made a master list of diatoms with indicator values, which were amended by the others in four roundtable sessions. Jako took care of much of the administration, and I wrote the draft text for the article. Martyn Kelly gave valuable suggestions on the draft.