When is a diatom like a London bus?

This post takes us back to Semer Water (see “Lake Lakelake Lake”) for a closer look at some of the diatoms that live in the littoral zone.   I studied this lake back in 2012 as part of a project funded by Natural England, and recently used one of the samples collected as part of that study for a “ring-test” amongst diatomists involved in routine ecological assessment in the UK and Ireland. This provides an opportunity for a number of experienced analysts to take a detailed look at the diatoms present in a sample and compare notes.

Between us, we found over 90 different species that we could name in a single sample, amongst which there were a number that caused us no disagreement and a few where we struggled to achieve a consensus.   And there was one group where we achieved a consensus of sorts insofar as the entities that we found matched the pictures in the latest reference works, but shared a sense of misgiving about whether the names we used told the whole story.   The diatoms that caused these problems were representatives of a group of small diatoms that, for a long time, were grouped in the genus Fragilaria but which, following a paper by David Williams and Frank Round in 1987, were split between several different genera including Pseudostaurosira, Staurosira and Staurosirella.   This generated some controversy although, as the dust settled, most people agreed that these taxa definitely did not belong in Fragilaria, even if the exact allocation of species between the genera continued to excite debate. The plate below shows three “species” that we found in Semer Water although the suspicion is that the seven forms I’ve called “Staurosirella pinnata” may represent several different species.


Small Fragilaroid diatoms from the littoral zone of Semer Water August 2012.   A. Pseudostaurosira brevistriata; b. Staurosirella leptostauron; c. – i. different morphotypes within the “Staurosirella pinnata” complex. Scale bar: 10 micrometres (= 1/100th of a millimetre).

However, my purpose today is not to get bogged down in the minutiae of these nomenclatural squabbles but to take a step back and look at their ecology.   Members of this complex are widely distributed across a wide range of water type but they are particularly abundant in hard waters, both standing and running.   What interests me is that they often seem to occur together.   Hence the title of the post: like London buses, you wait ages for one small Fragilaroid species to appear in a sample, and then three come along at once…

In order to test this association, I searched my database and found 705 records where at least one of the more common species in the complex were found (out of a total of 3838 records).   309 (44%) of these contained more than one species, but this figure leapt to 82% when I only considered samples where the sum of these species exceeded two per cent, and a mighty 98% when I just included the 90 samples where the sum exceeded ten per cent.   You can see this trend clearly in the graph below where I have plotted the percentage of one species against the sum of the percentages of the other four that I have considered.   The issue gets more complicated when we consider that some of the so-called species could, themselves, be complexes of several species that can co-exist (as in the case of Staurosirella pinnata in the plate above).


Co-variation in relative abundance of common Fragilarioid species. Each plot shows the percentage of one species (or species complex) on the x axis, and the combined percentage of the other four species on the y axis. Log scales are used for the sake of clarity.   a. Pseudostaurosira brevistriata; b. Staurosira construens (and varieties); c. Staurosirella leptostauron; d. Staurosira elliptica ( = Pseudostaurosira trainorii); e. Staurosirella pinnata complex.

What is going on here?   If they all live in the same habitat then, in theory, the “competitive exclusion” principle should work to eliminate all but the fittest.   That two or more appear to co-exist on a regular basis suggests that they are occupying distinct niches that we are not recognising with our usual methods of studying the microscopic world of lakes and rivers (see “Baffled by the benthos (1)” and “Baffled by the benthos (2)”).   It might be a physical niche, but the niche could also be defined by chemical or biological factors.   Could it be, for example, that these different but (fairly) closely-related forms differ in their levels of resistance or tolerance to fungal or viral infections?   Or do they have subtle variations in the amount of light that each needs, but which are obliterated by our unsubtle sampling methods?

The result is the diatom equivalent of a “perfect storm”: neither the taxonomy nor the ecology are described with any great clarity.   It is possible that a better understanding of the taxonomy will lead to a more nuanced appreciation of their ecology. Taxonomists often dangle this bait, but the sad truth is that there are few cases where ecologists emerge blinking into the light after the taxonomists have finished their fiddling. But ecologists must take their share of the blame, being very conservative in the methods that they use to deduce ecological differences between species.   I could, in fact, use my 1930 edition of the Süsswasserflora vön Mitteleuropa to identify this group of species and, apart from the name changes, reach exactly the same  conclusions as I might have done 85 years ago.

Thanks to Ingrid Jüttner (National Museum of Wales) for the diatom photographs, and Eduardo Morales (Bolivian Catholic University, Cochabamba, Bolivia) for taxonomic assistance.


Williams, D.M. & Round, F.E. (1987).   Revision of the genus Fragilaria. Diatom Research 2: 267-288.

A flavour of the subsequent debate is given in:

Morales, E.A. & Trainor, F.R. (2008). A new paradigm for freshwater fragilarioid diatom classification? A critique of Lange-Bertalot’s new system. Journal of Phycology 37 (supplement): 36-37.

3 thoughts on “When is a diatom like a London bus?

  1. Pingback: Hard science in hard water? | microscopesandmonsters

  2. Pingback: Older … but not necessarily wiser? – microscopesandmonsters

  3. Pingback: The devil lies in the detail … – microscopesandmonsters

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