I’ve written about Platessa oblongella, a small diatom common in low alkalinity environments, before (see “A tale of two diatoms …” and links therein) but my travels around west Cumbria are gradually revealing more and more about the ecology of this organism, so bear with me as I explain my latest findings.
My first graph shows how the distribution of this diatom varies in different types of water body in the Ennerdale catchment. I have analysed 223 samples from this small area over the past few years and, within this dataset, there is a very clear distinction between situations where Platessa oblongella is abundant and situations where it is very rare. I have very few records from Ennerdale Water itself (present in just two out of 27 samples, and never comprising more than 2.7% of all diatoms in the sample) nor from the River Ehen, which flows out of the lake (present in just 16 out of 164 samples, and always £ 1% of all diatoms). By contrast, in Croasdale Beck and in streams that flow into the north-west corner of the lake, it is present in 28 out of 32 samples, with a maximum relative abundance of 69%. In ten samples it forms more than 10% of all diatoms present. Several of my samples from the small streams were collected from just a few metres above the point where they joined the lake, which makes the distinction between these streams and the lake that much more intriguing.
My theory – based on data I showed in A tale of two diatoms is that Platessa oblongella is a species of disturbed habitats and that the littoral zone of a lake, whilst subject to some turbulence, is less disturbed than the rough world of an unregulated stream. The contrast between the River Ehen immediately below the dam at the outfall of the lake and the various small tributary streams also supports this idea.
Differences in percentage of Platessa oblongella (including P. saxonica) in epilithic samples from Ennerdale Water and associated streams. Data collected between 2012 and 2018 (along with one sample from River Ehen collected in 1997). The photograph at the top of the post shows Ennerdale Water, photographed in January 2018.
Some of the populations I looked at seemed to consist of two distinct forms, one broader than the other. This variability is quite common in Platessa oblongella and Carlos Wetzel and colleagues recently published a paper which suggests that these are, in fact, two distinct species. When I first started looking at diatoms, John Carter, my mentor, used the name Achnanthes saxonica, but Krammer and Lange-Bertalot, in the revised Süsswassserflora, regarded this as a synonym of Achnanthes oblongella, a species first found in Thailand. Wetzel’s study shows, as well as the difference in valve width, differences in the fine details of the striae between the two species. They also decided that both species belonged in the genus Platessa, rather than Achnanthes.
Platessa oblongella (top) and P. saxonica (bottom) from Croasdale Beck, October 2017. Scale bar: 10 micrometres (= 1/100th of a millimetre).
Valve width is, however, a very useful criterion, as the histograms below show. The left hand graph shows a distinctly bimodal distribution of widths in specimens from Croasdale Beck, whilst the right hand graph shows a much tighter, and clearly unimodal, range. This comes from another tributary stream flowing into the Ehen about 500 metres below the lake itself. Quite why two species can co-exist in one stream but only one is present in another is not clear.
The modes of these populations are very close to the median widths for P. saxonica (narrow, ± 4/5 – 5 mm) and P. oblongella (broader, ± 6.5 mm) respectively but, as the left hand histogram shows, there is some overlap. You might have trouble, for example, deciding whether a valve that was 5.5 mm wide was a “fat” P. saxonica or a “thin” P. oblongella. My standard advice in situations such as this is that we should identify populations not individuals although, in the case of Croasdale Beck, this will still leave a grey area between the “fat” and “thin” valves where a judgement call is necessary. In this case I think I could have done it because the P. saxonica valves in this stream tended to have a greater length:breadth ratio than those of P. oblongella, though I have not actually quantified this.
Width of valves in populations of “Achnanthes oblongella” from a) Croasdale Beck, and b) an unnamed tributary stream of the River Ehen, October 2017.
There is more to say about the ecology of these species, but I have probably written enough for now. I will leave you, for now, to bask in the rare sensation that occurs when diatom taxonomists make a situation clearer rather than more opaque, and return to this subject in a future post.
Carter, J.R. (1970). Observations of some British forms of Achnanthes saxonica Krasske. Microscopy: Journal of the Quekett Microscopical Club 31: 313-316.
Wetzel, C.E., Lange-Bertalot, H. & Ector, L. (2017). Type analysis of Achnanthes oblongella Østup and resurrection of Achnanthes saxonica Krasske (Bacillariophyta). Nova Hedwigia, Beiheft 146: 209-227.