When is a “record” not a record?

Just before Christmas, I reported on the first record of Achnanthidium catenatum for the UK, from a lake in Wales and pondered, in this post, what we meant by a “record” of a diatom.   Simply finding a few empty valves (the silica cell walls) is not, by itself, evidence that there ever was a viable population of A. catenatum in Llyn Padern.   What, indeed, should we use as criteria for regarding this as a new addition to the UK flora?

My instinct is that the more valves I find in a sample, the greater the probability that these indicate the existence of a viable population.  Even so, it could still just indicate the presence of a population at one point on one occasion.   If I were able to find A. catenatum at several locations around the lake, or at the same point over an extended period of time then, perhaps, we can conclude that it is an established part of the flora of this lake and, therefore, of the UK.

What if I cannot demonstrate this, even after visits to Llyn Padern?   We know that it was present (albeit as dead cells) in the summer of 2013 but is this enough to add it to a list of UK records?   Perhaps those of us who study algae need to adopt the practice of ornithologists of distinguishing between species that nest and breed in the UK (“British Residents”) and those that are “Summer/Winter Visitors” or “Passage Migrants”?   (apologies to Northern Irish readers, but the website www.birdsofbritain.co.uk does not appear to make any distinction between UK and Great Britain).

In other words, we should aspire to a two-tiered checklist that accounts for the often erratic occurrences of microorganisms that are readily dispersed by a variety of physical and biological agents.   Then again, a further complication is that populations may lay dormant for long periods and then proliferate for short periods, so our one-off record may actually have been present for much longer, but was missed.

And there’s a final problem: too often our records simply reflect the activities of the small number of people who collect and analyse diatoms and other microscopic organisms, making it very hard to understand how distribution patterns vary in space and time.

And, finally, you might well be asking whether these questions are relevant.   In my opinion, yes they are, partly because it puts the diversity of the microbial world into context.   According to www.rspb.org.uk there are 574 bird species recorded from the UK.    There are at least four times as many species of freshwater diatoms, and about 10 times as many species, if we include all freshwater algae.   There is a huge amount of diversity tucked away in a group of organisms that is largely overlooked or ignored.


First record of Achnanthidium catenatum for the UK?

I came across a few strangely-shaped diatoms whilst examining a sample from Llyn Padarn in Snowdonia a couple of weeks ago.   The girdle (side) view has a characteristic “spoon” shape whilst the outline in valve view (i.e. seen from above) is lanceolate with expanded (“capitate”) ends.   I had previously seen this only as illustrations in continental Floras and once in a sample from Corsica.

Leica Picture

Valves of Achnanthidium catenatum from Llyn Padarn, Snowdonia, September 2013; left hand view: two girdle (side) views; right hand view: valve view.   Scale bar: 10 micrometres (= 1/100th of a millimetre).

To the best of my knowledge, this is the first record of the species in the UK.  I helped to compile a checklist of freshwater algae about 10 years ago and found no records then.  A lot of samples have been analysed since this checklist was published, but I have seen no reports of this species being recorded.   There are, however, two problems with knowing that this really is the first record for the UK.  The first is that there are no formal structures for validating new records, such as exist for higher plants.  If a species is found in a part of the country where it had not previously been recorded, then there are specialists, appointed by the Botanical Society of the British Isles, and similar bodies, who can check the identity of the specimen.  The algal world still lacks this level of organisation.   I can send my images or slides to colleagues who can confirm my hunch, but this is all very informal.  More significantly, no single individual holds a truly definitive list of UK algal records.

The second problem with knowing that this is the first record of Achnanthidium catenatum is that I have not, actually, seen this species alive.  My sample from Llyn Padwan contains a few dead shells from which we can infer that there is probably a population of A. catenatum present.   However, as this lake lies in a popular tourist area, we cannot wholly exclude the possibility that these simply washed off the boot of a visitor from continental Europe.   This picks up the issue I discussed in A Christmas Turkey .. that reliance on the dead shells is dangerous.   A useful New Year’s resolution might be to visit Llyn Padwan, or find a colleague who can visit it for me, and see if I can find some living populations of A. catenatum.   Only then will we be sure that this really is a new addition to the UK algal flora.