The devil lies in the detail …

Our latest ring test* slide took us on a vicarious journey to the beautiful River Don in Aberdeenshire.  Maybe because I have been doing this job for so long, but the quality of the landscape was clear to me as I peered through my microscope 500 kilometres away: the range of diatoms that I could see would not have thrived anywhere with more than the lightest touch from humankind.

One of the clues for me lay in some of the smallest diatoms on the slide.   It took some discussion amongst my fellow experts, but we eventually came up with a list of five different species of Achnanthidium (all illustrated below) which, together, constituted about a third of all the diatoms on the slide (admittedly, because they are small, they constitute rather less than a third of the total volume of diatoms, but that is another story ….).   The mere presence of several Achnanthidium species is, in my experience, usually a sign of high habitat quality (see “Baffled by the benthos (2)”) but unravelling the identities of the different species with a light microscope is challenging.

Achnanthidium-minutissimum-Medwin_WaterAchnanthidium minutissimum from Medwin Water, Scotland. Photographs from the Diatom Flora of Britain and Ireland by Ingrid Jüttner.  Scale bar: 10 micrometres (= 1/100thof a millimetre). 


Achnanthidium pyrenaicum from the River Don, Towie, Aberdeenshire.  Photographs by Lydia King.  Scale bar: 10 micrometres (= 1/100thof a millimetre). 

The genus Achnanthidium is a good example of the delicate co-existence between “identification” and “taxonomy” in the world of diatoms.   Individuals from this genus are usually small so anyone using a light microscope for routine analyses will be working right at the optical limits of their equipment whilst anyone with a serious interest in taxonomy will depend upon a scanning electron microscope for the insights needed for critical differentiation between species.

This divergence between the working methods of “identifiers” and “taxonomists” means that it is rarely possible to name every individual of Achnanthidium with complete confidence.  The ones that present clearly in valve view (i.e. face-up) can mostly be assigned to a species based on features we can see with a light microscope, but it is not always straightforward for those seen in girdle view (i.e. on their side) or which are partly obscured by other diatoms or extraneous matter on the slide.   In this example from the River Don, we also noticed that smaller individuals of A. gracillimum lost their characteristic rostrate/sub-capitate ends and were, as a result, not easy to differentiate from A. pyrenaicum.


Achnanthidium gracillimum from the River Don, Towie, Aberdeenshire.  Photographs by Lydia King.   Scale bar: 10 micrometres (= 1/100thof a millimetre). 

What continues to mystify me is why so many closely-related species can live in such close proximity. It is Achnanthidium that prompt this question here, but other genera display similar tendencies (see “When is a diatom like a London bus?”).  And this immediately generates another question: why are more people not asking this question of diatoms and, indeed, microscopic algae in general?

The answer to that question falls into two parts. The first is that understanding the precise ecological requirements of microscopic algae is not a trivial task, and assumes that you are able to get several closely-related species to live in culture (which, itself, assumes you know the precise ecological requirements of each … you see the problem?).   There is, as a result, a tendency to avoid experimental approaches and, instead, look for how species associate with likely environmental variables in datasets collected from sites exhibiting strong gradients of conditions.   However, this assumes that the forces that drive the differentiation between species work at the same scale at which we sample (see “Our patchwork heritage …” for more on this).

Underlying this, however, is a deeply-held belief, dating back at least forty years, that the niches of freshwater diatoms are determined primarily by the chemistry of the overlying water.   This is a dogma that has served us well when using diatoms for understanding the effects of environmental pollution but which is, ultimately, a limitation when trying to explain why we found five separate Achnanthidium species in a single sample, all exposed to the same stream water.


Achnanthidium lineare (first three images from the left) and A. affine (two images on the right) from River Don, Towie, Aberdeenshire.  Photographs by Lydia King.  Scale bar: 10 micrometres (= 1/100thof a millimetre). 

I will go one step further: this dogma is so deeply held that referees rarely challenge the weak evidence that is produced to demonstrate different responses to environmental conditions between closely-related species.  There are certainly variations in environmental preferences between Achnanthidium species, but these are best expressed as trends rather than unambiguous differences and I have never seen such trends subject to rigorous statistical testing.

I blame better microscopes: greater magnification and resolution has revealed such a baffling amount of diversity that all the energy of bright diatomists is absorbed unravelling this rather than trying to explain what it all means (see “The meaning of … nothing”).  If we were bumbling along with the quality of equipment that Hustedt depended upon, then maybe we would be cheerfully lumping all these forms together and focussing on functional ecology instead.   Maybe.

* see “Reaching a half century” for more about the ring test scheme

More algae from Shetland lochs …


I’m taking you back in the Shetland Islands for this post, and onto the remote moorlands of northern Mainland.   When I visited this particular loch in 2016, I noticed a lot of slippery filaments of Batrachospermum attached to the sides of the cobbles in the littoral zone (see “Lucky heather …”).   This time around, I explored further around the edge of the loch and, in the south-west corner noticed prolific growths of algae in the shallow peaty water.  Closer inspection showed that these, too, were the red alga Batrachospermum and, though they were not fertile, Dave John suggests that they are likely to be B. turfosum Bory.


Tufts of Batrachospermum turfosumin the littoral zone of Lamba Water, north Mainland, Shetland Islands, May 2019.   The picture frame is about 15 centimetres across. 

If you have a hand lens you can just about make out a bead-like structure when observing Batrachospermum in the field; however this becomes much clearer with higher magnification.   I think it looks like a bottle-brush when seen under the microscope at low magnification, with whorls of side-branches arising from the central filament.  At higher magnification, these filaments can be seen to have a bead-like structure, with cell size gradually reducing with distance from the centre.

What you cannot do in the field is separate Batrachospermumfrom the closely-related genus Sheathia(see “News about Batrachospermum… hot off the press”).   I usually tell people that, for a general overview of the condition of a stream or lake (for example, as part of the UK macrophyte survey technique), then simply recognising that you have “Batrachospermum” (meaning Batrachospermum or Sheathia) should be enough.   In my experience, the presence of Batrachospermumis usually a good indication that the water body is in a healthy condition.  However, I have been told that Batrachospermumis often found growing prolifically in very enriched conditions in southern chalk streams, which would challenge this assumption.   This may be because the species that are found in southern chalk streams are different to those that I encounter in my more usual haunts in northern England and Scotland.  But it is also possible that the factors I described in “The exception that proves the rule …” pertain in those cases too.


Filaments of Batrachospermum turfosum from Lamba Water, north Mainland, Shetland Islands, May 2019.   The upper photograph shows a low magnification view of a filament (about 350 micrometres, or 0.35 millimetres, wide) whilst the lower image shows a whorl of side branches arising from the main stem.  Scale bar: 20 micrometres (= 1/50thof a millimetre).  

We often run into this dilemma with filamentous freshwater algae: it is reasonably straightforward to identify the genus but we need reproductive organs to determine the species.  As they seem to survive quite happily in the vegetative state our understanding of the ecology of individual species (rather than the genus as a whole) is scant so it is hard to tell whether there is value in that missing information or not.   In a few cases – this is one – better taxonomic understanding has revealed that we may not even be dealing with a single genus but the lists used for applied ecological surveys still persist with the old concepts.

This creates a toxic spiral of consequences: it is hard to split into species so most people don’t bother. Because we don’t bother, our interpretations are based on generalisations drawn from the behaviour of the genus.  This means we don’t generate the data needed to demonstrate the value (or otherwise) of the effort required to go from genus- to species-level identifications.   So we carry on lumping all records to genus (or, in this case, a pair of genera) and accept a few records that our out of line with our expectations as “noise”.  The situation is probably worse in the UK than in many places because there are very few people in universities specialising in these organisms and, as a result, no-one is producing the data that might break us out of this spiral.

We found Batrachospermum turfosum in a few other locations during our visit, but nowhere, even in nearby lochs, was it in such quantity as we saw in Lamba Water.   Chance might play a part in determining its distribution on a local scale but that ought to be the explanation of last resort rather than the go-to answer when we are worryingly short of hard evidence.



How to make an ecologist #7


Casting a plankton net to collect algae, somewhere in Scotland (possibly Loch Earn), April 1985.

At some point between leaving Westfield as a rookie ecologist with an enthusiasm for Sphagnum, and finishing a PhD on mosses at Durham I started the slow metamorphosis into a phycologist.   Brian Whitton expected his PhD students to help out in undergraduate practicals and my lack of phycological training up to that point was not regarded as sufficient reason to excuse me from this duty.   It was a steep learning curve but, in turn, it opened windows onto new worlds that have kept me fascinated ever since.

Brian had an old school natural historian’s approach to undergraduate practicals.   Technicians were sent out to local ponds and came back with handfuls of vegetation which were squeezed and scraped to yield rich harvests of algae. At the start of the practical, no-one had any idea which species might be present; three hours later, with the help of a handful of books in a range of languages (we just looked at the pictures) and cajoling from Brian, the demonstrators, at least, emerged older and wiser.

Straight after Easter, the third year botany students were taken on a week-long field trip to Loch Lomond, staying at University of Glasgow’s Rowadennan Field Centre, and learning about algae at a time when most of them would really have preferred to be getting on with revision for their finals.   However, once they arrived at the field centre, set amidst the forests on the east shore of Loch Lomond in the shadow of Ben Lomond, they usually mellowed.   It was a glorious location. We went out to various lochs and streams, sampled different habitats, collected a few environmental measurements, and then spent time in the laboratory trying to name what we had found.   In the evenings most of us made the three kilometre walk to Rowardennan Hotel for a pint of beer.

On one of the days we made a long excursion, down the east shore of Loch Lomond, then up the west shore, making a short diversion at Tarbet to Loch Long, the only sea loch we visited during the week. Then it was back into the vans and up to the north end of Loch Lomond, stopping at a stream in Glen Falloch before sampling Loch Lubhair and Loch Linhe. The final leg swung south past Loch Venachar to Lake of Menteith in the Trossachs (‘the only lake in Scotland’) before returning to Rowardennan in time for dinner. In one long day we had seen marine and freshwater habitats, sampled hard and soft streams and lakes, planktonic and benthic habitats and seen seaweeds as long as our arms and microscopic algae a 100th of a millimetre in diameter.


Durham University botany undergraduatest getting to know freshwater algae at Rowardennan Field Centre, April 1985.

At this time, the Durham botany degree was strong on biochemistry and molecular biology and notoriously light on traditional botanical skills.   There was a running joke during my postgraduate years that some of our molecular biologist colleague’s plant identification skills ran no further than reading the label on a packet of seeds. Reductionism ruled, with teaching on whole plants and their interactions with the environment pushed to the edges of the course.   The honours botany students were taken on a two week field course to Austria at the end of their second year to learn about alpine plants. This week in Rowardennan dealt with the 75 per cent of UK’s plant diversity that has now dropped off most undergraduate curricula over the past couple of generations. And, once again, the demonstrators, acting as intermediaries between Brian’s extensive knowledge and the near complete ignorance of the students, were probably the principal beneficiaries.

There were other beneficial outcomes to the course. I spent long hours walking to and from the pub sharing our experiences of travelling in the Himalayas with one of the students.   This same individual (and her distinctive orange cagoule) cropped up in more of my photographs than a hypothesis concerning the random distribution of students on 35 mm film would predict.

Reader, I married her.


Durham undergraduates sampling a stream in Scotland during the algae field course, April 1985.