Pens are too light …

We (or, to be more accurate, my colleague Lydia King) made the first record of a diatom in the UK last year.   Achnanthidium delmontii is a diatom well-known in continental Europe, because it appears to be rapidly expanding its range.   However, it had not, hitherto, been recorded from Britain.  We are being circumspect with our claim because there were not many individuals, and few of these were clear enough to get unambiguous photographs.   A recent paper from Hungary described A. delmontii as an “invasive species” due to the increase in number of records in recent years, but events over the past month have led me to wonder if we apply this term too readily to diatoms that appear in locations where they were not expected.  

Very approximately, 3.7 million people in Europe have changed their location over the past month.   Of those, about six per cent are “invasive” (Russian soldiers in Ukraine) whilst 94 per cent are not (Ukrainian refugees in Poland, Hungary, Romania and other eastern European countries).   Equating migration with invasion is a trope of right-wing populist politicians, but ecologists can also get very animated on the subject of “invasive non-native species” (INNS).   Rightly, too, because there is plenty of evidence of ecosystem damage as a result of accidental (or, in some cases, well-intentioned) introductions.   But that doesn’t mean that every new arrival is “invasive”.   As the quotation from Basil Bunting at the top of the post alludes, tossing a toxic term such as “invasive” into a sentence is easy; justifying use of that word is much harder.    

Achnanthidium demontii from the River Elz, Baden-Württemberg, Germany. Photos: Lydia King.  The photograph at the top shows a slate engraved with a quotation from Brigflatts by Basil Bunting in Durham University Botanic Garden.

The IUCN defines a species introduced outside its natural past or present distribution as “alien species” whilst if this species becomes problematic it is an “invasive alien species” .   So, if we peer down a microscope and simply see a diatom that we haven’t seen in a particular location before, we should not really use the term “alien” unless we can also demonstrate that it was “introduced” (implying a human vector) rather than just being the product of natural phenomena.  And we should not use the term “invasive” unless we can demonstrate that it is “problematic” (which I interpret, loosely, as causing a change in ecosystem functioning).  

With this in mind, here are three scenarios where the distribution of diatoms might change:

1. Local environment changes due to human activity; “natural” diatom assemblage is displaced by species more tolerant to the new conditions.   Water pollution is the best-known example, but the process happens in reverse too: a key aim of the Water Framework Directive is to displace the pollution-tolerant floras that predominate across much of Europe and reintroduce assemblages that grew in these lakes and streams two or three centuries ago.   It is the same basic rationale as reintroducing beavers in the UK, albeit with a less charismatic group of organisms.  These are not alien species although they may be different genotypes to any that were present in the past.
   
2. A diatom is accidentally introduced to a waterbody where it was not hitherto found, is able to thrive in its new environment and gradually spreads.   Someone peering at a sample from one of these new locations adds it to the list of taxa.  If it is abundant then it is likely that there will be competitive interactions with other diatoms.  However, this change in the species present does not necessarily affect the way that energy flows between trophic levels.   This is an alien species, but is it invasive?   The short answer is that most people who claim diatoms to be invasive don’t perform these broader studies into ecosystem functioning.
   
   A variant on this scenario is when there is natural selection in favour of a particular ecotype of an existing species.  The best-known examples (diatoms and other algae) is the evolution of tolerance to heavy metals but there is no evidence, at present, that new species arise as a result, in the timescales I am considering in this post.  If the new ecotype arose through a mutation in situ, then it is in no sense alien but, rather, a case of real-time natural selection.
   
3. A diatom is accidentally introduced, as in the previous scenario, and spreads, but this time there is an alteration to the way that energy flows between trophic levels.  In this case, the term invasive is justified but only after this extra information from other trophic levels has been gathered.  The best documented example is Didymosphenia geminata in New Zealand.  

My argument is that, for most of the diatoms claimed to be invasive, there is simply little or no evidence to show that there has been any alteration to ecosystem functioning as a result of their arrival.   The term alien is much more appropriate but, even here, we have problems, because differentiating between scenarios 1 and 2 is very difficult.   In addition, climate change adds a very substantial complication because it can shift temperature and hydrological regimes, potentially altering species compositions in ways that we do not yet fully understand. We may think we are looking at scenario 2 when, actually, we are dealing with scenario 1. Finally, as I have pointed out before (see “Achnanthdium subhudsonis invades Britain?”), the statistical power of most monitoring networks is sufficiently low that it is sometimes hard to differentiate between a newly-arrived species and one that has hitherto been overlooked.   

The hot question is not whether diatoms change their distributions over time (they do) but whether we understand the reasons why and, perhaps more importantly, the implications for the rest of the ecosystem.    I suspect that many of these aliens do not actually change the way that ecosystems function and, therefore, they are not “problematic” in the sense implied by the IUCN definition.   Maybe, as a Brit, I am particularly sensitive to this topic, as claims about the impact of migrants on the economy (mostly nonsense) were a cornerstone of the Brexit campaign.   Maybe that is why I find the term “invasive” to be inappropriate and, in these troubled times, way too toxic to be applied indiscriminately to any organism whose distribution changes.  

References

Buzckó, K., Trábert, Zs., Stenger-Kovács, Cs., Tapolczai, K., Bíro, T., Duleba, M., Főldi, A., Korponai, J., Vadkerti, E., Végvári, Zs. & Ács, E. (2022).  Rapid expansion of an aquatic invasive species (AIS) in Central-European surface waters; a case study of Achnanthidium delmontii.  Ecological Indicators 135: 108547.

Coste, M. & Ector, L. (2000). Diatomées invasives exotiques ou rares en France: principals observations effectuées au cours des derniėres décennies. Systematics and Geography of Plants 70: 373-400. 

Jellyman, P.G. & Harding, J.S. (2016).  Disentangling the stream community impacts of Didymosphenia geminata: how are higher trophic levels affected?   Biological Invasions 18: 3419-3435.

Kilroy, C., Larned, S.T. & Biggs, B.J.F. (2009).  The non-indigenous diatom Didymosphenia geminata alters benthic communities in New Zealand rivers.   Freshwater Biology 54: 1990-2002. 

Pérėz, F., Barthės, A., Ponton, E., Coste, M., Ten Haag, L, & Le-Cohu, R. (2012).  Achnanthidium delmontii sp. nov., a new species from French rivers.  Fottea (Olomouc) 12: 189-198.

Some other highlights from this week:

Wrote this whilst listening to: Nordic women: Jenny Hval’s Apocalypse Girl and and Björk in her Sugarcubes days.

Currently reading: Lee Schofiled’s Wild Fell, about the restoration of uplands around Haweswater in Cumbria.

Cultural highlight:  Not sure I can think of one.  Too much else going on just now.

Culinary highlight:   Probably a meze from a local Lebanese takeaway: enjoying some final meals with our son before he heads back to China.

Springing into life …

Spring officially started on 1 March, as if the officialdom has any say in the natural order.    But then officials also say that rivers should stay in their channels and that fallen trees should be removed because they make streams “untidy”.  All “official” pronouncements on the seasons (if not the natural order generally) deserve to be treated by aquatic biologists with a pinch of scepticism.   A peek below the surface of the Lake District’s streams, for example, suggested that spring was already well underway even before officials had approved its start, with the upper surfaces of cobbles and boulders in streams on my regular beat smothered with thick orange-brown diatom films.  But, then, algae have always had a difficult relationship with officialdom.  

Back home, after a fieldtrip beset by equipment problems and leaky waders, I had a chance to look at the organisms responsible for these growths.  First observation was that whilst the presence of films was almost universal, their composition differed from stream to stream.  On the other hand, there were some consistencies that helps pull these differences together to produce a coherent narrative.   One final confession before going any further: my fieldwork issues necessitated staying an extra night, and the algae collected on the first day did not look as healthy when examined under the microscope as those collected on my second day.   

Broadly speaking, all of the biofilms looked like the photograph below, with a mix of Achnanthidium and Gomphonema species, along with a seasoning of Fragilaria and Tabellaria.  They differed, however, in the details.  The films in Croasdale Beck, a small stream flowing off the fells around Ennerdale Water (most recently discussed in “The algae that got away …”) were dominated by the diatom Gomphonema parvulum, whereas those from the Rivers Ehen and Irt had one or more relatives.  In “Diatoms and dinosaurs” I referred to this as “Gomphonema gracile” but then went on to explain why this name was problematic.   Since then, the name G. gracile has been kicked into the long grass for different reasons, so I’ll call it “G. graciledictum” but still retain my original scepticism along with the further proviso that identifying diatoms from live material is not always straightforward.  

Diatoms from the River Irt, February 2022.  Scale bar: 20 micrometres (= 1/50^th of a millimetre).   The photograph at the top of the post shows the diatom films smothering cobbles on the bed of the River Irt at Lund Bridge.  

Gomphonema parvulum from Croasdale Beck, near Ennerdale Bridge, Cumbria in late February 2022.  All valve views, with the exception of the rightmost image.  Apologies for the shrivelled chloroplasts.  Scale bar: 20 micrometres (= 1/50^th of a millimetre).  

Gomphonema cf graciledictum from the River Ehen, just above Ennerdale Bridge, February 2022.  The main image shows cells attached to branched stalks whilst the bottom row shows valve (left) and girdle (right) views.  Scale bar: 20 micrometres (= 1/50^th of a millimetre). 

I don’t think it is a coincidence that these thick biofilms are dominated by species of Gomphonema.  You can see stalks emerging from the bases of several of the cells I have photographed.  These help the cells exploit space vertically by creating dense “bushes” and, in turn, means that these species are “ecosystem engineers” on a tiny scale, creating habitat and opportunities for other algae too.  Just as terrestrial bushes such as hawthorn can act as windbreaks, so these tiny underwater bushes will also slow the flow of water, both reducing physical stress and making nutrients more accessible to the algae.   And, just as hawthorn bushes provide branches on which birds can perch, as well as for lichens to grow, other organisms grow in and around Gomphonema.   I’ve written about this before (see “The curious life of biofilms …”) but it is easy to overlook because most of us count cells and ignore the stalks that are so important in this process of habitat creation.  

The abundance of algae we observed, however, is likely to be due to the low numbers of grazers at this time of year. I did see a few chironomid and simulidae larvae during this trip, but fewer large grazers.  From this point of the year onwards, the quantity of algae in these rivers generally declines, probably because invertebrates become more active as the water warms, and the algae are a valuable food resource for them.  This period, however, represents a window of opportunity when – cold water aside – they can grow with relatively few constraints.   The constraint, as I’ve commented before, lies with the ecologists themselves, relatively few of whom want to venture out to explore these habitats in winter.  And, with the right foot of my wader getting gradually damper as streamwater drips in, I’m wondering if they might just have a point.  

Gomphonema cf graciledictum from the River Irt at Lund Bridge, February 2022.  Scale bar: 20 micrometres (= 1/50^th of a millimetre). 

Reference

Reichardt, E. (2015).  Gomphonema gracile Ehrenberg sensu stricto and sensu auct. (Bacillariphyceae): a taxonomic revision.  Nova Hedwigia 101: 367-393.

Some other highlights from this week:

Wrote this whilst listening to: Gregorian Chant for Lent and Easter by the Gregorian Chant Schola of Saint Meinrad Archabbey, Indiana, USA.

Currently reading: Chronicles from the Happiest People on Earth by Wole Soyinka.

Cultural highlight: J’ai perdu mon corps (I lost my body).  French animated film from 2019.   Unique. Also, wonderful new British film Ali and Ava, directed by Clio Barnard and set in Bradford.

Culinary highlight: Sichuan-style fish head and frogs leg soup with plenty of whole chillis, served “hot pot”-style using a laboratory hot plate.