The River Wear in January

The series of events that eventually gave birth to this blog started with a visit to the River Wear at Wolsingham on the first day of 2009.  I had visited on a whim, intending to blow away the cobwebs after lunch on New Year’s Day, but with no real plan.  But I thought it would be interesting to pull on my waders and have a look at the river bed and, while I was there, I may as well collect a sample too.   Those observations and that sample must have triggered something in my mind, because I returned every month after that and, on each occasion, the samples and observations generated sketches which, in turn, made me curious about the factors that drove the algal communities in our rivers.

I thought it would be interesting to repeat that exercise during 2018 as my thinking has moved on over the past nine years.  I’m essentially visiting the same site and making the same observations but, this time, filtering them through deeper beds of experience.   The River Wear at this point is about 30 metres wide, a broad, shallow, riffled stretch, skirting the small town of Wolsingham roughly at the point where Weardale broadens out from a narrow Pennine valley to the gentler landscape of the Durham coalfield.  There are a couple of small towns upstream but the ecological condition of the river is still good.  Although there are still concerns about concentrations of heavy metals arising from the mines that are scattered around the upper parts of the valleys, I can see no serious effects of toxic pollution when I look at the plants and animals that live at Wolsingham.

If you follow this blog you will not be surprised to hear that, even in the depths of winter, algal communities in the River Wear are thriving Most of the larger stone surfaces are covered with a discernible brown film, up to a couple of millimetres thick.   The very top layer is dark brown in colour, with a lighter brown layer beneath this.   When I put a sample of this under my microscope, I saw that it was dominated by gliding cells of Navicula lanceolata, though other diatoms were also present (described in more detail in “The ecology of cold days”) and there were also a few thin filaments of a blue-green alga.

A submerged cobble photographed in situ in the River Wear at Wolsingham, January 2018, covered with a thick diatom-dominated biofilm.

I’ve included a picture of the view down my microscope because one of the questions that I’ve been trying to answer over the past few years is how we construct an understanding of the microscopic world using microscopy (see “The central dilemma of microscopy” and “Do we see through a microscope?”).   Of course, a single view field of view does not convey all the information I require, so my understanding is actually built up from observations of a large number of separate fields.  The boat-shaped cells of Navicula lanceolata were almost ubiquitous in these, as were patches of amorphous organic matter (“fine particulate organic matter” – see “A very dilute compost heap …”).  In total, I found 15 different species of algae in my preliminary analysis, of which Navicula lanceolata comprised about half of the total, with thin filaments of the cyanobacterium Phormidium and the diatom Achnanthidium minutissimum each constituting about 15 per cent.

A view of the biofilm from the River Wear, Wolsingham in January 2018.

However, my earlier comment about the biofilms having distinct layers means that simply observing what organisms are present will not tell us the whole story about how those organisms are organised within the biofilm (see “The multiple dimensions of submerged biofilms …”) so the next step is to hypothesise how these organisms might be arranged in the biofilm before I disrupted their microhabitat with my sampling.   The schematic diagram below attempts to capture this, but with a few provisos.  First, I said that the biofilm was a couple of millimetres thick but my portrayal only shows about a tenth of a millimetre; second, there is considerable spatial and temporal variation in biofilms and my depiction amalgamates my direct observations in January 2018 with information gleaned from a number of other visits.   Gomphonema olivaceum (probably a complex of two or three species in this particular case), for example, is often more prominent than it was last week, and I have also omitted Achnanthidium minutissimum altogether.   I suspect that this is less abundant in the mature biofilms but that the cobble surface is a patchwork of different thicknesses, reflecting different types of disturbance.   That raises another issue: the scale at which we generally collect samples is greater than the scales at which the forces which shape biofilms operate.   The whole image below, for context, occupies about the same width as a single bristle on the toothbrush that I used to collect the sample.

It is difficult to convert what we “see” back to the original condition when working under such constraints and, inevitably, decisions are guided by what others before us have written.  That brings a different set of problems: Isaac Newton may have seen further by “standing on the shoulders of giants” but Leonardo da Vinci’s usually rigorous objectivity lapsed on at least one occasion when his eye was led by assumptions he had inherited from earlier generations (see “I am only trying to teach you to see …”).   What my picture is actually showing, in other words, is a mixture of what I saw and what I think I should have seen.   This two-way process in art extends from the very earliest drawings we make through to the most sophisticated Old Masters so I am in good company.  In truth, I am not trying to depict a particular point in space or time so much as to encapsulate the idea of a biofilm from that river that is more than a random aggregation of cells.

A schematic view of the vertical structure of a submerged biofilm from the River Wear, Wolsingham, January 2018.   a., Navicula lanceolata (valve view); b., N. lanceolata (girdle view); c. Navicula gregaria (valve view); d. N. gregaria (girdle view); e. Gomphonema olivaceum (valve view); f. G. olivaceum (girdle view); g. Phormidium; h. inorganic particles; i. fine particulate organic matter.  Scale bar: 20 micrometres (= 1/40th of a millimetre).

References

You can find out more about the condition of the River Wear (or any other river or lake) using the Environment Agency’s excellent Catchment Planning webpages

Three good books that discuss the relationship between pictorial representation and the mind are:

Cox, Maureen (1992).  Children’s Drawings.   Penguin, Harmondsworth.

Gombrich, E.H. (1977) Art and Illusion: a study in the psychology of pictorial representation.   5th Edition.  Phaidon, London.

Hamilton, James (2017).  Gainsborough: a Portrait.   Weidenfield & Nicholson, London.

 

 

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Change is the only constant …

The diatoms I saw in my sample from the littoral of Lake Popovo (described in the previous post) reminded me of an assemblage that I had seen in another lake which, apart from its location, has much in common with Popovo. This lake is Wastwater, in the western part of the English Lake District (see “The Power of Rock …”).  Like Popovo, it is situated in a remote a region of hard volcanic rocks and, as such, has very soft water and is subject to few of the pressures to which most of our freshwaters are subject.  The photograph above shows me sampling Wastwater in about 2006 (more about this photograph, by the way, in “A cautionary tale …”).

I wrote about Wastwater when I was writing my book Of Microscopes and Monsters, the precursor of this blog.   I focussed, in particular, on an experiment that my friend Lydia King had performed as part of the research towards her PhD.  Her previous work had established that there were relationships between the types of algae that she found in lakes in the Lake District and the amount of nutrients that they contained.  She also saw that the types of algae she found depended upon how acid or alkaline the water was.  But the water chemistry only explained a part of the variation in the algae and now she wanted to find out about the variation that was not explained by this.   In particular, she wanted to know how much of the variation was due to the way that the algae interacted with each other.

Lydia’s experiment involved putting clay pots into the shallows at the edge of Wastwater and then watched how the algal communities changed over the course of six weeks.  She also examined small parts of the pots at extremely high magnifications using a scanning electron microscope.   These micrographs, and subsequent conversations with her, had inspired some of my early paintings and I returned to this subject several times, finally producing a series of three pictures that showed changes in the algae over time.

The microbial world of the littoral zone of Wastwater after two weeks of colonisation showing unidentified small unicellular blue-green alga,  unidentified small unicellular green alga; thin filaments of Phormidium,  Achnanthidium minutissimum and Gomphonema parvulum.

The first of these shows the surface of the plant pot after being submerged in Wastwater for two weeks.   You could think of this as a patch of waste ground that was, at the start of the experiment, bare of vegetation.   If we watched this patch over a number of weeks, we would notice some plants appearing: scattered stalks of grass, perhaps some rosebay willow herb, dock or plantains. A gardener might dismiss these as “weeds”, although this term has no ecological meaning but ecologists prefer to think of these as “pioneers”: plants adapted to colonising new habitats, growing quickly (which might mean producing lots of seeds in a short space of time or producing rhizomes or runners) and covering the ground.  This same process has taken place on Lydia’s plant pot in Wastwater: the “weeds” in this case are scattered thin filaments of the blue-green alga Phormidium, the diatoms Achnanthidium minutissimum and Gomphonema parvulum plus a number of spherical green and blue-green cells that she couldn’t identify.   Such is the scale that we are working at that this open landscape still contains about 92000 cells per square centimetre.

The microbial world of the littoral zone of Wastwater after three weeks of colonisation.   The composition is similar to that in the previous figure but the density of cells is greater.

When she came back a week later, much of the empty space had been infilled; there were now about 300,000 cells per square centimetre.  These mostly belonged to the same species that she had found the week before.  The difference is that they are now rubbing up against each other and this has some important consequences.  All plants need light and nutrients to grow and algae are no exceptions.   Sunlight provides the energy for photosynthesis but now, at week three, the density of algae is such that there is a chance that some of the light will be intercepted by a neighbouring cell.   The total amount of sunlight that filters through the water to the pot surface is already much lower than that available at the lake surface; now it has to be shared out between many more cells.   At this point, properties such as fast growth rates that helped our pioneers to colonise the plant pot become less relevant, and it is algae that are better adapted to capturing the limited light that will survive.

So when Lydia came back to Wastwater after six weeks, she saw a very different community of algae on her pots.   There was still a lot of Achnanthidium minutissimum, but rising above these was the elegant art deco shape of Gomphonema acuminatum (also found in Lake Popovo) which, importantly for our story, grows on a long stalk.  There are also cells of “Cymbella affinis” (the correct name at the time that Lydia was working but see comments in the previous post about the nomenclatural history of this species).   This, too, grows on a long-stalk, the better to grow above the Achnanthidium and other pioneers.   If we continue to use the analogy of a patch of wasteland, then it has now reached the point where it has been invaded by shrubs such as hawthorn and blackthorn.   However, in a terrestrial habitat this would happen two or three years after the first pioneers had arrived, not six weeks as Lydia had observed for the algae.   She also found the diatom called Tabellaria flocculosa which forms filaments.  These often start out loosely-attached to the substratum but more often break free and become entangled around the other algae.   In our “wasteland” analogy, these would be the brambles.

The microbial world of the littoral zone of Wastwater after five weeks of colonisation.  Gomphonema acuminatum, “Cymbella affinis” and Tabellaria flocculosa have now joined the assemblage seen in the two earlier dioramas.

The experiment finished shortly after this, terminated when the apparatus was overturned.  Whether by a wave or by vandalism, Lydia will never know but this event is, itself, a metaphor for the harsh world in which benthic algae have to survive.  In real life, the many cobbles in the littoral zone will be rolled by wave action or, as we have seen in other posts, invertebrate grazers could have removed much of the “shrubbery”, leaving a “pasture” composed of the tough, fast-growing species such as Achnanthidium minutissimum to dominate samples.   The “successions” we see in the microscopic world not only take place much more quickly than those in the macro world, but they also rarely have a stable “climax”: just a brief pause before the next onslaught from the physical, chemical and biological processes that shape their existence.

References

King, L., Barker, P. & Jones, R.I. (2000). Epilithic algal communities and their relationship to environmental variables in lakes of the English Lake District. Freshwater Biology 45: 425-442.

King, L., Jones, R.I. & Barker, P. (2002). Seasonal variation in the epilithic algal communities from four lakes of different trophic state. Archiv für Hydrobiologie 154: 177-198.

Diatom hunting in the Pirin mountains

I started 2018 peering down my microscope at a sample that I collected whilst in Bulgaria back in the summer.   I have written about my trip to the Pirin mountains before (see “Desmids from the Pirin mountains”) but the diatom sample that I collected from Lake Popovo had remained unexamined since I got back.

I had waded into the littoral zone of this steep-sided corrie lake and picked up a few of the smaller stones, which I had then scrubbed with the toothbrush stowed in my rucksack to remove the thin film of diatoms.  These, like most of the algae that I collect on my travels, get treated to a bath in local spirits to ease the journey back to the UK.  This is not an ideal preservative for soft-bodied algae but is not a problem when your primary interest is diatoms with their tough silica cell walls.  Once I got back, I had them prepared and mounted ready for inspection, but then got distracted by other things and have only just got around to having a proper look.

The two most abundant taxa were the Achnanthidium minutissimum complex (probably at least three species) and Cymbella excisiformis.  Together, these constituted over eighty per cent of all the diatoms in the sample.  Ten years ago, I would have called the organism I was calling Cymbella excisiformis by a different name, Cymbella affinis, but opinions have shifted more than once.  The original Diatomeen im Süsswasser-Benthos von Mitteleuropa has images of C. affinis that are actually C. tumidula, and also describes C. excisa as a separate species.   However, the most recent view is that C. affinis and C. excisa are two names for the same species, with C. affinis taking precedence.   To confuse matters yet further, the population illustrated below shows a gradation of features from “C. affinis” to “C. excisiformis”, suggesting that the use of length:width as a discriminating factor is over simplistic.  Krammer tried to explain his rationale for distinguishing between these species in his 2002 monograph but he uses the name “C. excisa” for the organism called “C. affinis” in our 2017 English edition.  Confused?  You will be ….

Cymbella excisiformis” from Lake Popovo, Pirin Mountains, Bulgaria, August 2017.   Based on Lange-Bertalot et al. (2017)’s criteria of length:breadth 4.2-5.3 in C. excisiformis compared to 3.1 – 3.8 in C. affinis, images a., b. and c. are C. excisiformis whilst d., e., f. and g. are C. affinis.   Scale bar: 10 micrometres (= 1/100th of a millimetre).

This is another good example of points that I have made several times before: that we should always try to identify populations rather than single cells, and that we should treat dimensions stated in the literature as indicative rather than definitive (see “More about Gomphonema vibrio”).    Length:width, in particular, can change a lot during the life-cycle of the diatom.

Species of Gomphonema were also present in the sample.  Though not numerically abundant (none constituted more than one per cent of the total count), they included some large cells which, in addition, have extensive mucilaginous stalks, so their contribution to total biomass is greater than their low abundance suggests.   I’ll write more about the ecology of these species in the next post.   Finally, I also found other Cymbella species, as well as some Encyonema and Encyonopsis, and a few valves of Eucocconeis flexella, a relative of Achnanthidium and Cocconeis which has a distinctive diagonal raphe.

Gomphonema spp. from Lake Popovo, Pirin mountains, Bulgaria, August 2017.   h., i.: G. acuminatum; j.: G. truncatum; k.: unidentified girdle view; l.: G. pumilum.  Scale bar: 10 micrometres (= 1/100th of a millimetre).

There is, at this point in time, no official Bulgarian method for assessing the ecological status of lakes using diatoms so I have evaluated Lake Popovo as if it were a low alkalinity lake in the UK instead.  Using the method we developed, this one sample has an Ecological Quality Ratio of 0.92, which puts it on the border between high and good status.   Looking around the lake, I see no reason why it should not be firmly in high status but, at the same time, I am using an evaluation method that was designed for lakes 2000 kilometres away, so maybe we should not expect perfect results.    However, I have performed similar exercises at other lakes far from the UK and also got similar results (see “Lago di Maggiore under the microscope”) which points to a basic robustness in this approach.

The outflow of Lake Popovo leads into a cascade that ends in the first of a series of lakes, the “Fish Popovski” lakes.   I wrote about the desmids in this lake back in September (see “”Desmids from the Pirin mountains”) and will return to this sample in order to describe the diatoms in another post.   But, meanwhile, the assemblage at Popovo reminded me of the littoral algae in another lake that I really should tell you about …

Miscellaneous diatoms from Lake Popovo, Pirin mountains, Bulgaria, August 2017.   m.: Cymbella sp.; n.: Encyonema neogracile; o. and p.: Eucocconeis flexella (raphe valve and girdle view respectively).  Scale bar: 10 micrometres (= 1/100th of a millimetre).

Lake Popovo, photographed from close to the location from which my sample was collected.  The brass plate on the rock at the right hand side gives the altitude as 2234 metres above sea level.  The photograph at the top of the post shows Lake Popovo against a backdrop of the Pirin mountains.

References

Hofmann, G., Werum, M. & Lange-Bertalot, H. (2011).   Diatomeen im Süßwasser-Benthos von Mitteleuropa. A.R.G. Gantner Verlag K.G., Rugell.

Krammer, K. (2002).  Diatoms of Europe volume 3: Cymbella.   A.R.G. Gantner Verlag K.G., Ruggell, Germany.

Lange-Bertalot, H., Hofmann, G., Werum, M. & Cantonati, M. (2017).   Freshwater Benthic Diatoms of Central Europe: Over 800 Common Species Used In Ecological Assessment (edited by M. Cantonati, M.G. Kelly & H. Lange-Bertalot).   Koeltz Botanical Books, Schmitten-Oberreifenberg.

The UK lake diatom assessment method is described in:

Bennion, H., Kelly, M.G., Juggins, S., Yallop, M.L., Burgess, A., Jamieson, J. & Krokowski, J. (2014).  Assessment of ecological status in UK lakes using benthic diatoms.  Freshwater Science 33: 639-654.

Details of the calculation can be found in the UK TAG method statement.

The multiple dimensions of submerged biofilms …

My recent dabbling and speculation in the world of molecular biology and biochemistry (see “Concentrating on carbon …” and “As if through a glass darkly …”) reawakened deep memories of lectures on protein structure as an undergraduate and, in particular, the different levels at which we understand this.   These are:

  • Primary structure: the sequence of amino acids in the polypeptide chain;
  • Secondary structure: coils and folds along the polypeptide chain caused by hydrogen bonds between peptide groups;
  • Tertiary structure: three-dimensional organisation of protein molecules driven by hydrophobic interactions and disulphide bridges; and,
  • Quaternary structure: the agglomeration of two or more polypeptide groups to form a single functional unit.

This framework describes journey from the basic understanding of the nature of a protein achieved by Frederick Sanger in the early 1950s, to the modern, ore sophisticated awareness of how the structure determines their mode of action. I remember being particularly taken by a description of how sickle cell anaemia was caused by a change of a single amino acid in the haemoglobin molecule, altering the structure of the protein and, in the process, reducing its capacity to carry oxygen.

There is a metaphor for those of us who study biofilms here. To borrow the analogy of protein structure, the basic list of taxa and their relative abundance is the “primary structure” of a biofilm. Within this basic “name-and-count” we have various “flavours”, from diehard diatomists who ignore all other types of organisms through to those who go beyond counting to consider absolute abundance and cell size in their analyses. Whatever their predilection, however, they share a belief that raw taxonomic information, weighted in some way by quantity, yields enough information to make valid ecological inferences. And, indeed, there are strong precedents for this, especially when the primary goal is to understand broad-scale interactions between biofilms and their chemical environment.

But does this good understanding of the relationship between biofilm “primary structure” and chemistry comes at the expense of a better understanding of the inter-relationships within the biofilm. And, turning that around, might these inter-relationships, in turn, inform a more nuanced interpretation of the relationship between the biofilm and its environment? So let’s push the metaphor with protein structure a little further and see where that leads us.

The “tertiary structure” of a submerged biofilm: this one shows the inter-relationships of diatoms within a Didymosphenia geminata colony.  Note how the long stalks of Didymosphenia provide substrates for Achnanthidium cells (on shorter stalks) and needle-like cells of Fragilaria and Ulnaria.   You can read more about this here.  The image at the top of the post shows a biofilm from the River Wyle, described in more detail here.

We could think of the “secondary structure” of a biofilm as the organisation of cellular units into functional groups. This would differentiate, for example, filaments from single cells, flagellates from non-flagellates and diatoms that live on long stalks from those that live adpressed to surfaces. It could also differentiate cells on the basis of physiology, distinguishing nitrogen-fixers from non-nitrogen fixers, for example. We might see some broad phylogenetic groupings emerging here (motility of diatoms, for example, being quite different from that of flagellated green algae) but also some examples of convergence, where functional groups span more than one algal division.

Quite a few people have explored this, particularly for diatoms, though results are not particularly conclusive. That might be because we cannot really understand the subtleties of biofilm functioning when information on every group except diatoms has been discarded, and it might be because people have largely been searching for broad-scale patterns when the forces that shape these properties work at a finer scale. General trends that have been observed include an increase in the proportion of motile diatoms to increase along enrichment gradients. However, this has never really been converted into a “take-home message” that might inform the decisions that a catchment manager might take, and so rarely form part of routine assessment methods.

Next, there is a “tertiary structure”, the outcome of direct relationships between organisms and environment, interdependencies amongst those organisms to form a three-dimensional matrix, and time. This is the most elusive aspect of biofilm structure, largely because it is invariably destroyed or, at best, greatly distorted during the sample collection and analysis phases. This has been little exploited in ecological studies, perhaps because it is less amenable to the reductive approach that characterises most studies of biofilms. But I think that there is potential here, at the very least, to place the outcomes of quantitative analyses into context.  We could, in particular, start to think about the “foundation species” – i.e. those that define the structure of the community by creating locally stable conditions (see the paper by Paul Dayton below).   This, in turn, gives us a link to a rich vein of ecological thinking, and helps us to understand not just how communities have changed but also why.

The tertiary structure of a Cladophora-dominated biofilm from the River Team, Co. Durham.  Cladophora, in this case, functions as a “foundation species”, creating a habitat within which other algae and microorganisms exist.   You can read more about this in “A return to the River Team”.

Finally, if we were looking for a biofilm “quaternary structure” we could, perhaps, think about how the composition at any single point in space and time grades and changes to mould the community to favour fine-scale “patchiness” in the habitat and also to reflect seasonal trends in factors that shape the community (such as grazing).   Biofilms, in reality, represent a constantly shifting set of “metacommunities” whose true complexity is almost impossible to capture with current sampling techniques.

Some of this thinking ties in with posts from earlier in the year (see, for example, “Certainly uncertain”, which draws on an understanding of tertiary structure to explain variability in assessments based on phytobenthos communities).  But there is more that could be done and I hope to use some of my posts in 2018 to unpick this story in a little more detail.

That’s enough from me for now.  Enjoy the rest of the festive season.

Selected references

Foundation species:

Dayton, P. K. (1972). Toward an understanding of community resilience and the potential effects of enrichments to the benthos at McMurdo Sound, Antarctica. pp. 81–96 in Proceedings of the Colloquium on Conservation Problems Allen Press, Lawrence, Kansas.

“secondary structure” of biofilms

Gottschalk, S. & Kahlert, M. (2012). Shifts in taxonomical and guild composition of littoral diatom assemblages along environmental gradients.  Hydrobiologia 694: 41-56.

Law, R., Elliott, J.A., & Thackeray, S.J. (2014).  Do functional or morphological classifications explain stream phytobenthic community assemblages?  Diatom Research 29: 309-324.

Molloy, J.M. (1992).  Diatom communities along stream longitudinal gradients.  Freshwater Biology, 28: 56-69.

Steinman, A.D., Mulholland, P.J. & Hill, W.R. (1992).  Functional responses associated with growth form in stream algae.  Journal of the North American Benthological Society 11: 229-243.

Tapolczai, K., Bouchez, A., Stenger-Kovács, C., Padisák, J. & Rimet, F. (2016).  Trait-based ecological classifications for benthic algae: review and perspectives.  Hydrobiologia 776: 1-17.

“tertiary structure” of biofilms

Bergey, E.A., Boettiger, C.A. & Resh, V.H. (1995).  Effects of water velocity on the architecture and epiphytes of Cladophora glomerata (Chlorophyta).  Journal of Phycology 31: 264-271.

Blenkinsopp, S.A. & Lock, M.A. (1994).  The impact of storm-flow on river biofilm architecture.   Journal of Phycology 30: 807-818.

Kelly, M.G. (2012).   The semiotics of slime: visual representation of phytobenthos as an aid to understanding ecological status.   Freshwater Reviews 5: 105-119.

More about Gomphonema vibrio

Gomphonema vibrio is part of a complex of species that has only begun to be unravelled in the past few years.   In the first edition of the Süsswasserflora von Mitteleuropa in 1930, Hustedt included it as one of three varieties of G. intricatum, along with G. pumilum and G. dichotum.  By the time of the second edition (1986), however, Krammer and Lange-Bertalot had subsumed G. intricatum into G. angustum, creating a single species that spanned an enormous range of size (see their Plate 164 if you don’t believe me).   A few years later they revised this opinion, and unpicked the G. angustum complex, reinstating several of the taxa that they had originally subsumed and also recognising some more recently described species (many by Erin Reichardt).   There may well be more changes to come as this group has not yet been subjected to critical study by molecular geneticists.

One of the other species in this melange is Gomphonema pumilum, a much smaller diatom that is common in both running and standing waters (Hustedt’s comment on the species complex only referred to a preference for “stagnant waters”).   We have met it a few times previously (see, for example, “Pleasures in my own backyard”) and I also found it in a 1999 sample from Croft Kettle whilst searching for G. vibrio.   However, I then turned to an older slide, based on a sample collected in 1872 and given to me by John Carter (see “Remembering John Carter”).   This had some cells of G. pumilum but also some that exceeded the quoted dimensions for G. pumilum (length: 12 – 36 mm; width: 3.5 – 5.5 mm) and which fell within the size range for G. vibrio.   I suspect that we are, in fact, dealing with a mixture of the two species and if this is a common situation then it may explain why Hustedt had difficulties unpicking the two species.   When I arranged the images of G. vibrio and G. pumilum that I found in this sample in order of diminishing size, there is a continuum between the two forms.  We now know that width is a better discriminator than length and, armed with this, we can see a difference between the two species. But that is one of the benefits of hindsight.

Gomphonema pumilum from Croft Kettle, May 1999.  a. – e.: valve views; f., g.: girdle views.   Scale bar: 10 micrometres (= 100th of a millimetre).

Gomphonema vibrio (h. – k.) and G. pumilum (l. – m. [and n.?]) from “Hell Kettles”, 1872.  Scale bar: 10 micrometres (= 100th of a millimetre).

This raises a question about the reliability of the size ranges quoted in the literature   A couple of the smaller valves of G. vibrio were less than 7 mm wide.  Yet, in other respects, they were more similar to the “true” G. vibrio valves than to those of G. pumilum.  The answer will vary from species to species but, as a general rule, we should not be too bothered if the extremes of a population stray a little beyond the values quoted in the literature.   These are usually based on the largest and smallest cells found in a thorough scan of one or more populations, but not necessarily on observations of an initial cell (the largest in a population) or of cells at the point immediately before sexual reproduction is initiated (the smallest).  We simply don’t have that information for most species so, as a result, should be prepared to accept larger and smaller valves into a species if they are qualitatively similar to, and quantitatively part of a continuum with, the rest of the population.  My post “Diatoms and the Space-Time Continuum”, also on Gomphonema, offers some further insights into this story.

Reference

Hustedt, F. (1930).  Susswasserflora von Mitteleuropa 10: Bacillariophyceae.  Gustav Fischer, Jena.

Krammer, K. & Lange-Bertalot, H. (1986). Susswasserflora von Mitteleuropa 2: Bacillariophyceae. 1 Teil: Naviculaceae.  Spektrum Akademischer Verlag, Heidelberg.

Krammer, K. & Lange-Bertalot, H. (1991). Susswasserflora von Mitteleuropa 2: Bacillariophyceae. 4 Teil: Achnanthaceae. Kritische Ergänzungen zu Achnanthes s.l., Navicula s.str., Gomphonema. Spektrum Akademischer Verlag, Heidelberg.

Reichardt, E. (1997).  Taxonomische revision des Artencomplexes um Gomphonema pumilum (Bacillariophyceae).  Nova Hedwigia 65: 99-129.

Reichardt, E. & Lange-Bertalot, H. (1991).  Taxonomische revision des Artencomplexes um Gomphonema angustum – G. intricatum – G. vibrio und ähnliche taxa (Bacillariophyceae).  Nova Hedwigia 53: 519-544.

Note

In my post on Gomphonema rhombicum, I mentioned that the location on the type slide is given as “Appleby”, which was not very precise.   My 1872 slide is labelled “Hell Kettles, Durham”.  “Hell Kettles” is the name for the pair of ponds, of which Croft Kettle, which I described in my earlier post, is the larger.   However, the location “Durham” is not very illuminating.   The closest town to Croft Kettle is Darlington, whilst Durham City is 40 km to the north.   “Durham”, in this context, could refer to the county, which covers 2721 square kilometres and habitats from calcareous ponds such as these to moorland pools.   A slide label offers very little space to give precise details of location but, in both these cases, a little more information would be useful.   The likelihood is that Firth had more detailed notes elsewhere but these have been lost over time, so we are left with these scant words.   There is a lesson here for all of us in how we record the meta-data that accompanies our samples.

Meetings with remarkable Gomphonema …

Having written about Gomphonema rhombicum in my previous post, I thought it would be worth staying with Gomphonema and showing some images of G. vibrio.   This is a diatom that I had rarely encountered previously but which cropped up in separate email conversations with Chris Carter and Geoff Phillips in the space of a couple of months.  Chris’ samples come from a small man-made pond at Yardley Chase, an SSSI in Northamptonshire (photographed above), whilst Geoff’s was from Phragmites stems in a Norfolk marsh dyke.  Both have hard water (Geoff’s location: pH: 7.6; alkalinity: 275 mg L-1 CaCO3; conductivity: 700 mS cm-1) and good water quality (TP: 60 mg L-1; TN: 1.5 mg L-1).   This set of conditions prompted me to dig out some samples from Croft Kettle, a location I wrote about a couple of years ago (see “The desert shall rejoice and blossom …”) where I had a vague memory of having seen something similar.

Valves of Gomphonema vibrio are relatively large (30 – 95 x 7 – 10 mm, according to Hofmann et al., 2017) and club-shaped with a slight swelling at the centre.  Overall, the valves are more slender than was the case for G. rhombicum (see illustrations in the previous post).   The striae are coarse (7 – 10 in 10 mm) and mostly radiate, but there is a distinct central area where there is a single stria on each side more distantly spaced from the adjacent striae than in the rest of the valve.  On one side, this stria is very short (sometimes it can be hard to see); on the other side, it is longer and ends with a distinct stigmoid (an isolated pore).    The central endings of the raphe are often turned to the same side.

Cleaned valves of Gomphonema vibrio from a pond at Yardley Chase, Northamptonshire.  Yardley Chase is shown in the image at the top of the post.   Images are in pairs, each at a slightly different focus plane.   All photos by Chris Carter.

Chris also sent me some photographs of the living cells, showing a clear stalk protruding from the narrower “foot” pole, as well as a beautifully-clear H-shaped chloroplast.  The presence of a stalk in this species just doubles my annoyance at not having checked for the same in G. rhombicum before cleaning the valves.

There are, it seems, remarkably few records of Gomphonema vibrio from the UK.  I can find no other records from rivers and Helen Bennion found just two other records of recent samples in the UCL database, both from Scotland: Loch Levan and Loch Davan.  Three of the five records are from ponds, which may be significant, and two of these were epiphytes, though there are not enough records here to make any firm pronouncements about habitat preferences.  However, the picture that is emerging is of a species that definitely has a preference for moderately hard to hard water with relatively low nutrients. If that is the case, then it could well be a species that used to be more common that it is now, as many habitats such as these will have deteriorated in recent decades due to agricultural enrichment.   It is certainly a very different habitat from the soft water, fast-flowing stream from which I recorded G. rhombicum in Bulgaria.

Live cells of Gomphonema vibrio from a pond at Yardley Chase, Northamptonshire.  Photos by Chris Carter. 

That makes a total of five records from the UK which, even allowing for the muddled taxonomy (which I’ll talk about in the next post) and the fact that the diatoms of small ponds are rarely studied, suggests that this may be a genuinely rare. It is listed as an “endangered species with persistent risk factors” on the German red list, with a forecast of further decline over the next ten years.   I’ve voiced my concerns about “rarity” and red lists before (see “A red list of endangered British diatoms?”) but will stick my neck out on this one and suggest that Gomphonema vibrio might be a candidate.

Reference

Lange-Bertalot, H., Hofmann, G., Werum, M. & Cantonati, M. (2017).   Freshwater Benthic Diatoms of Central Europe: Over 800 Common Species Used In Ecological Assessment (edited by M. Cantonati, M.G. Kelly & H. Lange-Bertalot).   Koeltz Botanical Books, Schmitten-Oberreifenberg.

In pursuit of Bulgarian diatoms …

Our visit to Rila Monastery was one of the cultural highlights of our trip to Bulgaria in the summer (see “The art of icons …”) with the added bonus that it is set amidst some spectacular scenery.   The monastery was founded by St Ivan Rilski (“St John of Rila”) in the 10th century, though the present structures date from the 19th century.   St Ivan, I gleaned from my reading, shared a love of nature with St Francis and St Cuthbert and Rila’s remote location reflects his desire to live the life of a hermit.

Beyond the monastery the road twists and turns amidst the forest that lines the valley.   Occasionally, the canopy is broken by meadows and orchards of apricot and plum trees.  The apricots were ripe for harvest and at an easy height for foraging as we passed by.   We were too late in the year to find many wild flowers in these meadows but the absence of any of the paraphernalia associated with intensive agriculture made us suspect that these would have been a riot of colour in the spring.    Eventually, we came to a small hamlet set amidst a wider area of meadows, where we refuelled at a small café serving the delicious local bean soup (“bob chorba”) before making our way across the meadow to the stream.

A haystack in a meadow near Rila Monestery, August 2017.

The Rila stream itself flowed through a densely-wooded channel, with a variety of substrates from coarse sand to moss-covered boulders.   The larger stones were mostly granite, reflecting the underlying geology of the region, which almost certainly means that the water was very soft.  The surrounding vegetation and low population density also mean that the water is probably as pure as we are likely to find anywhere in Europe.   I had a toothbrush and sample bottle in the bottom of my rucksack and scrubbed a few cobble-sized stones to remove the thin surface film and stowed this for the journey back down the valley.

Sampling Rila stream at Kirilova meadows, a few kilometres upstream from Rila Monestery in August 2017.   The photograph at the top of the post shows the scenery around Kirilova meadows.

It has taken until now for me to get around to looking at this sample, which turned out to have a large population of Gomphonema rhombicum, along with quite a lot of Achnanthidium minutissimum and relatives, confirming my suspicion of a circumneutral, low nutrient environment.   G. rhombicium is a relatively uncommon diatom, so I was intrigued to have a closer look.  It has a similar outline to G. pumilum, which is very common, but is larger and has a distinct broad lanceolate axial area and, consequently, relatively short striae.   The axial area broadens out a little further at the central area, where there is also a single stigmoid.  I wish now that I had had a look at the sample before digestion as many of the larger Gomphonema species have long mucilaginous stalks, whereas G. pumilum and relatives tend to be attached to the substrate by short mucilaginous pads.   I can find nothing in the literature that alludes to the habit of the living cell and, on this particular occasion, am in no position to judge the shortcomings of my peers.

By coincidence, the type location for Gomphonema rhombicum is given as “Appleby, Westmoreland”, which is just over an hour’s drive from where I live.   The most obvious place to hunt for G. rhombicum would be the River Eden; however, the scant details on the ecology of G. rhombicum that I can find suggest a preference for softer water than found here.  This is a geologically-complex area so there is a possibility of suitable habitat existing in a stream in the vicinity.  If the Eden was the location from which the original population of G. rhombicum was collected, then I suspect that it may have been a casualty of the agricultural intensification that has taken place in this area since it was first described.

Cleaned valves of Gomphonema rhombicum from Rila stream at Kirilova meadows, August 2017.  a. – g. show valve views; h. shows a girdle view.  The scale bar is 10 micrometres (= 100th of a millimetre).

That leaves me with just one option: a return to the Rila National Park.  I think I can just about live with that.   We stayed at Hotel Pchelina, a few kilometres down the valley from Rila Monestery and our dinner that evening was the most delicious grilled trout that I have ever tasted.  If a return visit was needed to solve the mystery of Gomphonema rhombicum’s habit then I think that is a hardship with which I can just about cope …

Reference

Iserentant, R. & Ector, L. (1996).  Gomphonema rhombicum M. Schmidt (Bacillariophyta): typification et description en microscopie optique.   Bulletin Français de la Pêche et de la Pisciculture 341/342: 115-124.