Desmids from the Pirin mountains

Our travels in southern Bulgaria took us south from Rila Monastery to Bansko, on the edge of the Pirin Mountains and, from here, via a chair lift and a rather more strenuous walk than we had expected to Popovo Lake  situated in a corrie overlooked by rugged mountains soaring up to over 2800 metres.

The outflow from the lake cascades over the lip of the corrie and down the hillside to a series of smaller lakes, before merging with some other small streams to form the Mesta river which flows south from Bulgaria and through northern Greece to the Aegean Sea.   The footpath that took us back from Popovo took a gentler route down the hillside but brought us close enough to the first of this series of lakes for the bright green areas at the margins to pique our interest.  Getting closer, we found ourselves on soft, yielding Sphagnum bog, more familiar to us from the moorlands of northern England (see, amongst other posts, “Back to the bog“) than in southern Europe.   A lot of the rock that we had passed on our hike up from the chair lift terminus had been granite, so the water around us would clearly have been soft enough for Sphagnum and, I guess, the marshy land was testimony to a higher level of precipitation than the cloudless skies that we encountered suggested.

The first of the “Fish Popovski” lakes below Popovo Lake in the Pirin Mountains, southern Bulgaria, with the marshland area clearly visible in the foreground.

One of the pools in this bog attracted my attention: a mat-like portion of the substratum, had floated to the surface whilst still being loosely attached at one corner.   This is a good clue that the substratum is jam-packed with algae, doing a double job of binding the silty particles together into a cohesive whole and, at the same time, pumping out oxygen as a by-product of photosynthesis in order to make the mat buoyant.   I last wrote about this in 2013 when I found some mats of Oscillatoria limosa in my local river (see “More from the River Wear”).   The same phenomena seem to be at play here although, on closer investigation, it was desmids rather than Cyanobacteria which were responsible for the mat.   There were, in fact, far fewer filamentous algae in this particular mat, something that I quickly noticed as there was very little physical integrity to the mat, and it dissolved into a suspension of fine particles as soon as I tried to remove a piece.   Had I stayed until nightfall, I expect the mat would have gradually sunk again as the rate of photosynthesis declined and oxygen production ceased.

The desmid mat floating up from the bottom of the bog pool beside the first of the Fish Popovski lakes in the Pirin Mountains.

The most abundant desmids in the mat were Hyalotheca dissilens and Closterium baillyanum.  The former is a filamentous desmid, whose chains of cells are enclosed in a broad mucilage sheath and, whilst there were many fewer of these filaments than I have seen in more cohesive mats, I suspect that these played a role in trapping the other algae, plus organic and inorganic particulate matter to form the structure that I saw.  Closterium baillyanum, by contrast, has large, robust cells and, in this case, the cell wall has a distinct brown colour.  Other desmids that I found in my brief examination included Tetmemorus granulatus, which has cylindrical cells with a narrow incision in the broadly rounded apex (most clearly visible at the left-hand side of the illustration) and two species of Euastrum: E. humerosum and E. ansatum.  There were also a few large cells of Eremosphaera, a green alga though not a desmid (see “More from Loughrigg Fell”), and various assorted unicellular algae.  I’ll write about the diatoms in a separate post.

Some common desmids from the Pirin mountains: a. Hyalotheca dissilens; b. Closterium baillyanum prox.; c. Tetmemorus granulatus.  Scale bar: 25 mm (= 1/40th of a millimetre).

Unfortunately, the chloroplasts in these illustrations are not at their best.   My preference is always to keep algal samples fresh for as long as possible but, as I was moving around southern Europe in August I adopted my usual practice when travelling and added some vodka to the sample as a temporary preservative.   Nonetheless, Dave John, who identified the species, could find enough in the general morphology and cell wall characteristics to guide him.   Desmid species are considered to be cosmopolitan so he was able to use identification literature from Northwest and Central Europe in order to do this.   The search terms “desmid” and “Bulgaria” yielded no papers when I looked on Web of Science, so this looks like a relatively unexplored corner of Europe, as far as this group is concerned.   Having said that, all five of the species illustrated here are listed in the checklist of Romanian algae, and it is quite likely that there are local publications that have not made it onto the major bibliographic databases.

Incidentally, it was only after I had bought a miniature of vodka that I realised that I should have used this as an opportunity to buy a bottle of rakija, the local spirit.  For some reason, I had assumed that this was an ouzo-type spirit and that it would give the water an opaque milky-white appearance (caused by the lower solubility of essential oil of anise in water compared to alcohol).   Experimental studies later that same evening showed that the local rakija was closer to the Romanian ţuică, prepared from grapes, plums or apricots (the latter is especially good), and would have made a fine preservative.   I am older and wiser although, in the immediate aftermath of my experiment, that wisdom may not have been immediately apparent.

More desmids from the Pirin mountains: d. Euastrum humerosum; e. E. ansatum. Scale bar: 25 mm (= 1/40th of a millimetre).


Cărăuş, I. (2017). Algae of Romania.  A Distributional Checklist of Actual Algae.  Version 2.4.  Original print edition published by University of Bacău, Romania.  Latest version available online []


How to be an ecologist #4

The British university system somehow manages to train ecologists despite two major impediments: most universities are in cites and all teach during the periods of the year that are least enticing for the budding field scientist.   Together, these factors work as two strong selective factors: getting thoroughly damp in a forest in Hampshire on a wet Sunday (because it is easier to timetable a whole day trip at the weekend), following a two hour coach trip, is a fine way of weeding out those with romantic notions of ecology gleaned from David Attenborough documentaries.

Then, of course, there is the residential field course, where students are immersed in an alien environment for an entire week (two, in some cases). These days, field trips seem to function as part of the marketing blurb in the prospectus, with departments vying with each other to offer the most exotic location. It makes as much sense to the customer (and that is what students are, let’s be honest) as a financial advisors offering a free pen to anyone who signs up to a pension plan.   Back then, we were taken, by boat, to Blakeney Point in Norfolk, and dumped outside the wooden hut that served as University College, London’s field station. We camped amongst the sand dunes and spent the days learning about the saltmarsh and sand dune plants under the tutelage of Dickie Clymo.

I still contend that you can learn more in a week about plant ecology on a British salt marsh or sand dune than in twice that time in a more exotic habitat.   Sand dunes and salt marshes have the additional benefit of having only a limited number of species, which means that we were able to focus on why they grew where they did rather than having to memorise long lists of names.   But then I do not have to recruit students for a degree course.


Left: saltmarsh vegetation at Blakeney Point, Norfolk, October 2011; right: close up of Suaeda maritima (Annual Sea Bite) from Blakeney Point.

I also learned a second very important lesson about ecology: that week among the sand dunes generated enough data to keep our ecology class busy through the winter months, as we analysed the data in all sorts of ways.   I learned multivariate statistics – ordination and classification – the slow way, crunching the numbers by hand.   The process worked, I think, because we could relate the graphs we produced to the communities that we had seen just a few months earlier.   I found Dickie Clymo’s way of teaching ecology sufficiently inspiring that I opted for a plant ecology-based project in my final year, and Dickie set me to work on his favourite type of habitat (the peat bog) and plant (Sphagnum moss).

I dug my dissertation out of the loft to see how it had stood the test of time as I was thinking about the post and memories came rushing back. My aim was to look at how the density of Sphagnum plants in a peat bog affected their properties.   I had read parts of John Harper’s book Population Biology of Plants during Dickie’s ecology course and was interested in how organisms competed with one another.   Bogs are seemingly prosaic habitats but, like the salt marshes where we had honed our ecological skills, they were excellent testing grounds for new ideas.


Sampling Sphagnum at Thursley Common, Surrey, early in 1983.

I made two visits to lowland bogs in southern England, Cranesmoor in Hampshire and Thursley Common in Surrey, pushed a metal quadrat with 3 cm sides into the top of the bog at various locations, and removed 1 dm2 squares of Sphagnum to take back to the laboratory. There, I counted the number of shoots and measured the length and mass of the green parts of the shoots. It was slow, painstaking work, which I slotted around my other lectures during my final year at Westfield, sitting in one corner of a research laboratory that Dickie’s PhD student shared with Brenda Thake’s students.   I also tried to look at the amount of chlorophyll in Sphagnum plants, and have a scar from pushing a pipette bulb onto a pipette with so much force that the pipette snapped and drove into my finger. This happened on a Sunday afternoon, when I was working in the laboratory on my own.   Back in those unenlightened days, I can’t even remember filling in an accident form.


The effect of density on shoot mass in different species of Sphagnum from bogs in southern England. Pearson correlation coefficient, r = 0. 75, P < 0.001. From my undergraduate dissertation.

The first graph that I have included makes a point that may be rather obvious to anyone who has grown vegetables – that the greater the density of shoots, the smaller each shoot tends to be.   They are competing with each other for resources and, the more plants that there are, the fewer resources each shoot can acquire.   Having made that point, you’ll see that species that tend to be found on the hummocks in peat bogs – such as S. capillifolium – tend to be smaller than those found in bog pools – such as S. subsecundum and S. cuspidatum.   The crowding may actually favour the former as they rely on capillary action to draw up the water that they need from the bog.

However, each shoot of Sphagnum is not really an independent “plant”, simply one of many genetically identical clones (called “ramets”) of a much larger organism.   The individual, in the sense that you and I are unique beings with a distinct genetic identity (a “genet”), is hard to distinguish from it’s neighbours; however, we can infer the response of the individual to density by looking at the frequency with which branches fork (leading to an increase in the size of the genet.   Once again, there was a linear relationship to density, with the greatest frequency of forks observed at low densities, suggesting that density was influencing the size of genets, as well as ramets. Of particular interest to me was that the slope of the relationship (ignoring the two pool species) was -1.42, which is close to -1.5, the theoretical relationship between density and the size of genets proposed in a 1963 paper by Japanese workers (“the -3/2 self thinning rule” which, I confess, I wrote in my lecture notes as the “three tooths law”, as if it were some arcane Japanese philosophical notion). It suggests that an element of “survival of the fittest” was working within Sphagnum populations, in ways that it was not easy to perceive.


The effect of density on the probability of a shoot of Sphagnum forking. The two points in the lower left hand corner (circled) are both pool species. Pearson correlation coefficient, r = 0.551; P < 0.05 (ignoring these two points). From my undergraduate dissertation.

Work on my dissertation took me perilously close to the start of my final examinations and I remember when Brenda Thake, phycology lecturer and unofficial mentor, took me aside and told me bluntly to finish writing and get stuck into some serious revision.   On reflection, I had reached the stage when data that has taken a long time to acquire (long enough, indeed, for you to question the validity of your original hypotheses) suddenly comes together. After all that time, there is a delirious sense of intoxication as you get stuck into the analyses and see patterns emerge.   It was my first taste of research as a vocation rather than as just another job, and it confirmed my desire to continue with my studies.

But that’s a story for another day.


Clymo, R.S. & Hayward, P.M. (1982).  The Ecology of Sphagnum.  In: Bryophyte Ecology (edited by A.J.E. Smith).  Chapman & Hall, London.

Harper, J.M. (1977). Population Biology of Plants. Academic Press, London.

Yoda, K., Kira, T., Ogawa, H. & Hozumi, K. (1963). Self thinning in overcrowded pure stands under cultivated and natural conditions. Journal of the Institute of Polytechnics, Osaka City University, Series D 14: 107-129.

Back to the bog


The microscopic world of an Upper Teesdale Sphagnum bog revisited, with diatoms and desmids living on and in the spaces between Sphagnum leaves and diatoms The chlorophyllose cells are about ten micrometres in diameter (1/100th of a millimetre) whilst the desmid in the foreground (Cosmarium ralfsii) is about 100 micrometres (1/10th of a millimetre) across.

I showed my first attempt at portrayal of the microscopic life of a Sphagnum bog a few weeks ago (see “Swimming with desmids …“) but, at the same time, I felt that there were a few elements that could be improved, so here is my second effort. The first time I tackle a new subject, there are usually technical issues to address and, perhaps, the outcome was not quite as naturalistic as I would have liked. Not that I, or anyone else, really has a great insight into “natural” in this particular context, but then none of us have seen dinosaurs hunting in Jurassic forests, but that hasn’t stopped people producing “naturalistic” illustrations.   In my first picture, just capturing the underside of a Sphagnum leaf in something approaching linear perspective and including two desmids seemed like progress. This time, there are two Sphagnum leaves plus a couple of diatoms – a single cell of Eunotia implicata on the underside of one leaf, plus a couple of cells of Tabellaria flocculosa on the other leaf. Both of these specimens were present when I made my initial observations of the Sphagnum leaves back in December.

One additional issue that the composition of this picture raised, is that the morphology of the upper surface of a Sphagnum leaf differs from that of the lower surface. This relates to the relative size of the chlorophyllose and hyaline cells (see the schematic diagram in Swimming with desmids ...). There were moments, I promise you, when I interrupted my meditations on Sphagnum morphology to wonder if I should go and get a life.

My justification, if any is needed, is that peering down a microscope and compiling data about the species present without sometimes contemplating the organisms in their natural state seems like an equally bizarre way of spending one’s life. I write this post having peer-reviewed a paper for the journal Limnology and Oceanography this afternoon. The work was quite interesting but, at the same time, I felt that it was a very sterile, technical study that had abstracted the real world into long lists of diatom species and then processed these using complicated statistical methods, without giving much sense of a real understanding of the ecosystems that they were studying.

Swimming with desmids …

My sampling trip to Upper Teesdale in search of desmids (see “Hunting for desmids in Upper Teesdale”) has now yielded another picture, this time figurative rather than semi-abstract.   I have tried to depict the world inside a Sphagnum bog so have shown two desmids underneath a canopy of Sphagnum leaves.   The Sphagnum leaves have a characteristic structure, with chlorophyllose cells alongside water-filled “hyaline” cells. The desmids live, in effect, inside a glass-roofed conservatory although I have probably conveyed an overly bright impression of the subaquatic world of the bog.   The reality is that the slow decay of Sphagnum yields brown humic materials that create an altogether murkier environment.


The microscopic world of an Upper Teesdale Sphagnum bog, with desmids living in the space underneath Sphagnum leaves.   The chlorophyllose cells are about ten micrometres in diameter (1/100th of a millimetre) whilst the desmid in the foreground (Cosmarium ralfsii) is about 100 micrometres (1/10th of a millimetre) across.

I’ve tried to illustrate the structure of a Sphagnum leaf in the diagram below.   Compare this with the photograph in “More from Upper Teesdale” (showing the view from above) the leaf to get an idea of how the leaf is constructed.   It also demonstrates why Sphagnum moss is capable of absorbing so much water: two-thirds or more of the leaf is composed of empty space and there is even a convenient pore to let the water in.


A schematic cross-section through a leaf of Sphagnum showing the arrangement of hyaline and chlorophyllose cells.   The chloropyllose cells are about 10 micrometres (1/100th of a millimetre) across.

My illustration of the microscopic world of a Sphagnum bog is a step outside my comfort zone, as I tried to combine the various elements together from separate microscopic images.   Microscopy tends to flatten perspective, partly because specimens are squashed onto microscope slides but also because of the focal length of the lenses involved.   Added to this was the problem of depicting the sinuous chlorophyllose cells in an approximation of single-point perspective.   Almost as soon as I had finished the picture, I was thinking about how I could be improving the next version. Striving towards realism is, itself, an ongoing mind experiment that offers tantalising glimpses of an otherwise hidden world.

Baffled by the benthos (1)

We encountered G. Evelyn Hutchinson in two posts last year (see “Diatoms from the roof of the world” and “The Clear Mirror”). In the second of these, I made a passing comment to the “paradox of the plankton” which he both proposed and partially resolved.  He asked “how it is possible for a number of species to coexist in a relatively isotropic or unstructured environment all competing for the same sorts of materials” when, “according to the principle of competitive exclusion … we should expect that one species alone would outcompete all the others …”   The answer, he suggested, was that the world that plankton inhabit is far less uniform than may first appear to be the case, providing opportunities for a range of organisms, each with their own ecological specialism, to thrive.

Exactly the same question could also be asked of the benthic algae which seem to show an extraordinary diversity. It is not uncommon for me to find fifty or more algal species in a sample from one habitat on one day within a river. They are all bathed in water with the same chemical composition, so why is it that one competitive species does not overgrow all the others to produce a monoculture?

Whilst I was pondering this question, I recalled a paper written by Dicky Clymo, who supervised my undergraduate dissertation. He was a specialist on Sphagnum and peat bogs and, in 1973, wrote a paper showing how variations in the values of two physical factors (water table depth and light) and two chemical factors (hydrogen and calcium ions) across a peat bog could create a mosaic of habitats that allowed twelve different species of Sphagnum to thrive.   If we assume that a typical diatom sample covers at least 10 cm2 of the surface of a submerged rock, then this would be equivalent to surveying at least a square kilometre of peat bog, providing plenty of opportunity for this level of habitat variation.

Sampling microscopic algae is limited by our own senses: a rock surface that may look smooth to our eyes may actually have a microtopography that creates a range of habitats within which different species can survive. The image below shows small cells of the diatoms Amphora pediculus and Achnanthidium minutissimum nestling in crevices on a rock surface that, presumably, offers some protection to them from marauding grazers.

Why am I making this point?   I frequently encounter comments such as the following in papers about diatoms: “poor species discrimination can lead to the combination of morphologically similar, but ecologically disparate taxa” and the suggestion that this can “compromise ecological status assessment”.   There may be situations where this is the case, but also situations where these taxa can happily co-exist in within a very small area because the ecological factors that separate them operate at a much more localised scale than the pressures we are trying to assess. Diatomists, I am afraid, are often naïve ecologists who rarely go beyond correlating species distribution patterns with a few chemical and environmental variables that are relatively easy to measure. There is still a lot that we don’t know about how the diverse assemblages of diatoms that we encounter in rivers and streams interact with the other organisms around them, let alone with aspects of their physical environment that are invisible to the naked eye.

More about this in the next post.


A scanning electron micrograph showing the surface of a rock showing Amphora pediculus (a) and Achnanthidium minutissimum (b) living in crevices. Scale bar: five micrometres (1/200th of a millimetre). Photograph: Marian Yallop.


Clymo, R.S. (1973). Growth of Sphagnum: some effects of environment. Journal of Ecology 61: 849-869.

Hutchinson, G.E. (1961). The paradox of the plankton. American Naturalist 95: 137-145.


Hunting for desmids in Upper Teesdale


Cronkley Fell from near Widdybank Farm, December 2014

We had Upper Teesdale to ourselves on Saturday morning, most fellow-walkers having been deterred, perhaps, by the strong westerly winds.   They missed some spectacular lighting as the weak December sun briefly broke through the clouds to light the Pennine fells.   The open fire in the bar of the Langdon Beck Hotel, and the bowl of hot soup, were very welcome when we finally completed our regular 13 kilometre loop.

As ever, my eyes are forever adjusting between the grand panoramic landscapes of Upper Teesdale, and the small scale botanical wonders all around us. Today, my primary interest was the desmids that inhabit the blanket bog and I diverted off the boardwalks that mark the Pennine Way’s course along the Tees to squeeze the brown water from handfuls of Sphagnum moss into my sampling tubes. The peaty-brown water that I collect usually contains a diverse assemblage of desmids, which I’m collecting to form the basis of a new painting.

I wrote about the desmids from Upper Teesdale last year (see “More from Upper Teesdale”) but since then I have upgraded the camera on my microscope and also purchased focus stacking software (see “Now … with added depth of field …”) that makes a repeat visit worthwhile.   Many of the desmids I found were the same as those in my sample from March last year, though there were a couple of strangers and the long moon-shaped cells of Closterium striolatum did not fit neatly into a single field of view.   However, a quick scan of the slide revealed half a dozen abundant species and a few that were represented by just occasional specimens as well as plenty of diatoms, other green algae and protozoans.


Squeezing Sphagnum to collect desmids in Upper Teesdale, December 2014.   The photo doesn’t really capture the reality of the 30 km/hour winds and associated wind-chill.

When I look at desmids, I’m way out of my comfort zone, but there is something about their symmetrical, often intricate outlines that is beguiling and makes me want to continue scanning the slide in search of more.   This particular sampling trip is the first step of the research for a new painting and, like diatoms, the desmids have a beauty that transcends the limits of objective science. That’s my agenda for this painting: to use the microscopic life of Upper Teesdale’s boggy pools as the counterpoint to the rugged, panoramic beauty of the landscape itself.   I could use pictures of desmids from the books I have on my shelves, but I like my pictures to have a direct link with a particular place and time. It is veracity that, perhaps, few will appreciate, but without this the end-product would just be a collection of abstract shapes.


Upper Teesdale desmids. a. Netrium oblongum; b. Micrasterias oscitans (var. mucronata); c Eurastrum didelta; d. Desmidium cf. aptogonum; e. Cosmarium ralfsii.; f. Micrasterias truncata.   Scale bar: 50 micrometres (1/20th of a millimetre).

Bollihope Common

I spent part of last weekend wandering in the vicinity of a small reservoir on Bollihope Common in Weardale.   It is one of many small manmade water bodies in this part of the northern Pennines constructed to power the mills that served the lead mines in the region.

Rocks on the northern shore of the reservoir had tufts of a dark green, almost black, moss inhabiting the splash zone.   Under the microscope, I saw the characteristic wavy-edged cells which indicated that this was a Racomitrium.   This is Racomitrium aciculare, a semi-aquatic cousin of the species we encountered on rocks in Teesdale last year (see “Upper Teesdale in March”).   The southern shore of the lake, by contrast, was not fringed with rocks, but with rushes and Sphagnum moss, along with some Polytrichum.   This side of the reservoir receives the drainage from the fells above and, I suspect, the constant supply of sediment has led to the gradual infilling of the original shoreline.   There were at least a couple of species of Sphagnum present here, but I was most interested in the submerged moss, S. cuspidatum.


Looking north towards the unnamed reservoir on Bollihope Common (NY 989 348).   The road on the left hand side of the image leads to Stanhope.


Aquatic mosses from the unnamed reservoir on Bollihope Common.  The left hand image shows Racomitrium aciculare on the tops of boulders and the right hand image shows Sphagnum cuspidatum from the boggy areas on the southern shore.

I shook portions of both mosses vigorously in a small amount of water from the reservoir to dislodge the attached algae.   The clear water quickly turned brown and I sucked up a few drops of each with a pipette and dropped them onto a microscope slides.  First up was the sample from the Racomitrum.  This was dominated by the small diatom Achnanthidium minutissimum (a – e in the figure below).  When I had looked at the Racomitrium leaves under the microscope, I had seen many of these attached to the leaves by short stalks.   These comprised just over half of all the diatom cells that I counted.  Long needle-like cells of Fragilaria rumpens (or something similar) which attached to the leaf by their base formed another 27% and another genus, Gomphonema (one or more forms in the G. parvulum complex), formed about 16%.  Most interesting to me were a few gracefully-curved cells of Hannaea arcus, as these are good indicators of a relatively pristine habitat.

Next up was the sample I had obtained from the Sphagnum.   Sphagnum usually favours acid habitats so I was intrigued to see what diatoms would be associated with it, having seen that the diatoms associated with Racomitrium, a hundred metres or so away, mostly suggested neutral or slightly alkaline conditions.

Once again, it was Achnanthidium, Fragilaria and Gomphonema that comprised the majority of the diatom cells (54, 19 and 16% respectively) but this time, about 8% of the total belonged to at least three species of a different genus, Eunotia, which is often associated with acid habitats, and the curved cells of Hannaea were conspicuous by their absence.   Interestingly, Sphagnum does not only favour acid conditions, peculiar features of its cell wall chemistry also helps to create those acid conditions and the diatoms living in the microhabitats around the submerged Sphagnum were clearly indicating a slight change in conditions, compared to those I found on the Racomitrium.


Diatoms growing on and around mosses in the unnamed reservoir at Bollihope Common; a – e: Achnanthidium minutissimum complex; f,g: Gomphonema parvulum complex; h. Eunotia spp (probably E. implicata); i. Navicula (probably N. cryptocephala); j. Fragilaria (probably F. gracilis); k. Hannaea arcus.  Scale bar: 10 micrometres (1/100th of a millimetre).   Note, particularly for h and k, healthier specimens were present in the samples but none presented in a manner amenable to photography.

There was much more Sphagnum underfoot as I walked over Bollihope Common.  Given time – a couple more centuries, maybe – and the gradual invasion of Sphagnum from the moorland around the reservoir might continue and, we can hypothesise, the acid-loving diatom species might become more abundant.  Indeed, we could even argue that this would simply be nature re-establishing its influence, the reservoir being an unnatural and – in the grand scheme of things – temporary intrusion into the landscape.