The big pictures …

If you read this blog regularly you will, I hope, have some sense of just how varied are the algae that live in our freshwaters.   It occurred to me, however, that, in cataloguing this diversity, I don’t often step back and give you some idea of how these many forms relate to one another. I drop terms such as “diatom” and “green algae” into my posts but have not, perhaps, discussed the meaning of these terms in very much detail for some time.

One of the problems is that the meaning of these terms can vary, as knowledge unfolds.  For the early part of my career, for example, I could define “green algae” quite easily, and point to several authoritative textbooks to support my case.   Depending on who wrote the book (and when), green algae were either a separate division (“Chlorophyta”) or a class (“Chlorophyceae”).  There was some dispute about whether Chara and relatives belonged in this group or formed a separate group (“Charophyta”) but that was pretty much the end of the story and taxonomists then got down to arguing about how the many genera and species of green algae should be arranged within this broad heading.

Opinion has, however, shifted over the last couple of decades, with the green algae now split between two separate phyla within the kingdom Plantae.   One of these phyla is the Chlorophyta and the other is the Charophyta, which includes not just Chara and relatives but also some quite important Classes of green algae.    We have met representatives from many of the Classes from both of these phyla in this blog over the years, with the exception of the Prasinophytes, which is an important group of marine plankton with only a few freshwater representatives, and the Trebouxiphyceae.


The organisation of the “green algae” subkingdom (“Viridiplantae”) showing division into two Phyla, and the major Classes found in freshwaters within each Phylum.   The organisation follows Algaebase and the Tree of Life website (see also Lewis & McCourt, 2004). 

Back in the summer I described a number of green algae that I found in the River Wear.   In “Summertime blues …” I wrote about algae that belong to the Chlorophyceae whilst, later in the summer, I explained how these had been joined by a number of desmids, which belong to the Conjugatophyceae (see “Talking about the weather …”).  The plate in that post includes a cell of Pediastrum boryanumbeside some of the desmids; if I was to put together a plate of animals sharing a similar level of kinship, I might include a human and a slug – representatives of two separate phyla within the same kingdom, Animalia (see “Who do you think you are?”).  That is a remarkable amount of diversity to pack into a group of microscopic cells.

The next figure shows the organisation within the Conjugatophyceae, one of the Classes of Charophyta.  The biggest group, in terms of number of species, is the Desmidales, which have featured in quite a few posts (see “Desmid diversity …”), but this class also includes Mougeotia and Zygnema, which we met in the previous post.  Again, just to give you some idea of the scale of the differences, Mougeotia and Zygnema are as closely related as we are to chimpanzees (different genera, same family), whilst their kinship to a desmid is on a par with ours to a warthog (different families, same order).

If you think that you are rather more different to a warthog than one microscopic green alga is to another, there are two things you need to remember: the first is that humans are, relatively speaking, rather good at knowing what features set different types of mammal apart, and that the absence of two short tusks protruding from the sides of the mouth, coupled with a bipedal gate, are highly relevant factors when struggling to decide whether or not the organism in front of you is a man or a warthog.  When trying to understand microscopic organisms such as algae, there are fewer obvious characters, and some of the most useful (such as the presence of flagellae during the reproductive stages) may be present only for a short period of the life cycle.   Straightforward observation, quite simply, is not so useful when trying to determine relationships between microscopic organisms.


Organisation within the Conjugatophyceae, showing division into two Orders and Families.  After Algaebase and the Tree of Life website.

The other point to bear in mind is that algae having had far longer to evolve than mammals.   The two green algae lineages may have separated before the end of the Precambrian era, whilst the primates, the Order to which humans belong, split from other mammals only 65 million years ago.   That means that the green algae have had eight times as long to evolve subtle differences as humans have had to ensure no confusion with warthogs.   Just because these differences are not manifest in obvious features such as tusks does not mean that they are not there.

This brief overview of the green algae has had a side-benefit for me, as it has highlighted a couple of groups I have not previously written about.  One of these groups (the Prasinophytes) is uncommon in freshwaters but the other (Trebouxiphyceae) is quite common and I can even see a green patch formed by a member of this Class from my window as I write this post.   At least I know now what I should write about next …


Lewis, M.A. & McCourt, M.M. (2004). Green algae and the origin of land plants.  American Journal of Botany91: 1535-1556.

Leliaert F, Smith DR, Moreau H, Herron MD, Verbruggen H, Delwiche CF & De Clerck O (2012) Phylogeny and molecular evolution of the green algae. Critical Reviews in Plant Sciences 31: 1-46.


Links to posts describing representatives of the major groups of green algae.  Only the most recent posts are included but these should have links to older posts.

Group Link
Chlorophyceae Keeping the cogs turning …

Summertime blues …

Ulvophyceae Includes many important filamentous and thalloid genera from freshwaters:

Chaetophorales: Life in the colonies …

Cladophorales: Cladophora and friends

Oedogoniales: More about Oedogonium

Trentepoliales: Fake tans in the Yorkshire Dales

Ulothrichales: Spring in Ennerdale

Ulvales: Loving the low flows

Trebouxiphyceae Watch this space …
Prasinophyta Watch this space …
Charophyceaee Life in the deep zone …
Conjugatophyceae Desmidiales: Desmid diversity

Zygnemetales: Fifty shades of green

Klebsormidiaceae The River Ehen in November



Talking about the weather …

September is here.  When I visited this site two months ago we were in the midst of the heatwave and the samples I collected from the Wear at Wolsingham were different to any that I have seen at this location before, dominated by small green algae (see “Summertime blues …”).   As I drove to Wolsingham this time, I could see the first signs of autumn in the trees and the temperatures are more typical of this time of year.   We have had rain, but there has not been a significant spate since April and this means that there has been nothing to scour away these unusual growths and return the river to its more typical state.

That does not mean, however, that there have been no changes in the algae on the submerged stones.  Some of these differences are apparent as soon as I pick up a stone.  Last month, there was a thin crust on the surface of the stones; that is still here but now there are short algal filaments pushing through, and the whole crust seems to be, if anything, more consolidated than in July, and I can see sand grains amidst the filaments.   Biofilms in healthy rivers at this time of year are usually thin, due to intense grazing by invertebrates, so I’m curious to know what is going on here this year.

A cobble from the River Wear at Wolsingham, showing the thick biofilm interspersed with short green filaments.   Note, too, the many sand grains embedded in the biofilm.  The bare patch at the centre was created when I pulled my finger through it to show how consolidated it had become.  The cobble is about 20 centimetres across.

Many of the organisms that I can see when I peer at a drop of my sample through my microscope are the same as those I saw back in July but there are some conspicuous differences too.   There are, for example, more desmids, some of which are, by the standards of the other algae in the sample, enormous.   We normally associate desmids with soft water, acid habitats but there are enough in this sample to suggest they are more than ephemeral visitors.   And, once I had named them, I saw that the scant ecological notes that accompanied the descriptions referred to preferences for neutral and alkaline, as well as nutrient-rich conditions.  Even if I have not seen these species here before, others have seen them in similar habitats, and that offers me some reassurance.    In addition to the desmids, there were also more coenobia of Pediastrum boryanum and Coelastrum microporum compared to the July sample.

A view of the biofilm from the River Wear at Wolsingham on 1 September 2019. 

There were also more diatoms present than in my samples from July – up from about 13 percent of the total in July to just over 40 per cent in September.   The most abundant species was Achnanthidium minutissimum, but the zig-zag chains of Diatoma vulgare were conspicuous too.  The green filaments turned out to be a species of Oedogonium, not only a different species to the one I described in my previous post but also with a different epiphyte: Cocconeis pediculus this time, rather than Achnanthidium minutissimum.   I explained the problems associated with identifying Oedogonium in the previous post but, even though I cannot name the species, I have seen this form before (robust filaments, cells 1.5 to 2 times as long as broad) and associate it with relatively nutrient-rich conditions.  That would not normally be my interpretation of the Wear at Wolsingham but this year, as I have already said, confounds our expectations.   I did not record any Cladophora in this sample but am sure that, had I mooched around for longer in the pools at the side of the main channel, I would have found some filaments of this species too.

Desmids and other green algae from the River Wear at Wolsingham, 1 September 2019.  a. Closterium cf. acerosum; b. Closteriumcf. moniliferum; c. Cosmarium cf. botrysis; d. Closterium cf. ehrenbergii; e. Coelastrum microporum; f. Pediastrum boryanum.   Scale bar: 50 micrometres (= 1/20th of a millimetre).  

It is not just the differences between months this year that I’m curious about.  I did a similar survey back in 2009 and, looking back at those data, I see that my samples from August and September in that year had a very different composition.   There was, I remember, a large spate in late July or early August, and my August sample, collected a couple of weeks later had surprised me by having a thick biofilm dominated by the small motile diatom Nitzschia archibaldii.   My hypothesis then was that the spate had washed away many of the small invertebrates that grazed on the algae, meaning that there were few left to feed on those algae that survived the storm (or which had recolonised in the aftermath)..   As the algae divided and re-divided, so they started to compete for light, handing an advantage to those that could adjust their position within the biofilm.   This dominance by motile diatoms was, in my experience of the upper Wear, as uncommon as the assemblages I’m encountering this summer, though probably for different reasons.

Other algae from the River Wear at Wolsingham, September 2018.    The upper image shows Diatoma vulgare and the lower image is Oedogonium with epiphytic Cocconeis pediculus.   Scale bar: 20 micrometres (= 1/50th of a millimetre).

I suspect that it is the combination of high temperatures and low flows (more specifically, the absence of spates that might scour away the attached algae) that is responsible for the present state of the river.  This, along with my theory behind the explosion of Nitzschia archibaldii in August 2009, both highlight the importance of weather and climate in generating some of the variability that we see in algal communities in rivers (see “How green is my river?”).   The British have a reputation for talking about the weather.   I always scan the weather forecasts in the days leading up to a field trip, mostly to plan my attire and make sure that I will, actually, be able to wade into the river.  Perhaps I also need to spend more time thinking about what this weather will be doing to the algae I’m about to sample.

Desmid diversity …

Back in September, I wrote about a joint British Phycological Society and Quekett Microscopical Club field weekend looking at desmids in the Lake District (see “Desmid Masterclass”, “Lessons from School Knott Tarn” and “Different tarn, different desmids …”).  Dave John sent some of the samples that we collected to David Williamson, the UK’s leading expert on desmids but, at 92, too frail to join us, and he has now sent back some fine drawings illustrating the range of desmids that he encountered.

Two of the tarns (Long Moss Tarn, Kelly Hall Tarns) are already recognised as Internationally Important Plant Areas (IPAs) for desmids because of their desmid diversity and containing internationally very rare desmids (based largely on David Williamson’s records) so their diversity is not a complete surprise to us.  Nonetheless, David found a total of 129 desmid taxa in the three tarns, whilst another desmid specialist, Marien van Westen, identified almost 160 desmids in another set of samples from the same tarns.

The drawings are arranged in three plates, one for each tarn.   Desmids identified by David Williamson from the three tarns are illustrated.  The desmids have been numbered and the captions prepared by David John who is analysing the findings and comparing them with surveys dating back to the 1970s.   David Williamson has drawn the taxa at different scales to roughly balance the arrangement on the collage, and adjusted the sizes so important details are visible.   No details of the chloroplasts are given since all samples had been preserved in formalin.  A few of the desmids, particularly those that are very long, have not been included in the plates.

Desmids from Long Moss Tarn (SD 292 936), September 2017.   Long Moss Tarn is shown in the photograph at the top of this post.

Desmids from Kelly Hall Tarn (SD 289 933), September 2017.

Desmids from School Knott Tarn (SD 427 973), September 2017.


1-Actinotaenium diplosporum; 2-Actinotaenium turgidum;  3-Bambusina borreri;  4-Closterium acerosum var. borgei; 5-Closterium angustatum;  6-Closterium archerianum var. pseudocynthia;  7-Closterium archerianum; 8-Closterium attenuatum;  9-Closterium baillyanum var. alpinum; 10-Closterium baillyanum; 11-Closterium closterioides; 12-Closterium costatum; 13-Closterium dianae var. arcuatum; 14-Closterium dianae var. minus;  15-Closterium didymotocum; 16-Closterium incurvum; 17-Closterium intermedium; 18-Closterium kuetzingii;  19-Closterium lunula; 20-Closterium navicula;  21- Closterium setaceum; 22-Closterium striolatum; 23-Cosmarium amoenum; 24-Cosmarium anceps; 25-Cosmarium binum; 26-Cosmarium brebissonii; 27-Cosmarium contractum;  28-Cosmarium davidsonii; 29-Cosmarium debaryi;  30-Cosmarium depressum; 31-Cosmarium formosulum; 32-Cosmarium hostensiense; 33-Cosmarium incrassatum var. schmidlei; 34-Cosmarium margaritatum; 35-Cosmarium margaritiferum; 36-Cosmarium monomazum var. polymazum;  37-Cosmarium obtusatum;  38-Cosmarium ornatum; 39-Cosmarium ovale;  40-Cosmarium pachydermum; 41-Cosmarium pachydermum var. aethiopicum; 42-Cosmarium perforatum var. skujae; 43-Cosmarium portianum; 44-Cosmarium punctulatum;  45-Cosmarium quadratum; 46-Cosmarium quadrum; 47-Cosmarium subochthodes var. majus; 48-Cosmarium subtumidum var. groenbladii;  49-Cosmarium subundulatum; 50-Cosmarium tetragonum var. ornatum ; 51-Cosmarium tetraophthalmum; 52-Cosmarium variolatum;  53-Cosmocladium tuberculatum; 54-Desmidium aptogonum; 55-Desmidium swartzii; 56-Docidium baculum; 57-Euastrum ampullaceum; 58-Euastrum ansatum;  59-Euastrum bidentatum var. speciosum; 60-Euastrum gemmatum; 61-Euastrum luetkemulleri; 62-Euastrum oblongum; 63-Euastrum pectinatum; 64-Euastrum pulchellum; 65-Euastrum verrucosum; 66-Gonatozygon aculeatum; 67-Gonatozygon brebissonii; 68-Groenbladia undulata; 69-Haplotaenium minutum;  70-Hyalotheca dissiliens;  71- Micrasterias americana var. boldtii; 72-Micrasterias compereana; 73-Micrasterias crux-melitensis; 74-Micrasterias denticulata; 75-Micrasterias furcata; 76-Micrasterias pinnatifida;  77-Micrasterias radiosa; 78-Micrasterias rotata; 79-Micrasterias thomasiana; 80-Micrasterias truncata; 81-Netrium digitus; 82-Netrium digitus var. latum; 83-Netrium interruptum;  84-Penium exiguum; 85-Penium margaritaceum; 86-Pleurotaenium coronatum var. robustum;  87-Pleurotaenium ehrenbergii; 88-Pleurotaenium truncatum; 89-Sphaerozosma filiforme; 90-Staurastrum arachne;  91-Staurastrum arctiscon; 92-Staurastrum bieneanum; 93-Staurastrum boreale var. robustum; 94-Staurastrum cristatum; 95-Staurastrum dilatatum; 96-Staurastrum inconspicuum; 97-Staurastrum kouwetsii; 98-Staurastrum lapponicum; 99-Staurastrum maamense; 100-Staurastrum polytrichum; 101-Staurastrum productum; 102-Staurastrum quadrangulare; 103-Staurastrum striolatum; 104-Staurastrum teliferum; 105-Staurastrum tetracerum; 106-Staurodesmus convergens; 107-Staurodesmus convergens var. wollei; 108-Staurodesmus cuspidatus var. curvatus; 109-Staurodesmus megacanthus; 110- Xanthidium antilopaeum; 111-Xanthidium antilopaeum var. laeve; 112-Xanthidium antilopaeum var. polymazum; 113-Xanthidium cristatum.

The underwater world of Ennerdale Water …

I’ve tried to capture the world of microscopic benthic algae many times but never, until now, attempted the same effect with plankton.   The picture below illustrates the problem that I face: whereas the benthic flora are organised with, for the most part, a clear three-dimensional structure and known dependencies amongst organisms (species A, for example, being epiphytic on species B), plankton are randomly distributed in a very dilute solution.   My picture  below, which is based on four phytoplankton samples collected by the Environment Agency in the summers of 2014 and 2016.

A representation of the phytoplankton of Ennerdale Water with cells of Rhodomonas and Kephyrion depicted at a realistic density (c. 1000 – 2000 cells per millilitre).

I had to address two issues in producing this image, which is based on four phytoplankton samples collected by the Environment Agency in the summers of 2014 and 2016: depicting the phytoplankton cells at approximately the correct density and making sense of the list of names that appeared on the list.  Ennerdale Water is a very nutrient-poor lake and cell concentrations during the summer are in the order of 1000 to 2000 per millilitre.  That sounds a large number until you consider the scale at which we are working.   For simplicity, I assumed spherical cells of about 20 micrometres diameter (= 1/50th of a millimetre) at a density of 1000 cells/ml.    That equates to one cell per micrometre which is 1 mm x 1 mm x 1 mm.   Using these assumptions, each cell is 50 diameters distant from its nearest neighbour, which means the foreground of a picture should contain only two small cells and a lot of blue paint.

Next, I need to know what algae to paint and the problem here is that 85 per cent of the cells in the Environment Agency phytoplankton analyses were described as “picoplankton < 2 micrometres diameter” or “nanoplankton 2-20 micrometres diameter” (the latter divided into flagellates and non-flagellates).  There are, apparently, big difficulties in naming many of the cells found as preservation with Lugol’s Iodine coupled with the long time in storage before analysis can lead to loss of useful diagnostic features.   Cells in the nanoplankton category can, in theory, belong to any one of a number of groups of algae but If I focussed just on those organisms that could be named, I see that the Cryptophyta Rhodomonas lacustris var nannoplanctica (formerly R. minuta var. nannoplanctica) predominates, followed by Chrysophytes, of which Kephyrion is the most abundant.   So these are the two cells that I have put in the foreground.

I subsequently turned up a paper from 1912 by the father and son team of William and George West who looked at the phytoplankton of Ennerdale Water and a number of other lakes in the Lake District and Scotland.  The range of taxa that they found was quite different to that recorded in these recent surveys with samples dominated by desmids and almost no Chrysophytes or Cryptophytes recorded at all. That may, in part, be due to differences in methods – they collected samples using a “silken tow net”, which would probably have missed the very small Chrysophyta and Cryptophyta (an earlier paper by them tells us of the size of the nets but not the mesh itself) .  Some desmids that they found were found in the recent surveys but in much smaller quantities and it is possible that this was partly an artefact of the differences in sampling technique.  The idea of comparing count data from old papers with modern records is appealing but, in most cases, separating genuine changes in composition from differences introduced by sampling and analytical methods is always difficult.

Excuse these ramblings … there is, as you can see, not a lot of pictorial interest in the underwater world of an oligotrophic lake.   If you want excitement, tune into Blue Planet II, David Attenborough’s latest series for the BBC You will find sex and violence galore there.  The underwater world of Ennerdale Water is a quieter, more serene and certainly less televisual place.  Maybe that’s not such a bad thing …


Lund, J.W.G. (1948) A rarely recorded but very common British alga, Rhodomonas minuta Skuja. British Phycological Bulletin, 2:3, 133-139.

West, W. & West, G.S. (1909). The British freshwater phytoplankton, with special reference to the desmid-plankton and the distribution of British desmids.   Proceedings of the Royal Society of London Series B 81: 165-206.

West, W. & West, G.S. (1912).  On the periodicity of the phytoplankton of some British lakes.  Journal of the Linnaean Society, Botany 40: 395-432.

Taking desmids to the next dimension …

Participants at the British Phycological Society / Quekett Microscopical Club field weekend at the Freshwater Biological Association, September 2017.  Scale bar: one metre (= 1,000,000 micrometres).

A theme that has run through this blog over the years has been that what you see down a microscope is often a highly distorted view of reality and at the end of our weekend of desmid hunting, Chris Carter gave a talk that also made this point, using desmids as a case study.  In essence, we had spent much of Saturday and Sunday morning peering down microscopes at three-dimensional objects that appeared, as a result of the very shallow depth of field that is characteristic of high magnification images, two dimensional.   We were then matching these to two-dimensional representations in the Floras and identification guides that we had to hand.  Dave explained a few tricks that experts use, such as applying gentle pressure to a coverslip with a fine needle, to turn desmids in order to see them from other angles but, mostly, we were restricted to very flattened views of desmids.

Chris has tackled this problem from several directions over the years, including experiments with anaglyphs (see “Phworrrrhhh …. algal sex in 3D!“) as well as the very careful manipulation of a long, cylindrical Pleurotaenium that won the Hilda Canter-Lund prize earlier this year.   He has also produced a number of plates with desmids laid out almost as if on an engineer’s drawing board, with front, top and side views.   Several of these are on Algaebase, but one example is reproduced below.  Microscopists learn to use the fine-focus control to appreciate the depth of the objects that they are examining and Chris also shows how it can reveal the nature of surface ornamentation on different parts of the cell.  The temptation, given a series of photos such as these (excluding the side view) would be to use “stacking” software to produce a single crisp image.  This is appropriate in some situations but you are, in truth, just producing a crisp two-dimensional image rather than offering any insights into the true shape of the cell.

Staurastrum furcatum from Botswana, photographed by Chris Carter.

Another technique that can be used to generate three-dimensional images is, of course, scanning electron microscopy.  However, this is beyond the budget for anyone outside a major institution.  This has helped greatly get a better understanding of the morphology of diatoms, in particular, but the third dimension comes at a price.   Scanning electron micrographs take us to an opaque, monochrome world, purged of the vivid colours that the microscopic world usually offers us.

Chris’ pièce de résistance, however, was a three-dimensional model of a Staurastrum, produced by the 3D printing company Shapeways and loosely-based on various pictures of S. furcatum and presented to him as a 70th birthday present by his son.  The main point is to demonstrate the symmetry and gross features of a typical Staurastrum rather than to be a taxonomic blueprint. The designers were very helpful but it does hint at what is possible with modern technology.

Chris Carter’s three-dimensional model of Staurastrum.  It is about six centimetres across.   You can buy your own copy from Shapeways by following this link …

The missing ingredient in this recipe is imagination.  Or, to be more precise, the viewer’s imagination as Chris has clearly demonstrated that he is not lacking in that department.  Once you have a sense of the three-dimensional form of a Staurastrum, you be able to use that knowledge every time you look at a two-dimensional image of a desmid through a microscope.   Seeing, as Ernest Gombrich reminds us in his great book Art and Illusion, is as much about using prior experiences to interpret the raw data collected by our optic nerves as it is about the patterns of light that stimulate our retinas.   Just as a child can look at a two-dimensional image of a cat in a book and match this to the real creatures that he or she encounters, so knowing about Staurastrum’s third dimension helps us to interpret the flat shapes that we see.

At a more basic level, all identification is a matter of matching the objects we see either to schemata stored in our memory or to patterns in books.  This, in turn, helps us to understand why the microscopic world seems so strange and mysterious to those who do not study it.  It all comes down to having (or not having) the prior experiences that generate recognition.   At one level, there are gasps of astonishment as people with none of these schemata in their memories encounter the beauty of desmids for the first time.  And then there is Frans Kouwets, another speaker at the meeting , who is busy cataloguing 750 different species of one genus, Cosmarium.   And in between there are the rest of us …

Frans Kouwets explains his fascination with Cosmarium to the British Phycological Society / Quekett Microscopical Club field meeting at the Freshwater Biological Association in September 2017.

Different tarn, different desmids …

Geoff and Chris, two of our band of desmid hunters, chose to stay in the FBA’s brand new holiday apartments and, rather than cross the lake to join us on Saturday morning they headed out to Moss Eccles Tarn, in the area between Esthwaite Water and Windermere.   One of Dave’s first dips into one of their samples yielded an almost pure monoculture of another filamentous desmid, Spherozosma vertebratum which presented some beguiling abstract patterns on my computer monitor.

Spherozosma vertebratum from Moss Eccles Tarn, September 2017.   Scale bar: 25 micrometres (= 1/40th of a millimetre).

Curiously, after our first encounter with Spherozosma vertebratum we did not see it in any of our other dips into the Moss Eccles samples although there were plenty of other desmids on display.   The most abundant of these was Staurastrum productum and, usefully, there were examples showing both apical and side views.   The three arms are distinctive (and distinguish it from relatives such as S. arachne which have five) and you can also see the knobbly “verrucae” on the spines as well as a broad mucilaginous envelope around the cells.

Staurastrum productum in side (left) and apical (right) views.  Images photographed from a computer monitor so apologies for their poor quality.  Scale bar: 25 micrometres (= 1/40th of a millimetre).

Another desmid with spines and mucilage was quite common.  This was Staurodesmus bulnheimii.  Spines slow the rate of sinking so are associated with several genera of predominately planktonic desmids.   The star-shaped arrangement of colonies of the diatom Asterionella formosa play a similar role (see “Little bugs have littler bugs upon their backs to bite ‘em”).   There were also several cells  of a small Cosmarium species, including some that had recently divided and the image shows how one cell has split down the central isthmus and a new semicell is growing back on each of the two daughter cells.   Finally, I have included an illustration of Micrasterias radiosa.  To the uninitiated this may look little different to M. compereana, illustrated in the previous post, but if you look closely you will see that the incisions between the lobes are much deeper in M. radiosa.

One sample from Moss Eccles Tarn kept me busy for half the morning and this account describes only part of the diversity.   Note how the differences between this and the School Knott Tarn sample are not just in the genera and species present but also in the life-forms I found.  The School Knott sample was from a Sphagnum squeezing whilst the Moss Eccles sample was from a plankton net.  That explains why I saw more spine-bearing desmids in the latter.  If I had looked at a plankton sample from School Knott and a Sphagnum squeezing from Moss Eccles, I might have found a different balance of life-forms between the two tarns.   But time was running out and I had to move on …

More desmids from Moss Eccles Tarn, September 2017: a. Staurodesmus bulnheimii; b. Cosmarium quadrifarium var. hexastichum; c. Euastrum cf. gemmatum.   Scale bar: 25 micrometres (= 1/40th of a millimetre).

Micrasterias radiosa from Moss Eccles Tarn, September 2017.   Scale bar: 25 micrometres (= 1/40th of a millimetre).

Lessons from School Knott Tarn …

As not everyone could join us on our excursion on Friday afternoon, we repeated the exercise on Saturday morning, heading to a small tarn just a short walk from Windermere and Bowness.   Despite its proximity to two of the busiest towns in the Lake District, there were very few other people around to disturb our peace whilst we collected samples.   As at Kelly Hall and Long Moss Tarns, Dave had his plankton net out, but we also explored a boggy region at one end, finding more patches of Sphagnum but also extensive growths of Utricularia minor (Lesser Bladderwort), one of a small number of aquatic carnivorous plants.   Dave was particularly pleased by this find as he associates this particular plant with rich hauls of desmids.

It was tempting to linger in the sunshine beside School Knott Tarn but the green tinge of the water that dripped out of the Sphagnum squeezings in particular was enough to lure us towards the Freshwater Biological Association’s laboratories in order to start examining our samples.

Utricularia minor (Lesser Bladderwort) from School Knott Tarn, near Windermere, September 2017.   Several of the spherical bladders which trap small invertebrates are visible on the plant.

My selection of photographs below shows just a part of the diversity that we encountered during our microscopic examinations.  I was using a borrowed set-up and the images are all from photographs of the desmids displayed on computer monitor, which is far from ideal.   Some of the larger desmids – one large Closterium species in particular – were too large to fit onto the screen and have had to be omitted from this account.  There were also a number of cells of Eremosphaera (see “More from Loughrigg Fell”) and some Cyanobacteria (Merismopedia was quite common) so this is a very partial description of our microscopical adventures in School Knott Tarn.

The first two desmids, Spirotaenia condensata and Cylindrocystis gracilis, belong to a group of desmids called “saccoderm desmids”.  These are more closely related to filamentous green algae of the Zygnemetaceae that are old friends of this blog (see “Concentrating on Carbon, for example) and, in fact, we could think of these genera as being unicellular analogues of their filamentous cousins.   Spirotaenia, with its helical chloroplast, for example, recalls Spirogyra whilst Cylindrocystis’ two star-shaped chloroplasts is reminiscent of Zygnema.  Mesotaenium, which we did not see in this sample, has a plate-like chloroplast similar to that in Mougeotia.

The next two illustrations both show species of Micrasterias.  Of these, M. compereana generated a vigorous discussion amongst our experts. This would have been described as M. fimbriata using the latest British floras but a paper has been published recently which uses molecular data to demonstrated the need to split the species. Finally, we have representatives of Euastrum and Haplotaenium, two genera that we also met at Dock Tarn (see “Damp days in search of desmids …”) although the species are different.   Haplotaenium differs from Pleurotaenium in the number and form of the chloroplasts and also because it lacks a terminal vacuole.

Desmids from Sphagnum squeezings from School Knott Tarn, September 2017: a. Spirotaenia condensata; b. Cylindrocystis gracilis; c. Micrasterias compereana; d. Micrasterias crux-meltensis; e. Euastrum oblongum; f. Haplotaenium rectum.  Scale bar: 25 micrometres (= 1/40th of a millimetre).

Four more desmids are illustrated on the lower plate.   Of these, we have seen Netrium digitus in Dock Tarn and the illustration there is better than this one, showing the undulating nature of the chloroplast margins quite clearly.   The desmid below this, Closterium closterioides caused some confusion at first.   We usually associate Closterium with lunate (moon-shaped) cells (see “More from Loughrigg Fell”) but this species is straight, sending me towards the section on Netrium in my Flora.  However, Netrium lacks terminal vacuoles whereas this specimen has prominent vacuoles at both ends.   We also found a variety, C. closterioides var. intermedium, in the same sample.

The final desmid that I have illustrated is a filamentous form: Desmidium schwartzii.  In contrast to Hyalotheca dissilens (see “Desmids from the Pirin mountains”) there is no obvious mucilaginous sheath around this specimen, but this may be an anomaly of this population or an artefact of the microscopy set-up.   We are looking at the side view of a chain of cells but if we were to look at the end view of one cell it would be triangular in this particular species.  The chloroplast fills most of the cell and has projections running into the corners of the cells.  However, as the filaments of the cells are slightly twisted, these projections appear to shift in position from cell to cell, giving a helical appearance.  I’ve tried to illustrate this with a schematic diagram.

More desmids from Sphagnum squeezings from School Knott Tarn, September 2017: g. Netrium digitus; h. Closterium closterioides var. closterioides; i. C. closterioides var. intermedium; j. Desmidium schwartzii Scale bar: 25 micrometres (= 1/40th of a millimetre).

This short post gives some idea of the diversity in a single sample from a single Tarn.   Dave handed all the samples we collected over to David Williamson on his way back south and we’ll get a fuller list of their diversity in due course.  This one sample occupied me for the latter part of Saturday morning and all of the afternoon.   On Sunday, I moved on to look at another sample and I’ll write about that in another post very soon.

A schematic view of a chain of Desmidium cells, showing the arrangement of the chloroplast seen in apical view (k.) and the implications of slight twisting of the filament on appearance (l.).  Diagram adapted from John et al. (2011).


John, D.M., Whitton, B.A. & Brock, A.J. (2011). The Freshwater Algal Flora of the British Isles. 2nd Edition. Cambridge University Press, Cambridge.

Neustupa, J., Šťastný, J. & Škaloud, P. (2014). Splitting of Micrasterias fimbriata (Desmidiales, Viridiplantae) into two monophyletic species and description of Micrasterias compereana sp. nov.  Plant Ecology and Evolution 147: 405-411.