More winning ways …

Having heard Davis Laundon’s tips on photographing the microscopic world in the previous post, we now turn to Michel Vos, the joint winner of the 2020 competition, who specialises in the algae and other life found in rock pools around our shore.  He takes a very different approach to Davis, proving that there is no single winning formula and each of us is free to experiment.   I’ve taken a couple of images from Michel’s blog An Bollenessor to illustrate his answers.  The image at the top of the post was taken whilst snorkelling off the coast of Cornwall whilst the image at the end shows stands of Himanthalia elongata off the coast of St Martin, in the Scilly Isles.   The kelp forests which form the backdrop to some of his images remind me of the Oscar-winning documentary film, My Octopus Teacher, available on Netflix, which explores the marine life in shallow water off the Cape coast in South Africa.

  • “Decisive moment” or carefully planned composition?
    My strategy is to snorkel around, stopping when something catches my eye, and take a whole bunch of photographs. Digital cameras make this incredibly easy, but I try to limit it a bit, because I do not want to wade through too many photos or delete too many either.
  • What photo editing software do you use?
    I have not invested in dedicated software, I just use the generic photo editor in Windows, which is pretty decent I must say.
  • What routine editing steps do you apply to your image (e.g. cropping, adjusting levels/curves/brightness etc, stitching, stacking)?
    I often crop a bit and I reduce highlights for pretty much any photo. I drag the clarity slider a bit to the right most times. I try to not fiddle with it too much because it is very obvious when someone has been too enthusiastic touching up and the picture will lose its natural feel. 
  • Do you ever “retouch” images to remove blemishes and improve their appearance?
    I occasionally use ‘spot fix’ to get rid of particles but this can leave distortions in the photo so I do not use it very much.
  • How important are the title and legend when you present an image?   
    I only have to formally present images when entering a composition which is very infrequent and where it is more of a requirement from the organisers. I do provide captions on Instagram I guess, and I could spend a little more time on the biology of my subjects probably. 
  • Are there any photographers who particularly inspire you?  (not necessarily photographers of algae)?
    I follow a lot of great underwater photographers on Instagram. Despite social media being on one level a big advertising and dopamine trick it has been a great medium to find inspiration and to exchange tips with likeminded people you would otherwise never meet. I really like Angel Fitor who is based on the Spanish Mediterranean coast (@angelfitor / who photographs anything from fish behaviour to jellyfish to fisheries and conservation issues. His photos are very atmospheric and I like his thoughtful, at times philosophical, captions. 
  • Who’s going to win the Euros?
    The Netherlands of course!

If you want to learn more of Michiel’s secrets, an online talk he have to the Cornish Wildlife Trust is available here.

Winning ways …

What’s the secret behind an image worthy of winning the Hilda Canter-Lund prize?   Over the next few weeks, I’m going to ask former winners some questions that should give some insights into their approach and techniques.   First to be grilled is 2020 winner Davis Laundon, currently finishing off his PhD at the Marine Biological Association in Plymouth, and winner of the 2020 competition.   

Davis was a worthy winner for many reasons, not least being his interests being so close to those of Hilda Canter-Lund herself.   I’d love to eavesdrop on a conversation between them because technology has moved on so much since Hilda took the images in Freshwater Algae: Their Microscopic World Explored.  All her photos were taken before digital cameras were available and she would have been intensely curious about Davis’ equipment.  However, I suspect that she would have agreed with Davis’ approach to taking great photographs and been interested in what his experiments have shown about the way that fungi can affect carbon flow through aquatic ecosystems.  

  • “Decisive moment” or carefully planned composition?
    For me the truth lies somewhere in the middle. Normally the first time I find out about the beauty of an organism or a subcellular structure is entirely by accident, either as part of a bigger microscopy experiment or simply by playing around. However, as a result the first images I take of the subject typically are not great and the acquisition settings need optimising. The reality is that once the ‘inspiration’ for an image is stumbled upon it is followed by a lot of technical fine-tuning.  
  • What photo editing software do you use?
    I use the same software for scientific research that I do for my ‘display’ images, namely Fiji/ImageJ alongside Inkscape if I need to tile or annotate multiple images. If working with complex 3D or 4D images, then I will also use Imaris and Blender.   
  • What routine editing steps do you apply to your image (e.g. cropping, adjusting levels/curves/brightness etc, stitching, stacking)?
    When capturing fluorescent microscopy images for quantitative analysis, it is important to avoid the extremes of the intensity range so that you neither saturate parts your image nor drive certain pixels to absolute zero. This is not always possible when adjusting the brightness and contrast for an image to be seen by the naked eye but an important thing to aim for. Likewise, when false-colouring your channels avoid a red-green colour pair which is not easily distinguished by colour-blind people and opt for colour blind friendly pairs like red-cyan or blue-yellow instead.
  • Do you ever “retouch” images to remove blemishes and improve their appearance?
    Never! The natural world is beautiful enough, warts and all.  
  • How important are the title and legend when you present an image?
    I think the title and legend serve very important purposes complementary to the image itself. For me, the title is the summary of the image’s purpose – what biological reality does it convey? The legend is an extension of this, but also the place to properly disclose your technical details and acknowledge those important people without whom the image wouldn’t be what it is.  
  • Are there any photographers who particularly inspire you? 
    As a microscopist focussing on aquatic fungi and algal interactions, I can honestly say that one has been Hilda Canter-Lund herself! Before I even started my own PhD work, I became familiar with her stunning images and meticulous illustrations of these organisms, at a time when the technical tools available were far behind what we have now.  
  • And, finally, who’s going to win Euro 2020?
    I’m more of a rugby person, I’m afraid.
Davis Laundon: Fluorescent image of chytrid fungi (asterisks) growing on an Asterionella formosa diatom. Red = chlorophyll, Cyan = DNA, Yellow = mitochondria. Imaged with a Leica DMi8.   The picture at the top of the post shows maximum projection of cell wall fluorescence in the fungus Paradendryphiella salina, false-coloured by z-depth. Imaged with a Leica SP8.

Hilda Canter-Lund competition 2021

The British Phycological Society’s 2021 Hilda Canter-Lund competition is now underway, with one important change from previous years.  In the past, entries had to be emailed to us and only the shortlist was available for general viewing.  This year, however, there is a facility on the BPS website that allows entries to be uploaded directly by competitors [], and for these entries to be displayed for all to see in our gallery.  A screenshot of the gallery at the time of writing is at the top of the post.  Click on any of the images in the gallery and you get some more details about the image and you can even add your own comments.

In previous years, the shortlist was chosen by a panel of art-inclined phycologists.   This year shortlist selection will be by a vote of the BPS membership, again via the BPS webpages.   In the past, the judges have strived for a balance of themes and techniques amongst the shortlisted images.   This year, the most highly-rated images may reflect the inclinations of the BPS membership so the rules allow the popular vote to be overridden to ensure balance.   The winning entry will be chosen, as in previous years, by a vote amongst the BPS Council.   The practice of awarding a second prize to an image in a contrasting style (i.e. of a microalga if an image of a macroalga wins, and vice versa) will continue, and this is one reason that there needs to be scope for a discretionary override of the popular vote.  It may not be needed, but it is there if necessary.

Whilst you do not need to be a BPS member to submit an entry to the competition, you will need to be a member if you want to vote for the shortlist.   If you want to join the society, go to the membership pages of the BPS website. it only costs £10.50 to join as an ordinary member without the journal, so it is not a large investment.  

You do, of course, need a good image to upload.  Head to to look at previous shortlists, and check out some of the posts I’ve written over the past few years for ideas on what makes a great image.   The most recent of these is How not to win the Hilda Canter-Lund competition and there is a list of older posts that address algal photography at the end of this.

The closing date is 25 July 2021 so you’ve got about six weeks to get a great image to submit.   

“Awesome Brown”: Erasmo Macaya’s image from the 2020 shortlist showing Macrocystis pyriifera, the giant kelp, the largest seaweed on earth.

Wrote this whilst listening to:   Path of Wellness, new album by Sleator-Kinney.  Guitarist Carrie Brownstein also stars in Portlandia, a very good comedy sketch show that used to be available on Netflix.

Cultural highlights:  The Sound of Metal: thought provoking film about deafness that, fortunately, contains less heavy metal music than the title might suggest.

Currently reading:  Maggie O’Farrell’s Hamnet.

Culinary highlight: Just finished lunch in the garden in friends, where the highlights were rhubarb crumble ice cream, from an old Delia Smith book, and a fresh watermelon.

More about Tabellaria

My statement in the previous post that “Tabellaria flocculosa is by far the most common species” [in the genus Tabellaria] is true except for one important point: Tabellaria flocculosa is not a species, in the biological sense.  We have good evidence from the UK, and my colleague Maria Kahlert has similar evidence from Sweden, that it is actually a complex of several species that no-one has yet taken time to dissect out and establish in their own right. 

The picture at the top of the post shows strains from eight different locations in Scotland collected for our library of molecular barcodes, with the black boxes delimiting three groups, based on differences in their rbcL gene.   All fit the description of T. flocculosa but the scale of the differences is such that they are highly likely to be different species.   However, it is hard to see morphological characteristics that set any of these groups apart.   It is possible that some features will be revealed by a more thorough study using scanning electron microscopy but that will not be particularly useful for routine identification using the light microscope.   Watch this space …

However, even if Tabellaria flocculosa is not a “species” in the strict biological sense, it is a “species” in a broader linguistic sense, insofar as it is a widely-used term that enables freshwater biologists to exchange information about the organisms that live in lakes and streams.   Biologists have a formal way of understanding species, with each and every Tabellaria cell being related to a physical “type specimen”.  However, philosophers and linguistics have a more abstract way of using the word “type” in relation to organisms, deriving from the work of Plato.   He argued that each and every ox was different but there must be something that unites all of these individual oxen.   This universal property could not necessarily be perceived directly with any human sense but must be understood with the mind.   The lay person does not need to link a beech tree back to a type specimen: they have the “type” lodged in their memory, enabling unambiguous identification, even from a distance.   

Not only is Tabellaria flocculosa recognisable as a distinct entity (using characteristics listed in the previous post), but it also conveys some useful information about the habitat.  If T. flocculosa is abundant in a sample (i.e. more than 10% of all cells), then you can be fairly sure that the habitat you are examining has relatively soft water (probably slightly acidic) and relatively low concentrations of inorganic nutrients (see graphs at the end of the post).   All this despite us knowing that it is not really a species in the formal biological sense at all.   There have been some attempts to split this species in the past, but none have stuck, at least in part, I suspect, to the proposed divisions not being sufficiently convincing – in either morphological or ecological terms – to usurp the longstanding Platonic “type” of T. flocculosa.   There are some hints in the work of both Brenda Knudson (see previous post) and John Koppen (see reference list) that there are morphologically distinct planktonic and benthic forms but, again, no-one has produced a convincing rationale for splitting these into separate species.

Linguists would argue that the words we use to describe the world around us are a consensus of usage, not an absolute, and also that meaning of words can change over time.   That’s certainly true for diatoms.  When I first started out, “Cocconeis placentula” was interpreted in much broader terms than it is now, and the same can be said for many other diatom species.  The same is true for genera: we all thought we understood what characterised the genus “Fragilaria”; now, we have a much more constrained definition, with many of its former constituents shifted to other genera.    So it is perfectly possible that, in 30 years time, we’ll have a different view of what “Tabellaria flocculosa” means.   

Distribution of Tabellaria flocculosa along pH and alkalinity gradients, based on analysis of the DARES dataset.   Vertical lines on the pH graph indicate thresholds for high (blue), good (green), moderate (orange) and poor (red) status.   Vertical lines on the alkalinity graph divide the scale into (from left) low, moderate, high and very high alkalinity water.  The arrow indicates the position of the present sample on the respective gradients.  
Distribution of Tabellaria flocculosa along nitrate-N and reactive P gradients, based on analysis of the DARES dataset.  
Relative abundance of Tabellaria flocculosa in lakes, regulated rivers and unregulated rivers in west Cumbria between 2019 and 2021.   


The graphs showing the distribution of Tabellaria flocculosa in relation to chemical variables are based on interrogation of a database of 6500 river samples collected as part of DARES project (Kelly et al., 2008, in reference list).  Vertical lines on the pH and reactive phosphorus graphs show UK environmental standards for conditions necessary to support good ecological status: blue = high status; green = good status, orange = moderate status and red = poor status.  Standards differ between water body types and thresholds for lowland high alkalinity rivers have been plotted here.  These indicate the maximum thresholds for particular ecological status classes for each variable and tighter standards will apply in many waters.

Phosphorus standards are based on the Environment Agency’s standard measure, which is unfiltered molybdate reactive P.  This approximates to “soluble reactive P” or “orthophosphate-P” in most circumstances, but the reagents will react with P attached to particles that would have been removed by membrane filtration. The current UK phosphorus standards are site specific, using altitude and alkalinity as predictor variables.  This means that a range of thresholds applies, depending upon the geological preferences of the species in question.  The plots here show boundaries based on the average alkalinity (50 mg L-1 CaCO3) and altitude (75 m) in the whole dataset.  

There are no UK standards for nitrate-N; thresholds in this report are based on values derived using the same principles as those used to derive the P standards and give an indication of the tolerance of the species to elevated nitrogen concentrations (see “This is not a nitrate standard”).  However, they have no regulatory significance.

The photographs of Tabellaria flocculosa are from cultures isolated by Shinya Sato and David Mann during the UK diatom metabarcoding project (Kelly et al., 2020, in reference list) 


Kelly, M.G., Juggins, S., Guthrie, R., Pritchard, S., Jamieson, B.J., Rippey, B, Hirst, H & Yallop, M.L. (2008).   Assessment of ecological status in UK rivers  using diatoms.   Freshwater Biology 53: 403-422.

Kelly, M.G., Juggins, S., Mann, D.G., Sato, S., Glover, R., Boonham, N., Sapp, M., Lewis, E., Hany, U., Kille, P., Jones, T. & Walsh, K. (2020).  Development of a novel metric for evaluating diatom assemblages in rivers using DNA metabarcoding.   Ecological Indicators 118: 106725.

Koppen, J.D. (1978).  Distribution and aspects of the eclogy of the genus Tabellaria Ehr. (Bacillariophyceae) in the Northcentral United States.   American Midland Naturalist 99: 383-397.

Wrote this whilst listening to:   Brilliant folk-blues guitarist Gwenifer Raymond, another artist who performed at Green Man 2019, but whose set I missed.   And Bruce Springsteen’s Girls in their Summer Clothes, because it caught the mood of the weekend so well.

Cultural highlights:  The Israeli TV show Fauda now available on Netflix, about life on the West Bank.   

Currently reading:  still on American Wife by Curtis Sittenfeld.

Culinary highlight: An impromptu French bistro evening at home, with coq au vin followed by tarte au citron.

The bluffer’s guide to Tabellaria …

Every now and then I write a post about how to identify algae.  Not about how to name algae (the job of taxonomists) or what algae live in which habitats (the job of ecologists) but of the nebulous area between these two specialities where optical nerves are stimulated by the light patterns seen through a microscope’s eyepiece, triggering impulses in the brain that relate particular patterns and shapes to a Latin binomial.   This process is integral to almost all ecological data collection, but we rarely stand back to consider how this happens.   Recent posts on this subject include “Disagreeable distinctions …” and “Dispatches from Plato’s cave …” – the latter making the point that bioinformatic pipelines used in metabarcoding are akin to the thought processes used when we identify organisms using traditional approaches.   Then, in “Identification by association” I pointed out how the mind can be conditioned during the identification process, leading to possible biases in the outputs.

I want to stay with this idea of the mind being conditioned in this post, using the genus Tabellaria as a case study.  This is a small genus with just four representatives, with overlapping ecological preferences (found mostly in soft water with low nutrient concentrations).  It is quite common to find two or more representatives of the genus in a single sample although, in benthic habitats at least, Tabellaria flocculosa is by far the most common species.   The question I want to ask is whether analysts approach each new specimen in the sample as a completely new identification task, or whether they carry forward a cache of information that they have built up about a sample.   Most biologists would probably claim the former, but I suspect that the latter is more common.  Moreover, I don’t think that there is any disgrace in admitting this.  In fact, I think it is a strength under some circumstances.

What is happening under such situations is a form of Bayesian reasoning: as we accumulate experience from a sample (and, over time, from many similar samples), we realise that most of the valves of Tabellaria belong to T. flocculosa.   We approach the next valve of Tabellaria with this prior knowledge along with the information we need to confirm or deny this (the “model”).   Equipped with this, we then check the next valve using this model and emerge with “posterior knowledge” which, in this case will be either confirmation that the valve belongs to T. flocculosa or an alternative diagnosis of T. fenestrata, T. quadrisepta or T. ventricosa.   

Tabellaria flocculosa (“short forms”) from Smerla Water, August 2019.  Scale bar: 10 micrometres (= 1/100th of a millimetre).  Photos: Lydia King.   The photograph at the top of the post shows live colonies of Tabellaria flocculosa.

How does this work with Tabellaria?  The genus is recognisable from its linear valves with central and terminal inflations, presence of septa and absence of a costa and a raphe.   That’s probably all the information you need when using most keys.  Now, assume that the individual is T. flocculosa unless there is evidence to the contrary.  What counterfactuals should we be looking for?

In valve view: 

  • If the width is > 10 micrometres at the central inflation and the rimoportula is at the end rather than at the centre   …. T. ventricosa;
  • If the axial area is narrow and linear throughout, and does not get broader towards the centre …. T. fenestrata;
  • If there are distinct marginal spines … T. quadriseptata.

However, Tabellaria does not always present in valve view (particularly when viewing live material), so we also need some characteristics to help us when it is in girdle view:

  • If there is a sharp kink in the septa just behind the point where they insert into the girdle band   …. T. fenestrata;
  • If there are no more than four (occasionally five) septa and marginal spines are not obvious … T. fenestrata;
  • If there are no more than four (occasionally five) septa and marginal spines are obvious … T. quadriseptata.

I’ve left colony formation out of this list because the literature on the extent to which this is a useful diagnostic criterion is contradictory.

Tabellaria species from Maa Water, Shetland Islands.   a.  zig-zag colonies of T. flocculosa which survived preparation.   b. – d.: three focal planes of a girdle view of T. fenestrata showing the kink in the septa close to the point of insertion (red arrow); e. T. ventricosa, with the terminal position of the rimoportula indicated by an arrow, in contrast to T. flocculosa (f. and g.) where it is close to the centre of the valve.   Scale bar: 10 µm.

Finally, if you are looking at an isolated girdle band (which is a frequent occurrence when Tabellaria are abundant in a sample), then all Tabellaria species except T. fenestrata have closed bands (i.e. a continuous ring of silica) whilst T. fenestrata’s girdle band is “open” (i.e. an incomplete ring of silica).   You should not include isolated girdle bands in a count, but this is evidence that T. fenestrata is present in a sample which should encourage you to look for other features from which you can make a reliable identification. 

One extra piece of advice I always give to beginners is that you should identify populations not individuals.   It may be that not every individual displays all the information you need to make a reliable identification, but that, through accumulating evidence from a range of specimens, in valve and girdle views, you can be confident of the population’s identity. Similarly, size ranges given in identification manuals are not always as reliable as they should be, and you should never reject a potential name solely because a specimen falls slightly outside a quoted range.  Taking measurements of a population to ensure that most specimens fall within these ranges again, gives you the confidence to include the few that exceed these measurements.   

Whilst taxononomy is a science, identification is a craft and, as such, something that individuals may develop and apply in different ways.   What works for me may not work for you so, I shall finish with a quotation George Orwell: “break any of these rules rather than say anything outright barbarous”


Knudson, B.M. (1952). The diatom genus Tabellaria. 1. Taxonomy and morphology.  Annals of Botany N.S. 16: 421-440.

Knudson, B.M. (1953a). The diatom genus Tabellaria. II. Taxonomy and morphology of the plankton varieties.  Annals of Botany N.S. 17: 131-155.

Knudson, B.M. (1953b). The diatom genus Tabellaria. III. Problems of infraspecific taxonomy and evolution in T. floccuosa.  Annals of Botany N.S. 17: 597-609.

Wrote this whilst listening to:   Otis Redding

Cultural highlights:   The film X+Y, sensitive portrayal of an autistic boy coming to terms with adolescence and coping with the world around him.

Currently reading:  American Wife by Curtis Sittenfeld.

Culinary highlight: spoilt for choice, but lunch at Square One in Great Dunmow, Essex (famous for nurturing Professional MasterChef winner Alex Webb) wins out over dinner at 56, our local Sichuanese restaurant in Durham.  Honourable mentions go to Wild Swan bakery in Wanstead and the Wanstead Tap.

Cassop Pond in May

A month on from the phycological debauchery that I wrote about in Promising Young Algae the Spirogyra flocs that covered quite a lot of the surface of Cassop Pond have disappeared.  With sexual reproduction over, the zygotes, I presume, have sunk to the bottom of the lake, where they will lay dormant until next Spring.   I searched around the margins of the pond, but only found a few wisps of Spirogyra hanging around some cattle hoofprints in the shallow water on the eastern side of the pond.   This, however, proved to be a different type of Spirogyra altogether, with broader and squatter cells than the main constituents of April’s flocs.  

The green tinge in this hoofprint is filaments of Spirogyra.   The picture at the top of the post shows Cassop Pond in May 2021: note the absence of flocs compared with the situation in April. 

Swimming around amongst the Spirogyra filaments were a number of very active green cells of Euglena.   Euglena is a genus that has not featured much in this blog over the years (see: “A visit to Loughrigg Fell”) but it is a genus closely associated with Cassop Vale, with 13 of the 36 species recorded from Britain and Ireland recorded from this location.  The story behind this richness is that there were no active experts in the Euglenophyta at the time when the Freshwater Algal Flora of the British Isles was being compiled.   Instead, a Polish expert, Konrad Wołowski, was invited to contribute and, to help him do this, he was taken on a tour of locations that Dave John and Brian Whitton, the editors, thought would be likely habitats.  Cassop Pond is well known to Brian and is conveniently located near Durham, so it was an obvious location.  As a result, it is probably the hot spot of recorded diversity for this genus in the UK but that is more due to the idiosyncrasies of biological recording than to anything about this location over many others that particularly favours Euglena

That said, cattle hoof prints are known to be a good location for Euglena and relatives, and I wrote about a relative of Euglena that I found in a puddle in Teesdale (see: “Puzzling puddles on the Pennine Way …”.  A hoofprint or a puddle is, from an alga’s point of view, a temporary pond and so long as you have a plan in place for when this dries up, it represents a potential habitat.   Cattle, as we have already seen, are allowed to graze on the reserve so there are plenty of damp hoofprints within which Euglena and relatives can thrive.  That’s also true of many other nature reserves around the country.  The one missing ingredient at all of those is naturalists within an inclination to search them out …

Spirogyra filaments from Cassop Pond, May 2021.  Scale bar: 20 micrometres (= 1/50th of a millimetre).


Wołowki, K. (2010).  Euglenophyta.  pp. 181-239.   In: Freshwater Algal Flora of the British Isles (edited by John, D.M., Whitton, B.A. & Brook, A.J.). Cambridge University Press, Cambridge.

Euglena sp. from Cassop Pond, May 2021.  Scale bar: 10 micrometres (= 1/100th of a millimetre).  

Some other highlights from this week:

Wrote this whilst listening to:   The soundtrack to The Pursuit of Love, available via BBC Sounds.   

Cultural highlights:   Emily Mortimer’s adaptation of Nancy Mitford’s The Pursuit of Love, available on the BBC iPlayer

Currently reading:  Pat Barker’s Noonday, the final part of the Life Class trilogy.

Culinary highlight: homemade digestive buiscuits.

The diatoms of Cassop Pond

We’ll stay at Cassop Pond for this next post, as I look back over the diatoms that I’ve found there.   So far, I have collected four samples although, due to the time it takes to prepare these for analysis, I’ve only got around to looking closely at three of these. Nonetheless, I’ve found a total of 98 species belonging to 39 genera in these three samples.  Here is a summary of the more abundant forms.

Araphid diatoms were particularly abundant in the sample I collected from reed stems in January.  Despite my comments in the post I wrote at the time (see: “A winter’s tale …”), the most abundant are Tabularia fasiculataand Ulnaria cf. acus., both of which grow singly or in small clusters, attached to the stem by a mucilage pad at one end.  The genus Tabularia is often described as a species of brackish and marine waters, but in my experience, it can be abundant in freshwater habitats where the water is quite hard.  

Araphid diatoms from Cassop Pond, 2021: a., b. Tabularia fasiculata; c. Ulnaria acus; d. Fragilaria tenera; e. Fragilaria cf. pectinalis; d. Fragilaria, unidentified girdle views; g. Diatoma tenuis; h. Tabellaria flocculosa.   Scale bar: 10 micrometres (= 1/100th of a millimetre). 

Cocconeis species were particularly abundant in the sample from Riccia fluitans and Lemna minor collected in February.   This genus is often abundant as an epiphyte on other plants and algae and it is common to find more than one species in the same sample, which would suggest that there are some subtle aspects of their niches that we do not yet fully understand.   Two of these species were also encountered growing on rocks in Croasdale Beck in Cumbria (see “Curried diatoms?”).  Lemnicola hungarica was also present in the samples, albeit in low numbers.  This species is often epiphytic on Lemna minor (see: “The green mantle of the standing pond …”) and I suspect that a sample composed mostly of Lemna and with less Riccia fluitans might have a higher proportion.   Finally, I have included five different focal planes of a single valve of Eucocconeis flexella, just to show the complexity of valve structure in this diatom.   Note the S-shaped raphe on the upper valve.

Monoraphid diatoms from Cassop Pond, 2021.  a., b. Cocconeis euglypta; c. C. lineata; d.,e. C. pseudolineata; f. Achnanthidium caledonicum; g. A. eutrophilum; h. A. saprophilum; i., j., k.Planothidium lanceolatum (3 focal planes); l. Platessa oblongella; m., n., o., p., q, Lemnicola hungarica; r., s., t., u., v. Eucocconeis flexella.  Scale bar: 10 micrometres (= 1/100th of a millimetre). 

One of the surprises of this sample was the relatively high proportion of Eunotia species that I found, particularly in the sample from February.  Eunotia is a species most often associated with soft water so I had not expected to find it to be frequent in a calcareous pond.   However, this sample was collected on the east side of the pond, where some spoil heaps form part of the shoreline.   Moreover, neither of the two species that were most abundant are particularly associated with very low pH.  However, these were a curiosity and there were also a few other species in the samples (e.g. Tabellaria flocculosa) which hinted that soft water might have some influence in the pond.   

Eunotia species from Cassop Pond, 2021.   a.,b. Eunotia bilunaris; c.,d.,e.,f. E. minor; g. E. incisa.   Scale bar: 10 micrometres (= 1/100th of a millimetre). 

The role of Epithemia adnata in the pond was considered in Working their passage so we don’t need to say much more here except that this is another species that is far more abundant on plants than on rock surfaces in this pond.   By contrast, the genus Nitzschia was much more common the rocks at the north end of the pond than on the plants.   These rocks were covered with a fine layer of marl (fine calcite deposits that has precipitated out from the water).  The most abundant species here was Nitzschia palea.

Epithemia adnata from Cassop Pond, 2021.   Scale bar: 10 micrometres (= 1/100th of a millimetre).
Nitzschia and Tryblionella species from Cassop Pond, 2021. a. Nitzschia paleacea; b. N. subtilis; c., d., e. N. palea; f. N. cf. archibaldii; g. N. capitellata; h. Tryblionella sp.  Scale bar: 10 micrometres (= 1/100thof a millimetre).

Another diatom that was common on the rocks at the north end was Cymatopleura solea.  Whilst this was less abundant in terms of numbers than Nitzschia palea, it has much larger cells, so the overall contribution to biomass and photosynthesis is probably the same or even greater than that species.   When I tried to describe Cymatopleura solea, with its central  constriction along with transverse undulations across the valve surface, to a class a few years ago, one participant suggested that it was a “voluptuous” diatom.   The presence of this along with the Nitzschia suggests that motility is an attribute that favours diatoms in this habitat, in contrast to the two samples from plants, which were both dominated by non-motile species. 

Cymatopleura solea from Cassop Pond, 2021.   One valve photographed at three focal planes.   Scale bar: 10 micrometres (= 1/100th of a millimetre).

The most diverse genus encountered was Navicula, with 14 species, although these were never found in great numbers.  As befits a motile genus, these were most abundant in the sample from the rock although they were also found in small numbers in the samples from plant surfaces.  Other biraphid symmetrical species found included Caloneis, Sellaphora, Hippodonta, Fallacia and Neidium.   Two of the images are described as “Sellaphora pupula” but we know that this is an aggregate of several species, barely distinguishable with the light microscope.  Both of these images are likely to represent different species. A significant omission from my list is Mastogloia (see “Structural engineering with diatoms”).  The habitat seems right for this species, and I have found it in other ponds in the area (see “Return to Croft Kettle”) so I suspect that it may turn up at some point during the year.

Biraphid symmetrical diatoms from Cassop Pond, 2021: a. Navicula cryptotenelloides; b., c. N. trivialis; d. N. subalpina; e. Caloneis amphisbaeana; f. Hippodonta capitata; g., h. Sellaphora pupula ag.; i. S .saugerresii; j. Fallacia pygmaea; k., l., m. Neidium dubium (three focal planes).  Scale bar: 10 micrometres (= 1/100th of a millimetre).

Finally, there were a few valves of Gomphonema, Cymbella, Amphora and Halamphora, but none present in significant quantities.  

To put the 96 diatoms I’ve recorded to date into perspective, Heather found 182 and 123 angiosperm species over the course of a year at two nearby nature reserves, both with similar geology to Cassop Vale.   That puts the diversity of the microscopic world into perspective.  Bear in mind, too, that the samples I’ve looked at to date were collected in the winter.  I fully expect the final count of diatoms to exceed that of angiosperms but we’ll have to wait and see.   Is an element of competition creeping into this natural history malarky?   Surely not …

Heteropolar and dorsiventral diatoms from Cassop Pond, 2021: a. Gomphonema cf. graciledictum; b. Gomphonema sp.; c. Cymbella affinis; d. Halamphora montana.  Scale bar: 10 micrometres (= 1/100thof a millimetre).


Mann, D.M., Thomas, S.J. & Evans, K.M. (2008).  A revision of the diatom genus Sellaphora: a first account of the larger species in the British Isles.  Fottea (Olumec) 8: 15-78.

Some other highlights from this week:

Wrote this whilst listening to:   Scottish singer-songwriter Karine Polwart, who I first encountered as the “hold” music on Triodos Bank’s customer service line.  The first and only time in my life I wish I was 8th in the queue, instead of 6th.

Cultural highlights:   Nomadland, winner of the Oscar for best film.   A Ken Loach vibe but set in the western USA rather than north east England.

Currently reading:  John le Carré’s Absolute Friends.

Culinary highlight: our first meal out for many months, at Whitechurch in Durham.  Apart from the cold and the damp, it was great.  

Pond politics …

We have not travelled away from Cassop Pond for this next post, as I try to summarise the earlier visits in a picture.  On the left-hand side, there is a stem of Phragmites australis, with epiphytic diatoms, dominated by Tabularia fasiculata (rather than the species I suggested on first examination – see “A Winter’s Tale”).   At the top right there is part of the thallus of the liverwort Riccia fluitans (see “Working their passage”) with different epiphytes: a combination of Cocconeis lineataRhoicosphenia abbreviata and Epithemia adnata. Then, towards the centre of the picture there is Lemna minor, with a floating leaf and a single root dangling below.   The leaf has some more Cocconeis on the underside, but also some Fragilaria (probably F. gracilis) on the root.   

Epithemia is a diatom often associated with nitrogen limitation and, interestingly, is one of a number of clues that Cassop Pond is nitrogen-limited for at least part of the year.   I also found some filaments of the cyanobacterium Aphanizomenon gracile, which can fix nitrogen via its distinctive heterocysts, and I also mentioned, in my previous post, that nitrogen limitation might be one of the triggers for conjugation in Spirogyra.   Interestingly, the Epithemia seems to be most abundant in the flocs of Riccia fluitans: a scarce resource, presumably, being even scarcer when there are plenty of other plant cells hoovering up any that is in the vicinity.   Why not also on Spirogyra?  Probably because the slimy mucilage that surrounds these filaments makes it difficult for an epiphyte to gain purchase.   The only time when epiphytes are abundant on Spriogyra and relatives is when the filaments are clearly unhealthly.

We can think of this in terms of the cost-balance sheets of the respective organism.   Spirogyra’s business model is focussed on maximising photosynthesis and, as such, it diverts some of its budget to produce mucilage.  That means that there are no pesky epiphytes to stand between the sunlight and its chloroplasts.   Riccia fluitans has a different approach: it sees epiphytes not as a “cost” but as a “benefit”: maybe the diatoms growing on the surface stop some sunlight getting to the liverwort’s photosynthetic cells but quite a few of these diatoms fix nitrogen and, as their cells are prone to “leakage”, some of the surplus nitrogen will be there to help the liverwort grow.   The diatoms provide a “subsidy” to the liverwort, to use ecological jargon.  Spirogyra is one of those right-wing algae that probably talks glibly about “trickle down economics” but, in practise, it is going all out for itself.   Don’t get me started on trickle-down economics.

Dinobryon sertularia, a living colony from Cassop photographed at four different focal planes.  Scale bar: 20 micrometres (= 1/50th of a millimetre).  

I also came across Dinobryon sertularia during my recent trips to Cassop Pond.  This is usually described as planktonic, although I found it growing in the brown film surrounding Phragmites stems at the pond’s margin.   The cells of this alga live in vase-shaped cases (termed a “lorica”) which are usually united to form colonies.  Each cell has two flagella – both clearly visible and busily thrashing around enough to make any attempt to produce a crisply-focussed image impossible.   You can see an excellent image by Hilda Canter-Lund here, almost certainly taken from fixed, rather than living, material.  Dinobryon is a member of the Chrysophyceae, which we last encountered in “The Little Tarn of Horrors”.  As explained in that post, many Chrysophyceae (including species of Dinobryon) are “phagotrophic” – capable of gaining energy and nutrients from bacteria and other particles they ingest.  The Dinobryon colony that I viewed was likely using its flagellae to create turbulence in the water that would waft bacteria in the direction of its gullet, as much as it was using them to move.   That’s another sign, perhaps, that Cassop Pond is, if not as nutrient-poor as Cogra Moss (where our previous encounter with Chrysophyceae took place), at least an imbalance in nutrients in the water that means that some “dietary supplements” will not go amiss.  

Four months into my visits to Cassop Pond and I am beginning to see the dynamics of the pond unfolding.   We’ve learnt about some of the “nouns” that occupy the pond but also, through these, are beginning to learn a little more about the “verbs”: the activities and functions that bind the other organisms into a living ecosystem.   We often think of ecosystems in terms of “survival of the fittest” but the picture that is emerging in Cassop Pond – and in countless other ecological studies – is that there are a lot of subsides and mutually-beneficial interactions between the organisms. Cassop Pond, like many of the villages around it, still leans to the left…


Caron, D.A., Sanders, R.W., Lim, E.L., Marrasé, C., Amarl, L.A., Whitney S., Aoki, R.B. & Porters, K.G. (1993). Light-dependent phagotrophy in the freshwater mixotrophic chrysophyte Dinobryon cylindricum .  Microbial Ecology 25: 93–111. 

Some other highlights from this week:

Wrote this whilst listening to:   Crosby, Stills and Nash’s 2009 set at Glastonbury via YouTube, which brought back some happy memories.   They played the day after Neil Young so I can stretch a point and say that I saw Crosby, Stills, Nash and Young that weekend.

Cultural highlights:   We watched both winners of the Oscars for best documentaries this week.  Both are good but we particularly recommend My Octopus Teacher, filmed in shallow waters off the South African coast and encapsulating the leitmotif of this blog: repeated visits to the same location yields unexpected insights into natural history.  The film also deserves the Oscar for Best Performance by Kelp in a Supporting Role, if such a category existed.  

Currently reading:  The Well-Gardened Mind, by Sue Stuart-Smith: a book about the therapeutic benefits of nature and gardening in the modern world.

Culinary highlight: Cauliflower steaks with a harissa sauce.  And Queen of Puddings.

Promising young algae …

Spring has arrived in Cassop Vale.  Leaves are appearing on many of the trees and the ground vegetation has the green flush of a new beginning.   More importantly, the herd of emo-fringed highland cows have been moved away, to give the plants more chance of flowering, and there is some warmth in the sun in the middle of the day.

From my point of view, the biggest change since I was last here is the appearance of an extensive floc of green algae covering much of the pond’s surface.   I had a hunch, from their appearance, that these would be predominately Spirogyra, but was not expecting the sight that greeted me when I put a small piece of a floc under the microscope. 

Flocs, predominately Spirogyra, in the margins of Cassop Pond, April 2021.

I find Spriogyra and its relatives quite regularly on my travels, but usually in the vegetative state.  It is relatively unusual to find them as they undergo sexual reproduction (see “Fifty shades of green …”).  But there was plenty of evidence of this process (termed “conjugation” in Cassop Pond’s green flocs.  There were plenty of vegetative filaments, each about 20 micrometres wide and with a single helical chloroplast.  But there were also many ellipsoidal zygotes apparent.   When I looked more closely, these were inside filaments which were linked to an adjacent filament by a narrow tube.   What started out as an early morning natural history trip has turned out to be the algal equivalent of Saturday night on Newcastle Quayside.   

For those of you unused to dating, Spirogyra style, here is a quick guide.   First, put on your best helical chloroplast (two or more, if you are daring), then head out to find a partner amongst the many other filaments in your particular floc.   Little is known about Spirogyra’s preferences, but we can assume that many species are not heterosexual, so don’t be shy: sidle up to any filament you fancy.   He/she/it might well play hard to get at first, so maybe you need to drop a hint.  Make sure your potential date gets a whiff of your aftershave (that’s what I assume “hormonal interactions between the paired filaments” means).  If he/she/it gets the hint, then you can indulge in a little mutual meiosis to get yourselves into the mood.    

Spirogyra from flocs in Cassop Pond, April 2021.   a. vegetative filament; b. two filaments undergoing sexual reproduction with zygotes in the lower filament.   Narrow filaments of Aphanizomenon gracile are also present.   Scale bar: 20 micrometres (= 1/50th of a millimetre).  

Now we’ve got that all-important emotional (okay … hormonal) connection, it is time to get physical.   An embarrassing bulge appears on the side of your filament but, fortunately, a similar one should appear on the side of your date’s filament at about the same time.   Eventually, these fuse to form a tube that links you both together.  The correct term for this is the “copulation canal” which is as frank as it is alliterative (it could also be called a “tupping tube”, I guess?). The protoplast of both cells now contracts and one (the “boy”, for want of a better analogy) crawls, amoeba-like, through the tube and fuses with the “girl” protoplast to form a zygote.  That’s as far as our frisky filaments in Cassop Pond have got.  If our phycological peep-show continued for longer, we would see the green zygotes gradually become brown in colour as thick, resistant walls grew around them, and the cell contents were processed into starch and lipid-rich food reserves.   They would then sink to the bottom of the pond and rest, dormant, until conditions were ripe for its germination.

Features of Spirogyra conjugation: a. a vegetative cell in one of the two aligned filaments; b. conjugation canals developing between the aligned filaments; c. a zygote.  Scale bar: 20 micrometres (= 1/50th of a millimetre).  

Why here, why now?   Nitrogen limitation has been quoted as one of the triggers for conjugation and the presence of a nitrogen-fixing cyanobacterium (Aphanizomenon gracile) plus nitrogen-fixing diatoms (Epithemia– see “Working their passage”) in the pond at the same time lends support to this hypothesis.  Also, the yellow-green appearance of the flocs is also a hint that they may be nitrogen-limited.   However, there are also reports of conjugation happening on a predictable annual pattern in some locations.  The two possibilities are not mutually exclusive, we should remember.  

Meanwhile, on dry land, there are plenty of other plants getting down to the complicated business of reproduction too.   We saw goat willow (Salix caprea) and hazel (Corylus aveana) as well as lesser celandine (Ficaria verna) in flower, and leaves of primroses yet to bloom.   You can read more about those here.   Just remember, when enjoying the sight of spring flowers, that the botanical bacchanalia takes place in less obvious ways in the water too.

Some other highlights from this week:

Wrote this whilst listening to:  Horses and Easter by the Patti Smith Group (see below).   And a 1977 BBC “Sight and Sound in Concert” recording of Jethro Tull, which I remembered seeing when it was first broadcast.

Cultural highlights:   The film Black Bear – a rather dark and challenging, but ultimately rewarding, film.

Currently reading:  Just Kids, by Patti Smith.  Best read with Horses and Easter as a soundtrack.  The geographer in me also reads it with a map of New York to hand, as it is a book with a very strong sense of place.

Culinary highlight:.our local Indian restaurant makes a rather good lamb shank, cooked in aromatic spices which, with basmati rice and a side order of bhindi, is just about unbeatable.

Of microbes and mountains …

I spent a few anxious days last week watching the weather forecast in anticipation of fieldwork.   It was not clear from the forecast whether there would be enough rain to lift river levels to a point where I could not safely collect samples or if, rather than rain, there would snow, which may make the roads slippery but would, at least, mean that river levels were low.  The best I thought I could expect was to be working in near freezing conditions amidst flurries of snow, so it came as an unexpected surprise to find clear skies and sunshine even if April water temperatures were still bitingly cold.   The photograph at the top of the post shows Wastwater, looking towards Scafell Pike and Great Gable on a cold but still April morning, but the focus is on the River Irt, about 500 metres or so downstream from the outfall from the lake.   The landscape overlooking the lake is spectacular but, on a different scale, the subaquatic landscape that I see when I peer at the bed of the River Irt through my aquascope, is equally spectacular, albeit on a vastly smaller scale. 

Underwater landscape in the River Irt, April 2021, with green algae (a.), mosses mixed with Cyanobacteria (b.) and diatoms (c.).   The foreground of the picture frame is about a metre in width.

The flocs of green algae are mostly composed of Spirogyra whilst the mosses and cyanobacteria combos in the background are the same as those that I wrote about in “Tangled up under blue skies” a couple of months back.   The diatoms in the front foreground are the most intriguing.   Here, as at several other locations, we could see and feel distinct biofilms on rocks, but our measurements of chlorophyll concentrations were nearly always very low.  The answer became clear when I looked at some of the biofilm under my microscope: the dominant diatom in these biofilms was a long-stalked form of Gomphonema; probably the same one that I wrote about in “Diatoms and the space-time continuum” a few years back.   The biofilm that covered the patches where other algae and mosses were not dominant was composed mostly of the polysaccharide stalks, with the photosynthetic cells forming a surface layer.  From the point of view of any subaquatic grazer, this roughly equates to a piece of bread with a thin smear of marmite on top.   However, the tangled mass of stalks also works well as a means of trapping organic and inorganic particles that are in transit through this part of the river, and most of the invertebrates that live on stream beds are not particularly fussy about precisely what is on the menu.   I have no data to back this up, but my hunch is that these diatoms make a bigger contribution to the diet of these bugs through these trapped particles than they do through the energy that they have created from trapped sunlight.  

Gomphonema on long stalks, along with a few Achnanthidium, from the River Irt, April 2021.  Scale bar: 10 micrometres (= 1/100th of a millimetre).  

With samples from the River Irt safely packed into the cool box, our field work was finished and, as it had already spilled into the weekend, we felt no compunction about heading back to explore the mountains that loomed over Wastwater.   Our target today was Great Gable, a steep pull up from Wasdale, but a climb that rewarded us with striking views of the lake and the famous screes.   Having started our walk with blue skies and cotton wool clouds, it also gave us a panoramic view of the changing weather as clouds blew in and showers fell along the west Cumbrian coastal plain.   We encountered a few flurries of snow as we approached the summit of Great Gable, but not enough to obscure a view which extended to Windermere to the east, Skiddaw, Helvellyn and Blencathra to the north-east and Crummock Water and the Liza valley to the north.  Somewhat to my surprise, Ennerdale Water was not quite visible from the summit.

The view back along Wasdale towards Wastwater from the flank of Great Gable, April 2021.

We made a steep descent to Styhead Tarn, then followed the track back to Wasdale, passing a group of intrepid mountain cyclists on the way up variously pushing and carrying their bikes.  We could still see showers in the distance but the snow flurries we encountered on the way up did not seem to have come to much.  Consequently, it was a surprise to us when, having driven out of Wasdale and back to our accommodation in Ennerdale, we saw the flanks of the hills around Great Gable to be covered with snow.   Last April, we were basking in unseasonably warm weather at the start of the first lockdown; a year on there is snow, and not just on the high fells either.   Nature continually surprises us, on a whole range of scales, from the minute to the enormous.  That’s what keeps pulling us back.

Wrote this whilst listening to:  Déjà vu, by Crosby, Stills, Nash & Young.

Cultural highlights:   The film The Mauritanean, about the longest serving prisoner at Guantanamo Bay.  

Currently reading:  Spring Has Not Been Cancelled: David Hockney in Normandy, by Martin Gayford.  Reflections on landscape painting by an undisputed master.

Culinary highlight: probably has to be the sandwiches we ate on the way up Great Gable, mostly because of the spectacular views that accompanied them.

Snow over the fells to the south of Ennerdale Water: Heather’s photo taken on the evening of 10 April after our ascent of Great Gable (just out of frame).