More about Croft Kettle

In my post on Croft Kettle, I commented on the long stalks possessed by Cymbella cymbiformis. These were difficult to capture with my camera, partly because the Cymbella cells readily detach themselves from their stalks and partly because the tangle of stalks exceeds the depth of field available to the microscopist. Instead, I have tried to capture the view through the microscope eyepiece in a drawing.


Croft Kettle, epiphytic algae associated with Chara hispida stems, May 2015. Drawn at x400 magnification.   The narrow stalks on the left hand side are about five micrometres in diameter.

There is a tangle of stalks on the left hand side, along with two cells each of Cymbella cymbiformis, Navicula radiosa and Rhopalodia gibba. Note, too, the narrow filament of Oedogonium, complete with oogonia (see “Love and sex in a tufa-forming stream …”).   The Rhopalodia cells have glided free from the tangle of stalks.

As I looked at these rich communities of algae I started to wonder if would make a good subject for a painting, so I have been continuing to examine the material I collected in order to build up a sense of what the three-dimensional community around the Chara stems would have looked like. One interesting observation came when I had a look at some of the narrow branchlets of Chara. I wanted to see which algae were directly attached to the Chara surface (more about this in a moment) but the feature that was most noticeable was the quantity of calcite crystals deposited around the stems.   These give Chara its characteristic stiff stems that are rough to the touch.   The calcite is deposited as a by-product of photosynthesis; intriguingly, Chara shares this property with many tufa-forming algae and bryophytes but not with its close relative Nitella, which is much softer to the touch (see “Finding the missing link in plant evolution…”).


Calcite crystals deposited around the tip of a branchlet of Chara hispida from Croft Kettle, May 2015.   Image composed using Helicon Focus stacking software. Scale bar: 10 micrometres (= 1/100th of a millimetre).

One of the questions that was puzzling me was the habit of the diatom Rhopalodia gibba within the community of algae on and around the Chara stems.   In many of my specimens, Rhopalodia seemed not to be attached to the Chara but, instead, glided amidst the tangle of Cymbella stalks growing around the Chara stems; however, I also saw a few cells directly epiphytic on the Chara stems, and this also seems to be the habit that Chris Carter has captured in some of his images of Rhopalodia. I suspect that Rhopalodia, and many other diatoms are opportunistic and can adopt slightly different habits depending upon the prevailing conditions. There is no point, for example, in doggedly sticking to an epiphytic habit if this m

eans sitting in the deep shade cast by a Cymbella forest. Whatever the textbooks say.


Rhopalodia gibba associated with Chara hispida stems in Croft Kettle, May 2015. b. is a valve view; e. is a girdle view and a.,c. and d. are intermediate between the two positions. Scale bar: 25 micrometres (= 1/40th of a millimetre).


Rhopalodia growing on Chara. Photographs by Chris Carter.

Rhopalodia is a genus with an unusual morphology. The raphe follows the dorsal margin (i.e. the left hand side of b. in the figure above) but this means that, in girdle view (i.e. looking from above), both raphes are on the same side of the valve.   I have often assumed that having raphe slits on opposite sides assists motility, by giving the cell two planes by which it may attach (much like a climber working his way up a narrow chimney).   It is possible that being attached to the surface is the preferred habit; motility would become an advantage only when the energy that this process consumes is offset by that supplied by the extra photosynthesis that can take place when it moves away from the shaded areas and into the canopy.   I have never seen any work done to address this topic, but it would make an interesting study.

Another interesting feature of Rhopalodia is the presence of cyanelles, organelles derived from cyanobacteria or similar prokaryotic algae. We also encountered these in Epithemia (see “A return to Cassop”) where I also mentioned that they may be involved in nitrogen fixation.   Cyanelles deserve a post all of their own at some point in the future, so I will just leave you for now with another of Chris Carter’s excellent photographs, in which the cyanelles of a Rhopalodia sp. are highlighted.   They are near-transparent, with thin membranes and are easy to confuse with the vacuoles that contain the polysaccharide chrysolaminarin (these tend to be more refractive). Very easy to overlook.

As I was putting this post together, I noticed that West and Fritsch noted that Rhopalodia gibba was “common in all kinds of localities.”   This surprised me, as I have only ever seen it at a handful of sites in the UK. It did make me wonder if West and Fritsch, writing in 1927, were right, and that it has declined significantly subsequently. A species that has a competitive advantage at low nitrogen concentrations will not have had an easy life in the period after West and Fritsch wrote, as agricultural intensification and widespread use of fertilisers led to increases in the concentration of nitrogen in surface water.


A girdle view of Rhopalodia sp. with cyanelles indicated by arrows.   Note, too, the characteristic lobed chloroplast. Photograph by Chris Carter.


West, G.S. & Fritsch, F.E. (1927). A Treatise on the British Freshwater Algae.   Cambridge University Press, Cambridge.


The Hilda Canter-Lund Prize


The Hidden World of Algae, showing winners and shortlisted entries for the Hilda Canter-Lund prize, 2009-2011, at the Great North Museum, Newcastle, January 2012.

The 2015 Hilda Canter-Lund competiton is well under way (deadline: 24 June, full details at and the winner this year has the honour of seeing their image displayed in the Angela Marmot Centre, at the Natural History Museum in London.   This will not be the first time that prize winners have had their images on public display: in 2012 the Great North Museum in Newcastle hosted the “Hidden World of Algae” exhibition, with 18 images from the shortlists on display along with four of Hilda Canter-Lund’s own images.

The staff at the Great North Museum did a great job, tactfully editing the captions to remove all the jargon that they knew their audiences would not understand, and using their expertise to put on a really professional display.   One of their suggestions was to place QR codes alongside some of the images so that visitors could listen to the photographers conveying their enthusiasm about their subject matter.   Great idea, I thought (once I had looked up “QR code” on my computer – this was 2012, remember, eons ago in the fast-moving world of the internet).   I asked some of the photographers to record a couple of minutes of audio and send it to me to pass on to the museum. They, then, uploaded the audio file to YouTube and set up QR codes to point to their locations.

The only problem was that the gallery where our images were displayed had very poor mobile ‘phone reception, which meant that few of our visitors got any further than pointing their ‘phones at the QR codes and waiting … and waiting … for something to happen.   Consequently, these audio files have sat on the Great North Museum’s YouTube channel ever since, without the audiences that they deserve.

So here they are, in all their multimedia glory:


Gordon Beakes talks about his personal memories of Hilda Canter-Lund and also about his image of algae from Langdale Tarn in the Lake District, shortlisted in 2010.


Chris Carter talks about the origins of his fascination with algae, and about his image of the red algae Balbiania, shortlisted in 2012. Chris won the competition in 2013.


Eileen Bresnan talks about her image Chaetoceros Chaos, shortlisted in 2010.


John Huisman talks about his image of the red alga Dictyomenia sonderi, which was shortlisted in 2011 (John was also the winner in 2014).


Mariano Sirioni talks about his image of a southern Right Whale swimming through an algal bloom off the coast of Argentina, the winning image in 2009.

Remember: the closing date for the competition is 24 June, so there is still plenty of time to find a favourite image to submit.   The BPS website has all the details.


The desert shall rejoice and blossom …


The Nordic Diatom Meeting, Ratnieki, Latvia, May 2015.  

The motivation for my recent trip to Latvia (see “Following in Arthur Ransome’s footsteps …”) was to attend the Nordic Diatom Meeting, which took place in the University of Riga’s conference centre, Ratnieki, set in the Latvian countryside. It was a small, informal and very friendly meeting, fuelled by enormous quantities of food and enlivened by an excursion that managed to compress an overview of Latvian history from the Iron Age to the present into one afternoon of sightseeing.

It is a little unfair to pick out one of the presentations, but there was a moment on the first afternoon when I sat up with a start as the story of a fossil lake in the Saharan desert unfolded.   There is, in the middle of the Western Great Erg (a huge sand-covered area of the Sahara in southern Algeria), within which there are several depressions. Some of these contain deposits that suggest that they were once lakes.   An Algerian PhD student, Nassima Yahiaoul, told us about her study of an outcrop in one of these depressions, Guern Touil, which was composed largely of diatomite, a rock consisting largely of the remains of dead diatoms.   This is good evidence that, in a moister period perhaps 7000 years ago (the precise date is not yet known), this area was not a bleak, unforgiving desert, but a freshwater or brackish lake.

What made me take particular notice, however, was the diatoms that she found when she analysed these deposits.   These included Cymbella cymbiformis, Epithemia argus, Denticula tenuis, three species of Mastogloia and Navicula oblonga, a very large and distinctive species.   None of these are particularly common in the streams and lakes that I study in temperate Europe but, curiously, several of these occur together in a small pond about 30 kilometres away from where I lived. This pond is, itself, botanically quite distinctive, and it was a strange sensation to sit in the Latvian countryside and hear about another with such an uncanny resemblance but which is so far distant in both space and time.


The outcrop of diatomite in the Guern Touil depression, Western Great Erg, Algeria, studied by Nassima Yahiaoul.

The place that Nassima’s description evoked for me is Croft Kettle, a small pond is fed by subterranean springs emanating from the Permian limestone.   It is fringed by the saw sedge, Cladium mariscus but the edges of pond then shelve very steeply and the submerged vegetation is dominated by Chara hispida and C. vulgaris.   Whether Nassima’s pond ever looked like the illustration below is debatable (there are fossil forests near Guern Touil so the idea of a tree-fringed oasis at some point in the Holocene is not wholly fanciful). The bare evidence that palaeoecologists produces often needs to be catalysed by the imagination, and the imagination, in turn, feeds off analogies. So long as we treat these speculations with a healthy dose of caution, all is good.


Croft Kettle, a Site of Special Scientific Interest in County Durham, just south of Darlington, photographed in May 2015.

As the pond is fed by subterranean springs, the water in Croft Kettle is very clear, allowing the dense Chara beds to extend into the depths.   I could only reach the very edge of these beds when I visited a few days ago, but I was struck by the large quantity of yellow-brown diatom growths that smothered the Chara.   Under the microscope, these proved to be composed of a dense tangle of a stalked diatom, probably Cymbella cymbiformis, within which other diatoms such as Rhopalodia gibba and Navicula radiosa were moving.   The Cymbella is the same one that Nassima found in Guern Touil and I could also see representatives of three of the other genera that she described. I have recorded some of the other species that Nassima recorded from here, but they were not showing themselves today.


An underwater view of the margins of Croft Kettle, showing the dense beds of Chara, smothered by growths of diatoms, May 2015.

The quantity of diatoms that I saw in Croft Kettle was surprising, especially as I normally expect grazers to be very active at this time of year.   The yellow-brown growths resembled those that I reported from the River Ehen in April (see “Diatoms and dinosaurs”). Those were of a Gomphonema species which, like Cymbella cymbiformis, grows on the end of long stalks. These, in turn, create a tangled matrix within which other species of diatom can live.

The Cymbella cells become detached from their stalks very easily, which means that it is easier to photograph isolated cells than the complete stem plus stalk complex.   The tangle of stalks is also difficult to capture in a photograph due to the very shallow depth of field available when you are using medium-and high-magnifications.   That brings me back to the topic of imagination: the microscopist needs this just as much as the palaeoecologist, if s/he is to gain an insight into the nature of communities that have been wrenched out of their natural habitat and squashed under a cover slip.   More so, indeed, for the diatomist, who habitually marinades samples in a sauce of oxidising agents to leave just the silica frustule behind.   But here I go again … droning on about the need to understand diatoms in their living state.   Forgive me …

Enough for today: Croft Kettle is a pond with many fascinating – and one or two very unexpected – stories to tell.   Plenty to keep me going for a few more posts …


Cymbella spp. growing on Chara in Croft Kettle, May 2015; a. – c.: Cymbella cf. cymbiformis; d. – e.: two as yet unknown Cymbella sp. Scale bar: 10 micrometres (= 1/100th of a millimetre).


Wheeler, B.D. & Whitton, B.A.(1971). Terrestrial and Sub-aquatic vegetation. The Vasculum 56: 25-37.

Hudson, J.W., Crompton, K.F. & Whitton, B.A. (1971). Ecology of Hell Kettles; 2. The Ponds. The Vasculum 56: 38-45.