Rotifers in the River Ehen

I promise that this is the final post on the River Ehen for a while. There is often quite a lot of the aquatic moss Fontinalis antipyretica at one of the sites we visit and it proved too tempting a subject for my new underwater camera. However, the real surprise came when I put a couple of leaves under the microscope on my return. I thought I should check that this really was F. antipyretica and not the related F. squamosa, and this necessitated a quick check of the leaf structure. What I saw when I looked down was that every leaf had several tiny rotifers attached to the surface, busily whirring their cilia to suck food particles into their gullets.


Fontinalis antipyretica in the River Ehen, March 2014.
Rotifers really are beautiful organisms to watch under the microscope. The ones I was looking at were mostly about 0.1-0.2 mm long (it is hard to give exact dimensions as their shapes were constantly changing). They were attached by their “foot end” to the leaf surface whilst the other end bears a simple mouth surrounded by a ring of cilia. These beat in synchronous waves to sweep food particles into the mouth; the impression to the viewer is of a rotating ring though, in fact, the cilia remain stationary. The rotifers I was looking at, Bdelloidea, actually possess two of these rotating “wheels” of cilia.
I wanted to take some photographs, and even videos, of these rotifers, because the visual effect of these beating rings of cilia is quite mesmerising. However, the rotifers were constantly in motion, moving through three dimensions making it impossible to keep the wheels in focus for long enough. There are ways of slowing rotifers down but all require experimentation to get it right. I tried adding alcohol and then some gum arabic, which should have made the water just viscous enough to slow down the cilia. Neither worked for me but it is probably just a matter of time and experience to get the concentrations and exposure times right. There is also a commercial preparation, ProtoSlow, which has much the same effect. In the end, I resorted to my pencils and paintbrushes though this does not really capture the full glory of the spinning wheels of cilia around the rotifer’s mouth.


A sketch of a Bdelloid rotifer feeding on the leaf of Fontinalis antipyretica in the River Ehen, March 2014. The main picture shows the rotifer in an upright position, extracting food particles from the water; the smaller picture, bottom right, shows the rotifer bent over to “hoover” up diatoms and other algae from the leaf surface. The scale bar is 25 micrometres (1/40th of a millimetre).

The presence of so many rotifers tells me a little more about the story of the River Ehen. So many organisms adapted to capturing tiny food particles from the water flowing past must be a sign that there is a plentiful supply of such particles. Perhaps these are washed out of the sediment, perhaps from the surrounding catchment. I suspect, too, that the rotifers are not the only bugs feeding on these particles, and that concentrations will be highest just after spates (which will flush much more of these particles into the river). There was a spate just a few days before we visited. And, I suspect, these particles also act as micro “compost heaps” for the algae which are my main interest in the river. This might partly explain the conundrum of why so many algae are growing in a river that apparently contains so few nutrients.

Can we use lakes to heat our homes?

Having made some light-hearted comments about the effect of insecurity in Russia on commercial interest in renewable energy in a recent post (“The truth is sometimes stranger than fiction …”), I was intrigued by an article in this week’s Independent on Sunday: “Renewable energy from rivers and lakes could replace gas in homes”. The premise is that the high specific heat capacity of water means that large bodies of water store a huge amount of thermal energy acquired from solar heating. An engineering company has now come up with an ingenious device, based on similar principles to refrigerators and air conditioners, which extracts this heat and stores it in a form that can be used to heat homes. The headline figure, probably widely optimistic, is that this could reduce household bills by 20 per cent.

But, hang on, what are the environmental consequences of this? Lakes, in particular, depend upon solar heating in many ways. What might happen if large quantities of the thermal energy in lakes are removed? Could it, for example, affect the ecology of species that depend upon particular ranges of temperature? Might this shorten the potential growth season for some species? Might it, even, counteract some of the consequences of climate change?

One reassuring aspect is that the most plausible locations for such schemes are lowland areas where rivers are already heavily modified and the standing water bodies are artificial reservoirs. The Independent on Sunday article described a scheme on the River Thames for example. Managers of lowland reservoirs, in particular, might find unexpected benefits: solar energy is responsible for the stratification of lakes in summer, when a warm surface layer sits over cooler water in the depths. When stratification is combined with abundant nutrients, as is often the case in lowland reservoirs, you have ideal conditions for toxic blue-green algae to thrive. Toxic algal blooms are neither natural nor desirable, as they pose health risks and incur higher treatment costs. Reducing nutrient concentrations is desirable but difficult. Maybe this type of energy-harvesting scheme would have unexpected spin-offs in terms of making conditions less favourable for toxic algae?

So, whilst I am not wholly convinced by the hyperbole in some of the statements in the article, I find myself intrigued to know exactly what the environmental costs and benefits are. And I wonder if, just for once, this might be a renewable energy scheme that does have some very positive spin offs too.

Lemanea in the River Ehen

The rocks in the fastest-flowing sections of the lowermost of our four sample sites on the River Ehen were all smothered with the coarse filaments of Lemanea fluviatilis. Lemanea is another red alga (see “The schizophrenic life of red algae …”) but one that grows to a much larger size than Audouinella which I wrote about back in early February. I wrote about Lemanea last year (“The River Ehen in April”) but that was before I had an underwater camera. However, most of the Lemanea is attached to large, stable boulders located in sections of the river where the fast current made it almost impossible to photograph safely. Instead, I hunted around and found a smaller stone that was wedged in amongst these, and moved this to a shallow area where it was easier to photograph.


Lemanea fluviatilis from the River Ehen in March 2014. Scale bar: one centimetre.
If you look closely you will see that each of the filaments has a series of nodes along its length. Under the microscope, these nodes form darker patches, composed of smaller cells than the rest of the filament. These are, in fact, the reproductive structures, spermatangia, of the plant as Lemanea has a similar life-cycle to that of Audouniella, which I described in my earlier post. There is also a closely-related genus, Paralemanea, which looks like Lemanea but which has these spermatangia in rings rather than in patches. Older books do not recognise the distinction between Lemanea and Paralemanea.


Lemanea fluviatilis from the River Ehen, March 2014. a. low-power image showing the knobbly stems; b. close-up of a single stem showing the spermatangia patches associated with these protruberences (scale bar: 20 micrometres; 1/50th of a millimetre); c. close-up of a patch of spermatangia.
Lemanea is, in my experience, a very useful indicator of good quality aquatic ecosystems. Looking back through my own records, I see 88 per cent are associated with “high status” or “good status” conditions and the few instances where it is found associated with poorer quality conditions, it is always quite sparse. There is a site quite close to Durham where we used to find Lemanea despite the water being quite enriched with nutrients: though low down in the catchment there was an extensive riffle area with fast currents and, I suspect, few other organisms able to compete for Lemanea’s favourite conditions. Remember, too, that there are enormous reserves of Lemanea in the upper catchment of the River Wear, and plenty of opportunities for this to be scoured off boulders and carried downstream. The wise ecologist always works on “balance of evidence”, rather than making categorical judgements on the presence or absence of a single organism. There is, simply, too much that we still don’t know about the biology of these species.

Finding the missing link in plant evolution …

I noticed some patches of a small, bright-green plant fully submerged in the slack waters beside the River Ehen a kilometre or so below the outflow from Ennerdale Water.   My first look suggested a tangle of narrow, flaccid grass-like stems but a closer examination showed that each of the narrow stems had whorls of branches arising from them at intervals (similar to the pattern seen in horsetails).   Their size and structure suggested an affinity to the vascular plants such as Myriophyllum and Juncus that I also saw in the river that day but, actually, these plants belong to an algal genus called Nitella.   I could not see any reproductive organs on these plants, but other evidence points to them being Nitella flexilis.

Nitella belongs to a group of algae called the Charophytes which have been the subject of vigorous debate by phycologists and evoloutionary scientists for a long time.    The author of one recent paper went so far as to claim that “no group in the plant kingdom has presented greater difficulties in classification …”.   They are usually placed in the green algae (Chlorophyta) although some people regard them as a separate division entirely.  Charophytes, in fact, have several characteristics, as well as their size and pigmentation, that suggest that they may be the closest algal relatives of land plants.   These similarities extend to the reproductive organs (see Chris Carter’s image of Chara virgata in a post from July last year).


Nitella flexilis photographed in the River Ehen, 18 March 2014.  Scale bar: 1 centimetre.

Under the microscope, some oddities of Nitella and other charophytes become clear.  The branchlets , despite their size, are composed of single giant cells, each with many tiny chloroplasts around the margins and a large central vacuole.  If you watch closely, you can see the cytoplasm close to the vacuole within these cells streaming around (though this proved to be hard to capture on video).   These giant cells made the charophytes very popular with physiologists, as they were easy to manipulate in the laboratory.


A cell of a branchlet of Nitella flexilis from the River Ehen collected on 18 March 2014.  Scale bar: 1/10th of a millimetre.

Their large size and distinctive appearance has meant that Charophytes were often recorded by mainstream botanists who usually ignore algae completely.   This means that we have a better understanding of their distribution than is the case for most freshwater algae.   These show records of Nitella flexilis and relatives to be particularly clustered around the Lake District and other areas associated with soft water, though more often in ponds and lakes than in rivers.   It is certainly not a common genus so this is one more feature that makes the River Ehen such a fascinating river to study.


Bennici, A. (2008).  Origin and early evolution of land plants.  Problems and considerations.   Communicative and Integrative Biology 1: 212-218.

Karol, K.G., McCourt, R.M., Cimino, M.T. & Delwiche, C.E. (2001).  The closest living relatives of land plants.  Science (New York) 294: 2351-2353.

Ruhfel, B.R., Gitzendanner, M.A., Soltis, P.S., Soltis, D.E. & Burleigh, J.G. (2014).  From algae to angiosperms – inferring the phylogeny of green plants (Viridiplantae) from 360 plastid genomes.  BMC Evolutionary Biology 14: 23.

Getting close to pearl mussels with my underwater camera …

I made one of my regular sampling trips to the River Ehen in Cumbria earlier in the week and took the opportunity to put my new waterproof camera through its paces.  There are plenty of waterproof cameras on the market but I chose a compact Olympus model for the very simple reason that I had been impressed by a colleague’s photos from a similar model.  Not only is it waterproof, but it also has a “super macro” mode that allows you to get to just a centimetre away from your subject.  There is also an LED light source for close-up illumination.  I’ve already shown some images of mosses taken with this camera; my trip to Cumbria gave me the opportunity to try it out underwater for the first time.


A “self-portrait” of my Olympus TG-2 camera

The River Ehen is a particularly good place to put this camera through its paces as it contains England’s largest population of pearl mussels (Margaritifera margaritifera) which have the added advantage for the rookie photographer as they are sessile.  Incidentally, as pearl mussels are protected under the Wildlife and Countryside Act, you need a licence from Natural England to work in the few rivers where they are still found.   I have a licence as I am involved in a scientific study on the river but unauthorised visits to view or photograph pearl mussels here or elsewhere could be construed as a criminal offence.

Both of these populations were photographed in about 30 centimetres of water.  The right hand population was located in a faster-flowing section of the river – indicated by the absence of silt and algae on the shells.   You can see the two siphons on the mussel at the centre of the left hand image through which the mussel inhales and expels water and particulate matter.  The gills are not only used for respiration; they also capture food particles from the water and pass these to the digestive system.


Pearl mussels (Margaritifera margaritifera) in the River Ehen, Cumbria, photographed in situ using an Olympus TG-2 camera.  The largest mussels are about 4-5 centimetres across.

The situation in the River Ehen has improved since I last wrote about the pearl mussels here (see “Pearl mussels in the River Ehen” from January 2013).   Back then, most of the mussels in the river looked like those in the left hand image; now the cleaner mussels on the right hand side are more typical in many parts of the river.   This is partly due to the higher discharge in the river which, we think, is gradually flushing the fine sediment out from the interstices between the gravels and pebbles, allowing oxygenated water to circulate through and, we hope, allowing the young mussels to thrive.  However, as the earlier post explained, the pearl mussel life cycle is very long and delicately balanced, so there is no reason to be complacent.   Over half of the world’s healthy populations of the pearl mussel are found in the UK so a lot of attention is being paid to the few remaining rivers where it thrives.

More about John Carter

John Carter, who I wrote about in my last-but-one post (“Remembering John Carter”) had a great interest in Achnanthes, the old genus to which Platessa bahlsii (see post of 12 March 2014) would have once belonged.   In the early 1960s he wrote a series of papers for the journal The Microscope entitled “The genus Achnanthes as it occurs in British fresh waters”, and which included a key and many of John’s highly detailed drawings (see figure).  “I feel”, he wrote, “that many forms are overlooked simply because of their minute size and the extreme fineness of sculpture on the valve.”   He spent many hours carefully drawing what he could see down his microscope and, indeed, described several new species.  I have long believed that drawing is an important discipline for focussing attention on the properties of organisms and John was an exemplar of this.  Within a few years of his death, however, the advent of high-resolution digital imaging made it much easier to put several individuals from each of several populations alongside one another in order to study this variation more objectively (see the images of Platessa bahlsii as an example).


An illustration from John Carter’s 1961 paper on the genus Achnanthes.  These drawings show several species most of which would now be placed in the genus Planothidium.

He attempted, along with two other amateur microscopists, to draw all the diatoms then known from Britain, and their Atlas of British Diatoms was published three years after his death in 1996.  It is still sits on the bookshelf beside my microscope, as much for the aesthetic pleasure that I gain from his meticulous drawings of the larger diatoms such as Pinnularia.  The book is now, alas, out of print.

My favourite memory of John Carter? I mentioned to him how I found the German in Hustedt’s 1930 edition of the Susswässerflora von Mitteleuropa much easier to understand than that in Krammer and Lange-Bertalot’s revised version.   He smiled as he recalled a conversation with Lange-Bertalot: “Horst, I said, you speak wonderful English but you write terrible German.”   I recounted this anecdote to a German colleague last year who looked horrified and told me that Lange-Bertalot is admired in Germany for his very meticulous and precise technical German.  This story sums up for me John Carter’s archetypal no-nonsense Yorkshire attitude that treated everyone from students to eminent professors in exactly the same manner.


Carter, J.R. (1961).  The genus Achnanthes as it occurs in British fresh waters.  The Microscope 12: 320-325.

Hartley, B., Barber, H.G. & Carter, J.R. (1996).  An Atlas of British Diatoms (edited by P.A. Sims).  Biopress, Bristol.

The truth is sometimes stranger than fiction …

Just before Christmas I had an idea for a story: a group of campaigners battling to save the last polluted river in the country before the evil utility company ceased to pour in their effluents and a unique and unusual ecosystem was lost forever.  It was, obviously, not a very serious topic but there were serious ideas behind it.  There are also precedents, with some former industrial land now protected as Sites of Special Scientific Interest because of its distinctive flora.

The thinking behind the story was that the factor most likely to lead to widespread reduction in pollution is not better regulation but the profit motive.   A few months earlier, I had watched a TV news story whilst in a hotel room, describing how Thames Water was able to extract phosphorus from sewage, process it into fertiliser then sell it to farmers.   Much of my professional work addresses the better regulation of phosphorus in the environment but I also know that there is a global shortage of phosphorus, which has stimulated considerable commercial interest in recovering phosphorus from sewage effluent.   The market, in other words, may ultimately play as large – or even a larger – role than legislation in controlling phosphorus releases to the environment.

I ran with this idea a little further: suppose utility companies found other ways of making money from sewage?   This is already happening on a small scale, with capacity to store and use methane released during the decomposition of sewage.  The limiting factor, as in most aspects of waste disposal, is economics.  Imagine, however, that the costs of energy were to shift dramatically … suddenly the opportunities presented by the huge quantities of sewage – which is just a semi-liquid form of the cow pats that half a billion Indian farmers traditionally used as fuel – look more attractive.  How might utility companies react?

So I needed a plot device that pushed up the price of energy and, in the process, stimulated utility companies to invest in energy production on sewage treatment plants, along with the infrastructure to connect this to the grid.   Suppose, I speculated, relations between Russia and the West deteriorated, threatening the huge natural gas supplies on which central European countries such as Germany depends?   This, in turn, would create greater demand for other sources of energy and push up prices to the extent that alternative sources of fuel might look more attractive.

All I needed was a geopolitical scenario that would create this east-west tension and my plot synopsis would be complete.   On cue, the crowds gathered in Kiev to overthrow Viktor Yanukovych and suddenly my bright idea for a work of fiction looked a whole lot more plausible …

Remembering John Carter

I wondered, as I re-read the previous post, what my late mentor and friend John Carter would have called the diatom I was writing about.   When I first started looking seriously at diatoms in the early 1990s, there was no-one in my laboratory in Durham with any experience from whom I could learn, and Brian Whitton suggested I went up to visit John at his home near Hawick, in the Scottish Borders.   I rung to arrange a date and, a couple of weeks later, made the two hour drive up through Northumberland and across the border to his house in the small village of Denholm.   Stepping into John’s study was like stepping back in time fifty years: it was dark and dusty, with piles of books and file boxes lining the walls and stacked on tables, along with boxes of microscope slides.  In the fireplace there was some of the equipment that he used to digest his diatom samples – apparatus that really belonged in a laboratory fume cupboard.   And, on a narrow desk against one wall, an old brass microscope equipped with a tilting mirror rather than its own light source.


John Carter in his study at Denholm, in Scotland.  An uncredited photograph from his obituary in Diatom Research

We spent the day in this study, John peering down his microscope and calling out the names of the diatoms he saw along with a commentary on the diatomists he had known (Hustedt, I remember him telling me, was a member of the Nazi party, which made it difficult for him to re-integrate with the scientific community after the war).  I perched on a chair beside him taking notes and occasionally squinting down the microscope to see for myself what he was describing.  I often, too, got a pithy assessment of the state of the slides that I had brought with me.   After a couple of hours of this, we would be summoned by his wife into the dining room for a hearty casserole, and while we ate they would quiz me about my children and talk about their years in the Borders.

Later, back in Durham, I would go back through the slides and try to reconcile my notes with what I could see under the laboratory’s much more modern microscope, ever marvelling at just how much detail John had been able to see with his old equipment.   It was a steep learning curve but, after half a dozen visits to John, it all began to make sense, and I gradually gained the confidence I needed to identify diatoms on my own.   Not long after that we had a telephone call in the laboratory to say that John Carter had died.  I felt that a door onto an older, more civilised, way of doing science had closed.

First record of Platessa bahlsii in the UK?

I have spent the past two or three weeks looking at diatoms from an almost unique perspective.   Rather than trying to name them by looking down a microscope and observing their physical properties, I have been looking at their barcodes (see “When a picture is worth a thousand base pairs …”).   We have over 100 different known diatoms in a “library” of barcodes, each of which has been grown in the laboratory, named from photographs and then had its DNA extracted and sequenced.  However, we also, now, have results from some mixed samples collected from rivers and analysed by a new approach called “Next Generation Sequencing”.   The challenge is trying to match the sequences found in the field samples with the diatom species that they match.  Sometimes there is clear agreement, but in some cases I had to go back to my microscope slides to check the names of things that should, in theory, have matched something in our library.

The diatom in the illustration below is one such example of a “near miss”, which made me scratch my head and have another look.   As it did not appear to match any of the illustrations in my books, I also sent photographs off to a colleague, Luc Ector, in Luxembourg.  He pointed me towards some pictures in a recent paper from the USA, leading to a provisional identification of Platessa bahlsii Potapova 2012.



Platessa cf bahlsii Potapova 2012 from the River Teise at Caddingford, Kent (TQ 691 488), 29 September 2011.

Though I call this a “new record” I am fairly sure that I have seen it before and suspect that it is probably easy to misidentify.  These diatoms are less than 10 micrometres (1/100th of a millimetre) long and, mostly, occur in relatively small numbers only sporadically, so this is the taxonomic equivalent of a “perfect storm”.   It is only in the relatively rare instances when a larger population is encountered along with good facilities and enough time that it is possible to track down the true identity.  

Indeed, is Platessa bahlsii the “true identity” of this organism?  Here we stray into deep water.   P. bahlsii was first described from the USA and there is a vigorous debate on the extent to which diatoms are cosmopolitan or have restricted geographical distributions.   Another problem is that the genus Platessa is, itself, only 10 years old and not everyone would agree with the decision to create this genus.  A colleague once pointed out that whilst a species is a rigorous and objective biological concept, all higher levels of taxonomy are, to some extent, negotiable.   Twenty years ago, this species would have been placed in the genus Achnanthes.   Since then, freshwater species of Achnanthes have been split into ten separate genera, two of which have quietly fallen out of favour already.  Looking at the results from our barcode study, I suspect that another shake-up may be necessary before we reach a classification of this group of diatoms that is biologically realistic.   That will mean more names changing and more confusion in the meantime. Hey ho …

More about mosses

In 1795, the explorer Mungo Park set off from what we now know as The Gambia to try to discover the course of the River Niger.   He describes his many adventures vividly in Travels into the Interior of Africa including, at one point, being robbed of absolutely everything he possessed, including his clothes.   He sat, naked, in the bush, and contemplated his situation.  “Whichever way I turned,” he wrote, “nothing appeared but danger and difficulty.  I saw myself in the midst of a vast wilderness in the depth of the rainy season, naked and alone; surrounded by savage animals, and men still more savage.”    Then, he wrote an extraordinary passage: “At this moment, painful as my recollections were, the extraordinary beauty of a small moss in fructification, irresistibly caught my eye. I mention this to show from what trifling circumstances the mind will sometimes derive consolation; for though the whole plant was not larger than the top of one of my fingers, I could not contemplate the delicate conformation of its roots, leaves, and capsula, without admiration.   Can that Being (thought I) who planted, watered, and brought to perfection, in this obscure part of the world, a thing which appears of so small importance, look with unconcern upon the situation and sufferings of creatures formed after His own image? – surely not!”

I was thinking of this passage this afternoon, as I walked passed a wall and noticed several hemispherical cushions of a moss, Grimmia pulvinata.  This is one of the more distinctive of the mosses of wall-tops.    Just as for Bryum capillare (“Wonders in my own backyard …”), the leaves gradually taper to long, fine hair-points, creating the fine, downy “cloud” around the cushion .  If you look closely, you’ll also see the capsules, which contain the spores, buried inside the cushion, although it will rise above the cushion later on.  Mosses such as these are easy to overlook but, when you adjust your focus and pay attention to them, you can start to appreciate their beauty, and understand Park’s rhapsody.


Cushions of Grimmia pulvinata on a wall-top in Durham, March 2014.   Both cushions are about two centimetres across.

Incidentally, Mungo Park was wrong in one respect, as mosses do not have roots.  It is an easy mistake to make when sitting naked in the bush.  Most modern bryologists work fully clothed, which has the additional advantage of providing pockets for storing specimens, notebooks, hand lenses and cameras.   This, actually, raises an interesting question: the moss that Park observed, a species of Fissidens, is now in the herbarium of the Natural History Museum in London (registration number: BM000871833).   How on earth did he get it back?