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.

A multicoloured rock stop …

Our route to Cassop Pond is rural but certainly not natural.   That’s the way with much of County Durham’s landscapes: the economic history of the area is based around extractive industries and those parts that were not mined for coal were most likely quarried. That’s particularly the case in the immediate vicinity of our house, overlooked by the scarp face of the Permian limestone.  There is a large working quarry beside the footpath we follow but, even when we have left that behind, it took some time to realise that another path we followed, overhung by trees on both sides to form a natural tunnel, was an ancient, long-abandoned wagonway.  This leads to the edge of Cassop Vale, where there are also signs of quarrying but, en route, passes through what is either another abandoned quarry or a man-made cutting for the wagonway.  Or both.  Why not get a return from the back-breaking endeavours of flattening out the route that rock-laden wagons will have to take?  

Splashes of colour on the exposed rock in the cutting also drew our attention.  Some of this was the orange-yellow of the alga Trentepohlia aurea that I have written about before (see “Fake Tans in the Yorkshire Dales”).  However, there were also patches with a brick-red colour that, under the microscope resolved into a different species, Trentepohlia umbrina (see “Cassop Vale, Febrary 2021”).   Above there are blackish patches of Gloeocapsa alpina (see “The mysteries of Clapham Junction …”) and, if you look closely, you’ll see some green patches too (see “Little round green things …”).  That’s at least four different terrestrial algae within about a metre of one another.

The quarry / cutting bordering the wagonway near Cassop Vale (NZ 3369 3820).   The image on the right shows a close-up of the exposed limestone.   a. orange patches of Trentepohlia aurea; b. brick-red patches of T. umbrina; c. black patches of Gloeocapsa alpina.

I confess to having walked along this footpath several times without ever really noticing what was right in front of my eyes.  That brings to mind a quotation from Henry David Thoreau: “It’s not what you look at that matters, it’s what you see”.   Seeing is a process that is as much to do with the brain as it is to do with patterns of light falling on the retina.   And, at the same time as I notice that there are four species of algae on this outcrop, I have to acknowledge that there is so much more to this narrow track than I have realised.  I am aware of the rich diversity of limestone plants, largely because Heather points them out.   We notice several of the birds, either by sight or by their song and she is getting better at spotting and naming butterflies.  Then there are the fungi: the larger ones on trees we notice but we read Merlin Sheldrake’s Enchanted Lives recently and this made us conscious of the rich diversity of this group – and their many important roles in ecosystems.  

Gloeocapsa alpina from the rock face near Cassop Vale.   Scale bar: 10 micrometres (= 100th of a millimetre).  

Our local countryside, in other words, offers us both a sense of stability, via nature’s predictable rhythms, but also the opportunity to encounter the unexpected.   The unexpected arises from two directions.   The first is an intrinsic capacity for nature to change and surprise us – for a species of plant or bird to disappear or another to appear at a location for the first time.  But there is also a capacity that lies more with us, as observers, than with the habitat itself.  That is for us to notice details that had been there all along but which we had overlooked, perhaps because we were rushing along, perhaps because we lacked the awareness that allowed the patterns that fell on our retinas to make the necessary connections in our brain.   Noticing that we’ve noticed something that we should have noticed before is doubly important, especially when this happens in our own backyards, because it makes us realise how little we know, even about the habitats we think we know best.

Wrote this whilst listening to: various live sets on YouTube by P.J. Harvey.

Cultural highlights:   Brilliant new film, Munari, about a Korean family’s attempts to farm in a remote corner of Arkansas.   Little House on the Prairie, but with Asian, rather than European settlers.  

Currently reading:  Lila, by Marilynne Robinson.  

Culinary highlight: homemade samosas, eaten al fresco on Stannage Edge, Derbyshire with daughter and partner: our first face-to-face meeting this year.

The Natural History of Upper Weardale

A very short post this week, having been knocked for six by my first dose of the Covid vaccine.  I’m going to use it to publicise The Natural History of Upper Weardale, a new book published by Durham Wildlife Trust that provides an accessible insight into the geological, geomorphological, climatic, ecological and human influences that have shaped the dale.  I contributed to the chapter on freshwater life and Heather wrote (and illustrated) the chapter on the plants of Weardale.  You can buy it (and its companion volume, The Natural History of Upper Teesdale) from the DWT website.

At some point in the not-too-distant future, you’ll also be able to access the book via the Living Uplands website.  The Upper Teesdale book is already here, with the individual chapters all downloadable as pdfs.  Weardale and Teesdale both have their own individual characters and putting the chapters of the two books alongside each other helps to understand the reasons behind this.   No-one would readily associate the northern Pennines with volcanoes, but it is two igneous rocks that are largely responsible for the differences between the two dales.   Whin Sill outcrops much more in Teesdale and is responsible for the dramatic waterfalls at High Force and Cauldron Snout which have no counterparts in Weardale, whilst the “Weardale Granite” was responsible for the mineral veins and associated mining activity which, though present in both dales, is more widespread and obvious in Weardale.    

David Attenborough said: “no-one will protect what they don’t care about; and no one will care about what they have not experienced”, and Buddha said: “true love is born from understanding”.  I hope this book helps people to understand Weardale better and, with this understanding, to have better experiences in Weardale.  That will give us a firm foundation for conserving and protecting this beautiful landscape.  

Some other highlights from this week:

Wrote this whilst listening to:  Lana Del Rey’s new album Chemtrails Over the Country Club.  And some vintage Mahavishnu Orchestra with John McLaughlin.

Cultural highlights:  Unusual low-key Canadian film Mouthpiece and, whilst crashed out with post-vaccination blues, the 2018 film about a dysfunctional all female punk rock band, Her Smell

Currently reading:  Matt Haig’s Midnight Library, having finished Penelope Fitzgerald’s The Bookshop.  More time than usual for reading this week. 

Culinary highlight:  Persian New Year feast of tahdig served with cauliflower roasted with harissa and tahini.

Around the village in 80 days …


Mildy riled by an “explorer” talking on the radio about derring-dos in far-flung tropical forests, I decided to write this post about an inauspicious back lane about 300 metres from my house.   The whole point of this blog, after all, is been to point out that there are more strange plants and creatures in your own neighbourhood than you are ever likely to encounter in the jungles of Borneo.   Much of what I write about is microscopic and, therefore, there is no disgrace in not noticing it.  But sometimes, we nonchalantly ignore nature because it is out of our sightline or simply too small and prosaic to attract our interest.

A patch of liverwort on a narrow strip of wasteland between garden fences and footpath in our back lane is a case in point.   It is possible to walk this past hundreds of times without noticing but, once you care to take a closer look, the sights in front of you are as weird as anything you see on a David Attenborough documentary.   Liverworts are relatives of mosses, and I talked briefly about a couple in a post in 2019 (see “A thousand little mosses …”).  Like these, today’s liverwort, Marchantia polymorpha, does not have a distinct stem or leaves but, rather, the whole plant looks like a lobed leaf, albeit lying flat on the ground.   The technical term for this is “thallus”.    If you pull one up, you’ll see thin white hairs (“rhizoids”) on the underside which anchor it to the soil, but there are no roots.    Peering even closer, you’ll see that the upper surface is dotted with white spots which, if you have a hand lens, resolve into air pores.   These look superficially like stomata (see “Whatever doesn’t kill you …”) but differ in that they are permanently open, allowing air to penetrate to the inner layers of the thallus.


Left: a thallus of Marchantia polymorpha with a young female reproductive structure present (approximately 3 cm across); right: close up of thallus surface showing air pores. 

Several of the thalli had structures that looked like minuscule palm trees arising from them.  These are the female reproductive structures.  The plant is dioecious, meaning that there are separate male and female plants but, curiously, all those in our back lane appear to be female.  On the underside of the umbrella-like structure there are a number of flask-like archegonia into which a sperm cell will, if very lucky, swim.   This all happens before the stalk elongates (as in the left-hand image above) and when the liverwort is damp.   Another whole phase of the life cycle takes place in these sacs, resulting in the production of spores which are eventually ejected with the assistance of spring-like structures called “elators” but only after the stalk has raised itself a couple of centimetres above the thallus.   The reason for the apparent absence of male reproductive structures from our back lane is something of a mystery, as it seems to make the possibility of congress even more remote than usual.   Fortunately, Marchantia polymorpha has a Plan B.


Left: female reproductive structures on the surface of a Marchantia polymorphus thallus and, right, close-up of the underside of the female reproductive structure showing the antheridia (photos: H. Kelly).

If you look at the left-hand image below you’ll see a circular shape on the thallus near the base of the palm-tree like female reproductive structure.  This is a gemma cup which contains small bundles of cells called gemmae which can also disperse to form new thalli.  These new thalli will be genetically-identical to their parents (clones, in other words).   That has some disadvantages but, as we have seen, it is a common phenomenon in the algae (see “The River Ehen in March”) and is a good strategy for organisms that want to occupy a lot of vacant ground in a short period of time.   What this also means is that the patch of Marchantia polymorpha is not necessarily a lot of individual plants but, rather a few (possibly even a single) “plant”, albeit not one formed of contiguous units.


A close-up of a gemma cup on the surface of a thallus of Marchantia polymorpha, with some gemmae visible inside. 

Finally, I’ve included n ant’s eye view of the Marchantia patch to finish off the post.  On the right foreground you can see the surface of the leaf with an air pore; on the left-hand side there is a gemma cup and, dominating the centre of the image, there are female reproductive bodies towering over the micro-landscape.   Where, I still wonder, are the male reproductive bodies?  Could it be that a female Marchantia polymorpha needs a male Marchantia polymorpha like a … a fish needs a bicycle?


An ant’s eye view of a Marchantia polymorpha thalli. 

Some other highlights from this week:

Wrote this whilst listening to:  live music!   Well not “live” as in happening in the same room, but in the sense that it was happening in real time as we listened via a YouTube link.  It was from local musician Emma Fisk and the Hot Club du Nord- playing Django Reinhardt / Stephane Grappelli -inspired jazz.

Cultural highlights:  The film Green Book.  Winner of three Oscars in 2019.   And the BBC adaptation of Vikram Seth’s A Suitable Boy.  Beautifully filmed and very evocative of our visits to India.

Currently reading:  A Single Thread by Tracey Chevalier.

Culinary highlight:  Butter beans in salsa verde.  Meera Sodha’s recipe from last week’s Guardian.  And Cumberland sausage, mash and a rich onion gravy from The Shepherd’s Arms in Ennerdale Bridge, our fieldwork home-from-home.



A reasonable excuse for exercise ..

High_Mill_falls_Apr2020A redefinition of the travel restrictions hereabouts means that “driving to the countryside and walking (where far more time is spent walking than driving)” it is now “likely to be reasonable” within the terms of Regulation 6 of the The Health Protection (Coronavirus, Restrictions) (England) Regulations 2020. That means that, rather than plan another post about the fascinating ecology of Lough Down, I can look a little further afield.   As both Heather and I are writing chapters for a forthcoming book on the Natural History of Weardale, we turned our eyes to the hills, largely for exercise and a change of scenery, but also as part of our background research for these chapters.

We parked the car at Westgate and followed a path alongside Middlehope Burn, a tributary of the Wear with a long history of lead mining and, as such, a case study in how man has shaped the ecology of Weardale, both terrestrial (Heather’s domain) and aquatic.  The first part of the walk is through Slitt Wood, where the stream cascades over a series of low step-like waterfalls, alternately sandstone and limestone, illustrating the bedrock geology of the area.   The air is full of birdsong and there are patches of primroses feasting greedily on the light that is still plentiful on the forest floor at this time of year.   However, this idyll is short-lived as, passing through a gate we emerge into a grassed area surrounded by derelict mine buildings.  Early on a Saturday morning in the midst of the pandemic, we have the place to ourselves and it is a struggle to imagine this place as a busy industrial site.   Similar sites are scattered throughout Weardale and the surrounding dales; all are now closed but once they would have employed large numbers of people.  There would have been the miners, working underground, of course, but also gangs of people (including women and children) breaking down and sorting the ore as it was brought to the surface, plus ancillary workers involved in construction, both above and below ground.

A couple of hundred metres beyond the site of the main shaft at Slitt Mine, I spot an adit (a shaft driven horizontally into a hillside) and make my way towards it.  These are intriguing habitats for ecologists interested in the interactions between man and nature and I was intrigued to see what was growing in this one, White’s Level.  The mine’s levels and shafts act as natural drainage channels, collecting water that has percolated through the rocks but, because the miners have driven the levels along the mineral veins, the water comes into contact with lead, zinc and cadmium during the course of its underground journeys, emerging with concentrations far in excess of those deemed safe by toxicologists.  However, the channel immediately downstream of the entrance of White’s Level was lush with vegetation.   I could see thick wefts of filamentous algae giving way to beds of water-cress (Rorippa nasturtium-aquaticum) and bog pondweed (Potamogeton polygonifolius).  The latter two were surprising as, in my experience, most of the streams draining north Pennine adits are dominated by algae rather than by higher plants.


The stream flowing from White’s Level to Middlehope Burn, April 2020.  The left-hand image shows the beds of water-cress very clearly whilst the right hand image shows the filamentous algae growths immediately below the entrance.   The picture at the top of the post shows Middlehope Burn at High Mill Falls, just upstream from Westgate.

The water cress had a distinctive purplish tinge which is probably a response to stress.  We’ve encountered this type of colour-change in response to stress elsewhere (see “Escape to Southwold”).   In this post, and in “Good vibrations under the Suffolk sun …” I talked about how plants have to regulate the amount of energy from sunlight in order that their internal photosynthetic machinery is not overwhelmed.  Those posts were both written after a hot weekend in July, but this was a chilly and overcast April morning in the Pennines where the prospect of plant cells being overcome by heat seems faintly ludicrous.   Here, instead, is my alternative hypothesis.

Although White’s Level and the other mines in the northern Pennines were driven by the demand for lead, lead is a relatively insoluble element and zinc, which is found alongside the lead in the metal-rich veins of the northern Pennines, is more soluble and, therefore, has a greater toxic influence on the plants and animals in these streams.  Zinc affects the metabolism of plants in several ways, one of the most important of which is to reduce the effectiveness of the chlorophyll molecules which are responsible for photosynthesis.  It does this by nudging the magnesium atom, which lies at the heart of every chlorophyll molecule, out of place.


Macrophytes in the stream flowing from White’s Level to Middlehope Burn, April 2020.   Left: Potamogeton polygonifolius; right: Rorippa nasturtium-aquaticum.

What this does, then, is alter the balance of the equation that tries to balance energy inputs and photosynthesis.   If your chlorophyll molecules are hobbling along, then the point at which they are overwhelmed by even the meagre Pennine sunlight shifts so that  the need for the plants to manufacture their on-board sunscreen kicks in sooner.   Just a hypothesis, as I said: if you have a better explanation, please let me know.

A few hundred metres further on, there is another lush growth of water cress in the stream flowing out of another adit, Governor and Company Level, this time even extending beyond the metal grille designed to keep the curious from harm.  I most associate watercress farms with the headwaters of chalk stream, which are characteristically spring-fed and, therefore, have very stable conditions.   The adits of the northern Pennines are, this respect, very similar to springs insofar as their flow, temperature and chemical conditions vary little over the course of a year.   In that respect, it is perhaps less of a surprise that we find water cress growing so prolifically here.   The zinc, admittedly, is a complication we don’t find in most springs but, that apart, the adits could be thought of as man-made springs, creating a series of almost unique, but largely overlooked habitats.

In the next post, I’ll talk about the algae that I found in the White’s Level channel.


A prolific growth of water cress in the drainage channel below Governor and Company Level, April 2020. 

Some other highlights from this week:

Wrote this whilst listening to: Still working through Dylan’s back catalogue: John Wesley Harding, , Nashville Skyline and Self-Portrait, the latter a blip in an otherwise superb run of albums.   Next up is New Morning but I want to re-read the chapter in Dylan’s Chronicles Volume One where he describes the genesis of this album before listening.

Cultural highlights:  My book group looked at Pride and Prejudice but, being deep into The Mirror and The Light, I did not had time to read this.   We watched the 2005 film version starring Kiera Knightly instead.   Turned out that three of the six participants in the book group had also watched the film the night before our Zoom meeting, rather than (re-)reading the book itself.

Currently reading:  Finally finished The Mirror and The Light which was, definitely, worth the effort.  Started Kate Atkinson’s Big Sky.

Culinary highlight:   Home-made tortellini filled with mushroom paté, served with a consommé made from turkey stock from the freezer.  Culinary ambition hereabouts always goes sky high in the week of the MasterChef finals.

More about Gomphonema vibrio

Gomphonema vibrio is part of a complex of species that has only begun to be unravelled in the past few years.   In the first edition of the Süsswasserflora von Mitteleuropa in 1930, Hustedt included it as one of three varieties of G. intricatum, along with G. pumilum and G. dichotum.  By the time of the second edition (1986), however, Krammer and Lange-Bertalot had subsumed G. intricatum into G. angustum, creating a single species that spanned an enormous range of size (see their Plate 164 if you don’t believe me).   A few years later they revised this opinion, and unpicked the G. angustum complex, reinstating several of the taxa that they had originally subsumed and also recognising some more recently described species (many by Erin Reichardt).   There may well be more changes to come as this group has not yet been subjected to critical study by molecular geneticists.

One of the other species in this melange is Gomphonema pumilum, a much smaller diatom that is common in both running and standing waters (Hustedt’s comment on the species complex only referred to a preference for “stagnant waters”).   We have met it a few times previously (see, for example, “Pleasures in my own backyard”) and I also found it in a 1999 sample from Croft Kettle whilst searching for G. vibrio.   However, I then turned to an older slide, based on a sample collected in 1872 and given to me by John Carter (see “Remembering John Carter”).   This had some cells of G. pumilum but also some that exceeded the quoted dimensions for G. pumilum (length: 12 – 36 mm; width: 3.5 – 5.5 mm) and which fell within the size range for G. vibrio.   I suspect that we are, in fact, dealing with a mixture of the two species and if this is a common situation then it may explain why Hustedt had difficulties unpicking the two species.   When I arranged the images of G. vibrio and G. pumilum that I found in this sample in order of diminishing size, there is a continuum between the two forms.  We now know that width is a better discriminator than length and, armed with this, we can see a difference between the two species. But that is one of the benefits of hindsight.

Gomphonema pumilum from Croft Kettle, May 1999.  a. – e.: valve views; f., g.: girdle views.   Scale bar: 10 micrometres (= 100th of a millimetre).

Gomphonema vibrio (h. – k.) and G. pumilum (l. – m. [and n.?]) from “Hell Kettles”, 1872.  Scale bar: 10 micrometres (= 100th of a millimetre).

This raises a question about the reliability of the size ranges quoted in the literature   A couple of the smaller valves of G. vibrio were less than 7 mm wide.  Yet, in other respects, they were more similar to the “true” G. vibrio valves than to those of G. pumilum.  The answer will vary from species to species but, as a general rule, we should not be too bothered if the extremes of a population stray a little beyond the values quoted in the literature.   These are usually based on the largest and smallest cells found in a thorough scan of one or more populations, but not necessarily on observations of an initial cell (the largest in a population) or of cells at the point immediately before sexual reproduction is initiated (the smallest).  We simply don’t have that information for most species so, as a result, should be prepared to accept larger and smaller valves into a species if they are qualitatively similar to, and quantitatively part of a continuum with, the rest of the population.  My post “Diatoms and the Space-Time Continuum”, also on Gomphonema, offers some further insights into this story.


Hustedt, F. (1930).  Susswasserflora von Mitteleuropa 10: Bacillariophyceae.  Gustav Fischer, Jena.

Krammer, K. & Lange-Bertalot, H. (1986). Susswasserflora von Mitteleuropa 2: Bacillariophyceae. 1 Teil: Naviculaceae.  Spektrum Akademischer Verlag, Heidelberg.

Krammer, K. & Lange-Bertalot, H. (1991). Susswasserflora von Mitteleuropa 2: Bacillariophyceae. 4 Teil: Achnanthaceae. Kritische Ergänzungen zu Achnanthes s.l., Navicula s.str., Gomphonema. Spektrum Akademischer Verlag, Heidelberg.

Reichardt, E. (1997).  Taxonomische revision des Artencomplexes um Gomphonema pumilum (Bacillariophyceae).  Nova Hedwigia 65: 99-129.

Reichardt, E. & Lange-Bertalot, H. (1991).  Taxonomische revision des Artencomplexes um Gomphonema angustum – G. intricatum – G. vibrio und ähnliche taxa (Bacillariophyceae).  Nova Hedwigia 53: 519-544.


In my post on Gomphonema rhombicum, I mentioned that the location on the type slide is given as “Appleby”, which was not very precise.   My 1872 slide is labelled “Hell Kettles, Durham”.  “Hell Kettles” is the name for the pair of ponds, of which Croft Kettle, which I described in my earlier post, is the larger.   However, the location “Durham” is not very illuminating.   The closest town to Croft Kettle is Darlington, whilst Durham City is 40 km to the north.   “Durham”, in this context, could refer to the county, which covers 2721 square kilometres and habitats from calcareous ponds such as these to moorland pools.   A slide label offers very little space to give precise details of location but, in both these cases, a little more information would be useful.   The likelihood is that Firth had more detailed notes elsewhere but these have been lost over time, so we are left with these scant words.   There is a lesson here for all of us in how we record the meta-data that accompanies our samples.

Rolling stones gather no moss …

Back in early July I mused on how rivers changed over time (see “Where’s the Wear’s weir?”) and reflected on how this shapes our expectations about the plants and animals that we find.  In that post, I compared a view of the River Tees today with the same view as captured by J.R.W Turner at the end of the 18th century.   The photograph above is taken about 40 kilometres further upstream from Egglestone Abbey and shows the River Tees as it tumbles along in a narrow valley between Falcon Clints and Cronkley Scar.   I’ve written about this stretch of river before (see “The intricate ecology of green slime” and “More from Upper Teesdale”) and it is an idyllic stretch.   It all looks, to the uninitiated, very natural, almost untouched by the hand of man.

However, a couple of kilometres beyond this point we turn a corner and are confronted by a high waterfall, Cauldron Snout, formed where the river cascades over the hard Whin Sill.   Scrambling up the blocky dolerite is not difficult so long as you have a head for heights but, on reaching the top, a wall of concrete comes into view.  This is the dam of Cow Green Reservoir, constructed between 1967 and 1971 and highly controversial at the time.  The purpose of the reservoir was to regulate the flow in the River Tees, in particular ensuring that there was sufficient flow in the summer to ensure a steady supply for the industries of Teeside (most of which have, subsequently, closed).  My first visit to Cauldron Snout was in the early 1980s on a Northern Naturalist Union field excursion led by David Bellamy.  As we scrambled down Cauldron Snout, Tom Dunn, an elderly stalwart of the NNU, told me how much more impressive Cauldron Snout had been before the dam was closed.

Now look back at the picture at the top of this post.   The dark patches on the tops of the boulders emerging from the water are growths of the moss Schistidium rivulare, which thrives on the tops of stable boulders that are occasionally submerged.    The old adage “a rolling stone gathers no moss” is, actually, true, leaving me wondering how much less of this moss an walker beside this river in the mid-1960s might have seen.   How many more powerful surges of storm-fuelled water would have there been to overturn the larger boulders on which Schistidium rivulare depends?   Bear in mind, too, that two major tributaries, the Rivers Balder and Lune, also have flow regimes modified by reservoirs and the potential for subtle alteration of the view that Turner saw at Egglestone increases.   I wrote recently about how differences in hydrological regime can affect the types and quantities of algae that are found (see “A tale of two diatoms …”).   I may have stood at exactly the same place where Turner had sat when he drew the scene at Egglestone, but I was looking at a very different river.

The dam of Cow Green Reservoir looming above the top of Cauldron Snout in Upper Teesdale National Nature Reserve, Co. Durham, July 2017.  The picture at the top of this post shows the Tees a couple of kilometres downstream from Cauldron Snout.

Trevor Crisp from the Freshwater Biological Association showed that the consequences of Cow Green Reservoir on the River Tees extend beyond alterations to the flow.  Impounding a huge quantity of water in one of the coolest parts of the country also affects the temperature of the river, due to water’s high specific heat capacity.  This means that there is not just a narrower range of flows, but also a narrower range of temperature recorded.   The difference between coolest and warmest temperatures in the Tees below Cow Green dropped by 1 – 2 °C, which may not seem a lot, but one consequence is to delay the warming of the river water in Spring by about a month, which delays the development of young trout.  However, Crisp and colleagues went on to show that any reduction in growth rate due to lower temperatures was actually offset by other side-effects of the dam (such as a less harsh flow regime) to result in an increase in the total density of fish downstream.   Others have shown significant shifts in the types of invertebrate that he found in the Tees below Cow Green, with a decrease in taxa that are adapted to a harsh hydrological regime, as might be expected.   Maize Beck, a tributary which joins just below Cauldron Snout, and which has a natural flow regime, shows many fewer changes.

One conclusion that we can draw from all this is that healthy ecosystems such as the upper Tees are fairly resilient and can generally adapt to a certain amount of change, as Trevor Crisp’s work on the fish shows us. The big caveat on this is that the upper Tees is relatively unusual in having no natural salmon populations, as the waterfall at High Force presents a natural obstacle to migration.  Had this not been present, then all potential spawning grounds upstream of the reservoir would have been lost.   A second caveat is that there is still a lot that we do not know.   The studies of the river that followed the closure of the dam focussed on lists of the animal and plant species found; a modern ecologist might have put more effort into understanding the consequences for ecological processes, the “verbs” in ecosystems, rather than in the “nouns”.  Who knows how different energy pathways are now, compared to the days before regulation, and what the long-term consequences of such changes might be?  Schistidium rivulare is a good example of the limitations of our knowledge: its presence offers insights into the hydrology of the river, but we know relatively little about the roles that these semi-aquatic mosses play in the river ecosystem.   Knowing that there is much that we do not know should, at least, keep us humble as we struggle to find the balance between preserving natural landscapes and their sustainable use in the future.


Twenty years ago, I would have recognised Schistidium rivulare, if not in the field, then at least after a quick check under the microscope.  Now, however, my moss identification skills are rusty and I had to turn to Pauline Lang to get this moss named.   I mentioned in “The Stresses of Summertime …” how the ecologist’s niche becomes the office not the field.  One danger is that we remain familiar with names (as I am with S. rivulare and other aquatic mosses) but, through lack of practice, lose the craft that connects those names to the living organisms.


Armitage, P.D. (2006).   Long-term faunal changes in a regulated and an unregulated stream – Cow Green thirty years on.  River Research and Applications 22: 957-966.

Crisp, D.T. (1973).  Some physical and chemical effects of the Cow Green (upper Teesdale) impoundment.  Freshwater Biology 7: 109-120.

Crisp, D.T., Mann, R.H.K. & Cubby, P.R. (1983).  Effects of regulation on the River Tees upon fish populations below Cow Green Reservoir.  Journal of Applied Ecology 20: 371-386.

Lang, P.D. & Murphy, K.J. (2012).  Environmental drivers, life strategies and bioindicator capacity of bryophyte communities in high-latitude headwater streams.  Hydrobiologia 612: 1-17.

No excuse for not swimming …


Lyon’s Lake, Hetton-le-Hole, County Durham, May 2015.

After my sojourns in the Lake District and Latvia, I find myself back home in north-east England for a few days.   Whilst I was away, a small packet had arrived in the post, containing a sample of algae collected from a local lake. The bottles contained globules of bright green jelly-like material, with enough integrity to pick up with the fingers and they were intriguing enough for me to drive across one lunchtime to take a closer look at the lake where they came from.

Hetton Lyons Country Park is on the Permian Limestone plateau about 10 kilometres from where I live. It is on the site of a former colliery which closed in 1960 and the surrounding land has been reclaimed and partially converted to a country park.   The lake – probably a hectare or so in size – is used for angling and water sports, and the paths around the edge were busy with cyclists and dog-walkers.   It is on the edge of Hetton-le-Hole, a small town whose odd name refers to its location in one of the more sheltered parts of the plateau.


A mucilaginous colony of Aphanothece stagnina (left) with (right) a microscopic view of the individual cyanobacterial cells embedded in mucilage. Scale bar: 25 micrometres (= 1/40th of a millimetre).

There were plenty of green algae around the margins of the shallow lake but, amidst this in a few locations, I could also see the small globules of the alga resting on the bottom which, like the colonies I had been sent, could easily be picked –up.    Under the microscope, these resolved into tiny cyanobacterial cells, mostly oval in outline and about five micrometres in diameter.   These belong to Aphanothece stagnina, a relative of the Gloeocapsa alpina, which we have seen in two other recent posts (see “The mysteries of Clapham Junction” and “Poking around amongst sheep’s droppings …”), albeit in very different habitats.

The word “cyanobacteria” alone is usually enough to make the manager of a recreational lake break out in a sweat.   Many cyanobacteria produce toxins that can affect the nervous system and the liver. This means that no contact water sports (swimming and canoeing, for example) can take place and dog-owners have to be warned not to let their pets drink from the water.   However, as far as I can tell from a brief search on the internet, Aphanothece is not a genus that is often reported in association with toxic blooms.   One less excuse, then, not to go wild swimming in a lake in north-east England on a breezy May afternoon …