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.

The dark side of the leaf …

Having mentioned in my previous post that the epiphytes on the top and bottom surfaces of a Potamogeton polygonifolius leaf were different, I have produced a companion piece to the painting I showed in that post.   The new painting is of the lower surface, and shows a greater number of diatoms than are present on the upper surface.  In order to explain why this is the case, it is helpful to look at the structure of the Potamogeton leaves.  The first image, therefore, shows a section through a leaf. It is quite a thick section but we can see the upper epidermis, the palisade mesophyll cells below this, which have plenty of chloroplasts in order to capture the sunlight that the plant needs for photosynthesis.  Below this, we can see parenchymous tissue arranged to create some large internal air spaces which contribute to the leaves buoyancy. Finally, at the bottom, there is a single layer of epidermal cells.   All this is crammed into a thickness of about half a millimetre.

Part of a section of a leaf of Potamogeton polygonifolius.  The leaf vein is on the left, thinning to the leaf blade on the right.  The leaf blade is about half a millimetre thick.   The picture at the top of the post shows an artist’s impression of diatoms and Chamaesiphon cf. confervicolus on the lower surface of a Potamogeton polygonifolius leaf. 

 Viewed from the underside, these parenchymous tissues create polyhedronal chambers, ranging from about 100 to 200 micrometres (a tenth to a fifth of a millimetre) along the longest axis.  There are also a few stomata scattered across the leaf surfaces (see the right hand image below).

With this in mind, take a look at my impression of the epiphytes growing on the lower surface of a Potamotgen polygonifolius leaf.   There are a number of cells of Chamaesiphon cf confervicolius, as seen on the upper surface, but there are several cells of the diatom Achnanthidium minutissimum, growing on short stalks, plus a few long, thin cells of Ulnaria ulna, growing in small clusters on the leaf surface (there were a few other species present, but such low numbers that I have not included them here).    It might seem strange to think of two surfaces of a leaf having such different communities of epiphytes but that’s because we’re thinking like large land-dwelling organisms, not like algae.   The longest alga visible in the image of the leaf underside is Ulnaria ulna, at about a 10^th of a millimetre in length.  Therefore, to get a realistic impression of the two images, we really need to put a distance of five of these between them, and then pack the gap with chloroplast-rich mesophyll cells inside the Potamogeton leaf.   Allowing for foreshortening, this distance is about five times the height of the image.

The structure of a Potamogeton polygonifolius leaf viewed from the underside.  The left hand image (100x magnification) shows a leaf vein running diagonally across the lower right hand side along with the polyhedron-shaped chambers; the right hand image (400x magnification) shows the outline of one of these chambers superimposed behind the epidermal cells with a stomata with two guard cells visible just above the centre.   Scale bar: 20 micrometres (= 1/50^th of a millimetre). 

The epiphytes on the upper surface of the leaf get first dibs at the meagre Pennine sunlight, which then has to pass through the upper layers of the Potamogeton leaf, where the mesophyll cells will continue to feast on the tastiest wavelengths, leaving relatively meagre pickings for the epiphytes that hang around on the underside of the leaf.

Chlorophyll, the molecule that makes plants green, absorbs light over a relatively narrow range of wavelengths – predominately red and blue – and this means that there are plenty of other wavelengths awaiting an organism with different pigments.   Diatoms have chlorophyll, but they also have some carotenoids (principally fucoxanthin) that grabs energy from the green part of the visible light spectrum (which is reflected, rather than absorbed by chlorophyll) and passes it to the cell’s photosynthetic engine.  Having this capability means that they can survive in relatively low light, which is why we see more diatoms on the underside of the Potamogeton leaf than on the top.

And that, best beloved, is the story of how Potamogeton got its epiphytes …

 

Some other highlights from this week:

Wrote this whilst listening to: more Bob Dylan.   I’ve got to the mid-70s, which means the live version of Like a Rolling Stone on Before the Flood plus the great Blood on the Tracks.  Also, as I was reading Ian Rankin, I listened to John Martyn’s Solid Air.

Cultural highlights:  we’re watching the BBC adaptation of Sally Rooney’s Normal People

Currently reading:  Ian Rankin’s Rather be the Devil.

Culinary highlight:   A rather fine vegetarian chilli, from Felicity Cloake’s column in The Guardian last week.   Served with corn bread, using a recipe we got from a hand-me-down American housekeeping magazine during our time in Nigeria.

 

Whatever doesn’t kill you …

The previous post focussed mostly on the higher plants that I found in the short stream that connects White’s Level with Middlehope Burn.  I mentioned the mass growths of algae that I found growing immediately below the entrance to the adit, but I did not talk about them in any detail, instead spinning off on a tangent while I mused on why the water cress had a purplish tinge.

When I did find time to examine the algal floc, I found it to consist of a mix of three different algae, the most abundant of which was Tribonema viride, but there were also populations of a thin Microspora (not illustrated) and Klebsormidium subtile.   I talked about Tribonema in the drainage from the Hadjipavlou chromite mine in Cyprus last year (see “Survival of the fittest (1)”) and both Microspora and Klebsormidium are also genera that are known to frequent these habitats.  Indeed, there is evidence that the populations that grow in these extreme habitats have physiological adaptations that help them to cope with the conditions.  Brian Whitton, my PhD mentor, led several studies on these adaptations in the streams of the northern Pennines in the 1970s, and Patricia Foster did similar studies in Cornwall at about the same time.   There is probably a mixture of physiological strategies involved, including the production of low-molecular weight proteins, which bind the toxic metals, and the production of extracellular mucilage.  Most of the populations I find in such habitats have a distinctly slimy feel due to the production of extracellular polysaccharides, and it is possible that these play a role in trapping the metal ions before they can get into the cell and cause damage.

Filamentous algae from the drainage channel below White’s Level, upper Weardale, April 2020.  a., b. & c.: Tribonema cf. viride, showing the characteristic H-shaped cell ends.   d.  Klebsormidium cf. subtile.  Scale bar: 10 micrometres (= 100^th of a millimetre).   The picture at the top of the post shows an artist’s impression of Chamaesiphon cf. confervicolus on the upper surface of a Potamogeton polygonifolius leaf. 

I also had a look at the algae growing on the submerged leaves of Potamogeton pergonifolius in the channel between the adit and Middlehope Burn.   One easy way of examining them is to add a small amount of stream water then shake the leaves vigorously in a plastic bag.  The result is a brownish suspension of algae that can be sucked up with a Pasteur pipette and placed on a microscope slide.  When I did this, I found a community that was dominated by a short cyanobacterium, closest in form to Chamaesiphon cf. confervicolus.  The other abundant alga in the sample was Achnanthidium minutissimum, which is often common in minewaters, along with smaller numbers of a few other species.  The total number of species in the sample was just 12, which is low by the standards of streams without metal pollution, but such suppression of all but the hardiest species is another characteristic effect of heavy metal pollution.

I’ve added a “cf” (from the Latin conferre, meaning “compare to”) to my identification of Chamaesiphon confervicolus because this is the closest name, based on a comparison with images in the Freshwater Algal Flora of Britain and Ireland.  However, it is not an exact match.  Whether this is because the metals have strange effects on Chamaesiphon (as we saw for diatoms in “A twist in the tale …”) or whether our knowledge of the species within this genus is imperfect is not clear.  But discretion is the better part of valour in this instance.  Chamaesiphon species fall into two groups: those that live on stone surfaces (see “Survival of the fittest (2)”) and those that live on algae and plants, such as the one we see today (another is illustrated in “More from the River Ehen”).   They consist of a single, elongate but gently tapering cell, attached at one end to the plant and enclosed in a sheath.   The upper end of the filament forms small spherical buds (technically “exospores”).  One reason that I am wary of calling this population C. confervicolus is that most illustrations of this species show a stack of exospores in the sheath, whereas the White’s Level population all had just a single exospore.

Chamaesiphon confervicolus, growing on Potamogeton polygonifolius in White’s Level outflow, April 2020.   Note the exospores at the end of the cell.  f. and g. show the sheath very clearly.  Scale bar: 10 micrometres (= 100^th of a millimetre). 

The picture at the top of this post shows an artist’s impression of the Chamaesiphon cf confervicolus on the upper surface of the Potamogeton leaf.   I wanted to get some idea of the size, shape and arrangement of the epidermal and stomatal cells on the Potamogeton leaves and resorted to the tried and tested technique of painting a layer of clear nail varnish onto the leaf surface, then peeling this off when it had dried.  This had the added (and unexpected) benefit of also pulling of the epiphytes, giving some idea of their arrangement on the leaf surface at the same time.   One extra observation that this yielded was that upper surface was dominated by Chamaesiphon, growing in clusters, whilst the lower surface had greater representation of diatoms.   I’ve also tried to portray the chloroplasts in the stomata guard cells.  Plant epidermal cells generally do not contain chloroplasts, as their purpose is to protect the mesophyll cells that are the main centres of photosynthesis.  Guard cells of stomata, however, need energy to open and close the stomata so these are the exception to this rule.  I had not even been sure that I would see stomata on the upper surface of the cell, as these are mostly found on the underside of leaves; however, Potamogeton appears to have stomata on both surfaces.  As ever, there is a certain amount of evidence along with a dose of extrapolation.   Imagined, but not imaginary …

You can find a description of the terrestrial plant life of Slitt Mine and its environs in this post on Heather’s blog.

References

Foster, P.L. (1982).  Metal resistances of Chlorophyta from rivers polluted by heavy metals. Freshwater Biology 12: 41-61.

Harding, J.P.C. & Whitton, B.A. (1976).  Resistance to zinc of Stigeoclonium tenue in the field and the laboratory. British Phycological Journal 11: 417-426.

Robinson, N.J. (1989).  Algal metallothioneins: secondary metabolites and proteins.  Journal of Applied Phycology 1: 5-18.

Say, P.J., Diaz, B.M. & Whiton, B.A. (1977).  Influence of zinc on lotic plants. I. tolerance of Hormidium species to zinc.  Freshwater Biology 7: 357-376.

Sorentino, C. (1985).  Copper resistance in Hormidium fluitans (Gay) Heering (Ulotrichaceae, Chlorophyceae).  Phycologia 24: 366-368.

(Note that Hormidium is the old name for the genus Klebsormidium.  There is an orchid genus called Hormdium and, as this was described first, it takes priority.)

 

Some other highlights from this week:

Wrote this whilst listening to: Bob Dylan’s New Morning and Pat Garrett and Billy the Kid.   Also, Samuel Barber’s Prayers of Kirkegaard.

Cultural highlights:  The Netflix series Unorthodox, about a young woman fleeing a Hassidic community in New York.

Currently reading:  Agatha Christie’s A.B.C. Murders.

Culinary highlight:   Arroz con leche (Spanish rice pudding) served with peaches poached in madeira.

A reasonable excuse for exercise ..

A 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.

Bollihope Bhavacakra*

My explorations of the biology of Ulothrix zonata have taken me from Bollihope Burn in Weardale (see “Bollihope Burn in close-up”) to upper Teesdale (see “The intricate ecology of green slime”) and one of the outcomes is this representation based on the diatom-smothered filaments that I observed in Bollihope Burn, close to the sink hole.   The picture illustrates the suggestion that I made in the post about Bollihope Burn – that the switch from “vegetative” to “reproductive” mode leads to less energy being available for the alga to manufacture the slime that it needs to stop epiphytes gaining a foothold.   By intercepting the limited light that penetrates into the water, these algae can shade the host plant to such an extent that it cannot gain the energy it needs to grow.   The mucilage is the equivalent of the “anti-fouling paint” that mariners use to stop barnacles encrusting their hulls.

My image shows a single healthy filament of Ulothrix zonata at the bottom right of the image and, on the left, two filaments of cells that are producing flagellated gametes that will eventually be released.  I write “gametes” with mild trepidation, as they may also be nascent zoospores associated with the asexual phase (see below).   A third filament, in the background, is composed mostly of empty cells that have already released their gametes.   There are no “male” or “female” gametes; any two can combine to form a zygote, so long as they come from different filaments.   This zygote then attaches to the substratum and does little more over the summer.

In my image, the Ulothrix filaments have been colonised by needle-like cells of Fragilaria gracilis, Achnanthidium minutissimum cells on short stalks, and a couple of cells of Gomphonema pumilum.   There are also a couple of cells of Ulnaria ulna and some zig-zag colonies of Diatoma tenuis.   The effect of these cells on the appearance of the Ulothrix zonata is marked, smothering the filaments entirely so that, with the naked eye, the assemblage appears brown rather than green.

The diagram below summarises the life cycle of Ulothrix zonata and emphasises the point that the green filaments that most people associate with this alga are only a small part of the story.  The cell contents divide in one of two ways.  The first produces zoospores, each with four flagellae, which are released, settle and grow directly into a new vegetative filament.  The second, however, produces a number of cells which are smaller but otherwise look similar to the zoospores except that each has two rather than four flagellae.  These gametes then fuse with gametes produced by another filament to produce a “zygote” which, in turn, germinates to produce several (typically eight) zoospores, each with four flagellae, from which new vegetative filaments grow (see illustration of putative “germlings” in “The intricate ecology of green slime”).

The life-cycle of Ulothrix zonata, following Lokhorst and Vroman (1974).   “2n” refers to diploid stages; “n” refers to haploid phases (note that the vegetative filament is also haploid).

The asexual phase can be produced at any time, but is stimulated by high temperatures; the sexual phase, however, is more strictly regulated.   The formation of gametes only occurs under “long day” conditions, which means that it will happen as daylight hours extend in the spring.   By contrast, the division of the zygote requires short day conditions and low temperature, meaning that the zygote is relatively inactive over the summer months, only dividing to produce zoospores, and ultimately, new filaments, in late autumn and winter.   This creates a useful niche for the organism during a period of the year when nutrients are relatively plentiful in upland rivers (as they are washed off the land following rainfall) and grazers are relatively inactive.   It also means that this apparently simple green filament actually has some sophisticated controls that regulates how and when it divides.

I’ve talked about algal life cycles in the past, commenting that the concepts behind these are not always easy to grasp (see “Reflections from the trailing edge of science …”).   The problem is that undergraduates of my generation were taught this as part of a broader overview of plant evolution and the variations between patterns in different groups tended to befuddle rather than enlighten students already struggling to grasp the big picture.   The interesting twist to my explorations of Ulothrix zonata is that it has shown how the idiosyncrasies of an organism’s life-cycle can have a practical significance that helps the organism survive in a particular habitat.   Knowing about the life cycle can, in turn, inform our understanding of processes occurring within a stream or river.  The problem is that these topics have largely fallen off the agenda both for teaching and research, so we are generally limited to interpreting descriptions from old journals, and often forget completely the role that these factors may play in creating the mosaic of algae in a stream.

Reference

Lokhorst, G.M. & Vroman, M. (1974).  Taxonomic studies on the genus Ulothrix (Ulotrichales, Chlorophyceae) III.  Acta Botanica Neerlandica 23: 561-602.

* “Bhavacakra” is a symbolic representation of the cyclical nature of existence used in Tibetan Buddhism.  The title of this post is also an affectionate tribute to Brian Moss, who died a few days ago.

Bollihope Burn in close-up

Bollihope Burn does not disappear dramatically down a single swallow hole in the way that Gaping Gill swallows up Fell Beck on the slopes of Ingleborough.  Rather, there is a gradual diminishment of flow, as the river percolates through the joints in the limestone, before the remnants of the stream swirl down a final sinkhole (see “Co. Durham’s secret Karst landscape”).   I was intrigued to see how the organisms that inhabited Bollihope Burn reacted to these stresses so got down on my knees close to this final sinkhole to get a closer look.

My waterproof Olympus TG2 (see “Getting close to pearl mussels with my underwater camera”) set to super-macro mode is equivalent to putting my head under the surface of the water and then peering at the rock through a magnifying glass … but gets fewer odd looks from passers-by.   Fortunately, this is an isolated corner of Weardale and passers-by were limited to a few rabbits, because sticking a camera into a stream to take a photograph of a stone is, itself, odd enough to attract stares from most people.

These close-up views of freshwater algae in their natural habitat continue to surprise me.  It is only in the last few years that waterproof digital cameras with macro facilities have fallen to an affordable price.  Before this, underwater photography required special kit that few freshwater biologists could afford.  Yet, removing a stone to photograph the algal growths meant that the algae were never photographed in their natural habitat, and were deprived of the buoyancy that the water afforded them.   I have plenty of photographs of green or brown gunk composed of different algae but, with the algae removed from their context, these photographs offer few insights into the biology of the stream bed.  The photograph below, however, shows a community with a distinct structure – a “turf” of near-vertical filaments waving in the gentle eddies of the stream as it swirls around before disappearing down the swallow hole.

A cobble in Bollihope Burn, close to the swallow hole, covered by a short “turf” of algae, April 2016.   Scale bar: approximately two centimetres.

Under the microscope, the structure of this “turf” starts to reveal itself.   The filaments appear to be aggregations of diatoms around dying filaments of the green alga Ulothrix zonata.   This is an alga that is common in Pennine streams in the winter and early Spring but which disappears as the weather starts to warm up. It often forms very conspicuous green patches on the river bed for a short period of time, as in the following picture, which I took a few kilometres away from my current location, in the River Wear at Wolsingham.   The difference in appearance between the alga in the two photographs is mostly due to the Bollihope population being smothered with diatoms whilst the Wolsingham population was virtually a pure growth of Ulothrix.   This may be partly due to the Bollihope picture being taken taken two months later than the Wolsingham image.   Ulothrix zonata produces copious quantities of mucilage and the Wolsingham population was slimy to the touch.  I rarely see epiphytes on this or any other slime-producing algae in their healthy state.   However, Ulothrix is a species that thrives in cold water.   Indeed, a study has shown that when the water starts to warm up and the day length increases, the Ulothrix filaments switch into their dispersal and reproductive modes and that is what may be happening here.   As the rate of photosynthesis declines, so there is less carbohydrate from which the slime molecules can be made and, as a result, less of a deterrence to any diatom looking for a perch.   From now until next winter, Ulothrix zonata will not be very obvious in the streams that I visit.  This is because the zygotes which are produced by sexual reproduction lie dormant until day length decreases and temperature drops.   At this point, they germinate and divide to produce zoospores which, in turn, grow into new Ulothrix zonata filaments.

Growths of Ulothrix zonata on cobbles in the River Wear at Wolsingham, February 2009. 

The photographs taken under the microscope illustrate this well.  On the left hand side there is one of the few healthy looking Ulothrix filaments that I found, with a chloroplast wrapped around the inside of the cell wall   On the right hand side you can see that the chloroplasts have gone, replaced by dark green blobs which are (I think) bundles of gametes awaiting release.   More significantly, you can also see several diatoms around the Ulothrix filament, taking advantage of it to lift themselves up above the rock surface.

The paradox is that these algae are entering their senescent phase just as most of the plant life in Weardale is flourishing.   This is probably not a coincidence: life in cold water means fewer grazing invertebrates and less shade to intercept the precious winter sunlight.   I suspect that algae, once masters of the planet, have gradually adapted and evolved to live a subordinate life, flourishing in those periods of the year when most of us are content to stay indoors.

Ulothrix zonata from Bollihope Burn, April 2016.  The left hand image shows a healthy vegetative filament; the right hand image shows zoospore production and colonisation by diatom epiphytes. 

References

Graham, J.M., Graham, L.E. & Kranzfelder, J.A. (1985).  Light, temperature and photoperiod as factors controlling reproduction in Ulothrix zonata (Ulvophyceae).  Journal of Phycology 21: 235-239.

van den Hoek, C., Mann, D.G. & Jahns, H.M. (1995).  Algae: an Introduction to Phycology.  Cambridge University Press, Cambridge.

Bollihope Common

I spent part of last weekend wandering in the vicinity of a small reservoir on Bollihope Common in Weardale.   It is one of many small manmade water bodies in this part of the northern Pennines constructed to power the mills that served the lead mines in the region.

Rocks on the northern shore of the reservoir had tufts of a dark green, almost black, moss inhabiting the splash zone.   Under the microscope, I saw the characteristic wavy-edged cells which indicated that this was a Racomitrium.   This is Racomitrium aciculare, a semi-aquatic cousin of the species we encountered on rocks in Teesdale last year (see “Upper Teesdale in March”).   The southern shore of the lake, by contrast, was not fringed with rocks, but with rushes and Sphagnum moss, along with some Polytrichum.   This side of the reservoir receives the drainage from the fells above and, I suspect, the constant supply of sediment has led to the gradual infilling of the original shoreline.   There were at least a couple of species of Sphagnum present here, but I was most interested in the submerged moss, S. cuspidatum.

Looking north towards the unnamed reservoir on Bollihope Common (NY 989 348).   The road on the left hand side of the image leads to Stanhope.

Aquatic mosses from the unnamed reservoir on Bollihope Common.  The left hand image shows Racomitrium aciculare on the tops of boulders and the right hand image shows Sphagnum cuspidatum from the boggy areas on the southern shore.

I shook portions of both mosses vigorously in a small amount of water from the reservoir to dislodge the attached algae.   The clear water quickly turned brown and I sucked up a few drops of each with a pipette and dropped them onto a microscope slides.  First up was the sample from the Racomitrum.  This was dominated by the small diatom Achnanthidium minutissimum (a – e in the figure below).  When I had looked at the Racomitrium leaves under the microscope, I had seen many of these attached to the leaves by short stalks.   These comprised just over half of all the diatom cells that I counted.  Long needle-like cells of Fragilaria rumpens (or something similar) which attached to the leaf by their base formed another 27% and another genus, Gomphonema (one or more forms in the G. parvulum complex), formed about 16%.  Most interesting to me were a few gracefully-curved cells of Hannaea arcus, as these are good indicators of a relatively pristine habitat.

Next up was the sample I had obtained from the Sphagnum.   Sphagnum usually favours acid habitats so I was intrigued to see what diatoms would be associated with it, having seen that the diatoms associated with Racomitrium, a hundred metres or so away, mostly suggested neutral or slightly alkaline conditions.

Once again, it was Achnanthidium, Fragilaria and Gomphonema that comprised the majority of the diatom cells (54, 19 and 16% respectively) but this time, about 8% of the total belonged to at least three species of a different genus, Eunotia, which is often associated with acid habitats, and the curved cells of Hannaea were conspicuous by their absence.   Interestingly, Sphagnum does not only favour acid conditions, peculiar features of its cell wall chemistry also helps to create those acid conditions and the diatoms living in the microhabitats around the submerged Sphagnum were clearly indicating a slight change in conditions, compared to those I found on the Racomitrium.

Diatoms growing on and around mosses in the unnamed reservoir at Bollihope Common; a – e: Achnanthidium minutissimum complex; f,g: Gomphonema parvulum complex; h. Eunotia spp (probably E. implicata); i. Navicula (probably N. cryptocephala); j. Fragilaria (probably F. gracilis); k. Hannaea arcus.  Scale bar: 10 micrometres (1/100^th of a millimetre).   Note, particularly for h and k, healthier specimens were present in the samples but none presented in a manner amenable to photography.

There was much more Sphagnum underfoot as I walked over Bollihope Common.  Given time – a couple more centuries, maybe – and the gradual invasion of Sphagnum from the moorland around the reservoir might continue and, we can hypothesise, the acid-loving diatom species might become more abundant.  Indeed, we could even argue that this would simply be nature re-establishing its influence, the reservoir being an unnatural and – in the grand scheme of things – temporary intrusion into the landscape.