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.

Note

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.

References

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.

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How to make an ecologist #5

Hindsight, curiously, confuses this exercise of looking back over my career, rather than aiding it. Looking back, I see a linear pathway from Harold Wood through to the present, losing sight of the crossroads where chance could have taken me off in entirely different directions.   Contemplating my visit to the site of the former Westfield College (see “How to make an ecologist #4) jogged my memory and reminded me that the process that brought me to Durham was, actually, far from straightforward.

My undergraduate project had convinced me that I wanted to do a PhD, and my first choice of a supervisor was Professor John Harper, at Bangor in North Wales, whose book Population Biology of Plants had been influential in determining the course of my project.   However, he had recently retired and my letter enquiring about opportunities had bounced around his colleagues.   I did go up to Bangor to meet someone in the Agricultural Science department with a view to doing a taught MSc leading into a PhD on the overlap between population biology and grassland agronomy. There was funding for the MSc but, after that, the situation seemed rather vague.

It was not until after I graduated from Westfield that I saw an advert in New Scientist for an MSc in Durham.   Though not concerned with population biology, it fitted in with my undergraduate studies in two ways: the focus was mosses (picking up on my work on Sphagnum) and heavy metals (a specialism of the department in Westfield).   There was also a strong likelihood of the MSc being converted to a PhD.   I applied.   I had never heard of Brian Whitton (my undergraduate years had involved a single first year lecture on algae) but Connie Allen, a postgraduate student in the lab where I had worked on my project, whooped with delight when I told her that I had an interview. He was, she told me, the leading expert on blue-green algae in the country.   I seem to remember that my application included a hand-written, photocopied CV which included a spelling mistake.   That I got the studentship probably says as much about the other candidates as it does about me.

This was one of those crossroads in my life where chance could have taken me off in several different directions.   It is not a simple process of finding a supervisor whose interests dovetail with your own.   You needed financial support and, in the sciences, that came mostly via a studentship that the supervisor had already been awarded.   It is supply-side economics: there was a pool of studentships, and a larger pool of candidates.   The candidates may have their own ideas on what they want to do, but there was no guarantee that a project on that precise topic would come up at the right time.   At the time of my interview I seem to remember that I was not wholly convinced that this was the right project for me but it seemed like the best opportunity at the time.   I booked a train ticket and headed to Durham for an interview.

Durham_from_station

Durham: the view from the station.

Durham entrances you before even leaving the station.   The train approaches through a wooded cutting, before the landscape drops away; the final approach to the station is across a viaduct, giving panoramic views of the ancient city, with castle and cathedral perched on an incised meander above the River Wear.   I walked down from the station to the town and through cobbled streets winding up towards the cathedral.   Several years later, when I visited Tuscany for the first time, I was struck by the similarity between this small northern English city and the Tuscan towns clustered on hillsides around a basilica.   I think I knew that I would accept the studentship, if offered, even before I got to the interview.

The science laboratories were a short walk away from the town centre, a zone of prosaic architecture to offset the glories of the peninsula.   The Botany Department was based in the Dawson Building, the oldest building on the site, which had once contained the entire science faculty and which is now home to the departments of Archaeology and Anthropology.   When I arrived, Brian Whitton had a suite of labs on the first floor, plus two satellite laboratories elsewhere on the site, relicts of a period just before I had arrived when the research group was much larger.   Although the work I was doing followed on from topics I had studied as an undergraduate, the Botany Department in Durham was a contrast to Westfield in many ways: an air of frantic industry pervaded the corridors in contrast to Westfield’s general serenity.   Research groups were larger, postdocs were more numerous and I got the sense that Durham academics were researchers who taught, rather than teachers who researched.

Dawson_Building

The Dawson Building on the University of Durham’s science site.  The phycology labs were at the right hand end on the first floor.  

The interview must have been early July 1983; one condition of the studentship was that I started on 1 August, so that I could get started on fieldwork straight away.   When I watched other postgraduate students turn up in October, and then read and plan experiments through the winter months before starting fieldwork the following Spring, I realised the sense of this step. It did mean, however, that I arrived in Durham at the quietest time of year, was dumped in the unprepossessing Parson’s Field House (now demolished), the postgraduate residence, and had little to do in the evenings. In those days before personal computers, however, Brian Whitton encouraged that all his students learned to type.   I bought a manual typewriter and a book on touch typing and spent the summer evenings banging out exercises to develop strength in my fingers.   The first transferable skill that I learned during my postgraduate days was, therefore, the ability to type without looking at the keyboard; a skill that has proved very useful over subsequent years.

Wonders in my own backyard …

One of my many half-worked out and not-fully-proven theories is that the golden age of Victorian microscopy coincided with an era when many educated British men were heading off to the colonies and sending back reports of weird and wonderful flora and fauna that they encountered.   The microscope was, for those left behind, a similar portal into hitherto unexplored worlds; one that, furthermore, could be found without leaving your own grounds.

A case in point: here is a photograph of some moss on my driveway.  I have walked past these mosses thousands of times without giving it a second thought.   Today, however, I have a point to prove.   The second photograph is a close up of the same moss, taken with the extreme macro lens on my new Olympus TG2 compact camera.   This reveals the colonies to consist of tongue-shaped leaves, each terminating in a long hair-like projection.   My somewhat dated guide to mosses tells me that these are plants of Bryum capillare.   Even at barely a millimetre across, these leaves are enormous compared to the algae I normally write about here.

Bowburn_driveway_mosses

A row of bright green colonies of Bryum capillare beside my driveway in County Durham, with a lens cap (five cm across) as an indication of scale.

The next step is to strip a few of the leaves off the plants using a pair of forceps and blade and mount these in a drop of water to examine under my high power microscope.   Ironically, the lowest magnification lens I have on this microscope (10x) is too powerful and I cannot get all the leaf into a single image, but we can see the hair point as an extension of the “nerve” that extends the length of the leaf.   Just visible, too, are the long, narrow cells which form a border around the leaf edge.  The cells, themselves, are just a single cell thick, each parallelogram-shaped, about 50 micrometres long and containing a number of small chloroplasts.

I wrote about the tops of boulders being like miniature deserts last year (“Upper Teesdale In March”) and the same applies to man-made habitats such as paths and driveways.  The cushion-like growth forms contains networks of tiny spaces which turn the whole plant into a miniature sponge, soaking up and retaining water, enabling it to continue to grow long after the ground around it has dried up.   In the past, I presume, mosses such as Bryum capillare would have been rare but, with our modifications to the landscape, including building walls and driveways, we have greatly expanded the habitat available to this species.   As a result, our sedentary Victorian naturalist had just as many opportunities to explore deserts as Richard Burton, Charles Montagu Doughty and Wilfred Thesiger.

Tortula_muralis_140302

Bryum capillare.  The left hand image is taken with a macro lens; the right hand image was taken under a microscope; the scale bar is 100 micrometres (1/10th of a millimetre).   The hair is roughly double the length of the portion included in the image.

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.

Bollihope_140208

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.

Bollihope_mosses

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.

Bollihope_diatoms

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 (1/100th 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.