Loving the low flows …

It is a long time since we have had a heatwave in the UK that has lasted as long as the present one.  The last that compares was in 1995, when the A1 near my house was busy with tankers ferrying water from Kielder Water in Northumberland to the drought-afflicted areas of Yorkshire.   Three weeks in and gardeners are staring anxiously at the parched soil and quietly praying for rain whilst, at the same time, trying to make the most of the rare luxury of warm weather.   Rivers, too, are showing the effects of the weather.  In some parts of the country, rivers are drying out and fish stocks are threatened.   That is not the case here in the north-east, but the River Wear is showing some signs.

The medieval Prebends Bridge is one of the most picturesque sights in Durham but, at the moment, the water underneath the bridge – and, indeed, throughout Durham – is bright green with flocs of algae.   Closer inspection showed this to be fronds of Ulva flexuosa: the cells are arranged to form a hollow tube, like a sausage skin, which traps the oxygen released by photosynthesis to give the alga an integral buoyancy aid.  You can see how this clearly in the image below. The common name for this alga is “gutweed”, which offers us another metaphor for the appearance of these semi-inflated sacs of cells.   This broad thallus is loosely attached to the river bed (see lower picture below) but is easily dislodged, after which the thalli drift downstream until they become entangled in other water plants or submerged branches.  In the current state, Durham’s rowers are grumbling that it is becoming entangled with their oars

We often see a little Ulva flexuosa in the Wear during the summer, but rarely as much as this.  It is a species that thrives under still, warm conditions and which also benefits from the weirs which regulate the flow of the river in Durham.   It is an alga that we tend to associate with nutrient-rich conditions, but this might be because the type of slow-flowing lowland rivers where it can become common are more likely to be nutrient rich than faster-flowing upland rivers where it is rarely found.   The current weather, in other words, creates the “perfect storm” for Ulva flexuosa.   Ironically, a storm – perfect or otherwise – will probably alter the flow regime in the Wear enough to flush it all downstream.  Another curiosity is that, despite being favoured by low flows and the near-standing water behind the weirs in Durham, Ulva flexuosa seems to be more likely to form mass growths in rivers rather than in lakes or ponds.

Ulva flexuosa in the River Wear, July 2018: the upper picture shows free-floating thalli, inflated by oxygen released by photosynthesis; the lower photograph shows thalli still attached to the river bed.

In my experience, the genus Ulva tends to be absent from nutrient-poor conditions which is subtly different to saying that it thrives when nutrients are abundant.  There are other factors – warm, stable conditions in particular that determine the success of the genus in any particular place.  The Wear has seen a significant decrease in nutrients in recent years yet here we are, in 2018, with a river full of Ulva.   I could say that, despite this reduction in nutrients, the Wear is still, relatively speaking, nutrient-rich, but the coincidence with an altered flow regime, a prolonged spell of warm weather and low flow conditions seems too great to ignore.   Ulva flexuosa is, in other words, a fine example of an alga where we need to think of a multifactorial “habitat template” rather than just in terms of single stressors.   We also need to think in terms of probabilities of mass proliferations increasing or decreasing as habitat factors vary, rather than a simple likelihood of finding Ulva at any particular location.

That means we need to look at climate change forecasts and, if there is a likelihood of more long, warm, dry summers, then we should expect more frequent blooms of Ulva in our rivers.  We may tinker with nutrient concentrations and even try to restore more natural flows (though Durham’s rowers will have a view if that was tried here!) but, ultimately, Ulva flexuosa is a species that enjoys a heatwave as much as any other resident of these islands.

A high magnification (x 400) view of the thallus of Ulva flexuosa from the River Wear.   Scale bar: 20 micrometres (= 1/50th of a millimetre).


Rybak, A.S. & Gąbka, M. (2017).  The influence of abiotic factors on the bloom-forming alga Ulva flexuosa (Ulvaceae, Chlorophyta): possibilities for the control of the green tides in freshwater ecosystems.  Journal of Applied Phycology https://doi.org/10.1007/s10811-017-1301-5


Pleasures in my own backyard


One of the delights of my part of County Durham is the range of natural history that is available without the need to travel great distances.  That, indeed, has been the theme of this blog right from the start (see “Cassop”) and today’s post continues the theme of nature on my doorstep, with a visit to a local nature reserve within walking distance of my house.  Like Cassop Pond, it is at the foot of the Magnesian Limestone escarpment and, at this time of year, the grassland is rich with Northern marsh and Common spotted orchids.   It is, of course, the ponds that draw my attention: they are rich in aquatic plants including, once I start to look closely, beds of the alga Chara, which I’ve written about before (most recently in “Everything is connected …”).  And then, once my eyes are adjusted to looking at natural history at this more intimate scale, I can see that the stones on the bottom of the pond are covered with tiny snails (probably Hydrobiidae) with shells coiled in the shape of an ice-cream cornet.  Freshwater snails crawl across submerged surfaces rasping off attached algae with their tough radula so I started to wonder what snails in this particular pond might be feeding upon.


Submerged stone from the pond at Crowtrees Nature Reserve, County Durham, covered in Hydrobiidae snails (left: the stone is about 10 cm across) and (right) a stone removed from the bottom of the pond showing the marl-covered part that was exposed and the marl-free part that was buried in the sediment. 

Viewed from just above the water, the surface of the stone looked as if it could be an algal film but, when I picked it up, the stone did not have the yielding texture that I associate with such films, but was a hard, mineral-rich marl.  More intriguingly, it was only present on the exposed surfaces, possibly, I suspect, due to the subtle interactions between chemistry and biology that I wrote about in “Everything is connected …”.

The calcite crystals make it hard to get a good view of the material under the microscope, but I managed to see a number of diatoms, mostly Gomphonema pumilum, or a relative, but also a good number of tiny, slightly asymmetric cells of a species of Encyonopsis, a genus that was, until recently, included in Cymbella, and which is usually a good indication that the water is about as untainted by human influences as it is possible to get.   It is, however, hard to get a really clear view of these under the microscope as they were scooting around.   With valves that are barely more than a hundredth of a millimetre long, I really needed to use an oil immersion objective to see them clearly, but the calcite crystals on the slide made it almost impossible to get a clear view of the live cells.  Not surprisingly, most of what we know comes from studies of carefully-cleaned preparations of the empty frustules.   Encyonopsis shares with Tyrannosaurus Rex the distinction of being an organism better known dead than alive.   It is rather ironic, given that healthy populations are living so close to my house, but that’s very often the case with diatoms.

There was one other abundant alga living amidst the rock (and, indeed, probably the major food source of the snails), but I am having some problems giving it a name, so a full account of that one will have to wait until another day.


Diatoms at Crowtrees Nature Reserve, July 2016: a.-d.: Gomphonema (possibly G. pumilum) in girdle and valve views; e.-g.: Encyonopsis sp.   Scale bar: 10 micrometres (= 1/100th of a millimetre).

You must be taking the piss?


You can find algae growing in the most unlikely places.   Look at the photograph above.   To the left you can just see The Swan and Three Cygnets, a popular pub beside Elvet Bridge in Durham.  The arches of Elvet Bridge extend onto the bank of the rivers, and one sits just beside the pub (hidden, in this photograph, by a large tree).  Under this arch, growing on the wall in a patch from approximately 30 centimetres above the ground, there is a lush patch of green algae.

Think about it: pub; arch nearby that is sheltered from the elements and from the gaze of passers-by; patch of algae from slightly below waist height downwards.  The technical term amongst ecologists for algae that like such habitats is “nitrophilous”: an alga that thrives in environments where the nitrogen concentrations is elevated.   Do I need to spell it out?  Pub, beer, sheltered archway, waist height …

I do not usually lurk around such unsavoury environments, but I was looking for a particular genus of green algae called Prasiola, which is known to favour habitats such as these.  Indeed, I have in front of me a paper from a learned journal that states “… consistent dampness and nitrogenous availability from animal wastes is likely to be the primary determining factor” for those trying to understand the distribution of this genus.


Left: the arch of Elvet Bridge with the Swan and Three Cygnets pub to the left; right: a patch of Rhizoclonium cf riparium on the north side of the arch.  Top image shows the Elvet Bridge with Durham Cathedral and Castle in the background.

To the naked eye, these patches of green algal growths on the damp, shaded arches, confirmed my pre-conceptions, so I was a little surprised when I put a small piece pulled from one of these under my microscope and saw unbranched filaments of cells, instead of the flat sheets that are characteristic of Prasiola.  The combination of unbranched filaments with a single net-like chloroplast indicates that this belongs to the genus Rhizoclonium, and Chris Carter, when he examined the material, suggests it is R. riparium, a common species of brackish and marine environments, including habitats such as salt-marshes where it will be periodically exposed to the atmosphere.  These patches of Rhizoclonium are like mini-vertical saltmarshes, subject to occasional immersion in squirts of high conductivity liquid, which will then be retained within the tangle of filaments.  Gradual evaporation from these patches (limited, due to the shaded microhabitat within the arch) will make the residual liquid yet more concentrated, and offering a selective advantage to an organism adapted to coping with salinity fluctuations in estuarine environments.


Rhizoclonium cf riparium from a damp arch of Elvet Bridge, Durham, April 2016 (photograph: Chris Carter)

Fabio Rindi and Mike Guiry made a particular study of algae associated with subaerial environments a few years ago but, curiously, their papers include no records of Rhizoclonium.   By contrast, I have so little experience of these habitats that I cannot say whether this was just a lucky (or unlucky – I was looking for Prasiola, remember) coincidence.  However, it does serve as an intriguing reminder that algae grow in some very unlikely places.  The problem is not that algae can’t grow on urine-soaked walls; it is that not many of us are interested in examining the natural history of such unsavoury habitats.


Rindi, F. & Guiry, M.D. (2003).  Composition and distribution of subaerial algal assemblages in Galway City, western Ireland.   Cryptogamie, Algologie 24: 245-267.

Rindi, F. & Guiry, M.D. (2004).  Composition and spatial variability of terrestrial algal assemblages occurring at the bases of urban walls in Europe.   Phycologia 43: 225-235.

Rindi, F., Guiry, M.D., Barbiero, R.P. & Cinelli, F. (1999).  The marine and terrestrial Prasiolales (Chlorophyta) of Galway City, Ireland: a morphological and ecological study.   Journal of Phycology 35: 469-482.

How to make an ecologist #6

My PhD studies were my introduction to the world of environmental monitoring, the shady netherworld of Jeremiahs who diagnose and pronounce upon the sorry state of our planet.   The roots of this activity lie in sound fundamental research; the form it takes, however, is strongly dictated by non-scientific circumstances, including politics and legislation.

In my case, the research group that Brian Whitton led had done some pioneering work on the effect of heavy metals on aquatic ecology from the late 1960s and throughout the 1970s. They had used the abandoned lead mines of the northern Pennines to establish general patterns between the concentration of heavy metal in the water and the number and types of aquatic plants and algae that were found.   The environmental and health consequences of heavy metals was one of the most prominent environmental/health issues of the time. This, however, turned out to be a mixed blessing for me.   On one hand, there was already a huge body of research addressing the basic questions; on the other hand, BP, who at the time had a mining subsidiary exploring the feasibility of zinc mining in the north of England, funded my PhD in order to manage any UK-based activities responsibly.   I was to investigate the use of aquatic mosses to monitor the concentrations of heavy metals, following on from the work of a recently-finished student, John Wehr.

Suffice it to say that I produced enough results over the three years of my studies to satisfy the examiners. It was workmanlike stuff, not terribly exciting and not work that I look back upon with great pride.   I got three papers in respectable journals without shaking the foundations of aquatic ecology and they got my career under way. However, even before I finished, I had the feeling that the rest of the world had moved on.   The issue of toxic pollutants in the environment had been simmering away in the scientific literature and popular press since Rachel Carson published Silent Spring in the 1960s.   In 1976, the European Economic Community had passed the Dangerous Substances Directive which limited the amounts of toxic pollutants (including heavy metals) that were allowed to be released into the environment.   By modern standards, it was not a very subtle piece of legislation, and it certainly did not have any ecological nuances that might have stimulated new directions in my research.   What it did mean, however, was that problems posed by heavy metals were on a wane in western Europe by the time I started. There were rivers that still suffered (see “A return to the River Team”) but more due to weak regulation than to a lack of understanding.   Staff in water authorities (forerunners to the Environment Agency) did use mosses to detect intermittent pulses of heavy metals but, as the legislation was written in terms of concentrations of chemicals, the role of biology was secondary.


Caplecleugh Low Level, an abandoned lead mine at Nenthead in the northern Pennines.   This is one of a very small number of slides from the period of my PhD studies, and their aftermath, that has survived.   You can just see the abundant algal growths (Mougeotia) in the channel in the foreground.

One area where heavy metals were still a problem, however, were the abandoned ore fields of the northern Pennines.   The regulators could only regulate extant businesses, but the mining companies who had exploited the minerals here had long since disappeared.   Polluted mine waters continued to pour from abandoned adits into rivers and these created a huge natural laboratory on which to test ideas. I had spent all my life until this point in London; I associated landscapes as majestic as those of the Pennine dales with holidays, not the everyday. Now, however, I had the excuse to get out and explore the hills, looking for suitable locations for experiments or to collect material to work on in the laboratory. I sometimes scheduled field work for the weekends and walk in the hills after I had finished what I needed to do.  The blurring of the boundaries between work and leisure is another recurring, and mostly positive, theme in my career. I know lots of people who need more rigorous compartmentalisation of professional and home life. I’m lucky, perhaps, in that the curiosity that sustains my work can also spill over into something as mundane as a stroll along a river bank.

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: 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.


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.

And the waters prevailed on the earth …

Conscious that this blog, which purports to be about natural history, has not actually had any posts reporting my own observations for some time, I had tentative plans to go out and collect some samples this weekend.   Storm Desmond, however, had other plans for me: high winds and heavy rain from Friday evening onwards pushed local river levels up to dangerously high levels, and I was left to contemplate the capricious nature of the British climate from a largely sedentary perspective.

A screenshot of the Environment Agency’s river level gauge shows how the spate developed. We had high winds in Durham, but not excessive amounts of rain, and the steep rise in levels during Saturday reflects the time-lag between rain falling on the Pennines and arriving in the Wear at Durham.   The peak in level occurred early on Sunday morning, and the subsequent pictures show the river in Durham at about 10:30 on Sunday morning. Though high, this is not the highest I can remember: that was in 2009 when the road into Durham was impassable at Shincliffe as the river had broken its banks.


A screenshot from the Environment Agency’s river level website, showing the passage of Storm Desmond through Sunderland Bridge, a few kilometres upstream of Durham City, from midday on Friday 4 December to midday on Sunday 6 December.


The River Wear in spate on 6 December 2015: the view downstream from Elvet road bridge.


The River Wear in spate on 6 December 2015: the view upstream from close to the Count’s House, showing the same clump of Japanese Knotweed that featured in my post “In praise of Japanese Knotweed” in early November. You can see how the river has risen up and covered the bankside footpath.

As I have some old-fashioned ideas about fieldworkers being part of the benthos not the plankton, I am watching river levels with interest. I have fieldwork planned in Cumbria at the end of the week and am intrigued to see what Storm Desmond has done to the algal communities. The irony being that to study the effect of spates on ecology, one has to first get into the river… or even get close to the river. This may be an issue as my route to the River Ehen passes some of the most affected parts of Cumbria.

However, questions about the effect of a spate on a river can be generalised into a broader question of how occasional catastrophic events alter ecosystems which, in turn, makes us think about problems that may arise from simplistic interpretations of the relationship between explanatory variables and response variables, discussed in the previous post.   The temptation is always to interpret ecology in terms of the variables that are easiest to collect and measure when you are collecting your samples whereas, looking back at “How green is my river?” we see how late summer conditions in the River Atna are at least partly the consequence of high flows in the Spring.

Biologists have suspected this for a long time, and there is anecdotal evidence of changes wrought by individual events. I was pleased, therefore, to see that a whole issue of Freshwater Biology had been devoted to this topic.   With the effects of flooding uppermost in our minds this weekend, the introduction to this issue, by Mark Ledger and Alexander Milner, reminds us that “catastrophic events” can also extend to droughts and heatwaves. One point that emerges, even from a quick scan through the contents of this issue, is the importance of long-term datasets – revisiting, again, topics that wrote about in “How green is my river?” It is more pertinent than ever because of the fiscal squeeze on our environment agencies and, ironically, the flooding that results from Storm Desmond is going to lead to calls for yet more DEFRA money to be diverted to flood defence, which can only mean less money for environmental regulation.   Therein lies the biggest irony of all: a government department that trumpets the virtues of “evidence-based policy” is planning to collect less evidence. Work that one out, if you can.


Ledger, M.E. & Milner, A.M. (2015). Extreme events in running waters. Freshwater Biology 60: 2455-2460.

In praise of Japanese knotweed …


Autumn colours on Pelaw Wood, photographed from Durham Racecourse, October 2015.

There, I’ve said it.   Now to redeem my professional reputation …

It has been a particularly fine autumn, with a long period of warm, dry, calm weather allowing leaves to stay on trees and giving us plenty of opportunities to stroll through Durham and enjoy their colours.   Some of the most vivid yellow-orange colours were right down beside the river, large heart-shaped leaves growing from a tangle of erect stems readily recognisable as Japanese knotweed, Fallopia japonica. There is not normally much good to say about Japanese knotweed, introduced to Britain in about 1850 via Kew Gardens, and now crowding out native species but the colours this autumn were so spectacular that I thought it deserved at least a brief mention here.   We can, perhaps, see the plant from the perspective of the Victorian horticulturists who thought that it would be an attractive addition to the garden flora.


Japanese knotweed on Durham riverbanks, November 2015.

It is all a matter of perspective: autumnal landscapes around Durham are enhanced by introduced maples and sycamores and, indeed, beech trees (not native to the north of England). Even larch, an introduced deciduous conifer, adds to the overall aesthetic, so long as it is planted in small, unregimented patches. But these are not vigorous hard-to-eradicate colonisers like Japanese knotweed or Himalayan balsam (see “The future is pink …”), so we do not damn them with the epiphet “weed”. “Weed” is not a botanical term; it is an anthropocentric word, a declaration of war, if you like, against a plant that intrudes into spaces where we do not think it belongs.   Just for a few weeks each Autumn, however, perhaps we should declare a truce?