Fade to grey …


Prudhoe is a small town in Northumberland whose most famous inhabitant doesn’t exist*.   I came here to have a look at a pond in Priestclose Wood, a nature reserve operated by Northumberland Wildlife Trust which hit the local headlines recently for a suspected pollution incident.  You can see the scum on the surface in the photograph above and it does have an oily appearance, so anyone might be forgiven for calling the Environment Agency and asking them what was going on.

The query worked its way through the Environment Agency and ended up in my in-box in the middle of last week, with a specimen falling onto my doormat a few days later.   Having had a good look at it through my microscope, I drove out to the pond on a damp afternoon to take a look myself.   It is just a small pond, perhaps 30 metres across, set amidst the oak, birch and rowan-dominated woodland, which means that much of the lake is in almost permanent shade and, perhaps more important for the development of surface films, sheltered from the wind.  The surface film was just as I had been led to expect, despite the efforts of folk from the wildlife trust tried to disperse it last week in case the newts which lived here were threatened.   It was greyish-brown in colour, and covered the entire surface.  When I stirred it with a twig, it broke up, quickly closing up again as the water settled.   I then skimmed a sample bottle across the surface layer and harvested a yellow-brown suspension which I brought home for a closer look.


The Chromulina scum on the surface of the pond in Priestclose Wood: the left hand image was taken after stirring the surface with a stick to break up the oily layer; the right hand image shows the golden-brown algae that I scooped from the surface.  The picture at the top of the post shows the pond with its covering of algae.

The fresh sample was dominated by large numbers of tiny cells darting around.  Closer observation showed this to be oval, with a single yellow-brown cup-shaped chloroplast and what looked like one flagellum.   Over time, however, these cells slowed down, became rounder and started to aggregate in groups.   These, rather than the motile cells, proved rather easier to photograph.   I suspected that we were looking at a Chrysophyte, and Dave John later confirmed it to be Chromulina ferrea (the chloroplasts lack a pyrenoid, otherwise it would be C. aerophila).   If that is the case then there will be a second, much smaller flagellum too, but which is much harder to see with the light microscope.

Both of these species were described by John Lund in 1942 from ponds in Richmond Park whilst he was a PhD student at Queen Mary College London.   They are “neustonic”, meaning that they are adapted to live at the air-water interface, which also explains why they form the surface film that we saw in the pond at Priestclose Wood.   John Lund gives a detailed description of just how the behaviour of the alga leads to the formation of these films.   However, apart from John Lund’s original observation, the only other record in the British Freshwater Algal Flora is from a pond near Orpington in Kent, close to Dave John’s house.   Such is the nature of phycological records: it is not necessarily the algae that are rare so much as the people who notice them.


Chromulina cf ferrea from the pond in Priestclose Wood, Prudhoe, Northumberland, July 2019.   The left hand image shows a clump of sessile cells, photographed at 400x magnification; the right hand images show sessile cells at x1000 using brightfield (upper) and phase contrast (lower) illumination.  Scale bar: 10 micrometres (= 1/100thof a millimetre). 

The local paper comments that the pond usually has a covering of duckweed at this year and blames the algae for killing this off.  The reality may be more complicated: duckweed (Lemna minor) can appear and disappear rapidly in a pond without any obvious cause (see “The green mantle of the standing pond …”) so it is equally possible that the duckweed disappeared for an unrelated reason (a virus, perhaps?) and this created an opening into which the Chromulina was able to expand.   We’ll probably never know the truth.   Maybe the duckweed will be back next year; maybe not.

Looking back at earlier posts, I see that the only other time a chrysophyte was the subject, I ended bemoaning circumstances where the these alga were both a “natural” part of the habitat’s biota whilst, at the same time, lacking in aesthetic appeal (see “A brief excursion to Norway”).   The same situation seems to apply here: an otherwise attractive woodland pond now covered with a greyish film which is, as far as I can tell, a “natural” phenomenon.  It is a shame if these are the only times that the lay-public encounter the chrysophytes as some of them are very beautiful under the microscope.   But, at the same time, the is no law that says nature has to be pretty.  Maybe it is our preconceptions that sometimes need adjusting …

* Ruth Archer, from the BBC Radio 4 series The Archers


John, D.M., Whitton, B.A. & Brook, A.J. (2011).  The Freshwater Algal Flora of the British Isles.  2ndedition.  Cambridge University Press, London.

Lund, J.W.G. (1942). Contributions to our knowledge of British Chrysophyceae.  New Phytologist41: 274-292.


The Martial Heavens

If you follow this blog you’ll know that I am interested in the interactions between art and science and in trying to understand the benefits that art can bring to science and vice versa. Art and science (or, for that matter, art and other academic disciplines) do not always dovetail neatly: science is evidence-driven, art deals with experiences.   Academic study, generally, requires there to be a critical distance between subject and investigator and there are situations where too much “experience” may compromise this. But, at the same time, the friction at the art-science divide can generate synergies that are mutually-beneficial.

There is a good example on show in Newcastle at the moment: Matthew Flintham was the Leverhulme Artist-in-Residence in the Geography Department at Newcastle University, working with my colleague Alison Williams, and has produced a series of depictions of the restricted airspace above military training areas in the UK.   His exhibition, the Martial Heavens, is currently displayed at the Ex Libris Gallery in the University and one of the pieces is illustrated below. In the foreground you can see a large-scale map of an area of Northumberland that includes the Otterburn training area. Resting on this is a to-scale wireframe model that shows the limits of the restricted airspace above the training area.


Matthew Flintham: the Martial Heavens exhibition. Ex Libris Gallery, Newcastle University, January 2015.

Broadly speaking, I see art-science interactions working in two ways: firstly, where the art acts as a creative adjunct that allows a scholar to examine the available evidence in new ways and open new perspectives and, second, as means of visualising the outputs of scholarship in order to make them more accessible.   Martial Heavens is a very good example of the latter.   I homed in on Matthew’s depictions of Northumberland because it is an area of the country that I know well. In particular, I could trace the upper section of the River Coquet, which has featured in my work (see “A journey to the headwaters of the River Coquet…”) on Matthew’s map as it forms one of the boundaries of the Otterburn training area.   The casual visitor driving along beside the Coquet enjoying the spectacular landscape is likely to be unaware of many facets of both the human and physical worlds that knit together to create these vistas.   Martial Heavens opens up one of these by extending our awareness of the military’s presence from the fluttering red flags and “Danger Area” signs on the hillsides to a series of virtual boundaries that extend 10000 metres into the sky.

And then you have to adjust your focus again from the clouds high above to the stream that flows alongside the road.   The same principle applies in my work, which draws on art-based approaches in order to bring facets of Coquetdale’s geography alive to audiences who would otherwise just drive past.   My own explorations were at the microscopic scale, highlighting a microscopic world that lives on every submerged stone but, once again, it is the chemistry between art and science, between evidence and experience, that allows us to clothe the bare bone of “data” or “evidence” in a manner that makes it more real to the wider world.


More about my own work in this publication:

Kelly, M.G. (2012). The semiotics of slime: visual representation of phytobenthos as an aid to understanding ecological status.   Freshwater Reviews 5: 105-119.

On the trail of Arthur Scott Donkin …

I was back at the River Team yesterday, pushing my way past Himalayan balsam, in order to collect some of the brown biofilm from the tops of some stones.   There was one species that I was particularly interested in finding, because of a strong historical link with north-east England. The species I was looking for was a small diatom called Navicula gregaria. “Navicula” is a Latin word which means “small ship”, an apt description as these small boat-shaped organisms were serenely gliding around the field of my microscope (see an earlier post “Coxhoe” for a short video of some close relatives in action).   Look out, too, for the two parallel chloroplasts which, in the case of N. gregaria, are slightly offset in respect to one another.   When the sample is digested to remove organic matter and a permanent slide is prepared, we can see more detail of the ornamentation on the silica cell wall (“valve”).


Navicula gregaria, from the River Team at Causey Arch (approximately 50 m downstream from the location photographed in my post of 31 July.   Scale bar: 10 micrometres (= 1/100th of a millimetre).


Navicula gregaria, cleaned valves from Bradgate Brook, Newton Linford, Nottinghamshire, October 2011.   Scale bar: 10 micrometres (= 1/100th of a millimetre).

Navicula gregaria is one of the most common freshwater diatoms in Britain, with a range that extends from very clean rivers to highly polluted streams such as the River Team.   It does not like very soft water or water that is acidic but, otherwise, is almost cosmopolitan, particularly in spring, though it can be found all year round.

I wanted to write about Navicula gregaria today partly because it gives me an opportunity to tell you about what we know about Arthur Scott Donkin, the Victorian microscopist who we met briefly in earlier posts (see “In the footsteps of a Victorian microscopist”, “Prime time diatoms”, “Sampling the surf at Alnmouth”). I mentioned in the last of these that we knew little about his life; however, along with colleagues at the Royal Botanic Gardens in Edinburgh, I have managed to unearth a few more details. As try to limit my posts to about 500 words, I’ll spread these across two posts.

The main source of information about his early life comes from a publication called “Men of Mark ‘Twixt Tweed and Tyne”, published in 1895, which has a chapter about his father, Samuel Donkin (1801-1888), a farmer and auctioneer based ,for most of his adult life at Felton, a village between Morpeth and Alnwick in Northumberland.   Arthur, the eldest of two sons and a daughter, chose a career in medicine whilst his younger brother continued the family business. The family farm at Felton is only about 10 kilometres from Druridge Bay, where he collected many of his specimens, as I mentioned in earlier posts. Towards the north end of Druridge Bay a small stream, Chevington Burn, flows through the sand dunes and into the Bay at Chibburn Mouth.   It was from here that Arthur Scott Donkin first recorded Navicula gregaria in 1861, describing it as “very abundant in localities where small streams pass over the sandy beach into the sea below the high-water level. Donkin pointed out that these localities would have wide daily fluctuations in salinity as the tide ebbed and flowed, and also commented that it was “the species which occurred in most abundance on our coasts.”   We now also know that it is very common in freshwaters as well, although there is a strong suspicion that that marine and freshwater forms may be different species (see Cox, 1987, listed below).   He described Navicula gregaria as “exceedingly minute”, though now, with the advances in optics, we now know that there are many species that are much smaller than N. gregaria but which Donkin, with his primitive equipment, could not see.


Cox, E.J. (1987). Studies of the diatom genus Navicula Bory. VI. the identity, structure and ecology of some freshwater species. Diatom Research 2: 159-174.

Donkin, A.S. (1861) On the marine diatomaceae of Northumberland, with a description of several new species. Quarterly Journal of the Microscopical Society 1: 1-15.

Welford, R. (1895). Men of Mark ‘Twixt Tweed and Tyne. Volume 2. Walter Scott, London


A journey to the headwaters of the River Coquet …

Friday morning arrived with blue skies, scattered cloud and sunshine over the Northumberland hills. More importantly, the river levels were much lower than on Thursday and my plans for fieldwork in the upper Coquet were back on course.


The River Coquet, just above the confluence with Rowhope Burn, May 2014.

I’m here in search of a remarkable diatom which grows luxuriantly on the rocks in this river and nearby rivers in Northumberland. The growths are visible with the naked eye in May; later in the year, they will be even larger. Each has the texture of damp cotton wool and, when you look at them through a low power microscope, you can see why: they are composed of dense masses of branched stalks, each topped by the distinctive, sarcophagus-shaped cells of Didymosphenia geminata. The left-hand image below shows the side (girdle) views of a group of cells which have recently divided; the right-hand image shows the front (valve) view. Each of the stalks was, in turn, smothered with many other diatoms.


A stone from the bed of the River Coquet, covered with colonies of Didymosphenia geminata, May 2014.

Didymosphenia has been the subject of a lot of interest over the last decade as mass growths (much larger than those I saw in the Coquet) have appeared in rivers in several parts of the world, notably New Zealand and Canada. It does not seem to have changed its distribution in Britain or Ireland markedly over this time. The paradox is that these huge biomasses seem to occur in rivers that are naturally low in nutrients.

Looking at Didymospenia down the microscope suggests a partial solution: the growths you can see smothering the stones are largely composed of the stalks which are made from carbohydrates which are just composed of carbon, hydrogen and oxygen and not phosphorus and nitrogen, the nutrients that normally limit growth in freshwaters. However, the Didymosphenia cells still need nutrients to survive and grow and some recent research has suggested an intriguing explanation for how low nutrients might actually be responsible for the high biomass that is often associated with Didymosphenia.


Didymosphenia geminata from the upper River Coquet, May 2014 (left hand image) and May 2006 (right-hand image). Scale bar: 20 micrometres (1/50th of a millimetre).

The first part of the story comes from a paper published by Brian Whitton and Neil Ellwood in 2007 which suggested that the stalk actually plays a role in helping the Didymosphenia cells scavenge phosphorus. Extracellular enzyme activity located in the upper part of the stalk helps the cells to liberate phosphorus that is bound into organic particles (from peat, for example). Even though routine measurements often indicate concentrations of phosphorus dissolved in the water are very low, there are occasional pulses of peaty water, associated with rainfall, that the Didymosphenia (and other algae) are ready to tap into.

The second part of the story follows on from this: Max Bothwell and Cathy Kilroy showed that low phosphorus actually stimulates growth of the stalk, presumably (my speculation here) to increase the potential to trap these organic phosphorus sources. They also lift the Didymosphenia colonies out of the narrow boundary layer close to the rock surfaces where it is exposed to any nutrients that might drift downstream.

The irony, as Bothwell and colleagues point out, is that most aquatic biologists associate high biomass of algae with high nutrients, whereas Didymosphenia actually seems to be associated with the opposite situation. Another irony, as I point out on my web pages is that, , when detached from the stream bed, these brownish masses floating downstream are often mistaken for raw sewage. So we have the rather unusual situation of an unsightly natural phenomenon (in the case of Northumberland, at least) being driven by the absence of pollution. So much for cleaner rivers!


Bothwell, M.L., Taylor, B.W. & Kilroy, C. (2014). The Didymo story: the role of low dissolved phosphorus in the formation of Didymosphenia geminata blooms. Diatom Research doi: 10.1080/0269249X.2014.889041.

Ellwood, N.T.W. & Whitton, B.A. (2007). Importance of organic phosphate hydrolysed in stalks of the lotic diatom Didymosphenia geminata and the possible impact of climate change. Hydrobiologia 592: 121-133.



Fieldwork in Northumberland

My fieldwork in Northumberland on Thursday was not particularly successful. After several days of dry weather, it started to rain on Wednesday night and, by Thursday morning, the rivers I wanted to sample were running very high. First rule of sampling: the ecologist is a benthic organism. Never planktonic. Second rule of sampling: discretion is the better part of valour. However, as I drove towards the raging torrent that was Wooler Water, salvation appeared in the guise of an Environment Agency van. The two guys whose van it was knew the Northumberland rivers well and assured me that, with no more rain forecast, the rivers would be low again by the next morning. Fortunately, I had decided to stay overnight in Rothbury, so I was able to reorganise my itinerary and return to Wooler Water and the nearby River Coquet in the morning.

This is a good example of the “streamcraft” I mentioned in my recent post (“Slow science and streamcraft”): I have a good idea of how my local rivers react to rainfall but, here in Northumberland, I needed to draw upon the experience of people who visited these rivers regularly. Once settled in my hotel, I was able to test their advice in real time, using the Environment Agency’s excellent system of hydrographs (see “The River Ehen in January”). The closest hydrograph whose results are available online was at Rothbury, a few hundred metres from my hotel and, assuming Wooler Water was behaving in a similar manner, my visit earlier in the day had coincided with the peak flow for this particular rainfall event. Over the next 12 hours, however, the flow gradually decreased and, by Friday morning, though the river was still higher than normal, it was possible to wade in and collect my samples. I should, in retrospect, have checked the hydrograph before I went out on Thursday, but it is useful to have some local knowledge in order to “calibrate” the readings in terms of the activities you want to perform.

So what was I looking for in these Northumberland rivers? You’ll have to wait until the next post to find that out, I’m afraid.

A screenshot of the Environment Agency’s hydrograph at Rothbury between 30 April and 2 May (see http://apps.environment-agency.gov.uk/river-and-sea-levels/120694.aspx?stationId=8173). My first visit was at 14:00 on 1 May; my return visit at about 10:00 on 2 May.