A river runs through it …


I made an journey via Paris to Orleans last weekend to wave off my wife and daughter as they walk part of the Camino de Santiago.   The part of this austere Medieval pilgrimage route that they chose to follow happens to be one that meanders along the Loire Valley, passing chateaux and wineries en route.   Apparently, blisters are providing an element of self-mortification to keep the spirit of pious ascetism alive.

Orleans has a beautiful Medieval old town, dominated by a cathedral, along with plenty of reminders that it was here that Joan of Arc whupped the English.   More importantly, for a freshwater ecologist, a river runs through it.   The mighty Loire – the longest river in France – rises in the Masif Central and then flows north until it reaches Orleans, then it swings round to flow west to join the Atlantic Ocean at Nantes, a total length of just over 1000 km.  The river at Orleans is broad and shallow, divided into two channels by a wooded island; the left-hand chnnel is braided, with many small gravel islands, some with grass and herbaceous vegetation, and the water is generally shallow.  However, the bed of the river itself was a dark green in colour.  The Loire Valley is known as the “Garden of France” and this gives a clue to the scale of nutrient enrichment that we might expect in the river.  In the backwaters, this green backdrop was enlivened by patches of red due to the aquatic fern Azolla (see “Escape to Southwold”).


Patches of Azolla floating over algae-smothered substrata in a backwater of the River Loire at Orleans.  The picture at the top of the post shows a view looking downstream from the left side of the Pont George V. 

I brought a sample of the algae from some stones that were just within reach of the shore home on the Eurostar in a Perrier Water bottle for a closer inspection and was surprised by the diversity. In particular, I noticed several clumps of a narrow cyanobacterium that proved hard to photograph (possibly Homoeothrix janthina: see “Algae from the Alto Duoro”) along with many green algae.  I also saw Cladophoraglomerata, which is one of the classic indicators of nutrient rich conditions, along with Stigeoclonium, two forms of Spirogyra, two forms of Oedogonium and myriad green unicells and coenobia.  Stigeocloniumis another good indicator of nutrient enrichment,as  the filaments narrow to long colourless “hairs” when key nutrients are scarce – these cells are physiological adaptions to scavenge phosphorus and their absence at Orleans shows that this nutrient is not in short supply (see “A day out in Weardale”).


Stigeoclonium cf. tenue (based on properties of erect filaments) from the River Loire at Orleans, September 2019.  Scale bar: 10 micrometres (1/100thof a millimetre). 

As well as green algae and cyanobacteria, there were also a lot of diatoms.  A few of these are illustrated below, and I’ll describe the diatoms in more detail in a future post.   As well as those I have photographed, I also saw long chains of a species of Fragilaria and another small araphid (possibly Staurosira) as well as Ulnaria ulna and some cells of Gomphonema and Navicula.   Note, in particular, the tube of Encyonema (possibly E. prostratum).   I’ve talked about tube-forming diatoms before (see “An excuse for a crab sandwich, really …”) but still can’t really explain what advantage this structure confers on a diatom.   What we can, perhaps, say, is that tube-dwelling is one of a several habits represented in the diatoms from Orleans – chains, erect, free-living motile, epiphytes  and more – and the mass of other algae create a rich diversity of microhabitats that the diatoms can exploit.


Some diatoms from the River Loire at Orleans: a. – d. Diatoma vulgare; e. Cocconeis pediculus; f. Encyonemasp.   Scale bar: 10 micrometres (= 1/100thof a millimetre).  The identity of the tube-dwelling form will have to wait until a cleaned sample is available. 

This abundance and diversity of green algae reminded me of some of the lush growths I had seen in UK rivers during the 2018 heatwave (see “Talking about the weather …”).  In a healthy river in the UK, I would expect to find less conspicuous growths than this, as invertebrate grazers would keep the algal biofilm shorn to a short stubble.   If, for any reason, the invertebrates cannot keep up with the algal growth, then a feedback loop is set up: the algae suck the valuable oxygen the invertebrates need from the water, the grazing reduces in intensity yet further, leading to a situation such as that I saw in the River Loire is the result.

Warm weather exacerbates the situation because water holds less oxygen at high temperatures.  In my posts about the River Wear last summer I commented that the plants in the river suggested that the river was more enriched with nutrients than was actually the case and I suspect that this was the result of these interactions.   The more southerly, more continental climate of the Loire Valley will experience these effects more often and it is possible that what I am looking at in Orleans may become the norm for UK rivers, as global warming intensifies.  Over the past decade I’ve worked on how to estimate the threshold concentrations of nutrients that a healthy river can endure.  However, nutrients rarely exert a direct effect on the plants and animals in a river but, instead, work through their effect on other factors such as oxygen. That will mean that global warming may wipe out any benefits of expensive nutrient reduction programs based on current estimates of the relationship between nutrients and river ecosystem health.  That’s a rather depressing prospect.

So I’ll end on a more cheerful note: the reason Heather and Rosie chose to start their Camino in Orleans was that they wanted to avoid a flight if possible.  At least that is how they sold it to me.   That they also chose to start their pilgrimage in a town close to the Sancerre vineyards may be pure coincidence.   Or maybe not ….


Surt, M.M., Jansen, M.A.K., Harrison, S.S.C. (2011).  Invertebrate grazing and riparian shade as controllers of nuisance algae in a eutrophic river. Freshwater Biology 56: :2580-2593

Wilco, C.E., Verbrak, P., Durance, I., Vaughn, I.P. & Ormerod, S.J. (2016).  Field and laboratory studies reveal interacting effects of stream oxygenation and warming on aquatic ectotherms.  Global Change Biology 22: 1769-1778.


Orleans cathedral, August 2019.


The intricate life of a colonial alga …


The annual Algal Training Course in Durham always has a field trip out to Cassop Pond, a small pond at the foot of the Permian Limestone escarpment in County Durham that has featured in a few of my posts over the years (see “A return to Cassop”).  This year, the group came back with some samples from the pond’s margins bearing a suspension of green dots just visible to the naked eye which, when examined under the microscope, turned out to be the colonial green alga Volvox aureus.  These are spherical, with the cells at the periphery, joined together by thin strands of protoplasm. The smaller colonies were scooting about, propelled by the pairs of flagellae borne by each of the cells that constitute the colony, whilst the larger ones (mostly “pregnant” with one or more daughter colonies) were sessile.


Volvox aureus colonies just visible to the naked eye in a drop of water from Cassop Pond, July 2019.   The drop is 13 millimetres across.


Colonies of Volvox aureus (each bearing daughter colonies) from Cassop Pond, July 2019.  Scale bar: 50 micrometres (= 1/20thof a millimetre).


A close-up of part of a colony of Volvox aureus from Cassop Pond, July 2019.  Scale bar: 20 micrometres (= 1/50thof a millimetre). 

Watching a Volvox colony swimming around under the microscope is a beguiling experience, but its movement is not random.  Consider: there may be a 1000 or more cells in the larger colonies, each with two flagellae.  If all beat their flagellae at random, the colony would not get anywhere, as the force in one direction would be cancelled out by forces in all other directions.   But Volvox colonies do actually move with intent.   Look closely at the individual cells in the photos below and you will see that each has a red-coloured eye spot (the light-detecting organelle actually lies beneath the red layer, which acts as a filter).   People with more patience than me have noticed that the eye spots in different parts of the colony differ in size, suggesting a level of organisation that may not be immediately apparent.  We also know that the daughter colonies tend to form at the posterior end of the colony (assuming “posterior” and “anterior” in a spherical colony are defined by the direction of travel) and also that only a small number of cells (larger than the others) are responsible for the division that produces these.

In theory, a spherical object is going to offer less resistance and so sink faster than an object of the same size that had a greater surface area : volume ratio. This should mean that they are not able to stay in the light-rich surface layers where they can photosynthesise and grow.   In practice, Volvox colonies are able to adjust their position by using their flagella but this requires them to pump some of the energy they have obtained from photosynthesis into the flagella’s motors.  Another advantage in Volvox’s favour is a relatively low density of the colony as a whole.  The individual cells are separated by strands of protoplasm which creates a lattice through which water can penetrate, so the overall density of the colony is closer to that of the surrounding water than would be the case if the cells were tightly packed.

Volvox is most often found in the summer in relatively nutrient rich lakes, where nutrients are sufficiently plentiful to support a rich crop of algae.  A motile colony that is not too dense is well-placed to adjust its position to stay in the surface layers and harvest the sunlight.  Moreover, the size of the colony probably means that it is too big for the filter-feeding zooplankton that grazes on the algae.   At the same time, however, Volvox begins to experience some of the problems associated with multicellular life (see references in “The pros and cons of cell walls …”).   As large multicellular organisms ourselves, a nuanced discussion about the pros and cons of multicellularity may seem to only have one possible outcome.   However, Volvox inhabits a world where plenty of single-celled organisms thrive and where a colonial lifestyle offers a small competitive advantage.  It means that it is quite happy drifting around at the time of year when many of us would like nothing better than to don swimming trunks and soak up some sun in a local pool.   Study algae for too long and you end up realising that only losers need to evolve.


Cells from a Volvox aureus colony from Cassop Pond, July 2019. You can see the red eye-spots in some of the cells in the left-hand image (bright-field) whilst the protoplasmic strands joining cells together can be seen in the right-hand image (phase contrast).   Scale bar: 10 micrometres (= 1/100thof a millimetre). 


Canter-Lund, H. & Lund, J.W.G. (1995).  Freshwater Algae: Their Microscopic World Explored.  Biopress, Bristol.

Reynolds, C.S. (1984). The Ecology of Freshwater Phytoplankton. Cambridge University Press, Cambridge.

The pros and cons of cell walls …


When I wrote about Vaucheria in a recent post I mentioned that it is “siphonous”, meaning that there are no cross walls dividing the filament into individual cells. Instead, the organism consists of branching tubes containing many separate nuclei and chloroplasts (see “When the going gets tough …”). What I did not do was explain why this might be of benefit to a stream-dwelling organism.

Many scientists over the years have considered the benefits that accrued when simple unicellular organisms banded together to form the first multicellular organisms. They’ve come up with a number of theories, all of which may apply in some cases. However, having accepted that multicellular organisms have a number of advantages over single cells, we have to ask why organisms such as Vaucheria seem to have gone one step further: not only have several cells banded together but they also seem to have lost the cell walls that usually separate the individual units. Most biologists, nurtured in the belief that the cell is the basic unit from which organisms are built, will find Vaucheria’s growth form to be a surprise.

The usefulness of cells, in an evolutionary context, is that this opens the way to specialisation and, over time, to the development of tissues within organisms dedicated to particular tasks. We could think of cells as tiny bags, each containing mixtures of enzymes most relevant to their function. Cells in a leaf, for example, will need to focus on producing enzymes required for photosynthesis whilst those in the trunk will be programmed to produce structural tissues. Cells, in other words, facilitate this division of labour within plants. However, a simple filamentous alga has far fewer needs than a mighty oak tree: the water around them provide both support (so complicated structural tissues are not needed) and a supply of nutrients (removing the need for internal plumbing). If there is less need for specialisation, then there is also less need to invest energy in building cell walls which, despite their advantages, also limit the capacity of cells to share resources. Cells of many algae and all higher plants have channels called “plasmadesmata” which link them together; however, these require energy to function.

If you will excuse a topical, somewhat teleological and rather tortuous metaphor, all plants have the choice of either a “trade deal” (investing energy in plasmadesmata in return for resources from neighbouring cells) or “open borders” (ditching cell walls and sharing resources). For most land plants, the former makes more sense; different lineages of algae, on the other hand, have dabbled in both strategies and Vaucheria represents a successful example of the latter. “Successful”, you sneer, “… it thrives in small clumps in polluted rivers. How is that successful?”. That, if I may say so, is a very terrestrio-centric view of the world: Vaucheria lives in a world where it does not need extensive investment in the tissues that comprise the organisms that are paraded before our eyes on the Living Planet. It has all it needs and neither boasts nor worries about tomorrow.


Siphon of Vaucheria with a side-branch, collected from the River Browney, Co. Durham and photographed at 400x magnification. Note the absence of cell walls. Scale bar: 20 micrometres (= 1/50th of a millimetre). The photograph at the top of the post shows more siphons, at 100x magnification. Scale bar: 100 micrometres (= 1/10th of a millimetre).

We can find siphonous algae not just in the Xanthophyta (the division to which Vaucheria belongs) but also in some lineages of the green algae, including the important marine genus Caulerpera. Cladophora, no stranger to these posts (see “Summertime blues …”) is siphonocladous, rather than strictly siphonous, meaning that it is divided into large cells, each of which contains many nuclei rather than the single nucleus that is characteristic of most cells throughout the plant and animal kingdoms. Balanced against this, there is a much greater number of multicellular algae that retain cell walls, some of which show considerable differentiation of tissue. Modern land plants arose from this latter largely because of the opportunities this differentiation offered. Being siphonous (or siphonocladous) has proved to be a good strategy under certain circumstances but, in turn, limits the options for the species to extend into new habitats.

In my earlier post I used the metaphor of a “sausage skin” to describe Vaucheria. Most of the interior of the organism is taken up by a vacuole, with the chloroplasts and other cell machinery pressed into a narrow band just inside the cell wall. If you watch closely, you can see the chloroplasts in living Vaucheria moving very slowly – a process called “cytoplasmic streaming”. In very bright light the chloroplasts gather at the sides, so protecting each other from harm (see “Good vibrations under the Suffolk sun …” for another way around this problem). The nuclei and mitochondria (the cell’s “batteries”) can also move around and studies have shown all three move by different mechanisms. Being a siphonous organism offers more prospects for this means of adaptation to local circumstances but, overall, the pros are outweighed by the cons, and there are far more genera of multicellular algae than there of siphonous or siphonocladous algae. Vaucheria and other siphonous algae are clearly very successful in a few habitats but the big picture suggests that being truly multicellular offered organisms far more options in the long term.

Coneva, V., & Chitwood, D. H. (2015). Plant architecture without multicellularity: quandaries over patterning and the soma-germline divide in siphonous algae. Frontiers in Plant Science. https://doi.org/10.3389/fpls.2015.00287

Herron, M. D., Borin, J. M., Boswell, J. C., Walker, J., Chen, I.-C. K., Knox, C. A., … Ratcliff, W. C. (2019). De novo origins of multicellularity in response to predation. Scientific Reports. https://doi.org/10.1038/s41598-019-39558-8

Canter-Lund, H. & Lund, J.W.G. (1995). Freshwater Algae: Their Microscopic World Explored. Biopress, Bristol.

Ott, D.W. & Brown, R.M. (1974). Developmental cytology of the genus Vaucheria 1. organisation of the vegetative filament. British Phycological Journal 9: 111 – 126.

Pennisi, E. (2018). The momentous transition to multicellular life may not have been so hard after all. Science, New York. doi:10.1126/science.aau5806

Raven, J.A. (1997). Minireview: multiple origins of plasmadesmata. European Journal of Phycology 32: 95-101.

Vroom, P.S. & C.A. Smith (2001). The challenge of siphonous green algae. American Scientist 89: 525-531.

Hilda Canter-Lund shortlist 2019

We’ve just announced the shortlist for the 2019 Hilda Canter-Lund but, unfortunately, the British Phycological Society’s webmaster is presently on a research cruise and has limited bandwidth so we haven’t been able to put them onto the BPS website yet.   Meanwhile, here is a sneak preview of what to expect when the shortlist finally does appear.  We had over fifty submissions this year, and it was a hard job to select the six images that make up the shortlist.  We always try to get a balance between different genres of images and, this year, we have two images of microalgae, two of marine macroalgae and two that sit in the middle ground – macroscopic images of microscopic organisms, if that makes sense.  If not, read on and all will become clear.

The first of the microscopic images is Cyanobacterial Entanglement, an almost abstract image of Cyanobacteria filaments taken by Forrest Leffler from the University of Florida. Alongside this we have Majestic Micrasterias, an image of the desmid Micrasterias furcata taken by William Murray from a sample from a lake in Delaware.   Whereas Forrest Leffler exploited abstract qualities in his image, William Murray achieves a sufficiently high level of detail that would not look out of place in an identification guide.   His image is very much in the tradition of Hilda Canter-Lund, which is one of the reasons why the judges recommended its inclusion on the shortlist.


Forrest Leffler’s Cyanobacterial Entanglement and William Murray’s Majestic Micrasterias.

A similar abstract versus representation tension is apparent in the two images of macroalgae on the shortlist.   Serial shortlist contender (and 2014 winner) John Huisman offers us a beautiful image of the red alga Martensia denticulata, photographed at Cape Perron, Western Australia, whilst Zoe Loffler from James Cook University in Queensland takes us to the other side of the continent to enjoy a riot of colour in her image of seaweed taken during a family camping trip.


John Huisman’s The next generation: Martensia denticulata, with cystocarpsand Zoe Loffler’s Symphony of Seaweed.

The final two images sit at the borderline between the macroscopic and microscopic worlds.  Damian Sirjacobs’ from the University of Liège in Belgium submitted an untitled image showing a bluish haze created by the diatom Haslea growing over macroalgae in shallow water in Calvi Bay, Corsica, whilst Wright State University’s Leon Kantona’s Pedestal of Productivity shows filaments of the Cyanobacteria Phormidiumand Oscillatoria amidst a yellow-brown mass of diatoms in an aquarium towards the end of a long-term photophysiology experiment.   You can also see oxygen bubbles surrounding the filaments due to the high rates of photosynthesis.


Damian Sirjacobs untitled view of the diatom Haslea growing amidst other algae, and Leon Katona’s Pedestal of Productivity.

The thumbnails in this post don’t really show the images at their very best; however, we hope to get them mounted on the BPS website within a few days, so that you can enjoy them all at higher resolution.  Meanwhile, the BPS Council are voting to decide the winners and I will be writing more about these just as soon as a decision has been reached.

When the going gets tough …


Two months after the visit I described in the previous post I was back at Castle Eden Dene.    The trees were now in leaf and the floor of the forest was carpeted with wild garlic.   The stream, however, had disappeared below the surface and, once again, I could walk along the channel without getting my feet damp.

Having found a rich crop of diatoms on my last visit when the stream was dry I was intrigued to see what was growing on the stones this time, so I used a toothbrush and some water that I had brought along to scrub a few and collected the dislodged material in my white tray.   I was intrigued to see that the suspension that collected in my tray had a distinct green tinge and, when I got a drop of it under my microscope, found it to be dominated by small green cells.  These were superficially similar to the cells of Desmococcus and Apatococcus that I found on the fence in my garden (see “Little Round Green Things …”) but this is a difficult group with not many clear morphological features with which to distinguish genera so I sent a sample off to Dave John for his opinion.

His view is also that groups such as this are almost impossible to identify unless you grow them in the laboratory or have access to DNA sequencing facilities.   He commented that Desmococcus and Apatococcus both have distinctive 2- or 4-celled packets of cells, which were not common in the Castle Eden Dene sample.  Likely candidates are the generaPleurastrumand Pseudopleurococcus, both of which are subaerial or terrestrial.   Perhaps “Little Round Green Things” is as close as we need to go in this particular instance?


A distinctly-green suspension of the biofilm on stones at Castle Eden Dene in May 2019 (left) along with a magnified view showing some of the green cells which dominated the sample (right).  Scale bar: 20 micrometres (1/5thof a millimetre).   

A short distance further on I found some mats of entwined filaments on the tops of stones which also piqued my curiosity.   Under the microscope, and with the addition of a drop of water to rehydrate them, these filaments revealed themselves to belong to Vaucheria (see “Who do you think you are?”).   Technically speaking, Vaucheria is not filamentous but “siphonous”, meaning that there are no cross walls but, instead, the organism consists of branching tubes containing many separate nuclei and chloroplasts.  The cell walls of Vaucheria, however, rupture easily releasing the chloroplasts and giving the appearance of an empty sausage skin. In this case, there are still quite a few chloroplasts but a healthy Vaucheria filament has a uniformly dense green appearance that none of those that I saw in Castle Eden Burn possessed.

There was more than just vegetative filaments of Vaucheria here: scattered amongst them were some larger, spheroid or jar-shaped cells, which are part of Vaucheria’s sexual reproduction apparatus.   I’ve talked before in this blog about how sexual reproduction is relatively rare in the filamentous algae that we find in lakes and streams (see “The perplexing case of the celibate alga …”) and Vaucheria is another case in point.   Put simply, many algae do not bother with sexual reproduction when conditions are favourable and they can grow through simple cell division.   If you subjected a Vaucheria filament to Freudian analysis, it would probably tell you that one outcome of sexual reproduction was a 50% dilution of its unique genotype. So why bother if you don’t have to?  On the other hand, sexual reproduction in these organisms usually results in a zygote with a thick wall that is capable of resisting tough conditions.   The complete absence of water in Castle Eden Burn would be one such circumstance.   To put it another way, when the going gets tough, the algae get frisky.


Mats of Vaucheria growing on a small boulder in Castle Eden Dene in May 2019.  The picture frame in the left hand image is approximately 30 centimetres. 


Cell walls of Vaucheria, with a few chloroplasts still present, from Castle Eden Burn, May 2019.  Scale bar: 20 micrometres (= 1/50thof a millimetre). 

However, the structures did not really match any pictures that I could find of oogonia or antheridia in Vaucheria.  I passed my images around some friends, and Gordon Beakes suggested that we might be looking at sporangia of chytrids, a group of fungi that have a string of previous convictions for infecting algae (see “Little bugs have littler bugs upon their backs to bite ‘em ….”).  As I was taking the photographs below, a cloud of tiny spores was released, prompting me to call out “come quickly if you want to see an alga ejaculating” before remembering that we had visitors in the house who might think this a little weird (and not just because I had not yet realised that they were, in fact, fungi).   I even took a video.  I’ll upload it to the Dark Web at some point.  There must be a site for fungal-themed pornography out there, if only I took the time to look…


Sporangia of chytrids on Vaucheria filaments from Castle Eden Burn, May 2019.  The one on the right was releasing spores (arrowed) at the time the photograph was taken.   Scale bar: 20 micrometres (= 1/50thof a millimetre).  

Out of my depth …


I was about to start writing up an account of my latest visit to Castle Eden Dene, when I realised that I had forgotten to describe my previous visit, back in March.   I’ve already described a visit in January, when the stream was dry (see “Castle Eden Dene in January” and “Tales from a dry river bed”) and promised regular updates through the year.   It seems that, amidst all the travel that filled my life over the last three months, I overlooked the post that I should have written about the visit that I made in early March.

Whereas the river was dry in January, rain during February meant that, when I returned to the Dene on 11 March, some rather turbid water was flowing down the channel on its short journey to the North Sea.   There is, finally, something more like a stream habitat from which I can collect some diatoms.

Many of the diatoms that I found in March belonged to taxa that I had also seen in January; however, the proportions were quite different.   In some cases, species that were common in January were less common now (e.g. Humidophila contenta*) but there was a small Nitzschia species with a slightly sigmoid outline that was very sparse in the January sample but which was the most abundant species in the March sample.  I’ve called this “Nitzschia clausii” but the Castle Eden Dene population does not fit the description of this perfectly.   A lot can change in a couple of months, especially when dealing with fast-growing organism such as these, as my posts on the River Wear showed (see “A year in the life of the River Wear”).  Castle Eden Burn’s highly variable discharge just adds another layer of complication to this.


Diatoms from Castle Eden Dene, March 2019:   a. – e.: Nitzschia cf clausii; f. Tabularia fasiculata; g. Tryblionella debilis; h. Luticola ventricosa; i. Luticola mutica; j. Ctenophora pulchella.  Scale bar: 10 micrometres (= 1/100thof a millimetre).   The picture at the top of the post shows Castle Eden Burn at the time that the sample was collected.   

Nitzschia clausii is described as being “frequent in brackish freshwater habitats of the coastal area and in river estuaries, as well as in inland waters with strongly increased electrolyte content”.   A couple of the other species from this sample – Ctenophora pulchella and Tabularia fasiculata (both illustrated in the diagram above) – have similar preferences.    My experience is that we do often find a smattering of individuals belonging to “brackish” species in very hard water, as we have in Castle Eden Burn.  Average conductivity (based on Environment Agency records) is 884 µS cm-1; however, values as high as 1561 µS cm-1.   The fluctuating discharge plays a role here, as any evaporation will serve to concentrate those salts that are naturally present in hard freshwater.   This should probably not be a big surprise: life in brackish waters involves adapting to fluctuating osmotic regimes so species that can cope with those conditions are also likely to be able to handle some of the consequences of desiccation.

Average values of other chemical parameters from 2011 to present, based on Environment Agency monitoring are: pH: 8.3; alkalinity: 189 mg L-1 CaCO3; reactive phosphorus: 0.082 mg L-1; nitrate-nitrogen: 1.79 mg L-1; ammonium-nitrogen: 0.044 mg L-1.   There is some farmland in the upper catchment, and the burn also drains an industrial estate on the edge of Peterlee but, overall, nutrient concentrations in this stream are not a major concern.   The Environment Agency classifies Castle Eden Burn as “moderate status” due to the condition of the invertebrates but does not offer any specific reason for this. I suspect that the naturally-challenging habitat of Castle Eden Burn may confound assessment results.

I’ve also been given some data on discharge by the Environment Agency which shows how patterns vary throughout the year.  The two sampling locations are a couple of kilometres above and below the location from which I collect my samples and both have more regular flow.  However, we can see a long period between April and September when discharge is usually very low.   The slightly higher values recorded in July are a little surprising, but are spread across a number of years.   It is also, paradoxically, most common for the burn to be dry in July too: clearly, a month of extremes.  As my own visits have shown, it is possible for the burn to be dry at almost any time of the year, depending on rainfall in the preceding period   The dots on the graph (representing ‘outliers’ – records that exceed 1.5 x interquartile range) show that it is also possible to record high discharges at almost any time during the year too.  I should also add that, as I am not a hydrologist, I am rather outside my comfort zone when trying to explain these patterns.  I would have said ‘out of my depth’ though that’s not the most appropriate phrase to use in this particular situation.


Discharge in Castle Eden Burn, as measured by the Environment Agency between 2007 and present.   Measurements are from NZ 4136 2885 (‘upstream’) and NZ 45174039 (‘downstream’).  

* Note on Humidophila contenta:it is almost impossible to identify this species conclusively with the light microscope as some key diagnostic characters can only be seen with the scanning electron microscope.   However, all members of this complex of species share a preference for intermittently wet habitats so these identification issues are unlikely to lead to an erroneous ecological interpretation.  It is probably best to refer to this complex as “Humidophila contenta sensu lato” rather than “Humidophilasp.” order to distinguish them from those species within the genus that can be recognised with light microscopy.


Lange-Bertalot, H., Hofmann, G., Werum, M. & Cantonati, M. (2017).  Freshwater Benthic Diatoms of Central Europe: over 800 Common Species Used in Ecological Assessment. English edition with updated taxonomy and added species.  Edited by M. Cantonati, M.G. Kelly & H. Lange-Bertalot.  Koeltz Botanical books, Schmitten-Oberreifenberg.

More algae from Shetland lochs …


I’m taking you back in the Shetland Islands for this post, and onto the remote moorlands of northern Mainland.   When I visited this particular loch in 2016, I noticed a lot of slippery filaments of Batrachospermum attached to the sides of the cobbles in the littoral zone (see “Lucky heather …”).   This time around, I explored further around the edge of the loch and, in the south-west corner noticed prolific growths of algae in the shallow peaty water.  Closer inspection showed that these, too, were the red alga Batrachospermum and, though they were not fertile, Dave John suggests that they are likely to be B. turfosum Bory.


Tufts of Batrachospermum turfosumin the littoral zone of Lamba Water, north Mainland, Shetland Islands, May 2019.   The picture frame is about 15 centimetres across. 

If you have a hand lens you can just about make out a bead-like structure when observing Batrachospermum in the field; however this becomes much clearer with higher magnification.   I think it looks like a bottle-brush when seen under the microscope at low magnification, with whorls of side-branches arising from the central filament.  At higher magnification, these filaments can be seen to have a bead-like structure, with cell size gradually reducing with distance from the centre.

What you cannot do in the field is separate Batrachospermumfrom the closely-related genus Sheathia(see “News about Batrachospermum… hot off the press”).   I usually tell people that, for a general overview of the condition of a stream or lake (for example, as part of the UK macrophyte survey technique), then simply recognising that you have “Batrachospermum” (meaning Batrachospermum or Sheathia) should be enough.   In my experience, the presence of Batrachospermumis usually a good indication that the water body is in a healthy condition.  However, I have been told that Batrachospermumis often found growing prolifically in very enriched conditions in southern chalk streams, which would challenge this assumption.   This may be because the species that are found in southern chalk streams are different to those that I encounter in my more usual haunts in northern England and Scotland.  But it is also possible that the factors I described in “The exception that proves the rule …” pertain in those cases too.


Filaments of Batrachospermum turfosum from Lamba Water, north Mainland, Shetland Islands, May 2019.   The upper photograph shows a low magnification view of a filament (about 350 micrometres, or 0.35 millimetres, wide) whilst the lower image shows a whorl of side branches arising from the main stem.  Scale bar: 20 micrometres (= 1/50thof a millimetre).  

We often run into this dilemma with filamentous freshwater algae: it is reasonably straightforward to identify the genus but we need reproductive organs to determine the species.  As they seem to survive quite happily in the vegetative state our understanding of the ecology of individual species (rather than the genus as a whole) is scant so it is hard to tell whether there is value in that missing information or not.   In a few cases – this is one – better taxonomic understanding has revealed that we may not even be dealing with a single genus but the lists used for applied ecological surveys still persist with the old concepts.

This creates a toxic spiral of consequences: it is hard to split into species so most people don’t bother. Because we don’t bother, our interpretations are based on generalisations drawn from the behaviour of the genus.  This means we don’t generate the data needed to demonstrate the value (or otherwise) of the effort required to go from genus- to species-level identifications.   So we carry on lumping all records to genus (or, in this case, a pair of genera) and accept a few records that our out of line with our expectations as “noise”.  The situation is probably worse in the UK than in many places because there are very few people in universities specialising in these organisms and, as a result, no-one is producing the data that might break us out of this spiral.

We found Batrachospermum turfosum in a few other locations during our visit, but nowhere, even in nearby lochs, was it in such quantity as we saw in Lamba Water.   Chance might play a part in determining its distribution on a local scale but that ought to be the explanation of last resort rather than the go-to answer when we are worryingly short of hard evidence.