Spheres of influence

Back to Moss Dub for this post because Chris Carter has sent me some stunning images of the filamentous desmid Desmidium grevillei that I talked about in my earlier post.   I mentioned that it is surrounded by a mucilaginous sheath, which was just apparent in my brightfield image.   Chris has added Indian ink to the wet mount.  The ink forms a dense suspension in the water but is repelled by the mucilage around the desmid cells, resulting in a much better impression of the extent of the sheath around the cell than is otherwise possible.


Desmidium grevillei from Moss Dub, photographed by Chris Carter using Indian ink to highlight the mucilage sheath around the cells. 

Indian ink is a negative stain, which means that it is the background, rather than the specimen itself, which takes up the colour.   This, in turn, alters the passage of light through the sample and appears to improve the contrast of the final image.   Chris’ images of the apical view show this well, and also illustrates the complicated three-dimensional arrangement of the chloroplasts within each semi-cell.   His photographs also show the pores through which the mucilage is secreted.

The curious thing about this negative stain is that, whilst it appears to emphasis a halo of nothingness around the Desmidium filament, it is actually drawing our attention to something important.   In his presidential address to the British Phycological Society in 1981 A.D. Boney referred to mucilage as “the ubiquitous algal attribute” and goes on to list the many functions that the slimes produced by a wide range of algal groups may perform.  Not all will apply to our Desmidium but Boney does use desmids as examples of some of the roles slime may play: it can be, for example, a buoyancy aid, keeping the desmids in the well-lit regions of a lake or pond and it can protect cells against desiccation if a pond or lake dries out.  It may also play a role in helping desmids adhere to their substrates and there is also evidence that mucilage layers may help to protect algae from toxins.


Apical view (at four different focal planes) of Desmidium grevillei from Moss Dub, photographed by Chris Carter, September 2019.

But that’s only part of the story.   There is two-way traffic across the membranes of algal cells, with essential nutrients moving into the cell but, in some cases, enzymes moving in the opposite direction.  If nutrients are in short supply then these enzymes can help the cell by breaking down organic molecules in order to release nutrients that can then be absorbed. Those enzymes take energy to manufacture, and the sheath of gunk around the filament means that there is a lower chance of them diffusing away before doing their job (see “Life in the colonies …”).   The same principle applies to sexual reproduction too, with mucilage serving, in some cases, as “sperm traps” or simply as the phycological equivalent of KY Jelly.

It is not just the algae that benefit from this mucilage: the outer layers, especially, can be colonised by bacteria which will also be hoovering up any spare organic molecules for their own benefit with, no doubt, some collateral benefits for the organisms around them.  The connection is probably too tenuous to count as a symbiosis with the desmids but we could think in terms of mutual benefits.

So that “nothing” really is a “something”, and that is before we consider the role of these extracellular compounds in the wider ecosystem.  I mentioned the role of similar compounds in consolidating the fine sediments on coastal mudflats in “In the shadow of the Venerable Bede” to give a flavour of this.   The least prepossessing aspect of the least prepossessing plants can, given time, change landscapes.  That should give us all pause for thought.


Close-up of Desmidium grevillei filament with focus on the left-hand cell adjusted to show the apical pores.   Photographed by Chris Carter from material from Moss Dub collected in September 2019.


Boney, A.D. (1981). Mucilage: the ubiquitous algal attribute.  British Phycological Journal 16: 115-132.

Domozych, D. S., & Domozych, C. R. (2008). Desmids and biofilms of freshwater wetlands: Development and microarchitecture. Microbial Ecology https://doi.org/10.1007/s00248-007-9253-y

Sorentino, C. (1985). Copper resistance in Hormidium fluitans (Gay) Heering (Ulotrichaceae, Chlorophyceae). Phycologia 24: 366-388. https://doi.org/10.2216/i0031-8884-24-3-366.1


The little tarn of horrors …

In addition to desmids, we found several other algae in the samples collected from Cogra Moss.  One of these consisted of colonies of cells in mucilaginous masses attached to floating mats of vegetation (which looked like terrestrial grasses).  We decided that these were probably Chrysocapsa epiphytica, the second representative of the Chrysophyta I’ve described in this blog this year (see also “Fade to grey …”).  As is the case for Chromulina, much of what we know about Chrysocapsa epiphytica is down to the patient work of John Lund who first described this species back in 1949.


Colonies of Chrysocapsa epiphytica growing on submerged vegetation at Cogra Moss, Cumbria, September 2019.  Cells are 7.5 – 15 micrometres long and 7.5 – 12 micrometres wide. 

He described the various mucilaginous lobes as “reminiscent of the …. human brain”.  The spherical, oval or ovoid cells form a layer, two to four cells deep, at the surface of the colony.   The cells have the typical yellow-brown colour of chrysophytes and, though it is hard to see the chloroplasts in this photograph, John Lund says that there are usually two, sometimes four, in mature cells.

Its presence in a soft-water lake probably means that it is a species that relies on dissolved carbon dioxide rather than bicarbonate as its raw material for phytosynthesis (see “Concentrating on carbon …” for some background on this).   We know, from laboratory studies, that most chrysophytes rely exclusively on carbon dioxide, and lack the capacity to use bicarbonate.  This confines them to water where the pH is low enough to ensure a supply of carbon dioxide (the chemistry behind this is explained in “Buffers for duffers”. It may also explain why Chromulina lives in surface films rather than submerged in the pond (the locations where we’ve it found are unlikely to have sufficiently low pH).

One extra twist to the story is that many chrysophytes are “mixotrophic”, meaning that they can switch between using photosynthesis as a means of getting the carbon they need to grow from inorganic sources, and “feeding” on other organisms.  Our Chrysocapsa epiphytica, in other words,  has parked itself beside a convenient supermarket of pre-packaged carbon in the form of decaying vegetation and associated bacteria which it then ingests by a process known as “phagotrophy”.

Phagotrophy is, in fact, a very ancient characteristic, insofar as the very first eukaryotic cells were the result of Cyanobacteria-type cells being ingested by larger heterotrophic cells and being retained as on-board “energy farms” rather than digested and treated as one-off vegetarian dinners.   However, the shift to a permanent role for chloroplasts within a eukaryotic cell involved a lot of rewiring of intercellular machinery, and effectively “switching off” the intercellular mechanisms involved in phagotrophy.   Retaining the ability to “feed” on bacteria alongside a capacity for photosynthesis is the cellular equivalent of a hybrid car: there is a lot more to cram under the bonnet.  Flexibility, in other words, comes at a cost.

On the other hand, phagotrophy does not just result in extra carbon for the Chrysocapsa cells in Cogra Moss.   In an oligotrophic tarn such as this, the extra nutrients that are obtained when the bacteria are absorbed will also be a useful boost.   Once again, though, you can see that, in environments where nutrients are more plentiful, the cost to the cell of maintaining the equipment required for phagotrophy outweighs the benefits.

I’m sure that a close inspection of the land around Cogra Moss would have revealed insectivorous plants such as Drosera(sundew) and we also recorded Utricularia minor, an aquatic insectivorous plant, in another tarn we visited whilst desmid-hunting (see “Lessons from School Knott Tarn”).  Chrysocapsa is, in many senses, a microscopic equivalent of these carnivorous plants.   OK, so it has a taste for bacteria rather than flesh but, somewhere out there, there must be a sub-editor in search of a headline …


Lund, J.W.G. (1949). New or rare British Chrysophyceae. 1.  New Phytologist48: 453-460.

Maberly, S. C., Ball, L. A., Raven, J. A., & Sültemeyer, D. (2009). Inorganic carbon acquisition by chrysophytes. Journal of Phycology 45: 1052-1061. https://doi.org/10.1111/j.1529-8817.2009.00734.x

Raven, J. A. (1997). Phagotrophy in phototrophs. Limnology and Oceanography 42: 198-205. https://doi.org/10.4319/lo.1997.42.1.0198

Saxby-Rouen, K. J., Leadbeater, B. S. C., & Reynolds, C. S. (1997). The growth response of Synura petersenii(Synurophyceae) to photon flux density, temperature, and pH. Phycologia 26: 233-243. https://doi.org/10.2216/i0031-8884-36-3-233.1

Saxby-Rouen, K. J., Leadbeater, B. S. C., & Reynolds, C. S. (1998). The relationship between the growth of Synura petersenii (Synurophyceae) and components of the dissolved inorganic carbon system. Phycologia 37: 467-477.  https://doi.org/10.2216/i0031-8884-37-6-467.1

Terrado, R., Pasulka, A. L., Lie, A. A. Y., Orphan, V. J., Heidelberg, K. B., & Caron, D. A. (2017). Autotrophic and heterotrophic acquisition of carbon and nitrogen by a mixotrophic chrysophyte established through stable isotope analysis. ISME Journal. https://doi.org/10.1038/ismej.2017.68


Desmids from Moss Dub


I’d like to say that this post is about an excursion I made beyond Ennerdale Water and along the valley of the River Liza in order to find some different habitats from those that I usually write about in this blog.  I’d like to but, in truth, I was sitting by a road about ten miles away waiting for roadside assistance whilst my compatriots on the Quekett Microscopy Club / British Phycological Society algae weekend went up the valley on a glorious mid-September afternoon without me.  Whilst I was sitting waiting for a tyre to be replaced in a garage in Egremont they were casting plankton nets and squeezing handfuls of Sphagnum beside Moss Dub, a small tarn set amidst woodland close by the River Liza.

Moss Dub is set within one of Britain’s oldest and most ambitious rewilding schemes, Wild Ennerdale, where nature is allowed to shape the landscape as far as possible free from human interference.  However, Moss Dub, as we found out, is far from a natural water body.  A path forks and the two arms act as bunds encompassing a shallow pond, now partly overgrown with aquatic vegetation.   There is evidence of past mining activity – for iron and copper – in the area and my guess is that Moss Dub was, in the far past, a reservoir associated with the Lingmell mine located on the hillside above the River Liza and active in the late 19thcentury.     Whatever its history, it proved to be a rich location for desmids, and we spent a happy Saturday dipping Pasteur pipettes into the vials of peaty water that they collected and peering through our microscopes (If you want to know more about how to collect desmids, look at the post I wrote after our last excursion to the Lake District: “Desmid masterclass”).

There were some conspicuous green growths suspended in the water at the margin of the pond.  Even without a microscope, their filamentous nature was obvious.  When magnified, we saw chains of green cells set within a distinct mucilaginous sheath.  Each filament was composed of short cells with a distinct notch on either side.   This is a representative of Desmidium, one of a relatively small number of filamentous desmids.  We met D. schwartziion our previous excursion (see “Lessons from School Knott Tarn”); that species was present here along with D. grevillei, which is similar in many respects but the cross-section is lemon-shaped rather than triangular.


Desmidium grevillei from Moss Dub, Ennerdale Valley, September 2019.  a. shows a macroscopic view of filaments in a Petri dish; b. shows a filament of cells, along with a distinct mucilaginous sheath whilst c. shows a cell in cross-section.  I forgot to bring my graticule so cannot add scale bars to any of the images in this post.   Instead, I will quote dimensions from the Freshwater Algal Flora of Britain and Ireland to give an indication of size.   Cells of D. grevillei are 30 – 56 micrometres wide (50 micrometres is 1/20thof a millimetre).   The photo at the top of this post is a view of Moss Dub, taken by Zeneb Henderson

There were numerous other desmids in the sample.  A couple are illustrated below, and we’ve sent the sample off to David Williamson for a more thorough examination, and some definitive names.  On the right-hand side of the plate there is a different green alga, Coelastrum pulchrum, a member of the Chlorophyceae that forms spherical colonies with a fixed number of cells (“coenobia”).  We met Coelastrum microporum in the River Wear last summer (see “More green algae from the River Wear”): cells of C. pulchrum, by contrast, have a blunt projection.


More algae from Moss Dub: d. Micrasterias radiosa (142 – 191 micrometres across); e. Euastrum pinnatum (65-75 micrometres across; 125 – 170 long); f. Coelastrum pulchrum (about 100 micrometres in diameter). 

The final desmid I’ve illustrated is Closterium lunula, large by desmid standards as it can reach half a millimetre or more in length.  Members of this genus have prominent vacuoles at each end of the cell within which small crystals can be seen.  Because C. lunula is so large it is easy to see both vacuole and watch Brownian motion move the crystals within.   Studies have shown that these are crystals of barium sulphate and also that the crystals are scattered throughout the cells, just happening to be easier to see I the vacuoles.  Quite what role they play remains speculation: barium is not required for plant nutrition and is, indeed, toxic in high concentrations.   It is also scarce in the soft waters where Closteriumis most often encountered, both in absolute terms and relative to other trace metals, which only adds further to the mystery.

That’s enough about Moss Dub for now.  A few words about Ennerdale Bridge, where we were based before I sign off from this post.  I usually stay at the Shepherd’s Arms when I am in the area and Keith and his staff hosted most of us and fed all of us.  It is a comfortable, unprententious inn, living mostly off walkers doing the Coast-to-Coast walk and with a menu that managed to put a smile on the faces of vegetarians and non-vegetarians alike.  Our daytime events took place in the community room of The Gather, a community-owned and run café and shop,  That gave us the satisfaction of knowing that the money we paid for the room was going to good use.   Their coffee keeps me going during long days of fieldwork in the area so I’m keen to make sure that they thrive!


Closterium lunula (400 – 663 micrometres long) from Moss Dub, showing the terminal vacuole (ringed) and (below) a close up showing rectangular crystals of barium sulphate inside the vacuole.


Brook, A. J., Fotheringham, A., Bradly, J., & Jenkins, A. (1980). Barium accumulation by desmids of the genus Closterium (Zygnemaphyceae). British Phycological Journal 15: 261-264. https://doi.org/10.1080/00071618000650251


Quekett Microscopy Club and British Phycological Society members getting stuck into analysis of samples from Moss Dub and the Ennerdale valley at The Gather, Ennerdale Bridge, September 2019.

Close to the edge in Wastwater …


I’m back in the Lake District for this post, standing beside Wastwater, the most remote and least disturbed of England’s lakes and, especially obvious on a sunny day in June, the most spectacularly-situated.  I stood on the western shore looking across to the screes and, beyond to the mass of Scafell Pike, England’s highest peak, looming up in the distance.

When I was done admiring the scenery I adjusted my focus to the biology of the lake’s littoral zone and some dark brown – almost black – marks on the boulders in the littoral zone.  In contrast to the grand vista stretching away to the north, these were beyond unprepossessing and my attempts to photograph them yielded nothing worth including in this post. However, I had seen similar looking marks in Ennerdale Water and there is a photograph in “Tales from the splash zone …” that should give you some idea of what I was seeing.

Under the microscope, my expectations were confirmed.  As in Ennerdale Water, these patches were composed of Cyanobacteria – gradually tapering trichomes of Calothrix fusca and more robust trichomes of Scytonema calcareum, both encased in thick, brown sheaths which, when viewed against the granite boulders on which they lived, resulted in the dark appearance of the growths.  To the untrained eye, these barely look like lifeforms, let alone plants yet they offer an important lesson about the health of Wastwater.


Calothrix cf fusca from the littoral zone of Wastwater, June 2019. Scale bar: 20 micrometres (= 1/50thof a millimetre)

Though hard to see amidst the tangle of filaments in these population, both Calothrix and Scytonema have specialised cells called “heterocysts” that are capable of capturing atmospheric nitrogen (you can see these in the photographs of Nostoc commune in “How to make an ecosystem (2)”.   Nitrogen fixation is a troublesome business for cells as they need a lot of energy to break down the strong bonds that bind the atoms in atmospheric nitrogen together.   That means that plants only invest this energy in nitrogen fixation when absolutely necessary – when the lack of nitrogen is inhibiting an opportunity to grow, for example.   The presence of these Cyanobacteria in Wastwater is, therefore, telling us that nitrogen is scarce in this lake.

The dogma until recently was that phosphorus was the nutrient that was in shortest supply in lakes, so attention has largely focussed on reducing phosphorus concentrations in order to improve lake health.   Over the last ten years, however, evidence has gradually accumulated to show that nitrogen can also be limiting under some conditions.   That, in turn, means that those responsible for the health of our freshwaters should be looking at the nitrogen, as well as the phosphorus, concentration and, I’m pleased to say, UK’s environmental regulators have now proposed nitrogen standards for lakes.   That marks an important shift in attitude as, a few years ago, DEFRA were quite hostile to any suggestion that nitrogen concentrations in freshwaters should be managed.   In this respect, the UK is definitely out step with the rest of Europe, most of whom have nitrogen as well as phosphorus standards for freshwaters.


Scytonema cf calcareum from the littoral zone of Wastwater, June 2019. Note the single and double false branches.   Scale bar: 20 micrometres (= 1/50thof a millimetre)

Wastwater flows into the River Irt and, a few kilometres down from the outflow, I found another nitrogen-fixing Cyanobacterium, Tolypothrix tenuis.  Once again, I could not get a good photograph, but you can see images of this in an earlier post from the River Ehen in “River Ehen … again”.   Nitrogen fixing organisms, in other words, are not confined to the lakes in this region, which raises the question why the UK does not have nitrogen standards for these as well (see “This is not a nitrate standard …”).   In rivers such as the Irt and Ehen that are already in good condition, it might only take a small increase in nitrogen concentration for the ecology to change.   Whether the loss of these nitrogen-fixing organisms will be noticed is another question.

For now, I am just happy to see that nitrogen in lakes has finally made it to the regulatory agenda.  It has taken about 15 years for the science to percolate through the many layers of bureaucracy that are an inevitable part of environmental management.  Give it another decade and maybe we’ll get nitrogen standards for rivers too.


Maberly, S. C., King, L., Dent, M. M., Jones, R. I., & Gibson, C. E. (2002). Nutrient limitation of phytoplankton and periphyton growth in upland lakes. Freshwater Biology. https://doi.org/10.1046/j.1365-2427.2002.00962.x

Moss, B., Jeppesen, E., Søndergaard, M., Lauridsen, T. L., & Liu, Z. (2013). Nitrogen, macrophytes, shallow lakes and nutrient limitation: Resolution of a current controversy? Hydrobiologia. https://doi.org/10.1007/s10750-012-1033-0

P.S. any guesses as to which 1970s prog rock group I was listening to over the weekend?  The clue is in the title.

Notes from Windermere


Just before the trip to the Shetland Islands I wrote about in the previous post, I spent two days in the Lake District teaching a course on identifying macroalgae for the Freshwater Biological Association.  It coincided with a period of gorgeous weather, showing Windermere at its absolute best (as the photo at top of the post shows).  Only a month ago my wheels were spinning in the snow on Whinlatter Pass (see “How to make an ecosystem (2)”).

Looking up Windermere towards the high peaks of the Lake District’s volcanic centre, I find myself reflecting on how geology creates the diversity in landscapes and aquatic features that, in turn, creates variety in the microscopic flora and fauna (see “The Power of Rock”).   A nuanced understanding of the aquatic world requires one to view the grand panorama at the same time as focussing on organisms that are scarcely visible with the naked eye.

One of the locations that we visited during the course was Cunsey Beck, which flows out from Esthwaite Water and, a few kilometres later, into Windermere.   Esthwaite is one of the more productive of the lakes in this region and we usually find a healthy crop of algae in the beck.   This year was no exception and, amongst the different forms we collected were some long straggly growths that had a slighty gelatinous feel.  Back at the laboratory we put part of one of these growths under the microscope and saw a large number of individual cells set in a jelly matrix.   This identified the alga as Tetraspora gelatinosa, a green alga that I have written about before (see “More from the Atma River …”) although not for some time.


Tetraspora gelatinosafrom Cunsey Beck, Cumbria, May 2019.   The picture frame is about five centimetres wide.

The genus Tetraspora gets its name from a mode of division that leaves many of the daughter cells in groups of four (visible in the lower illustration).  These, in turn, are embedded in mucilage, and repeated divisions can lead to growths becoming visible with the naked eye.   Three species have been recorded from Britain and Ireland, of which the Cunsey Beck population is most likely to belong to T. gelatinosa.   In the past, it might have been called Tetraspoa lubrica, which has a more tubular thallus; however, this is now thought to just be a growth form of T. gelatinosa that is associated particularly with fast-flowing rivers.  As far as I can tell, no-one has performed any detailed molecular genetic studies on this genus to better understand the relationships between these different growth forms so we will have to go with current convention for now.


Tetraspora gelatinosaunder the microscope.   Cells in the foreground are about ten micrometres in diameter.   Photograph by Hannah Kemp.

I’ve seen Tetraspora in a wide range of habitats – on stones in fast-flowing, relatively soft water rivers in Norway and growing on plant stems in the littoral zone of hard water ponds in Ireland.   Most of my records are from the spring, though I should add that spotting some of the smaller gelatinous colonies (barely more than near-transparent dots on the stone surface) does take some practice and I suspect that I have missed it on a few occasions too.

The microscopic image of Tetrasporawas taken during the course using a Carson Hookupz, a neat device which allows a smartphone to be attached to a microscope (or any other optical device).   It takes a little fiddling to get the set-up right but, once this has been achieved, the quality of pictures we obtained was excellent.   My microscope engineer tells me that he is selling large numbers of these to schools and colleges as it means that students can capture images during practical classes that they can subsequently use in reports or just (as was the case during our course) as an aide mémoire.


The Carson Hookupz 2.0 as it comes out of the box (left) and (right) in action during the Identifying Macroalgae course at the Freshwater Biological Association.


Looking north from Miller Ground towards the central Lake District peaks as the sun sets.  The photograph at the top of the post was taken from nearby but shows the view in early morning.  


More about measuring biomass …

The previous post showed how the proportions of green algae and diatoms changed as the total quantity of algae in the River Ehen waxed and waned over the course of a year.   The BenthoTorch, however, also measures “blue-green algae” and so let’s look at how this group changes in order to complete the picture.

Before starting, though, we need to consider one of the major flaws of the BenthoTorch: its algorithms purport to evaluate the quantities of three major groups of algae yet, in my posts about the River Ehen I have also talked about a fourth group, the red algae, or Rhodophyta (most recently in “The only way is up …”).  Having pointed a BenthoTorch at numerous stones with thick growths of Audouinella,we can report that Rhodophyta seem to be bundled in with the blue-green alga signal, which is no great surprise given the similarity in their pigments.  It is, however, one of a number of examples of the need to interpret any BenthoTorch results with your brain fully engaged, and not just to treat outputs at face value. Similar questions need to be asked of the Xanthophyta and Chrysophyta, though the latter tend not to be common in UK streams.


Relationship between the proportion of “blue-green algae” (Cyanobacteria and Rhodophyta) and the total quantity of benthic algae (expressed as chlorophyll concentration) in the River Ehen (c.) and Croasdale Beck (d.).  The blue lines show quantile regression fits at p = 0.8, 0.5 and 0.2.  

In contrast to the green algae and diatoms, the Cyanobacteria/Rhodophyta signal shows a strong negative relationship as biomass increases though, again, there is enough scatter in this relationship to make it necessary to approach this graph with caution.  I suspect, for example, that the data points on the upper right side of the data cloud represents samples rich in Audouinella, which tends to occur in winter when biomass, generally, is much greater.   On the other hand, Croasdale Beck, in particular, has a lot of encrusting Chamaesiphon fuscus colonies which are pretty much perennial (see “a bigger splash …”) but whose relative importance in the BenthoTorch output will be greatest when the other two groups of algae are sparse.   I suspect that encrusting members of this genus are favoured by conditions that do not allow a high biomass of other algae to develop, as these will reduce the amount of light that the Chamaesiphonreceives.

Thicker biofilms in the River Ehen often have some narrow Phormidium-type filaments as well as small bundles of nitrogen-fixing Calothrix, but the overall proportion is generally low relative to the mass of diatoms and green algae that predominate.    But that is not really telling us the whole story.  I finished my previous post with a graph showing how the variation in biomass increases as the biomass increases.  The heterogeneity of stream algal communities, however, cannot be captured fully at the spatial scale at which the BenthoTorch works: there is a patchiness that is apparent to the naked eye: one of our sites has distinct mats of Phormidium autumnale towards one margin, and dense Lemaneagrowths in the fastest-flowing sections, largely attached to unmovable boulders, which makes biomass measurement very difficult. I’ve also written about distinct growths of Tolypothrix and its epiphytes (see “River Ehen … again”), another alga which forms discrete colonies at a few locations. I try to collect a random sample of stones from a site but there are constraints, including accessibility, especially when the river rises above base flow.   In the River Ehen we also have to take care not to disturb any mussels whilst removing stones.

Whilst our sampling cannot really be described as “random” I do think that there is sufficient consistency in the patterns we see for the results to be meaningful. We could spend a lot more time finessing the sampling design yet for little extra scientific gain.   I prefer to think of these measurements as one part of a complex jigsaw that is slowly revealing the interactions between the constituents of the dynamic ecosystem of the River Ehen.   The important thing is to not place too much faith in any single strand of evidence, and to have enough awareness of the broader biology of the stream to read beyond the face value indications.

The only way is up …


How does an alga move upstream?   I’m curious because, I am now seeing populations of Lemanea fluviatilisabout four kilometres further upstream in the River Ehen than when I first started my regular visits in 2013.   I can explain the presence of the organism partly through changes in the hydrology of the river: a small tributary, Ben Gill, that had been diverted into the lake in Victorian times was reconnected to the river in 2014 and this introduced periodic pulses of intense energy to the river that had immediate effects on the substrate composition.  Lemanea fluviatilisis a species that thrives in the fastest-flowing sections of streams so I am quite prepared to believe that even a small shift in the hydrology of this very regulated river might make the habitat more conducive.

But that does not explain how it got there in the first place.   If the alga was occurring a few kilometres further downstream we would not have any such problems: the upstream populations would provide innocula and, if the habitat conditions changed at the downstream location, then some of those propagules might be able to establish at the downstream locations.   But what about movement in the other direction?

There has been relatively little published on this topic in recent years.  I have a review by Jørgen Kristiansen from 1996 that considers the dispersal of algae but most of the references that he cites are quite a lot older than this and I have not seen much published subsequently.   He lists our options: dispersal by water, by organisms, by air currents and by human activity.   Let’s consider each in the context of Lemaneain the River Ehen.   Lemanea, like most red algae, has a complicated life cycle with the potential for dispersal in both the haploid and diploid phases, but that is probably more detail than we need right now.  We’ll just outline the options in broad terms:

Water:the linear flow of the river means that it is almost impossible for the downstream population to provide inocula for the new upstream locations.  It may be possible for populations from further upstream in the catchment to seed the new locations.  I have not seen Lemaneain any of the streams that flow into Ennerdale Water (from which the Ehen emerges) but my knowledge of the catchment is not exhaustive.   Likelihood: very low to low.


Young shoots of Lemanea fluviatillis(bottom right) growing on a submerged boulder in the River Ehen at a location where I have not previously seen it.   These are growing alongside thick growths of diatoms (yellow-brown in colour) and patches of green filamentous algae.

Organisms:much of the older literature is concerned with the possibility of living algae or their propagules being transported in mud attached to bird’s feet or feathers and this cannot be ruled out.   There is also a recent study showing how mink may act as a vector for Didymosphenia geminata in Chile.  The Ehen also has aquatic mammals (such as otters) that could be acting as vectors for Lemanea, as well as migratory fish such as salmon and trout that could move propagules upstream.   There is also some evidence that some algae can survive passage through mammalian and invertebrate guts, and this, too, may provide a means for Lemaneato spread upstream.    Likelihood: low to medium.

Air currents / wind:quite a lot has been written about airborne dispersal of algae, with even Darwin making a contribution (see reference in Kristiansen).  The key hazard in airborne dispersal is desiccation so, in the case of Lemanea, the most likely lifecycle stages that could be dispersed in this way would be the diploid carpospores or haploid monospores. This, however, would assume that there were times during the year when the relevant life-cycle stages were exposed and, as Lemaneais a species that I usually find in the Ehen only fully-submerged, this is not very feasible.  Likelihood: low.

Human activity:there is evidence that Didymosphenia geminatacan be transported between sites attached to waders and new records often correspond with patterns of recreational use (references in Bergey & Spaulding – see below).   When we work in the Ehen we prefer to move downstream in order to minimise the risk of moving organisms on our kit, and we also clean our kit before we start.   However, a lot of people work in this part of the Ehen and it only takes one dirty wader to introduce a propagule.   Likelihood: low to medium.

We’ll almost certainly never know for sure why Lemanea fluviatilisis now thriving four kilometres further upstream than it was five years ago.  It is, however, worth bearing in mind that, given enough time, even a low probability may yield a positive result.   So none of the four hypotheses can be ruled out for sure.   Three of the possibilities are entirely natural, with one – movement by the stream itself – being constrained by the direction of flow.  Biological vectors look like a very plausible means of moving algal propagules around catchments but, for this to work, we need wildlife-friendly corridors around the river to support the animals and birds.  The upper Ehen has these, but many other rivers do not.

Actually, having a number of options all with a relatively low likelihood adds to the sense of mystery that every ecologist should have when they approach the natural world.  When cause and effect are too predictable, we tend to focus on engineering the right “solution”.  The truth, in our muddled and unpredictable world, is often that nudging several factors in the right direction will give us a more resilient outcome, even though we may have to wait longer for it to happen.


Bergey, E.A. & Spaulding, S.A. (2015). Didymosphenia: it’s more complicated.  BioScience65: 225.

Kristiansen, J. (1996).  Dispersal of freshwater algae – a review.  Hydrobiologia336: 151-157.

Leone, P.B., Cerda, J., Sala, S. & Reid, B. (2014).  Mink (Neovision vision) as a natural vector in the dispersal of the diatom Didymosphenia geminataDiatom Research29: 259-266.

Raven, J.A. (2009).  The roles of the Chantransia phase of Lemanea (Lemaneaceae, Batrachospermales, Rhodophyta) and of the ‘Mushroom’ phase of Himanthalia (Himanthaliaceae, Fucales, Phaeophyta).  Botanical Journal of Scotland46: 477-485.