Survival of the fittest (2) …

As well as the bright green flocs of Tribonema, the stream draining the Hadjipavlou chromite mine also had bright orange-red growths on some of the pebbles on its bed.  These seemed to be composed primarily of the Cyanobacterium Chamaesiphonthough I am still not sure what species.   Using the limited literature I have, from the UK and Germany, I would opt for either Chamaesiphon polymorphusor C. polonicus.   This particular alga was very easy to remove from stones, compared to other epilithic Chamaesiphon species (see “A bigger splash …”).  This is a feature of C. polymorphus, though the colour is more typical of C. polonicus*.  On the other hand, that bright colour could be the response to high solar radiation, so maybe my north European guides are not that reliable.  It could be something else altogether.

Chamaesiphon growths on pebbles in the stream draining Hadjipavlou chromite mine in the Troodos mountains, Cyprus, March 2019.

Colonies of Chamaesiphon from Hadjipavlou chromite mine under the microscope.   Scale bar: 10 micrometres (= 1/100^thof a millimetre). 

In addition to the Chamaesiphon, there were a few diatoms, mostly Achnanthidium minutissimumand Meridion circulare.   These are typical species of metal-rich streams, as is the general lack of diversity that was evident.   There were also a few filaments of the cyanobacterium Phormidium, along with quite a few Paramecium and Vorticella.  As these are both heterotrophs that feed on organic matter, their abundance is probably at least partly a reflection of the long time that the sample spent in my suitcase between collection and analysis.  The latter is a fascinating organism to watch: it is a goblet-shaped cell with a fringe of cilia around the lip (or “peristome”).  These beat in unison to create water currents that draw small particles towards the cell.   These particles mostly at least an order of magnitude smaller than the algae)  are then collected in food vacuoles where they are digested.   A few of these vacuoles can be seen in the image of Vorticella below.

Vorticella is attached to its substrate by a stalk which contains contractile filaments, giving it spring-like qualities.  Watching a Vorticella is a beguiling experience, with the undulating rows of cilia drawing food into the vestibule (as the opening is known).  At intervals, the whole cell lurched across the field of view as the “spring” in the stalk suddenly contracted, shortening the stalk.  After this, the stalk would gradually extend again, the cilia not having missed a beat meanwhile.   This process may simply be a device that enables the Vorticellato exploit its locality to the full, as well as creating some additional turbulence to keep a steady flow of particles towards the peristome.  To be honest, I haven’t seen a more convincing explanation but, even if we don’t know why it does what it does, Vorticella is a fascinating organism to watch, whether or not I understand what is going on.

I’ll be coming back to talk more about the diatoms in a future post, and writing these posts has also reminded me that I’ve never written about the interesting mine sites almost on my own doorstep.  I cut my ecological teeth looking at these habitats back in the 1980s and they are striking examples of natural selection in action.   So, plenty of potential for more left-field natural history …

Other organisms present in the Hadjipavlou chromite mine. a. – d.: Meridion circulare; e. Phormidiumsp.; f. Vorticellasp.   Scale bar: 10 micrometres (= 1/100^thof a millimetre). 

* Note: after I had written this post Brian Whitton confirmed that it was, most likely, Chamaesiphon polonicus.

Survival of the fittest (1) …

When I signed up to a trip to Cyprus in late March I was anticipating feeling some warm Mediterranean sun on my skin after the ravages of the British winter.  I did not expect snow and sleet.   However, as one of our destinations was the Troodos mountains, the location of Cyprus’ only ski resort, maybe it was a case of unrealistic expectations.   Fortunately, we realised our mistake just in time to pack some warm clothes, and the unseasonable weather did, at least, mean that the spring flowers at lower altitudes were, even by Cypriot standards, particularly impressive.

I was in Cyprus primarily as a camp follower on a reconnaissance trip for a geology and botany excursion next year.   Cyprus is, to put it in layman’s terms, the outcome of a collision between the African and European continental plates.   The Troodos mountains are a geologist’s paradise, having a wide range of features arising from this and from associated volcanic activity.   As the molten rocks cooled, minerals precipitate out and the resulting geological strata reflect differences in the melting points of these minerals.   Some of these minerals, such as chromite, are commercially valuable and have been mined for centuries.   Indeed, the name Cyprus itself is derived from cuprous, the Greek word for copper, which was mined here since 4000 BC.

The Hadjipavlou mine is set amidst pine forests close to the highest point of the Troodos.  It was an active chromite mine from 1950 to 1954 and from 1979 to 1982 but was abandoned when cheaper sources of chromite became available in South Africa.   Over a million tonnes of ore were extracted in the period when the chromite mines in the area were active, but now there are few obvious signs apart from this adit driven into the hillside.   A small stream bearing water that has percolated through the rocks and collected in the mine’s galleries emerges from the mine entrance and tumbles down the hillside to join the stream below.   This, on closer inspection, has some quite interesting microbial growths.

First of all, having been told that this is a chromite mine, you might expect the water to carry toxic concentrations of heavy metals.   So you might also be surprised to see abundant growths of bright green algae thriving in the stream immediately downstream of the mine entrance.   This is, in fact, a common phenomenon in mine waters and happens, we think, because the fast-growing algae evolve metal tolerance whilst the animals that feed on them are slower to adapt.   This is, literally, survival of the fittest and, with nothing to eat them, the algae grow prolifically.

These filaments belong to the genus Tribonemawhich, despite being bright green in colour, actually belongs to the yellow-green algae, the Xanthophyta, rather than to the green algae.  This group is actually more closely related to the diatoms than to the green algae, though it can be hard to understand why simply by peering through a microscope.  One useful test is to add a little iodine  solutionto the slide: this binds to the starch inside green algae cells, staining them a dark brown colour.   The Xanthophyta, by contrast, do not have starch as their storage product so the cells are not stained by iodine.   The only other member of this group that I have discussed in this blog is Vaucheria, a very different alga (see “Who do you think you are?”).

Tribonema cf affinein the channel draining the Hadjipavlou chromite mine in the Troodos mountains, Cyprus, March 2019.   a. close-up of the alga in situ; b.  microscopic view of filaments; c. fragments of disintegrated filaments showing the H-shaped cell endings.  Scale bar: 10 micrometres (= 100^thof a millimetre).   

Tribonemahas simple, unbranched filaments with two or more plate-like chloroplasts arranged around the cell periphery.   One other feature is the arrangement of the cell wall, which tends to consist of two overlapping halves.  When filaments disintegrate (as they often do) the fragments have an H-shape, with each end forming half the cell wall of a different cell.   The cells are, in fact, cylindrical but this is not apparent with the flattened perspective of a high magnification objective.   This feature is not universal in the Xanthophyta, nor is it unique to this group (a few green filamentous algae show the same characteristic) but it is a useful hint that you may be looking at Tribonema.

Whilst lush growths of algae is a common feature of streams draining mines, the species that form these growths can vary.   In the northern Pennines, I am used to seeing green algae in these habitats, but there are at least three different genera that I find.  Typically there is just one filamentous alga in this location, and they tend to be  constant over time: they are reliable sources for material for undergraduate practical classes as a result.  There is more to this story but I will have to come back to it at some point in the future.  .

There is also more to the algal flora of the Hadjipavlou chromite mine but, again, that will have to wait for another post.  I should also confess that, although I visited the mine briefly last year, these samples were collected by Heather, whilst I was sitting snugly below the snow line.

Hug a Brexiteer …

I was hoping to start this blog, written on the original date for Brexit, noting that, in contrast to most other UK citizens, I had begun the day outside the EU but had, during the course of the morning, re-joined the Union.  The delay in the date for Brexit messes up that neat little opener but the experience of walking across the Green Line in Nicosia, from the Turkish Republic of Northern Cyprus to the Republic of Cyprus is a sobering reminder of the way that festering resentments within communities can spiral out of control.

There are few certainties in UK politics at the moment but, based on voting patterns in the referendum, it is very likely that over 40 per cent of the population is going to be dissatisfied with the outcome of the Brexit negotiations.   I have made my own views clear in this blog and I know that some of my readers disagree with my views.   This post is not about the rights and wrongs of Brexit but about the aftermath, and how the country as a whole treats that large proportion who will almost certainly be disappointed by the outcome.

The situation in Cyprus is complex but there are parallels to Brexit in that, after 1945, the key political question concerned a union (with Greece in this case) that would have left a significant minority of the population feeling disenfranchised.  On the other hand, there is one key difference from the UK in 2019 in that the disenfranchised minority were ethnically distinct.  In 1974 the failure to find a mutually-acceptable settlement led eventually to invasion by Turkish forces and the partition of the island which persists to this day.   We in the UK should be thankful that there is no such clear “them” and “us” distinction as our politicians pick their way through the morass of possibilities.

But the absence of a physiognomic, linguistic or religious differentiator in the Brexit debate does not mean that differences – and resentments – will not persist long after a final settlement is agreed.  That means the country, once it has resolved the present Brexit stalemate, will need to think seriously about a reconciliation process to heal the divisions.   Time, alone, will not necessarily be enough; indeed, time may even sharpen the divisions, especially if the economy is not buoyant in the post-Brexit years.  I live in a liberal bubble where almost everyone I encounter is pro-EU and opposed to Brexit; however, if the UK does end up leaving the EU, there is no point in brooding over what might have been.  We will need to pick ourselves up, dust ourselves down and move on.   And hug a Brexiteer.  Judging by the press reports, many of them are going to be just as disappointed as the Remainers.   At least we will have that in common

 

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 complexities of measuring mass…

Once upon a time, measuring the quantity of algae growing on the beds of streams and rivers was a painstaking, slow process that invariably revealed large amounts of spatial and temporal variation that, very often, obscured the ecological signals you were looking for. That has changed in the last decade thanks to the availability of field fluorimeters such as the BenthoTorch.  This makes it much quicker and easier to measure chlorophyll concentrations, the usual proxy for algal quantity.  Thanks to devices such as this it is now much easier to discover that your ecological signal is masked by spatial and temporal variation.

We’ve generated a lot of data about the fluxes of algae in the River Ehen using a BenthoTorch over the past five years and are in a position where we can start to make some generalisations about how the quantity of algae vary over the course of a year.  In broad terms, the results I showed in “The River Ehen in January” back in 2014 have not varied greatly over subsequent years, with peak biomass in mid-winter and low biomass in the summer (due, we presume, to intense grazing by invertebrates).  Curiously, we see a much less distinctive seasonal pattern in the nearby Croasdale Beck, but that is a story for another day….

The BenthoTorch uses an algorithm to partition the fluorescence signal between three major algal groups and, though this is not without issues (see below), I thought it might be interesting to see how these groups varied with biomass trends, and consider how this links to ecological theory.  The first group I’m considering are the green algae which, in this river, are mainly filamentous forms.   The general pattern, seen in the graph below, is for a gradual increase in the proportion of green algae, which fits with the current understanding of thicker biofilms having greater structural complexity with filamentous algae out-competing attached single celled algae to create a “canopy” of algae that are more effective at capturing light and other resources.  The relationship is, however, strongly wedge-shaped so, whilst many of the thickest biofilms have a lot of green algae, there are also thick biofilms where green algae are scarce or even non-existent.  Croasdale Beck shows a similar, but less pronounced, trend.

Relationship between the proportion of green algae and the total quantity of benthic algae (expressed as chlorophyll concentration) in the River Ehen (a.) and Croasdale Beck (b.).   The blue lines show quantile regression fits at p = 0.8, 0.5 and 0.2.   The image at the top of the post shows Ben Surridge using a BenthoTorch to measure algal biomass beside Croasdale Beck in Cumbria.

The second graph shows that this pattern of a gradual increase in proportion is also the case for diatoms and, once again, there is a broad wedge of points with an upward trend.  But, once again, there are also samples where biomass is high but diatoms are present in very low numbers or are even absent.   What is going on?

The problem is clear I think, if one looks at the final image in “The only way is up …” where the very patchy nature of algal communities in the River Ehen (and, indeed, many other rivers).   There are plenty of algae on this boulder, but not organised in a homogeneous manner: some zones on the boulder are almost pure diatom whilst others are almost pure green algae (and there are also zones that are almost pure Lemanea– I’ll come to that in a future post).   We try to sample the stones as randomly as possible so you can see the potential for getting very different numbers depending on where, on a stone, we point the BenthoTorch’s sensor.

Relationship between the proportion of diatoms 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.  

With experience, you can make an educated guess about the types of algae present in a biofilm.  I’ve tried to capture this with my watercolours, using washes of raw sienna for the diatoms and a grass-green for the green algae, which roughly matches the colour of their respective growths in the photo in my earlier post.   The two groups of algae a are relatively distinct on that particular boulder.   The top row roughly matches the upper “edge” of the graph showing variation in diatoms, whilst the bottom row emulates the upper “edge” of the graph showing variation in green algae.  These are the two extreme situations; however, we also often see darker brown growths in the field, which can be recreated by mixing the raw sienna and grass-green together.  When I peer through a microscope I often see green algae smothered in diatoms: genera such as Oedogoniumare particularly prone as they have less mucilage than some of the others we find in the Ehen. Their filaments often host clusters of Fragilariacells as well as Achnanthidium minutissimum, whilst stalked Gomphonemaand chains of Tabellaria flocculosaoften grow through the tangle of green filaments.   The dark brown colour is deepened yet further by the colour of the underlying rock, so my effort on white watercolour paper is a little misleading.

A colour chart showing how different proportions of green algae and diatoms influence the colour of biofilms.

The final graph shows how, as the average biomass increases in the River Ehen, so the variability in biomass also increases.   The River Ehen is one of the cleanest rivers I know but I suspect that this pattern in benthic algal quantity could be reproduced in just about any river in the country. What I would not expect to see in any but the purest and most natural ecosystems is quite so much variation in the types of algae present.   Once there is a little enrichment, so I would expect the algae to become more of a monoculture of a dominant filamentous alga plus associated epiphytes.  Like much that happens in the microscopic world of rivers, it is easier to describe than it is to measure.

That, however, is only part of the story but I’ll come back to explain the patterns in the other main groups of algae in the Ehen and Croasdale Beck in my next post.

The relationship between mean chlorophyll density and the standard deviation (based on measurements from five separate stones) for samples from the River Ehen and Croasdale Beck. 

 

Secular icons?

I’ve got two pictures on display as part of an exhibition at Durham University Botanic Gardens, both showing abstract or semi-abstract views of algae.  One is my sextych of the alga Apatococcus(see “Little round green things …”) and the other is a triptych based on Haematococcus, an alga which I wrote about in “An encounter with a green alga that is red” back in 2013.   Both pictures were painted for my final degree show back in 2008 and both addressed questions about the boundaries between abstract and representational art.

The point that I was trying to make with these images is that the boundary between abstract and representational art depends partly on what the viewer knows about the subject matter and, in the case of algae, this is usually not very much.  In cases such as these, the legend becomes very important as a means of bridging the gap between abstraction and reality by providing just enough information to help viewers make sense of the content (see “How to win the Hilda Canter-Lund Prize (2)”).   In the case of microscopic images, this should always include some indication of scale, written in terms that non-biologists can easily understand (I would always write “1/100^thof a millimetre”, rather than “10 micrometres”, for example).

Haematococcus. Triptych.   2008 50 x 130 cm Acrylic, resin and PVA on canvas.

This issue of viewers being able to “unlock” the meaning of images extends beyond the abstract/representational boundary that I encounter when displaying images of the microscopic world.  Exactly the same challenges occur when, for example, secular western Europeans look at eastern Orthodox icons, a subject that occasionally creeps into this blog (see, for example, “Unorthodox icons”.   My own curiosity about this art form led me to spend a week studying icon painting at the Quaker College in Woodbrooke, in the suburbs of Birmingham.  About ten minutes away from Woodbrooke there is the Serbian Orthodox church of St Lazar (built after the second world war by Yugoslav refugees with financial support from the Cadbury family, the Quaker philanthropists who also established Woodbrooke).

I talked a little about the practice of icon painting in “The art of icons …”.  Today, I am more interested in the symbolism.   A secular westerner can look at many of the icons I’ve depicted and broadly catagorise the contents: most would recognise that Fr Nenad, the priest of the Selly Oak Orthodox church, is holding an icon that depicts the crucifixion, for example, or that the icon just to the left of the centre of the doors in the iconostasis in the lower image depicts the Virgin and Child.  However, the symbolism goes much deeper.   I have a spotter’s guide to icons (sad, I know …) and it lists twenty eight different variants on the basic depiction virgin and child, differing in the physical relationship of Mary and Jesus, their facial expressions and the setting.  Each of these have a slightly different meaning for the Orthodox faithful.   The westerner sees “virgin” and “child”, the eastern Orthodox devotee sees so much more.

The Serbian Orthodox Church of the Holy Prince Lazar in Selly Oak, Birmingham with, right, Father Nenad displaying an icon of the crucifixion. 

What’s all this got to do with painting algae, you may ask.   Scientific illustratation and icon painting are two branches of applied art, where the subject matter serves a higher purpose.  Both, in their own way, try to help viewers understand their place in the world.  If you are not religious, you may not be comfortable with this comparison but, for most of Europe, east and west, until the Enlightenment, this would have been the case.   In both cases, however, the image cannot be viewed in isolation, the viewer needs the appropriate “keys” to unlock meaning.   Even then, the viewer is not a passive observer.   The icon requires a response from the viewer, it is the focus for contemplation and meditation and, I suggest, scientific images, when displayed as “art” should play a similar role, inviting viewers to reflect upon the mysteries of the natural world and demanding a response.

The iconostasis at the Serbian Orthodox Church in Selly Oak.

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.

Reference

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 geminata.  Diatom 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.

Croasdale Beck in February

My latest trip to the west Cumbria coincided with the period of freakily warm weather that marked the end of February (in marked contrast to a year previously when we were in the midst of the “Beast from the East”).   It felt like spring had come early although the skeletal outlines of leafless trees were incongruous against the backdrop of blue skies and, despite feeling the warmth of the sun on our faces as we worked, the water still had a wintery chill when the time came to plunge in my arm.

There were thick growths of algae on the bed of Croasdale Beck: a quick check with my microscope later showed this to be mostly Odontidium mesodonand Gomphonema parvulumand this piqued my curiosity to see how different species responded to the fluctuations in biomass that we observe in the streams in this region. I’ve talked about this before (see “A tale of two diatoms …”), suggesting that Platessa oblongellatended to dominate when biofilms were thin whilst Odontidium mesodon preferred thicker biofilms.  That was almost two years ago and I now have more data with which to test that hypothesis, and also to see if any other common taxa had an equally strong preference for particular states.

A cobble from the bed of Croasdale Beck in February 2019 showing a brown biofilm (approx. 1.7 micrograms per square centimetre) dominated by Gomphonema parvulumand Odontidium mesodon.   The photograph at the top of the post shows Ennerdale Water photographed on the same day.

I should also be clear that, in Croasdale Beck especially, diatoms are the main algal component of the biofilm, so they are not so much responding to a particular state of the biofilm as actively contributing biomass to create that state.  The other photosynthetic organism that is obvious to the naked eye in this part of Croasdale Beck is the cyanobacterium Chamaesiphon fuscus (see “A bigger splash …”) but this forms crusts on stone surfaces rather than contributing to the superstructure of the biofilm itself. We do find other filamentous algae, but intermittently and in smaller quantities.

We’ll look at Platessa oblongellafirst, bearing in mind that this was shown to be a mixture of two species about halfway through our study (see “Small details in the big picture …”).   The graph below, therefore, does not differentiate between these two species although, from my own observations, I have no reason to believe that they behave differently.   What I have done in these graphs is to divide the biomass measurements and the percent representation of these taxa in each sample into three categories: low, middle and high.   In each case, “low” represents the bottom 25 per cent of measurements, “high” represents the top 25 per cent of measurements and “middle” represents all the rest. The left-hand graph shows biomass (as chlorophyll a concentration) as a function of the relative abundance of the diatom whilst the right-hand graph shows the opposite: the relative abundance of the diatom as a function of the biomass.  These graphs bear out what I suggested in my earlier post: that Platessa oblongella(and P. saxonica) are species whose highest relative abundances occur when the biofilm is thin.  So far, so good.

Relationship between relative abundance of Platessa oblongella (including P. saxonica) and biomass in Croasdale Beck, Cumbria.  a. shows biomass (as chlorophyll a) as a function of the relative abundance of the two species (Kruskal-Wallis test, p = 0.047) whilst b. shows the relative abundance as a function of biomass (p = 0.057).

My second prediction in my earlier post was that Odontidium mesodonpreferred moderate or thick biofilms; however, whilst there is a clear trend in the data, differences between low, middle and high values of neither biomass nor relative abundance are significant.   The explanation may lay in the strong seasonality that O. mesodondisplays, thriving in spring but less common at other times of year (see “More about Platessa oblongella and Odontidium mesodon”).  However, there are no strong seasonal patterns in biomass in Croasdale Beck, and this disjunction introduces enough noise into the relationship to render it not significant.

Relationship between relative abundance of Odontidium mesodon and biomass in Croasdale Beck, Cumbria.  a. shows biomass (as chlorophyll a) as a function of the relative abundance of O. mesodon (Kruskal-Wallis test, p = 0.568) whilst b. shows the relative abundance as a function of biomass (p = 0.060).

I then tried looking at the relationship between relative abundance and biomass for a few other common taxa but with mixed results.   None of Achnanthidium minutissimum, Gomphonema parvulum complex or Fragilaria pectinalis showed any clear relationship; however, when I looked at Fragilaria gracilis, a different pattern emerged, with a significant relationship between the quantity of biomass and the proportion of this species in the sample.  That, too, is not a great surprise as I often see clusters of Fragilaria gracilis cells growing epiphytically on filamentous algae within the biofilm.  Whilst Platessa oblongella, which sits flat on the stone surface, seems to be a species that thrives when the biofilm is thin, so Fragilaira gracilisis favoured by a more complex three-dimensional structure, where it can piggy-back on other algae to exploit the light.   I suspect, however, that in a stream such as Croasdale Beck, where the substratum is very mobile, Fragilaira gracilis will also be one of the first casualties of a scouring spate which will, in turn, open up the canopy allowing Platessa oblongella back.   Even though my results for Odontidium mesodonare not significant, I still think it plays a part in this sequence, occupying the intermediate condition when some biomass has accumulated.  It looks to me as if it also likes cooler conditions which then complicates interpretation of my results.

Indeed, I am being rather selective in the results that I have included here.  Three of the six species I investigated showed no response and one of the three that I did include showed a trend rather than a statistically-convincing effect.  I suspect that the situation will rarely be as simple as I have shown for Platessa oblongella and Fragilaira gracilis.  Nonetheless, there is enough here to make me want to scratch a little more and try to understand this topic better.

Relationship between relative abundance of Fragilaria gracilis and biomass in Croasdale Beck, Cumbria.  a. shows biomass (as chlorophyll a) as a function of the relative abundance of F.gracilis (Kruskal-Wallis test, p = 0.010) whilst b. shows the relative abundance as a function of biomass (p = 0.036).

Croasdale Beck, photographed in February 2019. 

How Craticula got its name

Here is a puzzle for anyone who is learning to identify diatoms: how many species are shown in the plate below?   All share the same size and outline but they are very different in other respects, including several that we would normally regard as important for separating different species.   The left-hand image is an isolated girdle band, so let’s leave that to one side for the moment.  What about the two middle valves?   Both have a raphe in two parts, that runs along the midline, but the arrangement of their striae is very different.   And how do these relate to the pair on the right, which seem to have stout silica bars which traverse the cell?

The answer is that all belong to the same species: Craticula cuspidata.   Image b. is the way that it is most often seen (although it is not a particularly common species in the UK).   You should be able to see the raphe and fine striae which are more-or-less parallel to one another and perpendicular to the midline of the valve.   If you look with a scanning electron microscope, you’ll see that each of the striae is composed of a series of round or elliptical pores, equidistantly spaced so that the striae may appear to be running longitudinally as well as across the valve.

Craticula cuspidatafrom Pitsford Water, January 2019.   a. isolated girdle band; b.  “normal” valve; c. valve at “heribaudii stage”; d., e.: valves at “craticulae” stage. Scale bar: 10 micrometres (= 1/100^thof a millimetre).  Photos: Chris Carter.

Although the genus Craticula was described in the 19^thcentury by Grunow, it was considered to be part of the genus Naviculafor most of the 20^thcentury.  We now regard the strictly parallel striae as one of the characteristics of Craticula but, if you think of it within in the broader realm of “Navicula” (basically, boat-shaped diatoms with a central raphe), many of which have radiate striae, then you might be happy to consider valve c. as being related to valve b.   In this case, it would have been called “Navicula cuspidata var. heribaudii”.   However, in 1979 Anne-Marie Schmid of the University of Salzburg, grew cultures of “normal” Craticula cuspidata in increasing salt concentrations and was able to show this (and the structures seen in images d. and e.) were responses to the stresses that this caused.

Under certain conditions, it seems, the normal process of cell division breaks down so that, rather than producing two daughter cells, each composed of two silica valves, just one “internal valve” is produced so that there are, in effect, three valves for two cells.  One of the cells then degenerates leaving a single functional cell albeit with one extra valve.   This phenomenon is not confined to Craticula but seems to be better understood for this genus than for others for reasons that I will come to shortly.   In this particular case, the internal valve has a similar outline to the parent, but a different arrangement of striae

Images d. and e. show another aspect of the same phenomenon: the formation of a “craticula” (from the Latin for “grid-iron”).  Schmid showed that this stage actually happens at lower salt concentrations than the “heribaudii” stage but that it, too, is related to the formation of these “internal valves”.   There is a thickening of silica along the central rib, after which transverse “buttresses” grow out and, finally, a silica band is laid down around the edges of the valve.  Schmid suggested that the resulting structures were resting stages, noting that she had found such structures in ponds in the Namib Desert that were only wetted for short periods every other year or so.  When they dried up, salinity increased very rapidly and these “resting spores” lay in the bottom muds protected by layers of “jelly” (i.e. extracellular polysaccharides).  About 11 days after she re-suspended them in distilled water, she observed viable cells gliding around again.

In the early 1990s, it became clear for other reasons that members of this genus were quite different from Naviculaso the original name was resurrected.  That leaves us with the unusual situation of a genus that is named after rarely-seen monstrosities.   It would be akin to naming Fragilaria “twisty diatoms” because, as we saw in “A twist in the tale …” a different form of stress causes a characteristic reaction in members of that genus.    Because Craticula is not a particularly common genus, and because “craticulae” valves are a relatively rare phenomenon within that genus, it is likely that most people have never seen the structure after which it was named.

References

Mann, D.G. & Stickle, A.J. (1991).  The genus Craticula. Diatom Research6: 79-107.

Schmid, A.-M. (1979).  Influence of environmental factors on the development of the valve in diatoms.  Protoplasma99: 99-115.

Life out of water …

Last time I wrote, I mentioned that those diatom genera that did not have to be permanently submerged in order to thrive (so-called “aerophilous diatoms”) often appeared together in samples.   Having seen some Luticola muticaearly in my analysis of the sample from Castle Eden Burn, it was no surprise to find Diadesmisand Simonsenialater in the same analysis.   If anything, the biggest surprise was that I did not also find Hantzschia amphioxys, another habitué of the damp fringes of diatom society.

A quick analysis of my database puts these thoughts into context.   There are 6500 samples in my database, so we can see, from the total number of records of each of the aerophilous genera that these are relatively scarce in the samples I encounter.  That is largely because my sampling approaches are biased against the habitats where these thrive (more about this below).   Aerophilous diatoms are more common than you might think; it is scientists with a yearning to learn more about them that is in short supply.

Hantzschiaand Simonseniaare both less frequent and less abundant than the other two genera, never occurring in numbers exceeding ten per cent of the total but, when they form more than one per cent of the total, there is a very high chance that you will also find other aerophilous taxa in the sample.   Humidophilaand Luticolaare sometimes found in higher numbers, and when this is the case, then the proportion of other aerophilous taxa is also often high: 75 per cent of samples where Humidophilais abundant, for example, have at least one other aerophilous taxon present at one per cent or more.

Frequency of other aerophilous genera in samples with Hantzschia, Humidophila, Luticolaand Simonsenia.    Each genus is represented by two rows: records where it formed 10 per cent or more of the total number of valves and records where it formed more than one per cent.   Similarly, records for other aerophilous genera are also stratified into those where they comprise more than 10 per cent of the total and those where they comprise more than one per cent.  

Genus	number of records		other aerophilous genera
			>10%	>1%
Hantzschia	147	>10%	n/a	n/a
		>1%	0.50	0.70
Humidophila	248	>10%	0.25	0.75
		>1%	0.09	0.29
Luticola	630	>10%	0.09	0.35
		>1%	0.05	0.16
Simonsenia	61	>10%	n/a	n/a
		>1%	0.50	1.00

Over the years, I have come to use this information informally as a way of knowing whether the results of an analysis are likely to be giving me useful insights into ecological condition.   Many of the samples I analyse are collected by other people and sent to me.   These samplers should have been working to protocols that ensure that they check that the stones they choose were fully submerged for some time prior to their visit.  However, the person collecting the sample may have to make a judgement about river and lake level fluctuations in the period before their visit.  Finding lots of cells of aerophilous taxa in a sample is a good hint that something is awry.

The German method for ecological status assessment actually uses the proportion of aerophilous taxa as a check on the reliability of an assessment.    I suspect that they are not the only ones, but They have a list of 46 species that they regard as aerophilous taxa, and use a threshold of five per cent in a sample as a threshold.   The genera I’ve discussed all feature prominently, along with representatives of 19 other genera. Most of these are represented by only one or two species, although there are seven species of Nitzschia, five of Pinnulariaand six of Stauroneis.   I suspect that some species on this list are more tolerant of desiccation than others. We do not know enough of the physiological mechanisms behind this tolerance but it would seem that a few genera (Hantzschia, Humidophila, Luticiola) have definitely got this hard-wired into their genotypes, whilst other genera have members which are mostly aquatic in their habit but with a few exceptions able to survive out of water for some time.   I, personally, would trust the five per cent threshold if it was restricted to the hardcore aerophilous genera, with other taxa on the list providing supporting evidence. I would also add the proviso that there should be more than one aerophilous taxon contributing to that five per cent.  I would be happier, too, if there were a few experimental studies behind these lists and thresholds but, as ever with the world of diatoms, taxonomists are several steps ahead of the physiologists and so we are heavily dependent on anecdotal information when interpreting results.

List of taxa regarded as aerophilous in the German system for assessing ecological status in rivers. 

Name	Authority
Achnanthes coarctata	(Brébisson) Grunow in Cleve & Grunow 1880
Chamaepinnularia parsura	(Hustedt) C.E.Wetzel & Ector in Wetzel et al. 2013
Cosmioneis incognita	(Krasske) Lange-Bertalot in Werum & Lange-Bertalot 2004
Denticula creticola	(Østrup) Lange-Bertalot & Krammer 1993
Diploneis minuta	Petersen 1928
Eolimna subadnata	(Hustedt) G. Moser, Lange-Bertalot & Metzeltin 1998
Fallacia egregia	(Hustedt) D.G. Mann 1990
Fallacia insociabilis	(Krasske) D.G. Mann 1990
Fistulifera pelliculosa	(Brébisson ex Kützing) Lange-Bertalot 1997
Halamphora montana	(Krasske) Levkov 2009
Halamphora normanii	(Rabenhorst) Levkov 2009
Hantzschia abundans	Lange-Bertalot 1993
Hantzschia amphioxys	(Ehrenberg) Grunow 1880
Hantzschia elongata	(Hantzsch) Grunow 1877
Hantzschia graciosa	Lange-Bertalot 1993
Hantzschia subrupestris	Lange-Bertalot 1993
Hantzschia vivacior	Lange-Bertalot 1993
Humidophila aerophila	(Krasske) Lowe, Kociolek, Johansen, Van de Vijver, Lange-Bertalot & Kopalová, 2014
Humidophila brekkaensis	(J.B.Petersen) D. Lowe, Kociolek, Johansen, Van de Vijver, Lange-Bertalot & Kopalová, 2014
Humidophila contenta	(Grunow) Lowe, Kociolek, Johansen, Van de Vijver, Lange-Bertalot & Kopalová, 2014
Humidophila perpusilla	(Grunow) Lowe, Kociolek, Johansen, Van de Vijver, Lange-Bertalot & Kopalová, 2014
Luticola cohnii	(Hilse) D.G. Mann 1990
Luticola dismutica	(Hustedt) D.G.Mann1990
Luticola mutica	(Kützing) D.G. Mann 1990
Luticola nivalis	(Ehrenberg) D.G. Mann 1990
Luticola nivaloides	(W.Bock) J.Y.Li & Y.Z.Qi 2018
Luticola paramutica	(W. Bock) D.G. Mann 1990
Luticola pseudonivalis	(W.Bock) Levkov, Metzeltin & A.Pavlov 2013
Luticola saxophila	(W.Bock ex Hustedt) D.G.Mann 1990
Mayamaea nolensoides	(W. Bock) Lange-Bertalot 2001
Melosira dickiei	(Thwaites) Kützing 1849
Muelleria gibbula	(Cleve) Spaulding & Stoermer 1997
Neidium minutissimum	Krasske 1932
Nitzschia aerophila	Hustedt 1942
Nitzschia bacillarieformis	Hustedt 1922
Nitzschia disputata	J.R. Carater 1971
Nitzschia harderi	Husedt 1949
Nitzschia modesta	Hustedt 1950
Nitzschia terrestris	(J.B. Petersen) Hustedt 1934
Nitzschia valdestriata	Aleem & Hustedt 1951
Orthoseira dendroteres	(Ehrenberg) Genkal & Kulikovskiy in Kulikovskiy et al. 2010
Orthoseira roseana	(Rabenhorst) Pfitzer 1871
Pinnularia borealis	Ehrenberg 1843
Pinnularia frauenbergiana	E. Reichardt 1985
Pinnularia krookii	(Grunow) Hustedt 1942
Pinnularia largerstedtii	(Cleve) Cleve-Euler 1934
Pinnularia obscura	Krasske 1932
Simonsenia delognei	(Grunow) Lange-Bertalot 1979
Stauroneis agrestis	J.B. Petersen 1915
Stauroneis borrichii	(J.B.Petersen) J.W.G.Lund 1946
Stauroneis gracillima	Hustedt 1943
Stauroneis lundii	Hustedt 1959
Stauroneis muriella	J.W.G. Lund 1946
Stauroneis obtusa	Lagerstedt 1873
Surrirella terricola	Lange-Bertalot & Alles 1996
Tryblionella debilis	Arnott ex O’Meara 1873

Reference

Schaumburg, J., Schranz, C., Steizer, D., Hofmann, G., Gutowski, A. & Forester, J. (2006).  Instruction protocol for the ecological assessment of running waters for implementation of the EC Water Framework Directive: macrophytes and phytobenthos.  Bavarian Environment Agency