Hilda Canter-Lund competition winners 2018

The winner of the 2018 Hilda Canter-Lund competition for algal photography is Rafael Martín-Ledo for “Drifting diatoms”, his phase contrast image of a fragment of a colony of the diatom Licmophora, seen in a sample collected from the Bay of Santander, northern Spain, in March 2018.   There are over twenty cells attached to this branched stem, each just over a 10th of a millimetre in length.   The frond itself was probably originally attached to a seaweed in the littoral zone (see “epiphytes with epiphytes …”) but Rafael found it drifting in open water whilst using a plankton net.

Rafael trained at the University of Extremadura in Spain and started his research career with Biodiversity and Ecology of Marine Invertebrates group at the University of Seville. His primary focus during this period was the taxonomy, symbiosis and biogeography of the ophiuroids (echinoderms, including brittle stars) of Antarctic waters. After that he worked with the British Antarctic Survey in Cambridge, examining thousands of specimens from several expeditions.

Rafael Martín-Ledo: 2018 Hilda Canter-Lund competition winner.

He currently lives in Santander, working as an independent researcher with a particular interest in marine plankton. A personal project to document the larvae of planktonic invertebrates has led to the production of hundreds of images shared through a personal website, a YouTube channel (his videos of marine organisms are also of a very high quality) and a Twitter account (@rmartinledo). The primary motivation is taxonomic but a by-product of this is to make people aware of the great morphological beauty of lesser-known marine organisms.   Some other examples of his work are reproduced below.


More examples of Rafael’s photomicroscopy skills:
a. Larva, nectochaete stage, of Glycera alba (polychaete). DIC microscopy, 200x magnification;
b. Pilidium larva, gyrans type, of nemertean worm. DIC microscopy, 200x magnification;
c. Ascidian embryo (tunicate). DIC microscopy, 400x magnification; and,
d. Cymbasoma thompsonii, female with eggs (copepod). Polarization microscopy, 40x magnification.

More examples of Rafael’s photomicroscopy skills:
e. Tripos candelabrus (dinoflagellate). DIC microscopy, 200x magnification; and,
f. Zoothamnium pelagicum (colonial ciliate). Phase Contrast microscopy, 200x magnification.

The second prize this year, awarded to the photographer of an image in a contrasting style, goes to John Huisman, an old friend of the competition who has been on the shortlist several times, winning in 2014.  John is based in Perth, Western Australia and this photograph was taken during a trip to Ashmore Reef off the northern coast of Western Australia.   His motivation is to document the marine flora of this remote region, and the image shows a new species from the red algal genus Ganonema.  Ganonema is a genus of calcified, often mucilaginous red algae, the calcification occurring as granules in the cortex and not forming a firm skeleton. At Ashmore the new species was growing in coarse coral rubble at 12 metres depth. The photograph was taken while SCUBA diving, with a Nikon Coolpix P7100 in a housing with twin Inon strobes providing fill flash.

A new Ganonema: John Huisman’s prize-winning entry for the 2018 Hilda Canter-Lund competition.

You can see these and all other winners and shortlisted images since the competition started in 2009 at the Hilda Canter-Lund pages of the British Phycological Society’s website.

John Huisman: 2014 winner and 2018 second prize winner




A question of scale …

It has taken some time to convert the observations from my last visit to the River Wear (see “Spring comes slowly up this way …”) into a picture.  Then, if you remember, the river was balanced between its “spring” and “summer” guises, the cool, wet weather that we experienced in March seems to have held the plants and animals that I usually see at this time of year back.   The result was a patchiness that was easy to see with the naked eye, but harder to visualise at the microscopic level.

First there were quite a few diatoms, Achnanthidium minutissimum in particular, – that suggested a thin biofilm subject to grazing by invertebrates (and I could see some chironomid larvae moving amongst the biofilm as I was sampling).   However, there were also diatoms such as Ulnaria ulna and Gomphonema olivaceum that suggested a thicker biofilm.    And finally there were filaments of the green alga Stigeoclonium tenue, mostly in discrete patches.   I never see healthy filaments of Stigeoclonium tenue smothered in epiphytes, which I have always assumed to be due to the copious mucilage that surrounds the plants.  However, I wondered if, nonetheless, Stigeoclonium contributes to overall habitat patchiness for the diatoms, as they subtly alter the way that water flows across the stone, reducing drag and shear stress in a way that favours Gomphonema and Ulnaria.   This is just speculation, of course…

That brings me back to a familiar theme: the problems of understanding the structure of the microscopic world (see “The River Wear in January” and “Baffled by the benthos (1)”) and, tangentially, to a paper on organisms’ responses to climate that was quoted at a scientific meeting I attended recently.   In this, Kristen Potter and colleagues demonstrated that there was typically a 1000 to 10,000 fold difference between the scale at which the distribution of organisms is studied and the size of those organisms.   That might be enough to draw out some coarse-scale patterns in distribution of species, but organisms actually live in microclimates, which may be patchy and which can often be quite different to the prevailing macroclimate (the difference between being exposed to full sun in open grassland and in the shade of a forest being a good example).   They suggested that the ideal spatial resolution is between one and ten times the organism’s length/height.

I see no reason why the same challenge should not also apply to the pressures faced by organisms in rivers where, again, we can get a certain amount of useful information from a coarse analysis of distribution in relation to (let’s say) average nutrient concentrations in a reach, but cannot really understand the reasons behind the spatial and temporal variation that we see in our data.  This mismatch between the scale at which organisms respond and the scale at which we study them is, I suspect, an even bigger problem for those of us who study the microscopic world.

A second illustration came at the same meeting in a talk by Honor Prentice from the University of Lund in Sweden.  She was dabbling in molecular biology years before this became a fashionable pastime for ecologists and has, over her career, developed some fascinating insights into how the structure of both plant communities and populations of individuals vary over short distances.  Her work has focussed on the island of Öland in Sweden which has the largest extent of alvar (limestone pavement) in Europe.   The system of grikes (the slabs) and clints (the fissures which separate the grikes) create quite different microclimates – the cool, moist conditions in the latter can create bog-like conditions with much lower pH than the limestone clints.   These differences influence not just the composition of the community but the genetic structure of species within these communities too.  I left thinking that if she could detect such differences at a scale barely more than one order of magnitude greater than the organisms, then how much more variation am I missing, with perhaps a five order of magnitude difference between organism size and sampling scale?

Based on these two studies, we would need to sample biofilms at a scale of about 1 mm2 in order to get a meaningful understanding of habitat patchiness in stream benthic algae.  That might just be possible with Next Generation Sequencing technologies, though I am not sure how one would go about collecting environmental data at that scale needed to explain what is going on.  Meanwhile, I am left with the coarse approach to sampling that is inevitable when you are five orders of magnitude bigger than the organism that you want to collect, and my imagination.


Potter, K.A., Woods, H.A. & Pincebourde, S. (2013).  Microclimatic changes in global change biology.   Global Change Biology 19: 2932-2939.

Prentice, H.C., Lonn, M., Lefkovitch, L.P. & Runyeon, H. (1995).   Associations between allele frequencies in Festuca ovina and habitat variation in the alvar grasslands on the Baltic island of OlandJournal of Ecology 83: 391-402.


The Catchment Data Explorer

One of the things I like to do on this blog is to draw out the links between the microscopic and human worlds, and also to explain how we measure the extent of human impacts on the aquatic environment, and what we can do to reverse significant negative impacts.   My professional life is largely concerned with how the evidence for these evaluations is gathered and used to arrive at decisions.  Lip service has always been paid to the importance of transparency in this process but it has not always been easy to find information about the condition of your local environment.  So a few months ago I was pleased to find a new website from the Environment Agency that makes this process a lot easier.

The Catchment Data Explorer starts with some intuitive navigation panes that let you search for your part of England, and then to locate particular streams, rivers and lakes and see how these match up to current environmental targets.   Navigating to my local river, the River Wear, and, more specifically, to the section closest to my house (“Croxdale Beck to Lumley Park Burn”), I find a table with drop-down tabs that give a brief overview of its state.    I see from this that the overall condition of the river is “moderate” and, then, by opening-up further levels, see that the various components of the ecology are all good (I’m not sure that I agree with that for the microscopic algae but that’s a story for another day) but that “physico-chemical supporting elements” are “moderate”.   Classification of rivers and lakes follows the “one out, all out” rule, so it is the lowest class that is measured that determines overall status.   In this case, opening up the physico-chemical elements levels in the table, I see that all is well, except for phosphorus, which is moderate and, therefore, determines the classification.

The home page of the Catchment Data Explorer

From here we can also download a file of “reasons for not achieving good status” in order to understand why phosphorus levels are elevated which tells us that it is waste water treatment and urban drainage that is the most likely source of phosphorus in the catchment.    Control that and, in theory, all should be well.   However, these are just two rows of 147 in a spreadsheet which deals with the lower Wear catchment and its tributaries, so the scale of overall challenge facing the Environment Agency becomes clear.    Moreover, the Wear has already had over £7M investment to install phosphorus stripping from the larger sewage works, to comply with the Urban Wastewater Treatment Directive, so the potential for further improvement is already limited.   Go back to the original table and look at the right hand column, which is labelled “objectives”.   The ecological target is “good by 2027”; however, if you hover the cursor over this, a box pops up telling you that this is “disproportionately expensive” and “technically infeasible”, invoking the WFD’s notorious “Get Out of Jail Free” card which lets countries bypass the need to achieve good status in certain specified situations (clause 4 paragraph 5 – “Less Stringent Objectives”).

Water body classification information from the Catchment Data Explorer for the River Wear, between Croxdale Beck and Lumley Park Burn.

All good so far.   The problems come when you start burrowing deeper into the Catchment Data Explorer and, in particular, when you download data.   There is a lot of information in Excel spreadsheets (which is great); however, it is riddled with jargon and not very well interpreted.   Then there are some apparent contradictions that are not explained. I searched for one stream that interested me, and found the overall ecological status to be moderate, despite the status of the fish being poor.  There is probably a good reason for this (perhaps there was low confidence in the data for fish, for example) but, again, it is not very well explained.

Then there are those water bodies that are, apparently, “good status” but, when you delve deeper into the Catchment Data Explorer, you find that there is no evidence to support this.   This is a surprisingly common situation, not just in the UK but across Europe.  The phrase “expert judgement” is invoked : probably meaning that someone from the local Environment Agency office went along for a look around and could not see any obvious problems.   It seems to be used, in the UK at least, mostly for smaller water bodies and is probably a pragmatic decision that limited resources can be better used elsewhere.

These are relatively minor niggles when set against the positives that the Catchment Data Explorer offers.   There is already quite a lot of information in the Help pages, and there is also a Glossary, so you should be able to work out the situation for your local water bodies with a little patience.   A struggle with terminology is, perhaps, inevitable, given the complexities of managing the environment.  We would all do well to remember that.


Hilda Canter-Lund shortlist 2018

We’ve just put the shortlist for the 2018 Hilda Canter-Lund prize onto the BPS website and voting is now underway to determine the winners.   As in previous years, we had a lot of images of marine macroalgae and rather fewer of microalgae, which is a problem that I’ve tried to address in a few posts over the years (see, for example, “How to win the Hilda Canter-Lund competition (3)”.  However, the best of the micrographs were stunning and two are included on the shortlist, though they have some stiff competition.  I’ll go through the shortlisted images in alphabetical order.  In each case, I have included a thumbnail, but you can see better quality images at http://www.brphycsoc.org/Canter_Lund_2018/index.lasso.

Kristen Brown of the University of Queensland kicks off our shortlist with an image of the green alga Chlorodesmis fastigiata on top of a coral.  Like many of our shortlisted images over the years it has both aesthetic qualities and tells a story as interactions between macroalgae and coral are believed to play fundamental role in the degradation of coral reefs.    Kristen is lucky enough to have a job that lets her dive on the Great Barrier Reef in Australia; our second shortlisted photographer, by contrast, dives in rather cooler waters.   José M. Fariñas-Franco was returning from a dive off Aillwee, in the Connemara region, Ireland when he took his photograph of a floating kelp forest at high tide.  The low light gives his image a particularly atmospheric appearance.

The next two images are both by photographers who have appeared on Hilda Canter-Lund shortlists before: Karie Holterman and John Huisman.   Karie Holterman, who was last on the shortlist in 2011, uses fluorescence microscopy to show cyanobacterial (blue-green algal) filaments growing amidst a mat of benthic diatoms from the bed of a lake in California.  The chlorophyll in the diatoms fluouresces with a red colour whilst DNA in the cyanobacteria pigments has been stained with Syber Green 1 to highlight its DNA.   The result is one of those intriguing images that crops up in the competition from time to time, balancing a fine line between representation and abstraction.

John Huisman is a regular fixture on the Hilda Canter-Lund shortlist, winning in 2014.  His entry this year takes us to a coral reef on the other side of Australia to the Great Barrier Reef and shows a calcified red alga, Ganonema, growing at a depth of 12 metres.

We return to the microscopic world for our next image: a frond of cells of the diatom Licmophora found drifting in the Bay of Santander in northern Spain by Rafael Martín-Ledo of the University of Extremadura in Badajoz.   Licmophora is a diatom most often found as an epiphyte on seaweeds in the littoral zone but Rafael’s frond was free-floating, perhaps having become detached from its substratum.   Rafael is a marine biologist whose interests stray well beyond algae and his Twitter feed (@martinledo) is well worth following for the wealth of beautiful images that he posts.

And finally we have an image by another old friend of the competition, Leah Reidenbach (shortlisted in 2016).  Leah’s photograph shows the green alga Ulva along with some mussels set against a background of pearl-white sand in the Bay of Fires Conservation Area in Tasmania.   There is nothing particularly exciting about any of the components of this picture but Leah’s photograph captures these in a pleasing, semi-abstract arrangement.

You may note one conspicuous absence from the shortlist this year: Chris Carter, who won in 2013 and 2017 and was shortlisted in 2010, 2011 and 2016 has now joined myself, Juliet Brodie and David Mann on the panel of judges who compile the shortlist.  The final step will be for the BPS council to vote for the winner and that should take a couple of weeks, so keep an eye out on ALGAE-L and Twitter for an announcement before too long.

And get photographing … the 2019 competition will be starting in just 10 months time…

Environmental governance post Brexit

It is some time since I have written a post about Brexit but the time has come to re-visit one of the issues which I identified early on as a critical to UK environment policy once we have left the EU: the role of European institutions as powerful enforcers of policy (see “Who will watch the watchmen now?”).  It was a concern reflected by the House of Commons Environmental Audit Committee and others (see “(In)competent authority”), leading to Michael Gove proposing a new environmental watchdog part to “hold the powerful to account” (see “Michael Gove has made a sensible suggestion”).   The document in which these ideas are laid out in detail, Environmental Principles and Governance after EU Exit Consultation Document, has just appeared and it is worth taking a closer look.

First point to notice is that this document applies only to England.   Responsibility for the environment was devolved to Scotland, Wales and Northern Ireland in the 1990s; however, as the key environmental legislation has come from Europe, their activities have had a common focus until now.   The UK administrations have worked together to ensure consistent application of the Water Framework Directive, in marked contrast to some other Member States (Belgium usually sends separate “Walloon” and “Flemish” representatives to meetings, for example).   Take away the constraint of European law and the devolved administrations are free, in principle, to develop their own environmental legislation as they feel appropriate.   There are moves towards a “common framework” for the environment but, as yet, no concrete mechanisms in place.

In many ways, this consultation document encapsulates one of the biggest contradictions of the whole Brexit process: the slow, bureaucratic and barely democratic structures of the European Union are actually rather good at managing the environment and this is precisely because they override national sovereignty.   A Member State has the right to implement EU legislation as it sees fit but the European Court of Justice acts as a powerful deterrent to any that might try to sidestep their responsibilities.   Taking back control from Brussels and recognising the sovereignty of Parliament means, in effect, that DEFRA are responsible not just for developing and implementing legislation but also for scrutinising the effectiveness of that implementation.   The risk, recognised by Michael Gove (though not in these words), is a self-affirming circle of smugness.

So the consultation document makes two important suggestions: first, that a new policy statement is made in which the environmental principles that guide policy-making and legislation are set out, and second, a new independent watchdog is set up to ensure that governments adhere to these.   The environmental principles might be set out in primary legislation or in a separate policy statement.  The former would have more teeth whilst the latter would be more flexible, allowing the evolution of these principles as scientific and technical knowledge improve.

The watchdog – “a new independent and statutory body holding government to account for the environment” – sounds like a good idea except, of course, that it is funded by, and must answer to the same government that it is holding to account.   The Government, in turn, must balance “… environmental priorities with delivering economic growth and other policy priorities such as housing” (paragraph 83) and, as a result, this new regulator would be primarily advisory, and would lack enforcement mechanisms.   It would not be an insignificant force but, at the same time, it would lack teeth.   If economic growth, for example, depended on trade agreements with the USA and China, for example, will the government be prepared to sacrifice stringent domestic environmental targets?

The irony is that one of the best indicators of the UK’s performance in any area of environmental policy is a comparison with that of other countries and the EU – via the European Environmental Agency – already provides a framework for doing this.   If the rhetoric is that the UK will perform even better after we leave then EU then, it follows, the UK’s relative position in the many “league tables” that the EEA produces should improve*.   A confident Michael Gove would, therefore, ensure that his new regulator worked as closely with the EEA as possible and, importantly, produced indicators that enabled such comparisons to be made.   My concern, again, is that, restoring UK sovereignty in these areas will create greater headroom for political meddling and spin.

Michael Gove’s appointment as Secretary of State for Environment, Food and Rural Affairs was greeted with caution by many working in the environment, given his mixed track record in his previous roles.   We were, however, cautiously optimistic as, for the first months of his tenure, he made all the right noises. He is a politician who likes the big picture and is not afraid of a grand rethink of entrenched ideas.   However, the environment is a sphere of policy where grand rhetoric is cheap and the devil lies in the detail.  The consultation on environmental principles and accountability for the environment shows up the weakness of the UK’s muddled thinking on Brexit: the EU has many faults but handing back control of the environment to a parliament whose members are slaves to short-term cycles of public opinion is unlikely to deliver a greener future any time soon.

* in practice, of course, the four administrations of the UK may need to report separately in the future.   England might well drop a couple of places whist Scotland and Wales could improve, simply due to the differences in population density.


The blog on the www.brexitenvironment.co.uk website is well worth a visit if you want to learn more about this topic.

Return to Cyprus …

A few weeks ago I described some of the algae that I found during a visit to the Avgás Gorge (pictured above) in Cyprus, including a chain-forming Ulnaria (see “Cypriot delights …”).   I’ve now had a chance to prepare cleaned valves from this material so we can take a closer look.

The chain-forming habit had already led David Williams to suggest Ulnaria ungeriana (Grunow) Compère 2001 and more detailed observations have confirmed this.  This is a species that was actually first described from Cyprus (actually Northern Cyprus) and it was also recorded quite extensively during a survey of the island’s diatoms a few years ago.   Unfortunately, some of the key diagnostic characters – such as small marginal spines and striae composed of single rows of pores – cannot be seen with light microscopy but the former, at least, can be inferred from the chain-forming habit.   Note, too, how the long chains that dominated the population in the live state, fell apart when the sample was cleaned with oxidising agents and I did not see more than three cells joined together in the new preparation.

Ulnaria ungeriana from Avgás Gorge, Cyprus, April 2018.   Scale bar: 10 micrometres (= 1/100th of a millimetre).

The Ulnaria ungeriana cells are mostly about 100 – 150 mm long and 7-8 mm wide, with a striae density of 9-10 / 10 mm.   They have parallel sides, narrowing to rostrate to slightly sub-capitate ends, and central areas that reach to the valve margin and which are slightly longer than they are broad.   Unfortunately, most of these characteristics overlap with those of Ulnaria ulna in all but most recent identification guides.   This species was first described by Nitzsch in 1817; it would have been one of the more conspicuous diatoms visible with the relatively basic equipment available at the time, with a magnification of about 150x.  His drawings are of live cells, mostly in girdle view, which means that many of the details which modern diatomists use to discriminate species are not apparent.   Moreover, the material on which these drawings are based is no longer available so we cannot go back to this in order to ascertain the characteristics of the original Ulnaria ulna and, to increase the confusion yet further, it is possible that Nitzsch has illustrated more than one species (see the reference by Lange-Bertalot and Ulrich below).

It would be, in short, very easy to look at a population of Ulnaria ungeriana in the cleaned state and match it to the descriptions of Ulnaria ulna which, under various names, have appeared in the identification literature over the past 100 years or so.   You might just detect the small marginal spines if you have a good microscope and know what you are looking for.  In the live state, however, the ribbon-like colonies are a very distinctive feature yet these do not survive preparation, putting anyone who only encounters this species on a permanent slide at a distinct disadvantage.   It is a good example of how examination of live material can add valuable information to an understanding of a diatom species yet, inevitably, many diatomists make little time for examination of their samples before dropping them into their bubbling cauldrons of oxidising agents.

High magnification views of the ends and central portions of Ulnaria ungeriana valves.   Scale bar: 10 micrometres (= 1/100th of a millimetre). 

What do we know about the ecology of Ulnaria ungeriana?   Our survey of Cypriot streams a few years ago yielded 11 records, forming up to four percent of all diatoms in the sample.  This means it is both less widespread and less dominant in samples than some other Ulnaria species.   It was often found along with other Ulnaria species, in particular U. mondii and, though generally not associated with reference sites (one out of the 11 records), it was mostly found in relatively clean conditions.   It was also associated with sites with high conductivity, which corresponds with the limestone geology that we saw in the Avgás Gorge.   On the whole, these environmental preferences are similar to those of other Ulnaria species from Cyprus that we’ve studied (see reference in earlier post).

The last question is perhaps the hardest to answer.  What benefit does the chain-forming habit confer upon Ulnaria ungeriana?   Ulnaria often forms tufts of upright cells sharing a common pad of mucilage at the base, and it is often (but not exclusively) found as an epiphyte on other plants.   We can’t rule out the possibility that the Ulnaria ungeriana chains are not also attached at one end, but it is also possible that the chain-forming habit means that they are easily entangled with the Chara and filamentous green algae that I described in the earlier post.   Both mucilage pads and entangled chains fulfil the same role of keeping the alga in the same spot in the stream, particularly where there are other plants and filamentous algae to offer extra protection from the current.

There is some speculation in the final couple of sentences but that’s never a bad thing for an ecologist.  If nothing else, it provides me with a reason to return one day …

Ecological preferences of Ulnaria ungeriana at running water sites in Cyprus.  a. pH; b. conductivity; c. total nitrogen (TN) and d. total phosphorus (TP).  Arrows indicate the mean value for each variable, weighted by the relative abundance of Ulnaria ungeriana in the sample.


Krammer, K. & Lange-Bertalot, H. (1991).   Süsswasserflora von Mitteleuropa 2 Bacillariophyceae, 3 teil: Centrales, Fragilariaceae, Eunotiaceae.   Spektrum Akademischer Verlag, Heidelberg, Berlin.

Lange-Bertalot, H. & Ulrich, S. (2014).  Contributions to the taxonomy of needle-shaped Fragilaria and Ulnaria species.   Lauterbornia 78: 1-73.

Spring comes slowly up this way …*

I took a few minutes out on my trip to Upper Teesdale to stop at Wolsingham and collect one of my regular samples from the River Wear.  Back in March, I commented on the absence of Ulothrix zonata, which is a common feature of the upper reaches of rivers such as the Wear in early Spring (see “The mystery of the alga that wasn’t there …”).   I put this down to the unusually wet and cold weather that we had been experiencing and this was, to some extent, confirmed by finding prolific growths of Ulothrix zonata in late April in Croasdale Beck (see “That’s funny …”).   Everything seems to be happening a little later than usual this year.   So I should not have been that surprised to find lush growths of green algae growing on the bed of the river when I waded out to find some stones from which to sample.

These growths, however, turned out to be Stigeoclonium tenue, not Ulothrix zonata (see “A day out in Weardale”): it is often hard to be absolutely sure about the identity of an alga in the field and, in this case, both can form conspicuous bright green growths that are slimy to the touch.   Did I miss the Ulothrix zonata bloom in the River Wear this year?   Maybe.   Looking back at my records from May 2009 I see that I recorded quite a lot of narrow Phormidium filaments then but none were apparent in this sample.   That taxon thrived throughout the summer, so perhaps, again, its absence is also a consequence of the unusual weather.

Growths of Stigeoclonium tenue on a cobble in the River Wear at Wolsingham, May 2018.  

The photograph illustrates some of the problems that ecologists face: the distribution of algae such as Ulothrix zonata and Stigeoclonium zonata is often very patchy: there is rarely a homogeneous cover and, often, these growths are most prolific on the larger, more stable stones.   I talked about this in Our Patchwork Heritage; the difference now is that the patchiness is exhibited by different groups of algae, rather than variation within a single group.   Ironically, the patchiness is easier to record with the naked eye than by our usual method of sampling attached algae using toothbrushes.   That’s partly because we tend to sample from smaller substrata (the ones that we can pick up and move!) but also because of the complications involved in getting a representative sample.   We have experimented with stratified sampling approaches – including some stones with green algae, for example, in proportion to their representation on the stream bed – but that still means that we have to make an initial survey to estimate the proportions of different types of growth.

Under the microscope, therefore, the algal community looks very different.   There are fewer green cells and more yellow-brown diatom cells, these dominated by Achnanthidium minutissimum, elegant curved cells of Hannaea arcus and some Navicula lanceolata, still hanging on from its winter peak.   The patterns I described in The mystery of the alga that wasn’t there … are still apparent although the timings are all slightly adrift.

A view of the biofilm from the River Wear, Wolsingham in May 2018.

The schematic view below tries to capture this spatial heterogeneity.  On the left hand side I have depicted the edge of one of the patches of Stigeoclonium.   Healthy populations of Stigeoclonium do no support large populations of epiphytes, probably as a result of the mucilage that this alga produces.  My diagram also speculates that the populations of Gomphonema olivaceum-type cells and Ulnaria ulna may be living in the shadow of these larger algal growths, as neither is well adapted to the fast current speeds on more exposed rock surfaces.  Finally, on the right of the image, there are cells of Achnanthidium minutissimum, small fast-growing cells that can cope with both fast currents and grazing.   I have not included all of the taxa I could see under the microscope, partly because of the space available.  There is no Hannaea arcus or Navicula lanceolata and I have also left out the chain of Diatoma cells that you can see on the right hand side of the view down the microscope.

The speckled background in the image of the view down the microscope is, by the way, a mass of tiny bacteria, all jigging around due to Brownian motion.  The sample had sat around in the warm boot of the car for a few hours after collection so I cannot be sure that these were quite as abundant at the time of collection as they were when I came to examine it.  However, some people have commented on the absence of bacteria – known to be very abundant in stream biofilms – from my pictures, so these serve as a salutary reminder of an extra dimension that really needs to be incorporated into my next images.

Schematic view of the biofilm from the River Wear at Wolsingham, May 2018.  a. Stigeoclonium tenue; b. Gomphonema olivaceum complex; c. Ulnaria ulna; d. Meridion circulare; e. Achnanthidium minutissimum.   Scale bar: 10 micrometres (= 1/100th of a millimetre).

* Samuel Taylor Coleridge, Christabel (1816)