My name is Legion …

I promised to write a little more about Gomphonema subclavatum, one of the diatoms we encountered in the previous post.   I picked this one out for more attention because it is one of many diatoms that have changed names in recent years and it is sometimes interesting to scratch around to understand why this has happened.

Had I seen this particular species fifteen years ago I would have called it Gomphonema clavatum without hesitation.  Although G. subclavatum was recognised as a distinct species back in the nineteenth century, for most of the twentieth century it was treated as a variety of G. longiceps, which Krammer and Lange-Bertalot then subsumed into G. clavatum.  If you look at their plate of G. clavatum, you will see a huge range of sizes and shapes so it is perhaps no surprise that people subsequently realised that there was more than one species lurking under this name.

Gomphonema subclavatum from Cregduff spring, Co. Mayo, Ireland, September 2017.  Photographs: Bryan Kennedy.  Scale bar: 10 micrometres ( = 100th of a millimetre).

When this happens, taxonomists ask which of the various contenders was the Gomphonema clavatum seen by the person who originally described the species.  This involves going back to the museum collection where that person deposited the material that they examined and taking another look.  This process of “typification” helps determine which of the forms is the rightful inheritor of the name.   Erwin Reichardt decided to have a go at this process for G. clavatum and went to examine the samples, now in the Museum für Naturkunde in Berlin, on which Christian Gottfreid Ehrenberg had based his original description.  However, he could find nothing that resembled G. clavatum, with the closest match being G. olivaceum.

I’m reading a biography at the moment that contains the warning that “history is always a matter of trying to think into the minds of people who think differently from ourselves”.  That serves as a useful reminder that Ehrenberg knew far less about the biology of diatoms than we do today, but was also limited by the technology available.  Not only were his microscopes far less sophisticated than ours but also capturing the essence of the organisms he saw in print was far from straightforward (see “Picture this?”).  The idea of Gomphonema clavatum that we had until Reichardt re-examined the type material was the result of a 180-year game of “Chinese whispers”: each generation matching their specimens to inadequate images and descriptions, then making their own images which, in turn, became the basis for their successor’s identifications.  By the time Krammer and Lange-Bertalot wrote their Flora, it was finally possible to reproduce high quality micrographs, rather than line drawings but over a century of taxonomic drift meant that their images are no longer connected to the right name.  Their plate actually shows two species: the larger forms with undulate margins belong to G. longiceps Ehrenberg 1854) whilst the smaller specimens are G. subclavatum.   That assumes, of course, that there are no further twists to come.  As I alluded in my previous post, morphology might not be telling us the whole story for this genus.

The unfortunate twist, also mentioned in my previous post, is that the taxonomic confusion in the past means that we don’t actually get any sharper ecological insights in the present as a result of unravelling these names.   Anyone looking at ecological data associated with “Gomphonema clavatum” from twenty years ago needs to know that this could represent either G. longiceps or G. subclavatum or one of a number of other species that have been split away in recent years.  There is always a hope that this better understanding of taxonomy will yield fruits as we go forward but I’m always suspicious that someone else might come along and rearrange things yet again…


Krammer, K. & Lange-Bertalot, H. (1986).  Süsswasserflora von Mitteleuropa. 2/1 Bacillariophyceae 1: Naviculaceae. Spektrum Akademischer Verlag, Heidelberg.

Reichardt, E. (2015). The identity of Gomphonema clavatum Ehrenberg (Bacillariophyceae) and typification of five species of the genus Gomphonema described by C.G. Ehrenberg.  Diatom Research 30: 141-149.

The biography to which I refer is Tom Wright’s new book on Paul (SPCK, 2018).



Baffling biodiversity …

Few of the participants in the UK / Ireland diatom ring-test that I described in my previous post felt any need to thank me for my choice of slide for our 50th test.  The slide came from a spring in County Mayo, Ireland, which is part of the Agricultural Catchments Programme, a large study into the effect of farming on water quality. The sample itself came from the stems and leaves of the submerged water cress (Nasturtium officinale*) plants which fill the entire channel.  It was a real stinker, with a mess of Gomphonema forms, several of which did not neatly fit any species description that we could find.   A conservative reckoning is that there were at least eight different Gomphonema “species” and that raises a further question about what it was about this habitat that led to so much diversity within a single genus within a single sample.

First, a quick tour around some of the Gomphonema forms that we found.   There was general agreement that the most common type was close to G. micropus Kützing 1844 but not a perfect match to published descriptions (the stria density, in particular, was too low).   The situation was further complicated because the status of G. micropus was questioned at times, with it being treated as a variety of G. parvulum and placed in the G. angustatum complex by different authorities during the 20th century.  Then there were a number of valves with more rounded ends and a higher striae density than G. micropus but which, if you look closely, are not symmetrical around the long axis.   We thought that these were close to G. cymbelliclinum Reichardt & Lange-Bertalot 1999.   Unfortunately, there were also quite a lot of valves that had intermediate properties, making it hard, in many cases, to say whether it was one species or the other.

Gomphonema cf micropus from Cregduff spring, Co. Mayo, Ireland, September 2017.  Photographs: Bryan Kennedy.  Scale bar: 10 micrometres ( = 100th of a millimetre).  The image at the top of the post shows Cregduff spring (photo by Lauren Williams)

Gomphonema cf cymbelliclinum from Cregduff spring, Co. Mayo, Ireland, September 2017.  Photographs: Bryan Kennedy.  Scale bar: 10 micrometres ( = 100th of a millimetre).

We also found some valves that were close to descriptions of Gomphonema utae Lange-Bertalot & Reichardt 1999 and some that were close to G. parallelistriatum Lange-Bertalot & Reichardt 1991.  We also found representatives of the G. parvulum complex, G. tergestinum and G. subclavatum (more about this one in the next post).

Gomphonema cf utae from Cregduff spring, Co. Mayo, Ireland, September 2017.  Photographs: Bryan Kennedy.  Scale bar: 10 micrometres ( = 100th of a millimetre).

Gomphonema cf parallelistriatum from Cregduff spring, Co. Mayo, Ireland, September 2017.  Photographs: Bryan Kennedy.  Scale bar: 10 micrometres ( = 100th of a millimetre).

So what is going on here?   There are, I suspect, two key elements to the story that we need to explain.  The first is the limits of species within Gomphonema.  I’ve touched on this before (see “Diatoms and dinosaurs”) and some recent studies that combine morphological and molecular biological evidence also cast doubt on our ability to differentiate within this genus using classical approaches.   Whilst I was struggling to disentangle the species in this sample, I had a conversation with an eminent taxonomist and she hinted darkly that Gomphonema was “over-described”.  There is a readiness to “split” established taxa and describe new species that, in her opinion, runs ahead of the evidence.

The limitations of taxonomy cannot explain all of the variation that we observed in this sample, so the second question to ask is what it is about the conditions here that allow so many representatives of one genus to thrive.   I’ve touched on this subject before (see “Baffled by the benthos (1)” and “Baffled by the benthos(2)”).  In these posts I introduced G. Evelyn Hutchinson’s “paradox of the plankton” in which he suggested that environments that look uniform, to mortals six orders of magnitude larger than algae are, in fact, considerably more heterogeneous  and, so offer more opportunities for “variations on a theme” to thrive.   In the second post I went on to suggest that this type of diversity imparts resilience to an ecosystem and so should be looked upon as a positive feature of the ecosystem when doing ecological status assessments.

There is, however, one final possibility that, to my knowledge, has not yet been explored.  The presence of transitional forms in the diatom assemblage at Cregduff may be an artefact of our inability to differentiate biological species based on a limited range of morphological criteria on offer. However, it is also possible that we are looking at a situation where the Linnaean species are not reproductively isolated from one another, allowing hybridisation.   The concept of a “hybrid swarm” is well known in some other groups (e.g. orchids) but has never been formally demonstrated in diatoms.  However, the wide morphological diversity within a single genus in one sample alongo with, in some cases, apparent continua of variation, does raise questions about speciation within thi genus.

The final twist to this story is that, from the point of view of current ecological status assessments, all this diversity has little effect.  Though everyone grumbled about the difficulties in naming the Gomphonema species, the results, as the box-and-whisker plot in the previous post show – were less variable than in many of our other ring tests.  What I suspect happened is that the underlying taxonomic confusion means that many of these taxa have “mid-range” scores for the TDI (and other indices), so the final calculation cancels out the identification issues.  Bear in mind that this may not always be the case!

* I understand that this is the correct name now, rather than Rorippa nasturtium-aquaticum.  See Al-Shehbaz, A. & Price, R.A. (1998).  Delimitation of the genus Nasturtium (Brassicaceae).  Novon 8: 124-126.


The two papers that deal with variation within Gomphonema to which I refer are:

Abarca, N., Jahn, R., Zimmermann, J. & Enke, N. (2014).  Does the cosmopolitan diatom Gomphonema parvulum (Kützing) Kützing have a biogeography? PLOS One 9: 1-18.

Kermarrec, L., Bouchez, A., Rimet, F. & Humbert, J.-F. (2013).  First evidence of the existence of semi-cryptic species and of a phylogeographic structure in the Gomphonema parvulum (Kützing) Kützing complex (Bacillariophyta).  Protist 164: 686-705.

Reaching a half century …

Last week saw a small career achievement as I sent out the result of the 50th diatom ring-test that I organise for the diatom analysts in the UK and Ireland.  “Ring-test” is the informal term for an inter-laboratory comparison, when two or more laboratories analyse the same sample and compare their results.  We started out doing regular ring-tests in 2007 for all the people who were analysing diatom samples for assessments associated with the Water Framework Directive, sending out five slides each year to staff in the UK and Irish environment agencies and contractors who worked with them. Now, a decade later, the scheme is still going strong, with participants from Germany, Sweden and Estonia joining the British and Irish contingents.

There are a number of similar schemes around Europe with the same basic model: the organiser sends out copies of a slide made from the same sample, all participants then analyse the slide and send in their results, which the organiser collates.   There is usually one or more designated “expert” against whose results everyone else is judged.  Most of the other schemes then organise a workshop at which participants gather to discuss the finer points of diatom taxonomy.   We have had workshops in the past, but these are not directly linked to the ring-tests.  Instead, we send out a report that summarises results and provides notes on the identification of difficult or unusual taxa.   The money we save on workshops means that we can circulate more slides.  I’m a great believer in “little and often” for this type of quality control.

A second feature of our scheme (which some of the other European schemes have also now adopted) is to use a panel of experienced analysts to provide the benchmark that other participants should achieve.   This means that we have an idea of both the average result and the scale of the variation associated with this.  We learned early on that some samples gave much less variable results than others, even when the analyses were performed by experienced analysts.  We use this knowledge to adjust the size of the “target” that participants must achieve.   The graphs below show the results for our most recent test.  The horizontal blue lines on the left hand graph show two standard deviations around the mean of the “expert” analyses (expressed as TDI).  This is the “warning limit”; if an analyst exceeds this then he or she should be looking at their results to see if they have made any mistakes.  The red line is the “action limit”, seven TDI units either side of the expert mean.   We know from other studies (see lower graph, left) that it is very unlikely that two replicate analyses have a greater difference than this, so analysts who exceed this should definitely be checking their results.

The results of the 50th UK / Ireland diatom ring test showing (left) difference in TDI and (right) number of taxa (N taxa) between experts and other participants.  Blue lines: mean TDI ± two standard deviations of expert panel’s mean; red lines: mean TDI ± 7.   Note that it is unusual for the between-analyst variability to be quite as narrow as it was for this slide.

The reason why we need flexible “warning limits” is illustrated in the right hand graph below.   This shows the similarity between two counts as a function of the diversity of the samples.   The relationship has a wedge-shape (illustrated by the blue line – the regression line through the 90th percentile of the data).   There are a number of reasons why two analysts are unlikely to get identical results, one of which is that they disagree on the identities of the taxa that they encounter (the reason why we are doing the audits in the first place).  But what a wedge-shaped relationship is also telling us is that there seems to be an upper limit to the similarity that can be achieved at any given diversity.   This is an inherent stochastic quality of the data and has nothing to do with the competence of the analysts.

Left: some of the data from which the “action limit” for the ring-tests was established.   These are the results of audits of 67 samples from Northern Ireland in which the original (“primary”) analysis was checked against the result of an independent (“audit”) analysis.   Right: The effect of diversity on the similarity between primary and audit analyses for the same dataset.

A further way in which our scheme differs from others is that no-one “passes” or “fails”.  That might seem counter-intuitive as this is supposed to be a test of competency.   A regular reader of this blog, however, should understand that there absolute truth is often elusive when it comes to identifying diatoms and other algae.  The hard objectivity needed for a real test of competency always has to be moderated by the recognition of the limitations of our craft.   Moreover, turning this exercise into a calibration exercise runs the risk of turning the analysts into machines.  Rather, we use the term “reflective learning”, encouraging participants to use the reports to judge their own performance relative to the experts, and to take their own corrective action.

Some of the organisations whose analysts participate use the ring-test as part of their own quality control systems, and will take corrective action if results stray across the action limit.  That seems to be a sensible compromise: quality control should be the responsibility of individual laboratories, rather than delegated out to third parties.   At the same time, organisations need to understand that the people who perform ecological analyses are professionals, not treated as if they are one more machine in a laboratory that needs to be calibrated.


If you are interested in joining the UK / Diatom ring test scheme, or just want to learn a little more about it, get in touch with me and I’ll do my best to answer your questions.


Kelly, M.G. (2013).  Building capacity for ecological assessment using diatoms in UK rivers.  Journal of Ecology and the Environment 36: 89-94.


3 minutes 59.4 seconds …

Back in 1995 I interviewed a number of eminent people about their first academic publications as part of an occasional series for the Times Higher Education Supplement.  I wrote about one of the more daunting of these in “An encounter with Enoch Powell”.   The hour or so I spent with Sir Roger Bannister, who died a couple of days ago, could not have been more different.   He was best known for three minutes 59.4 seconds on a running track in Oxford in May 1954 but went on to have a successful career as a neurologist and eventually became master of Pembroke College, Oxford.  He was, despite this sporting and academic prowess, one of the most charming people I have met.

One of the secrets of his success on the running track was that he was, to all intents and purposes, a sports scientist before that term had been coined.  He took time out from his medical degree to do research on the physiology of breathing and, more particularly, how the point of exhaustion could be delayed by feeding his subjects with different concentrations of oxygen.   As a medical student working in straightened times just after the war, his first task was to build his own equipment, including the treadmill on which he and an assortment of colleagues and friends ran in order to generate the data he needed.  Building this kit involved trips to RAF bases to strip meters and other parts from decommissioned bombers (John Hapgood, former Archbishop of York and also a physiologist by training told me a very similar story).

Whilst his experiments were not directly relevant to his running (his actual training time amounted to less than an hour a day), there was, clearly, a benefit from understanding how his body worked.  However, whilst a runner cannot alter the concentration of oxygen that he breathes, a mountaineer can, and Bannister’s work was used by the team that conquered Everest the following year (he commented that he was surprised at how unfit some of the Everest team were by the standards of track runners).

Whatever his other achievements, however, it was that afternoon in Oxford in May 1954 that defined Roger Bannister.  Three minutes 59 seconds works out at just over a quarter of Andy Warhol’s quotient of fifteen minutes of fame and would have ensured Bannister’s place in the history books.   However, as the obituaries in the newspapers show, he achieved far more than that in his life.   And he was a gentleman too.

If you have the patience to battle with News International’s paywall, you can read my original article by following this link.

Burnhope Burn’s beautiful biofilms …

I have continued the series of studies that I started in “In search of the source of the Wear” with a three-dimensional diorama of the biofilm that I found at the mouth of Burnhope Burn, and can now compare it with the corresponding study from Wolsingham (see “The curious life of biofilms”).   The two big differences are the greater number of green filaments at Burnhope and the large numbers of cells of Navicula lanceolata at Wolsingham.   I suspect the two are linked: the Wolsingham biofilm was a mix of diatoms and organic particulate matter along with associated bacteria whilst the Burnhope biofilm was green algae and organic matter with diatoms in a subordinate role.  I speculated, in my earlier post, that Burnhope Burn’s location below a reservoir may have altered the hydrology of the stream such that green algae were favoured.   I wonder, too, if the presence of green algae then subtly shifts the composition of the biofilm matrix such that dense aggregations of Navicula lanceolata are not able to develop in the way that they could at Wolsingham.

There is something about the ecology of a few Navicula species that leads to the development of these aggregations (see “The ecology of cold days” for more about freshwaters, whilst “An excuse for a crab sandwich, really” and “A typical Geordie alga …” describes similar phenomena in brackish habitats).   Conversely, Nitzschia dissipata, which was the most abundant diatom at Burnhope Burn, never seems to form these dense monocultures.   Nitzschia dissipata was also much less common in the biofilm from Killhope Burn, just a few metres away from where I collected the Burnhope sample and where filamentous green algae are scarce.  wonder if this, too, is more than a coincidence and that N. dissipata is actually adapted to living within matrices formed by filamentous algae rather than on top of matrices dominated by diatoms and organic particulates?

I have seen a few other motile diatoms – Denticula tenuis is one – that seem to be more abundant in the presence of filamentous algae.   There may also be species that thrive when the matrix is composed largely of inorganic particles, as well as other species (Navicula angusta and N. notha are two that spring to mind) that may be naturally “understory” species that are never especially abundant in biofilms.   All this is pure speculation, but it is worth remembering that most of the insights into diatom ecology come from studies on cleaned valves which removes all traces of non-diatom algae, and also that the prevailing dogma of diatom sensitivity to their chemical environment is such that non-chemical factors are largely overlooked in academic studies.   No evidence, in this case, may just mean that no-one has asked the right questions.

More about Platessa oblongella and Odontidium mesodon

As my last post used the conventions of figurative art to describe algal ecology, I thought I would stick to graphs – science’s very own school of abstract art – for this one.   I spent some time in “Small details in the big picture” discussing the ecology of Platessa oblongella (including P. saxonica) but without saying very much about the types of streams where these species were found.  So I am going to take a step away from the Ennerdale catchment in this post and, instead, collate environmental data a large number of sites to get a broader understanding of their habitat preferences.  As these species are often associated with Odontidium mesodon (see “A tale of two diatoms …”), I will summarise the preferences of this species at the same time (but see Annex 1 for a graph of this species’ preferences for still versus standing water).

The first set of graphs show the response of these species to pH and alkalinity and establish both as species typical of circumneutral soft water.  Platessa oblongella can be abundant in more acid conditions (i.e. to the left of the green vertical lines) but most of the records where it is abundant have pH values between 6.5 and 7.5.   Note that P. oblongella can also be found in humic waters, where lower pH thresholds apply (see Annex 2).

Distribution of Odontidium mesodon and Platessa oblongella (including P. saxonica) to pH and alkalinity in UK streams.   Vertical lines for pH indicate threshold values that should support high (blue), good (green), moderate (orange) and poor (red) ecological status classes.  See Annex 2 for more explanation.

The second set of graphs shows how these species respond to inorganic nutrients.   Both are most abundant when inorganic nutrients are present in low concentrations, though the trend is stronger for phosphorus than it is for nitrate-nitrogen.   The graphs for Platessa oblongella, however, both have a few outliers.   I have seen P. oblongella in a few situations where I did not expect it – I remember finding it in the Halebourne, a stream draining heathland around Aldershot and Bagshot in Surrey, where the water was well buffered (mean alkalinity: 61.3 mg L-1 CaCO3) and nutrient concentration were high (mean total oxidised nitrogen: 4.01 mg L-1; dissolved phosphorus: 0.25 mg L-1) and Carlos Wetzel and colleagues note some other anomalous records from the literature in their paper (cited in my earlier post), including a few from high conductivity and even brackish environments.   So we should treat these plots as indicative of the ecological preferences rather than definitive.

Distribution of Odontidium mesodon and Platessa oblongella (including P. saxonica) to nitrate-N and dissolved phosphorus in UK streams.   Vertical lines indicate threshold values that should support high (blue), good (green), moderate (orange) and poor (red) ecological status classes.  See Annex 2 for more explanation.

The final pair of plots show how the relative abundance of these two species changes over the course of the year.  These plots show the months when each taxon is abundant, by the standards of that taxon.  Because Platessa oblongella tends to be very numerous in samples, the threshold for this taxon (the 90th percentile of all records) is higher than that for O. mesodon.   This reveals a very clear pattern of O. mesodon thriving in Spring whilst P. oblongella is abundant throughout the year, but with a slight preference for summer and autumn.  We need to reconcile these patterns with the observations in A tale of two diatoms that show that P. oblongella is associated with thinner biofilms than O. mesodon and try to work out whether season is driving the patterns or whether the seasonal patterns are the manifestation of other forces.   My suspicion is that P. oblongella is a classic pioneer species but also has a low-growing prostrate habit which means that it should be resistant to heavy grazing, which may confer an advantage in the summer and autumn when grazers are most active.  However, I may be getting ahead of myself, as we are in the process of analysing data on grazer-algae interactions in the River Ehen and Croasdale Beck that may throw more light on this.  There are clearly more layers to this story yet to be revealed …

Distribution of Odontidium mesodon (i.) and Platessa oblongella (j., including P. saxonica). The solid lines represent relative sampling effort (i.e. the proportion of samples in the dataset collected in a particular month) and the vertical bars represent samples where the relative abundance of taxon in question exceeded the 90th percentile for that taxon (20% for P. oblongella/P. saxonica and 5% for O. mesodon).


The dataset used for these analyses is that used in:

Kelly, M.G., Juggins, S., Guthrie, R., Pritchard, S., Jamieson, B.J., Rippey, B, Hirst, H & Yallop, M.L. (2008). Assessment of ecological status in UK rivers using diatoms. Freshwater Biology 53: 403-422.

Annex 1: Odontidium mesodon’s preference for still or standing water

As I included a graph showing the preference of Platessa oblongella / P. saxonica for still or standing water in “A tale of two diatoms …”, I have included a similar graph for Odontidium mesodon here.   I have not included any data from the streams that flow into Ennerdale Water’s north-west corner in this graph as this would give a distorted picture.  To date, I have only seen a single valve of O. mesodon during analyses of 14 samples from these streams but I have not yet sampled these in spring which, as the graph above shows, is the time when O. mesodon is most abundant.   Like Platessa oblongella, O. mesodon is predominately a species of running, rather than standing waters.

Differences in percentage of Odontidium mesodon in epilithic samples from Ennerdale Water and associated streams.  Data collected between 2012 and 2018.

Annex 2: notes on species-environment plots

These are based on interrogation of a database of 6500 river samples collected as part of DARES project.  Vertical lines show UK environmental standards for conditions necessary to support good ecological status: blue = high status; green = good status, orange = moderate status and red = poor status.  Note that there are no environmental standards for alkalinity and the vertical lines show a rough split of the gradient into low alkalinity (“soft water”: < 10 mg L-1 CaCO3), low/moderate alkalinity (³ 10, < 75 mg L-1 CaCO3), moderate/high alkalinity (³ 75, < 150 mg L-1 CaCO3) and high alkalinity (“hard water”: ³ 150 mg L-1 CaCO3).

pH thresholds are for clear water (see UK TAG’s Acidification Environmental Standards.  The corresponding thresholds for humic waters are lower (high/good: 5.1; good/moderate: 4.55; moderate/poor: 4.22; poor/bad: 4.03).

Phosphorus thresholds are based on UK TAG’s A Revised Approach to Setting WFD Phosphorus Standards.   Current UK phosphorus standards are site specific, using altitude and alkalinity as predictors.  This means that a range of thresholds applies, depending upon the geological preferences of the species in question.  The plots here show the position of boundaries based on the average alkalinity and altitude measurements in the DARES database.

Note, too, that phosphorus analyses use the Environment Agency’s standard measure, which is unfiltered molybdate reactive phosphorus.  This approximates to “soluble reactive phosphorus” or “phosphorus as orthophosphate” in most circumstances but the reagents will react with phosphorus attached to particles that would have been removed by membrane filtration.

Nitrate-nitrogen: There are, currently, no UK standards for nitrates in rivers.  Values plotted here are derived in the same way as those for phosphorus (see “This is not a nitrate standard”)


In search of the source of the Wear …

Having investigated the microscopic world at Wolsingham (see “The River Wear in January” and “The curious life of biofilms), I decided that it would be interesting to head further upstream and see how much difference there was between the algae at the two locations.  I drove up to Wearhead on a cold Saturday morning to take a look but was immediately faced with a conundrum: the River Wear is formed from the confluence of two very different streams, both with extensive catchments on the moors of the northern Pennines.   One of these is Burnhope Burn, which is fed by Burnhope Reservoir, about a kilometre above Wearhead, and the other is Killhope Burn, which drains a large area of blanket bog, forestry and, importantly, abandoned metal mines.   Burnhope Burn is on the left of the photograph above whilst Killhope Burn comes in from the right.  I thought it might be rather interesting to take a sample from each and see how they compared.

The two streams look quite different to one another.   Burnhope Burn, its flow regulated by the reservoir, is the Cain of the pair whilst Killhope Burn is the unruly turbulent Abel.   This was apparent, too, when I was collecting the samples and, again, when I peered at them through my microscope.   Burnhope Burn’s biofilm was thicker and the most conspicuous algae that I could see were green filaments of Klebsormidium.  Killhope Burn’s was thinner and dominated by diatoms.   Many of the same diatoms were found in the two samples, but Burnhope Burn had more of the motile Nitzschia species that benefit from the tangled matrix of green algal filaments that thrived there.

Views of the biofilm from Burnhope Burn (a.) and Killhope Burn (b.) just above their confluence to form the River Wear, February 2018.

I’ve tried to capture the essence of the biofilm from Burnhope Burn in the schematic diagram below.  Compare this with the diagram of the biofilm from the Wear that I showed in my earlier post.   In both cases, we have a mix of organic and inorganic elements, with the organic matter further divided into living organisms and agglomerations of particulate matter.  A few of the species are common to both but there are also some notable differences.   The biofilm in the Wear, for example, had almost no green algae (though that may change over the coming months) whilst that from Burnhope Burn has many filaments of Klebsormidium.   There were motile diatoms at both locations but the species are different: Navicula lanceolata and N. gregaria at Wolsingham and Nitzschia dissipata at Burnhope Burn.  People usually describe differences in the ecology of diatoms in terms of their chemical environment but I sometimes wonder if, in the case of motile diatoms, the nature of the matrix within which they live also plays a role in determining which thrive.

The difference between Burnhope and Killhope Burns is a variation of the theme that I discussed in “Small details in the big picture …”.  Again, regulation of a river or stream plays a role in determining which species of algae can thrive.  However, whereas I found a lot of Platessa oblongella in the unregulated streams of the Ennerdale catchment, the more base-rich environment of the Pennines means that I am much less likely to find P. oblongella in these streams.  In fact, I don’t think I have ever seen it in north-east England (see distribution maps in “Why do you look for the living amongst the dead”).

That reminds me: I was going to write more about the ecology of Platessa oblongella before I was diverted by desmids and Wearhead.   Soon …

A schematic view of the vertical structure of a submerged biofilm from Burnhope Burn, Wearhead, February 2018.   a. Klebsormidium fluitans; b.  Phormidium; c. Nitzschia dissipata (valve view); d. N. dissipata (girdle view); e. Gomphonema cf. calcifugum (valve and girdle views); f. inorganic particles; g. fine particulate organic matter.  Scale bar: 20 micrometres (= 1/40th of a millimetre).