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.

Reference

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.

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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)

 

That’s funny …

The most exciting phrase to hear in science, the one that heralds new discoveries, is not “Eureka!” but “That’s funny”
Attributed to Issac Asimov

I have visited Croasdale Beck, in western Cumbria, twenty-eight times since 2015 and I thought I was beginning to understand it’s character (see “A tale of two diatoms” and “What a difference a storm makes”).   It is the unruly sibling of the River Ehen which, usually, offers a far less amenable environment for freshwater algae.  Last week, however, as we walked down the track towards the stream, we were confronted with the unexpected sight of a river bed that was bright green.  Our measurements, too, showed that not only was there a lot of algae in absolute terms, but there was far more here than we had measured in the River Ehen.  Usually, the situation is reversed, with the Ehen having more than Croasdale Beck.

Croasdale Beck at NY 087 170 looking upstream in April 2018.   The position of the gravel bar has shifted over the time that we have visited, with the wetted channel originally being at the right hand side, rather than being split into two.

It was hard to capture the extent of the algae growing on the river bed in a photograph, but the macroscopic image below captures the colour of the growths well, and you’ll have to use your imagination to scale this up to cover half of the stream bed.  Under the microscope, these growths turned out to be virtual monocultures of the green alga Draparnaldia glomerata.  This is common in clean rivers in spring time, and I often find it in the nearby River Ehen (see “The River Ehen in February”).  What my images do not show is the mucilage that surrounds the filaments.   In some cases, the growths can be almost jelly-like, so prolific is this mucilage.   One of the roles of this mucilage plays is to serve a matrix within which enzymes released by the fine hairs at the end of the filaments can act to release nutrients bound into tiny organic particles (see “A day out in Weardale …”).

Growths of Draparnaldia glomerata in Croasdale Beck (NY 087 170) in April 2018.  The upper image shows the filaments growing on submerged stones and the lower image shows the bushy side-branches growing from a central filament.  Scale bar: 100 micrometres (= 1/10th of a millimetre).

We also sample a site a couple of kilometres downstream on Croasdale Beck and, here again, the river bed was smothered in green growths.  I assumed that this, too, was Draparnaldia glomerata but, when I examined the filaments under the microscope, it turned out to be a different alga altogether: Ulothrix zonata (see “Bollihope Bhavacakra” and links therein).   There is little difference between the two sites that might explain this: the latter is slightly lower and is surrounded by rough pasture whilst the other is closer to the fells.   However, I have seen both Ulothrix zonata and Draparnaldia glomerata at several other sites in the vicinity, and a simplistic interpretation based on agricultural enrichment does not really work.

There were also a few obvious differences in the diatoms that I saw in the two samples.   In both cases, we sampled stones lacking green algae but, instead, having a thick brown biofilm.  Several taxa were common to both sites – Odontidium mesodon, for example (broadly confirming the hypothesis in “A tale of two diatoms …”) and Meridion circulare was conspicuous in both.   However, the lower site had many more cells of “Ulnaria ulna” than the upper site.   Again, there is no ready explanation but, at the same time, neither green algae or diatoms at either site suggests anything malign.

Filaments of Ulothrix zonata at Croasdale Beck (NY 072 161).   The upper filament is in a healthy vegetative state (although the cell walls are not as thickened as in many populations).  The lower filament is producing zoospores.   Scale bar: 25 micrometres (= 1/40th of a millimetre).

Diatoms in Croasdale Beck, April 2018.   a. upper site: note the abundance of Odontidium mesodon, plus cells of Gomphonema cf exilissimum, Achnanthidium minutissimum and Meridion circulare; b. lower site: note the presence of “Ulnaria ulna” as well as several of the taxa found at the upper site.   Scale bar: 25 micrometres (= 1/40th of a millimetre).  

So where does this take us?  I talked about the benefits of repeat visits to the same site in “A brief history of time wasting …” and I think that these data from Croasdale are making a similar point.  By necessity, most formal assessments of the state of ecology are based on very limited data, from which, at best, we get an estimate of the “average” condition of a water body over a period of time.  Repeat visits might lead to a more precise assessment of the “average” state but also give us a better idea of the whole range of conditions that might be encountered.  Here, I suspect, we chanced upon one of the extremes of the distribution of conditions.   Cold, wet weather in early spring delayed the growth of many plants – aquatic and terrestrial – as well as the invertebrates that graze them.   Then the period of warm, dry conditions that preceded our visit gave the algae an opportunity to thrive whilst their grazers are still playing “catch-up”.  I suspect that next time we visit Croasdale Beck will have its familiar appearance.   It is, nonetheless, sobering to think that this single visit could have formed fifty-percent of the evidence on which a formal assessment might have been made.

 

Cypriot delights …

I could not return from my visit to Cyprus without an algal sample and a fine opportunity presented itself early last week when we visited the Avgás Gorge, on the Akámas peninsula at the west coast of Cyprus, just north of Pathos.  This is a spectacular limestone ravine whose steep sides offered welcome relief from the Mediterranean sun.  At points, as in the photograph above, the ravine narrowed to just a few metres wide, reminiscent of the siq which guards the entrance to Petra except that instead of exquisite carvings we stumbled across a Russian team conducting a glamour shoot.

A small stream made its way down the gorge.  The presence of woody debris at intervals suggested a considerable head of water during the winter months but, at this time of year the water has reduced in power, tumbling across a series of boulders into stagnant pools, interspersed with short runs shaded either by the high cliffs or the vegetation that flourished away from the harsh glare of the sun.

In the sections where water was still running there were clumps of Chara tangled up with filamentous algae, with plenty of bubbles of oxygen as evidence that both were busily photosynthesising away.  The filamentous algae was a coarse unbranched filament that was clearly a relative of Cladophora but which did not match any of the genera or species that I had encountered before (see “Fieldwork at Flatford” for similar situation).   There were pebbles and cobbles between these clumps, their surfaces criss-crossed by the galleries of caseless caddis larvae – probably Psychomyiidae, according to Richard Chadd.

Chara growing in the stream at Avgás Gorge in western Cyprus, April 2018.

Cladophora or a relative growing in the stream at Avgás Gorge in western Cyprus, April 2018.  The scale bar is 50 micrometres (= 1/20th of a millimetre). 

There were a number of diatoms present too, the most abundant of which was a chain-forming Ulnaria.  Unfortunately, despite having just co-authored a paper on Ulnaria from Cyprus, I cannot name the species, as I saw no cells in valve view.  I will have to return to this subject once I have prepared a permanent slide from the sample that I brought back.   The chloroplasts in the illustration below are not in a very healthy state because the sample lived in a fridge for almost a week before I was able to get it under a microscope.  Had I looked at this sample 20 years ago, I would have assumed that I was looking at a species of Fragilaria, as most keys then stated categorically that Synedra (the former generic name) were solitary rather than chain-forming.  However, we now know that there are several Ulnaria species that form chains although most that I see in my regular haunts in the UK do not.   Our paper states that the species we describe form short chains although, as we worked from cleaned samples collected by other people, I now wonder if that was an artefact of the preparation process and whether these, too, formed longer chains in their living state.

A chain of Ulnaria from the stream at Avgás Gorge in western Cyprus, April 2018.  Scale bar: 20 micrometres (= 1/50th of a millimetre).

Though it is rare for me to stray into describing the invertebrate life of streams, the Psychomyiidae are actually an important part of the story here, as the larvae graze algae from the surface of the stones.  They create a silk tube and this, in turn, becomes covered with fine sediment to create the galleries that are visible in the picture.   We know that invertebrates can change the composition of the attached algae by grazing but I sometimes wonder if the caseless caddis larvae also change the composition by creating myriad patches of very fine sediment across the rock surface.   If you look closely you can also see a couple of Simuliidae (blackfly) larvae.  These attach themselves to the rock by a circle of hooks at their last abdominal segment (I think that is “bum” in entomological language) and then use fan-like structures around their mouths to filter tiny particles from the water.  However, I have also seen Simuliidae larvae bent double on rock surfaces in order to hoover up particulate matter and algae that live there.

That, I thought, was just about enough natural history for one day.   I put my toothbrush and bottle back into my rucksack and made my way back down until, turning the corner, I stumbled across the Russian glamour models.   I’ve often written about the similarities between freshwater ecosystems in different parts of Europe but you don’t often see a bikini – or less – in April in my cold, damp corner.

Galleries of Psychomyiidae larvae on the top of a limestone cobble from the stream at Avgás Gorge in western Cyprus, April 2018.

Reference

Cantonati, M., Lange-Bertalot, H., Kelly, M.G. & Angeli, N. (2018).  Taxonomic and ecological characterization of two Ulnaria species (Bacillariophyta) from streams in Cyprus.   Phytotaxa 346: 78-92.

Wallace, I. (2003).   The Beginner’s Guide to Caddis (order Trichoptera).  Bulletin of the Amateur Entomologists’ Society 62: 15-26.

A brief history of time-wasting …*

Having talked about diversity on a microscale in the previous post, I thought it would be interesting to place this in context by looking at the variations that I have observed in the River Wear at Wolsingham over the past decade or so.   The River Wear has seen some significant improvements in water quality over this period, but those have mainly affected sections of the river downstream from Wolsingham.  Most of the changes at Wolsingham are, therefore, giving us some insights into the range of natural variation that we should expect to see in a river.

I’ve got 31 samples from the River Wear at Wolsingham on my database, collected since 2005.  Over this period, nine different diatom species have dominated my counts: Achnanthidium minutissimum on 21 occasions, Nitzschia dissipata twice and Cocconeis euglypta, Encyonema silesiacum, Gomphonema calcifugum, Navicula lanceolata, Nitzshia archibaldii, N. paleacea and Reimeria sinuata once each.   I also have records for non-diatoms during 2009, during which time the green alga Ulothrix zonata, and two Cyanobacteria, Phormidium retzii and Homeothrix varians were the dominant alga on one occasion each.   In total, I have recorded 131 species of diatom from this one reach, although only I’ve only found 91 of them more than once, and only 59 have ever formed more than one percent of the total.   I’ve also got records of 22 species other than diatoms.

This – along with my comments in “The mystery of the alga that wasn’t there …” raises questions about just how effective a single sample is at capturing the diversity of algae present at a site.  .    In 2009 I collected a sample every month from Wolsingham and the graph below shows how the total number of species recorded increased over that period.   Typically, I find between 20 and 30 species in a single sample, and each subsequent month revealed a few that I had not seen in earlier samples.   Importantly, no single sample contained more than 40 per cent of the total diversity I observed over the course of the year.  Part of this high diversity is because of the greater effort invested but there is also a seasonal element, as I’ve already discussed.   The latter, in particular, means that we need to be very careful about making comments about alpha diversity of microalgae if we only have a single sample from a site.

Increase in the number of diatom taxa recorded in successive samples from the River Wear at Wolsingham.  In 2009 samples were collected monthly between January and December whilst in 2014 samples were collected quarterly. 

This seasonal pattern in the algal community also translates into variation in the Trophic Diatom Index, the measure we use to evaluate the condition of streams and rivers.  The trend is weak, for reasons that I have discussed in earlier posts, but it is there, nonetheless.   Not every river has such a seasonal trend and, in some cases, the community dynamics results in the opposite pattern: higher values in the summer and lower values in the winter.  It is, however, something that we have to keep in mind when evaluating ecological status.

Variation in the Trophic Diatom Index in the River Wear at Wolsingham between 2005 and 2015, with samples organised by month, from January (1) to December (12).   The blue line shows a LOESS regression and the grey band is the 95% confidence limits around this line.

All of these factors translate into uncertainty when evaluating ecological status.   In the case of the River Wear at Wolsingham, this is not particularly serious as most of the samples indicate “high status” and all are to the right of the key regulatory boundary of “good status”.  However, imagine if the histogram of EQRs was slid a little to the left, so that it straddled the good and moderate boundaries, and then put yourself in the position of the people who have to decide whether or not to make a water company invest a million pounds to improve the wastewater coming from one of their sewage treatment plants.

At this point, having a long-term perspective and knowing about the ecology of individual species may allow you to explain why an apparent dip into moderate status may not be a cause for concern.  Having a general sense of the ecology of the river – particularly those aspects not measured during formal status assessments – should help too.  It is quite common for the range of diatom results from a site to encompass an entire status class or more so the interpretative skills of the biologists play an important role in decision-making.   Unfortunately, if anything the trend is in the opposite direction: fewer samples being collected per site due to financial pressures, more automation in sample and data analysis leading to ecologists spending more time peering at spreadsheets than peering at stream beds.

I’ve never been in the invidious position of having to make hard decisions about how scarce public sector resources are used.  However, it does strike me that the time that ecologists used to spend in the field and laboratory, though deemed “inefficient” by middle managers trying to find cost savings, was the time that they learned to understand the rivers for which they were responsible.  The great irony is that, in a time when politicians trumpet the virtues of evidence-led policy, there is often barely enough ecological data being collected, and not enough time spent developing interpretative skills, for sensible decisions to be made.   Gathering ecological information takes time.   But if that leads to better decisions, then that is not time wasted …

Ecological Quality Ratio (EQR: observed TDI / expected TDI) of phytobenthos (diatoms) at the River Wear, Wolsingham) between 2005 and 2015.   Blue, green, orange and red lines show the positions of high, good, moderate and poor status class boundaries respectively.

* the title is borrowed from the late Janet Smith’s BBC Radio 4 comedy series

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…

Reference

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.

References

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.