The green mantle of the standing pond* …

One of the highlights of a wet and windy weekend at Malham Tarn Field Centre for the annual British Diatomist Meeting was a talk by Carl Sayer on the ecology of a small pond in Norfolk.  The work was not new to me, as I had been the external examiner for Dave Emson’s PhD thesis on which the work was based.  I remember, at the time, making a mental note to write a post once the work was fully in the public domain, and Carl’s talk has finally jogged me into action.

Carl’s starting point was the observation that small ponds are often covered with dense growths of floating aquatic plants such as duckweed (Lemna minor).  Repeated visits to ponds in north Norfolk, close to where he grew up, had shown that this cover of duckweed often lasted for a few years before disappearing, only to reappear some years later.   As this duckweed blocks out sunlight, periods of dominance are likely to have unfortunate consequences for other aquatic plants in the pond and, as these pump oxygen into the water as a by-product of photosynthesis, life for other pond-dwelling organisms – such as the Crucian carp (Carassius carassius) that Carl likes to catch from the pond – will also get tougher.

There’s a lot of questions that could be asked about what’s going on here, and not all can be answered in a single study, but establishing whether these periodic episodes of duckweed dominance were one-offs or if they were regular events is a good place.  Here Carl and Dave  were able to use a well-known association between a diatom – Lemnicola hungarica – and duckweed to track changes in Lemna over time.   Lemnicola hungarica grows attached to the roots of duckweeds and similar species and seems to be unusually fussy about its habitat compared to many diatoms, which means that when Lemnicola is found in the sediments of a pond, that is a fairly good indication that Lemna was abundant when those sediments were being laid down.   In the process, they also discovered another diatom, Sellaphora saugerresii, also seemed to be strongly associated with Lemna, at least in this habitat (it is also common in many rivers were Lemna is sparse or absent).

The relative abundance of a) Lemnicola hungarica and b) Sellaphora saugerresii in surface sediments of north Norfolk ponds with and without Lemna dominance.   The two species are illustrated on the right hand side (S. saugerresii is typically about 10 micrometres  (= 1/100th of a millimetre) in length).

Armed with this information, Dave and Carl went back to one of Carl’s local ponds and extracted a core of the sediments from the middle in order to see how numbers of Lemnicola hungarica and Sellaphora saugerresii changed through the length of the core.   Because they were also able to date the core, they were able to show that the period when there are documentary records of duckweed dominance coincides with both of these indicators being abundant in the pond sediments.  Below these levels (i.e. further back in time), the relative abundance of these two species waxes and wanes several times, suggesting that the duckweed cover, too, had come and gone over the years.

Left: Dave Emson and the core from Bodham Rail Pit; right: changes in the relative abundance of Lemnicola hungarica and Sellaphora saugerresii at different levels of the core.    The grey rectangle indicates the period during which Lemna is known to have been dominant in the pond.

Quite why this is so is not clear.   There are several species of floating aquatic plant (water hyacinth and Salvinia, the floating fern are two good examples) that are able to cover large areas of standing water bodies in a short period of time and they often do this by vegetative growth rather than by seed.   This means that the plants are mostly clones of a very small number of plants that first colonised the water body.   And this, in turn, may mean that a virus that infects one frond will be able to infect every other frond as well as there is a very narrow range of genotypes within the population.  That’s one possibility but there may be others.

But back to the story: knowing that Lemna abundance fluctuates is not quite the same as being able to describe the consequences of this for the rest of the organisms that inhabit these ponds.   The Crucian carp was the species that attracted Carl to the pond in the first place so it would be good to know whether this species can survive the dark, oxygen-poor years when the surface is covered with duckweed.   They did find scales of Crucian carp in the cores right through the pond’s dark ages suggesting that this tough little fish had managed to hang on.  In 2008, a few years after the most recent duckweed episode, they found just a single carp when they cast their nets out into the pond but there were three by the following spring and, in 2011 there were over 200 juveniles.  So it looks like the carp populations definitely retrench during the duckweed episodes but that they do, eventually, recover.   And, maybe, another generation of north Norfolk natural historians will become enthralled by the aquatic world as a result?

* King Lear Act III scene IV

References

Buczkó, K. (2007).  The occurrence of the epiphytic diatom Lemnicola hungarica on different European Lemnaceae species.  Fottea, Olomouc 7: 77-84.

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It’s all about the algae

Just a short post to point you all towards an article I wrote for Royal Society of Biology’s magazine The Biologist.  It is a broad overview of the reasons why we use algae to assess the condition of our lakes and rivers in Europe and is illustrated with three of Chris Carter’s beautiful images, and the print edition will have even more of these.  Take the figure legends with a pinch of salt (we didn’t write these!): neither Tolypella nor Chaetophora are particularly common in the UK.   Navicula, on the other hand, is common but the legend makes no mention of this.

Whilst I have your attention, I will also point you towards a short article that I wrote for the most recent Phycological Bulletin, the newsletter of the Phycological Society of America.  This offers a few more hints to anyone thinking about entering the Hilda Canter-Lund competition next year.

Certainly uncertain …

Back in May I set out some thoughts on what the diatom-based metrics that we use for ecological assessment are actually telling us (see “What does it all mean?”).  I suggested that diatoms (and, for that matter, other freshwater benthic algae) showed four basic responses to nutrients and that the apparent continua of optima obtained from statistical models was the result of interactions with other variables such as alkalinity.   However, this is still only a partial explanation for what we see in samples, which often contain species with a range of different responses to the nutrient gradient.  At a purely computational level, this is not a major problem, as assessments are based on the average response of the assemblage. This assumes that the variation is stochastic, with no biological significance.  In practice, standard methods for sampling phytobenthos destroy the structure and patchiness of the community at the location, and our understanding is further confounded by the microscopic scale of the habitats we are trying to interpret (see “Baffled by the benthos (1)”).  But what if the variability that we observe in our samples is actually telling us something about the structure and function of the ecosystem?

One limitation of the transfer functions that I talked about in that earlier post is that they amalgamate information about individual species but do not use any higher level information about community structure.  Understanding more about community structure may help us to understand some of the variation that we see.   In the graph below I have tried to visualise the response of the four categories of response along the nutrient/organic gradient in a way that tries to explain the overlap in occurrence of different types of response.   I have put a vertical line on this graph in order that we can focus on the community at one point along the pollution gradient, noting, in particular, that three different strategies can co-exist at the same level of pollution.  Received wisdom amongst the diatom faithful is that the apparent variation we see in ecological preferences amongst the species in a single sample reflects inadequacies in our taxonomic understanding.  My suggestion is that this is partly because we have not appreciated how species are arranged within a biofilm.  I’ve tried to illustrate this with a diagram of a biofilm that might lead to this type of assemblage.

Schematic diagram showing the response of benthic algae along a nutrient/organic gradient.  a.: taxa thriving in low nutrient / high oxygen habitats; b.: taxa thriving in high nutrient / high oxygen habitats; c.: taxa thriving in high nutrient / low oxygen habitats; d.: taxa thriving in high nutrients / very low oxygen habitats.   H, G., M, P and B refer to high, good, moderate, poor and bad ecological status.

The dominant alga in many of the enriched rivers in my part of the world is the tough, branched filamentous green alga Cladophora glomerata.   This, in turn, creates micro-habitats for a range of algae.  Some algae, such as Rhoicosphenia abbreviata, Cocconeis pediculus and Chamaesiphon incrustans, thrive as epiphytes on Cladophora whilst others, such as C. euglypta are often, but not exclusively, found in this microhabitat.  Living on Cladophora filaments gives them better access to light but also means that their supply of oxygen is constantly replenished by the water (few rivers in the UK are, these days, so bereft of oxygen to make this an issue).   All of these species fit neatly into category b. in my earlier post.

Underneath the Cladophora filaments, however, there is a very different environment.  The filaments trap organic and inorganic particulate matter which are energy sources for a variety of protozoans, bacteria and fungi.   These use up the limited oxygen in the water, possibly faster than it can be replenished, so any algae that live in this part of the biofilm need to be able to cope with the shading from the Cladophora plus the low levels of oxygen.   Many of the species that we find in highly polluted conditions are motile (e.g. Nitzschia palea), and so are able to constantly adjust their positions, in order to access more light and other resources.   They will also need to be able to cope with lower oxygen concentrations and, possibly, with consequences such as highly reducing conditions.  These species will fit into categories c. and d. in the first diagram.

A stylised (and simplified) cross-section through a biofilm in a polluted river, showing how different algae may co-exist.   The biofilm is dominated by Cladophora glomerata (i.) with epiphytic Rhoicosphenia abbreviata (ii.), Cocconeis euglypta (iii.) and Chamaesiphon incrustans (iv.) whilst, lower down in the biofilm, we see motile Nitzschia palea (v.) and Fistulifera and Mayamaea species (vi.) growing in mucilaginous masses.

However, as the cross-section above represents substantially less than a millimetre of a real biofilm, it is almost impossible to keep apart when sampling, and we end up trying to make sense of a mess of different species.   The ecologists default position is, inevitably, name and count, then feed the outputs into a statistical program and hope for the best.

A final complication is that river beds are rarely uniform.  The stones that make up the substrate vary in size and stability, so some are rolled by the current more frequently than others.  There may be patches of faster and slower flow associated with the inside and outsides of meanders, plus areas with more or less shade.   As a result, the patches of Cladophora will vary in thickness (some less stable stones will lack them altogether) and, along with this, the proportions of species exhibiting each of the strategies.  The final twist, therefore, is that the vertical line that I drew on the first illustration to illustrate a point on a gradient is, itself, simplistic.  As the proportions vary, so the position of that line will also shift.  Any one sample (itself the amalgamation of at least five microhabitats) could appear at a number of different points on the gradient.  Broadly speaking, uncertainty is embedded into the assessment of ecological status using phytobenthos as deeply as it is in quantum mechanics.  We can manage uncertainty to some extent by taking care with those aspects that are within our control.   However, in the final analysis, a sampling procedure that involves an organism 25,000 times larger than most diatoms blundering around a stream wielding a toothbrush is invariably going to have limitations.

The same schematic diagram as that at the start of this article, but with the vertical line indicating the position of a hypothetical sample replaced by a rectangle representing the range of possibilities for samples at any one site. 

A hidden world in a salty puddle …

An exchange of emails amongst a group of us preparing an obituary for Hilary Belcher led me to a short paper written by herself and Erica Swale on diatoms from a salty puddle close to a bridge under the M11 motorway in Cambridgeshire.  They had noticed some brown patches that looked like diatoms on the bottom of this puddle in 1979 and took a sample home to examine under the microscope. What they saw was an assemblage of diatoms that was more suggestive of a brackish habitat than freshwaters, leading them to conclude that the road salt that was spread on the M11 in winter was draining off the road and creating these mini salt lakes.  These were not one-off observations: they returned several times to find similar assemblages of diatoms in the same puddles.   Of these, only Surirella brebissonii is common in freshwaters.  Entomoneis and Cylindrotheca are two genera that I have written about before, both from marine or brackish habitats (see “A typical Geordie alga …” and “Back to Druridge Bay”).

Some diatoms associated with a puddle close to the M11 in Cambridgeshire: A: Entomoneis paludosa var. salinarum; B: Surirella brebissonii; C: Tryblionella hungarica; D: Nitzschia sigma; E: Nitzschia vitrea; F: Cylindrotheca closterium; G: C. gracilis.  From Belcher and Swale (1993).

I do occasionally find diatoms from marine habitats in rivers, and often suspect road salt to be the culprit.  One of the most extreme cases I encountered was a sample from the Ingrebourne, a small stream close to my childhood home where Bacillaria paxillifer constituted a third of all the diatoms present.  Bacillaria paxillifer is an intriguing diatom (see “The paradox that is Bacillaria” and links) but one that is very definitely a species that prefers saline rather than fresh water.  The Ingrebourne passes under the M25 motorway within about a kilometre of its source and crosses the busy A12 trunk road just upstream of the sampling location, so periodic pulses of salt are a possibility.

The ephemeral nature of these events, however, make them hard to prove and we are left with scattered notes such as this one in a small natural history journal.   These journals are, in many cases, struggling to survive in the modern age and I guess blogs such as this are taking over from them as records of botanical observations that are not structured in a way that makes publication in a mainstream scientific journal a possibility.  Hilary Belcher and Erica Swale made a number of substantial contributions to algal research over the course of their careers, but they were also consummate observers and recorders of their local environment – the wellspring from which an understanding of the natural world ultimately flows.

I am thankful to Hilary in one other way: she and her partner Erica Swale wrote a small (47 page) booklet with clear line drawings of the most common freshwater algae that was a required purchase for all undergraduates (and demonstrators) attending Brian Whitton’s algae practicals at Durham and it was through this book that I started to learn how to identify algae.  There are, I notice, just 17 genera of diatoms illustrated in this book but there was enough here to start putting names onto the shapes that floated – or flitted – through my field of view as I struggled to learn the rudiments of the craft.

Left: Hilary Belcher on a sampling trip to the Thames in the early 1990s (photo: Alison Love) and, right: the cover of her introductory guide to freshwater algae, co-authored with Erica Swale.

Reference

Belcher, H. & Swale, E. (1993).  Some diatoms of a small saline habitat near Cambridge.  Nature Cambridgeshire 35: 75-77.

A full appreciation of the life and work of Hilary Belcher, compiled by Jenny Bryant, will appear in the next edition of The Phycologist.

Unlikely neighbours …

One of the lessons I learned from writing “A tale of two diatoms …” is that we can often learn more about the ecology of a species by contrasting its behaviour with that of another species rather than by just relating the distribution of that species to features of its environment.  I came across another example of this when I was writing up the results of the latest “ring-test” that UK diatom analysts undertake to maintain their competence.

The sample came from a stream in east Devon (the one that had a walk-on part in “The challenging ecology of a freshwater diatom”).  This stream receives effluent from a small sewage works but our sample comes from just upstream of this works.   We know that the stream downstream of the sewage works is quite polluted but were also interested in the condition of the stream above the works.   This has proved to be challenging and, it seems, there are some pollution sources, including septic tanks and runoff from fields, that mean that the stream already shows signs of impact before it reaches the sewage works.   There are, however, mixed messages when we look at the aquatic flora, and some of the diatoms that are abundant are characteristic of low or only slight enrichment.

One feature of the stream that was quite unusual was a relatively large number of cells of Reimeria uniseriata, a relative of Reimeria sinuata which is quite common.  Both of these are illustrated below: note that R. uniseriata tends to be slightly larger and has distinctly punctate striae.  However, when I looked at the distribution of these species in response to water chemistry, I could see few differences, with most of the records suggesting a preference for water with low or slightly elevated phosphorus concentrations.   Reimeria sinuata is more common than R. uniseriata and when the latter is found, the former is usually present too.  They seem to be able to share their habitat quite comfortably.

Reimeria sinuata from Polly Brook, Devon, December 2016.   a. – f.: valve views; g.: girdle view focussed on ventral side.  Scale bar: 10 micrometres (= 1/100th of millimetre).  Photos: Lydia King.

Reimeria uniseriata from Polly Brook, December 2016.  h., i.: valve views; j.: girdle view focussed on dorsal side; k., l.: girdle views focussed on ventral side.   Scale bar: 10 µm (= 1/100th of millimetre).  Photos: Lydia King.

In other words, we cannot learn very much from looking at differences in the distribution of these two species of Reimeria, given our current state of knowledge.  There is, however, one other “compare and contrast” within the data that I collected from Polly Brook that is more intriguing.   If Reimeria sinuata, in particular, usually indicates a healthy stream, possibly with a little nutrient enrichment, Rhoicosphenia abbreviata is more often associated with enriched conditions.   We have met this diatom before (see “Cladophora and friends” amongst other posts) and I have explained that it is often found growing as an epiphyte on other algae.  We rarely see situations where both species are abundant at the same time, as the graph below shows.

The relative distribution of Reimeria sinuata and Rhoicosphenia abbreviata in the 6500 UK stream and river samples in the DARES dataset.   The horizontal and vertical lines indicate 10% relative abundance of each species.

When I started looking at stream algae there was a prevailing assumption that there were strong causal relationships between the species of diatom that were found at a site and the level of chemical pressures.  In the case of phosphorus, in particular, I am now not convinced that the evidence supports this whilst, at the same time, am more convinced that we should be able to, at the very least, describe what a healthy stream algal community looks like and give reasons.  I use the word “describe” because I think that many of us have been preoccupied with counting and measuring, often at the expense of a qualitative understanding.  These two species illustrate my point as when I look down a microscope and see Reimeria sinuata, I can usually assume that the stream where it was growing was reasonably healthy, even if the nutrients are a little higher than would be ideal.  On the other hand, seeing lots of cells of Rhoicosphenia makes me suspect that there has been a breakdown in the functioning of the healthy community.  These conclusions would be irrespective of what the chemistry or the values that biological indices told me.

Two species is barely enough to base a credible assessment upon but we could stir more into the mix: I often find Reimeria sinuata with Achnanthidium minutissimum, and that, in in summer especially, suggests strong top-down control by grazers, which means that pathways of energy flow have not been disrupted.   And Rhoicosphenia, as I have already mentioned, is associated with Cladophora which, in abundance, suggests a breakdown in these pathways, as shown by Michael Sturt and colleagues from University College, Cork, a few years ago.   That Polly Brook has both Reimeria and Rhoicosphenia in abundance suggests that it might just be at the tipping point between these two states.

The naïve answer to making sure that the upper stretches of Polly Brook do not cross this threshold would be to manage the nutrients.  However that is not quite as easy as it sounds in an agricultural catchment.   It could be that managing other aspects of the riparian environment are equally effective at keeping the stream in a healthy condition but that takes us into areas where the evidence is still accumulating.  It could be that the simplistic determinism that drove much of the development of biological assessment methods actually held back the gathering of that evidence for a long time.  Reimeria sinuata – and it’s cousin, R. uniseriata – stand as two reminders that there is more to the management of aquatic ecosystems than strong correlations.

Reference

Sturt, M.M., Jansen, M.A.K. & Harrison, S.A.C. (2011).  Invertebrate grazing and riparian shade as controllers of nuisance algae in a eutrophic river.  Freshwater Biology 56: 2580-2593.

A tale of two diatoms …

I’ve been writing about the River Ehen in Cumbria since I started this blog, sharing my delight in the diversity of the microscopic world in this small river along with my frustrations in trying to understand what it is that gives this river its character.   We know that the presence of a weir at the outfall of Ennerdale Water has a big influence so, in 2015, we started to look at a nearby stream, Croasdale Beck (photographed above), which is similar in many respects but lacks the regulating influence of a lake and weir.  Maybe, we reasoned, the differences we observed would give us a better understanding of how the regulation of flow in the River Ehen influenced the ecology.

Broadly speaking, any kind of impoundment – whether a natural lake or an artificial reservoir – removes a lot of the energy from a stream that might otherwise roll stones, move sediment downstream and, in the process, dislodge the organisms that live there.   We noticed quite early in our studies, for example, that Croasdale Beck generally had less algae growing on the stones than in the nearby River Ehen, and also that the algal flora here was less diverse.

There were also some quite big differences in the algae between the two streams.  I wrote about one of the Cyanobacteria that are found in Croasdale Beck in “A bigger splash …” but there are also differences in the types of diatoms found in the two streams.  Most diatomists think about ecology primarily in terms of the chemical environment within which the diatoms live but I think that some of the differences that I see between the diatoms in the River Ehen and Croasdale Beck are a result of the different hydrological regimes in the two streams.

Several diatom species are common to both streams but two, in particular, stand out as being common in Croasdale Beck but rare in the River Ehen.  These are Achnanthes oblongella (illustrated in “Why do you look for the living amongst the dead?”) and Odontidium mesodon.  However, a closer look at the data showed that, whilst both were common in Croasdale Beck, they were rarely both common in the same sample.   If Achnanthes oblongella was abundant, then Odontidium mesodon was rare and vice versa, as the left hand graph below shows.   There were also a few situations when neither was abundant.

Odontidium mesodon from Croasdale Beck, Cumbria, July 2015.  Photographs by Lydia King.

The story got more interesting when I plotted the relative proportions of these two taxa against the amount of chlorophyll that we measured on the stones at the time of sample collection (see right hand graph below).   This gives us an idea of the total biomass of algae present at the site (which, in this particular case, are dominated by diatoms).   Achnanthes oblongella was most abundant when the biomass was very low, whilst Odontidium mesodon peaked at a slightly higher biomass, but proportions of both dropped off when the biomass was high.   I should point out that “high” in the context of Croasdale Beck is relatively low by the standards of other streams that we have examined and this adds another layer of complexity to the story.

When the biomass exceeds two micrograms per square centimetre, both Odontidium mesodon and Achnanthes oblongella are uncommon in the biomass, and the most abundant diatoms are Achnanthidum minutissimum, Fragilaria gracilis or, on one occasion, Cocconeis placentula.   A. minutissimum and F. gracilis are both common in the nearby River Ehen but C. placentula is very rarely found there.

The difference between River Ehen and Croasdale Beck is probably largely a result of the very difernt hydrological regimes, though this is an aspect of the ecology of diatoms that has been studied relatively rarely.   The differences within my Croasdale Beck samples is probably also a result of the hydrology, but reflects changes over time.   I suspect that Achnanthes oblongella is the natural “pioneer” species of soft-water, hydrologically-dynamic streams, and that Diatoma mesodon is able to over-grow A. oblongella when the biomass on stones increases due to prolonged periods of relative stability in the stream bed.  That still does not explain what happens when biomass is high and neither are abundant: the dataset is still small and we need to collect some more data to try to understand this. But the point of the post is mostly to remind everyone of the dangers of trying to interpret the ecology of attached stream algae solely in terms of their chemical environment.   And to make the point that a little more understanding of a natural system often fuels, rather than removes, the sense of mystery that is always present in nature.

a. The relationship between representation of Achnanthes oblongella and Odontidium mesodon in samples from Croasdale Beck between May 2015 and January 2017. Both axes are presented on square-root-transformed scales; b. relationship between representation of Achnanthes oblongella and Odontium mesodon and total epilithic biomass (as chlorophyll a). Lines show a locally-weighted polynomial (LOESS) regression fitted to the data.

Taxonomic note

Odontidium mesodon is the correct name for Diatoma mesodon (see “Diatoms from the Valley of Flowers”).   The name Odontidium had fallen out of popular usage, but Ingrid Jüttner and colleagues made the case to resurrect this genus for a few species that would hitherto have been classified in Diatoma.

Achnanthes oblongella, by contrast, is definitely not the correct name for this organism.  Three other names have been proposed: Karayevia oblongella, Psammothidium oblongella and Platessa oblongella.  The first two are not convincing and I have not yet been able to see the paper describing the third.  It will be interesting to see what a combined morphological and genetic study of this species (or, more likely, complex) reveals.

Reference

Jüttner, I., Williams, D.M., Levkov, Z., Falasco, E., Battegazzore, M., Cantonati, M., Van de Vijver, B., Angele, C. & Ector, L. (2015).  Reinvestigation of the type material for Odontidium hyemale (Roth) Kützing and related species, with description of four new species in the genus Odontidium (Fragilariaceae, Bacillariophyta).  Phytotaxa 234: 1-36.

Wetzel, C.E., Lange-Bertalot, H. & Ector, L. (2017): Type analysis of Achnanthes oblongella Østrup and resurrection of Achnanthes saxonica Krasske (Bacillariophyta). Nova Hedwigia Beiheft (in press).

 

Theme and variations

Following our visit to the cities of the Silk Road (see “Daniel and his den of diatoms …”) in April we turned our eyes in the opposite direction and, within an hour of leaving Tashkent, we had left the flat plains behind and climbing into the foothills of the Tien’shan mountains.   The intensive agriculture of the lowlands gave way to pine forests and, as the road started to twist and turn up the slopes, we started to get tantalising glimpses of the snow-capped mountains which straddle Uzbekistan’s eastern border with Krygyzstan.

As ever, I looked for opportunities to combine business and pleasure, collecting one sample from a small calcareous seepage in the hills near the village of So’qoq and another from a stream running through mixed geology near the village of Kumyshkang, where we were staying in a Soviet-era dacha.   Sampling the seepage drew some curious looks from two women who were collecting water mint from further downstream, and yielded an almost pure growth of a diatom that is either Achnanthidium pyrenaicum or a close relative.   This would have been, by the way, the diatom that I would have expected to find were I to sample a remote, unpolluted calcareous stream in the UK.

Achnanthidium cf pyrenaicum from a calcareous stream near So’qoq in eastern Uzbekistan (41°18’45.6” N 69 ° 51’40” E).  a. – d.: rapheless valves; e. – g.: raphe valves; h.: girdle view.  Scale bar: 10 micrometres (= 1/100th of a millimetre).

Later in the day, we explored a side valley of an unnamed river that flows through the village of Kumyshkang.  The steep landscape on the south side of the valley had a thin cover of scrubby vegetation (in contrast to the wooded slopes on the other side) and the stream tumbled off the hillside towards the river below.  The biofilm, partly as a result of this harsh environment and partly, I suspect, due to grazing by invertebrates, was very thin but, nonetheless, quite diverse, with Achnanthidium minutissimum predominating.  There were a lot of outcrops of pink granite in the hillsides around the stream, but there were other rocks too, including shales and slates.   The flora here, as a So’qoq, would not look out of place in samples I find in the UK although the mix of taxa is not what I would expect if granite was the predominant rock in the catchment.   I travel light, without meters to check the chemical composition of the water, so there is no way to confirm this.  Except by going back one day better prepared …

Diatoms from a stream near Kumyshkang, Uzbekistan (41°18’45.6” N 69 ° 51’40” E, approx. 1400 m above sea level).  .   i.: Ulnaria ulna; j. – l.: Achnanthidium minutissimum; m.: A. cf. pyrenaicum; n., o.: A. cf caledonicum; p.: Achnanthidium girdle view; q.: Navicula tripunctata; r. Navicula sp.; s. Gomphonema gracile; t. Gomphonema sp.; u. Surirella brebissonii var. kutzingii; v. Diatoma moniliformis; w. Nitzschia sp.; x. Planothidium lanceolatum; y. Reimeria sinuata; z.: Encyonema ventricosum; aa.: Encyonopsis sp.   Scale bar 10 micrometres (= 100th of a millimetre).

I should add a caution about names applied to Asian diatoms using identification literature written for European freshwaters, especially after my comments in “Back to the Himalayas …”.   Until the 1980s there was a widespread belief that diatom species were cosmopolitan and could be found all around the world.  This belief became self-fulfilling as, armed with this assumption, biologists set out with books written by and for Europeans and blithely applied the names to the diatoms that they found.  From the 1980s, however, papers started to appear in which people took a closer look at variation in some of these apparently cosmopolitan species and argued that there were, in fact, substantial differences between forms from different locations, and that there were, in fact, much greater numbers of diatoms than previously thought, and that many of these were restricted to particular geographic regions.   But then, in 2002 Bland Finlay and colleagues challenged this emerging view by arguing that it was not diatoms that were restricted in their distributions, it was the locations where these detailed studies had been performed that were rare.   In other words, given enough time and effort on the part of diatomists, we should expect to see these so-called endemic species cropping up in samples from all over the world.

This created a brouhaha within the diatom world which resulted in some further papers that questioned Finlay’s assertions and argued from theoretical grounds that there was no reason why diatoms should not be restricted to a limited geographical area.  As the new century progressed, diatomists added molecular barcoding to their armouries and this offered partial support for both positions: some diatoms – or at least some strains of some diatoms, Nitzschia palea and Gomphonema parvulum, for example – do appear to be genuinely cosmopolitan whilst others do not.  Of course, Finlay and colleagues always hold the trump card in this respect: it is not possible to disprove the existence of any so-called endemic species elsewhere in the biosphere until every conceivable habitat has been examined. But a truce, of sorts, does seem to be emerging.

Sampling the calcareous seepage near So’qoq, April 2017.  The picture at the top of the post shows the valley upstream of Kumyshkang.

The truth may well lie between the two extreme positions.  Maybe many diatoms really are widely distributed because random dispersal mechanisms for microscopic organisms are highly effective, as Finlay and colleagues argue.  But every time a few viable cells of a diatom species land on a suitable habitat, their small pool of genetic variability will either thrive or disappear.   When they thrive, the story of Darwin’s finches will be replayed and a combination of genetic drift and selective pressures will create variations on the original theme, just waiting for an observant biologist to come along and discover the new species.

The question that intrigues me is whether or not the bugs that crawl across the submerged stones in search of food ever notice the difference.   One of my perennial bugbears is that the careful taxonomic work that has resulted in the discovery of all this diversity within diatoms is rarely accompanied by ecological analyses of similar rigour.   In particular, do these different forms of what we once regarded as “cosmopolitan” species actually have any effect on how energy flows through the ecosystem?  Do they, in other words, taste different to the invertebrates that crawl across the stones in search of food?  Or, as Bland Finlay hinted in a subsequent review article, are these different genotypes, in effect, variations on the same basic “ecotype”?   In which case, a casual observer crouching beside a foreign stream may not know the precise name of every species he encounters but still may have a pretty good idea of how these fit into the bigger picture of aquatic diversity.

References

Finlay, B.J. (2002). Global Dispersal of Free-Living Microbial Eukaryote Species.  Science (New York) 296: 1061-1063.

Finlay, B.J. (2004). Protist taxonomy: an ecological perspective.  Philosophical Transactions of the Royal Society Series B 359: 599-610.

Finlay, B.J., Monaghan, E.B. & Maberly, S.C. (2002). Hypothesis: the rate and scale of dispersal of freshwater diatom species is a function of their global abundance. Protist 153: 261-273.

Kemmarec, 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.

Mann, D.G. & Droop, S.J.M. (1996).  Biodiversity, biogeography and conservation of diatoms.  Hydrobiologia 336: 19-32.

Telford, R.J., Vandvik, V. & Birks, H.J.B. (2006). Dispersal limitations matter for microbial morphospecies. Science (New York) 312: 1015.

Trobajo, R., Clavero, E., Chepurnov, V.A., Sabbe, K., Mann, D.G., Ishihara, S. & Cox, E.J. (2009). Morphological, genetic and mating diversity within the widespread bioindicator Nitzschia palea (Bacillariophyceae). Phycologia 48: 443-459

Vyverman, W., Verleyen, E., Sabbe, K., Vanhoutte, K., Sterken, M., Hodgson, D.A., Mann, D.G., Juggins, S., van de Vijver, B., Jones, V., Flower, R., Roberts, D., Chepurnov, V., Kilroy, C., Vanormelingen, P. & de Wever, A. (2002). Historical processes constrain patterns in global diatom diversity. Ecology 88: 1924-1931.

A view of the Tien’shan mountains from near So’qoq, Uzbekistan.