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.

How green is my party?

It seems like only a short time ago that I was considering the manifestos for the 2015 General Election (see “The political landscape isn’t very green …”).   And it seems like a lifetime ago, such are the potential ramifications of the Brexit vote (see “What has the EU ever done for us?”).   So what are the main parties saying about the environment in their manifestos this time around?

First impressions are important and, looking at the contents pages, I see that neither of the two largest parties regard the environment as sufficiently important to merit a chapter to itself.  There are crumbs of comfort tucked away in the text of both: both, for example, commit to improve natural flood management and to take a lead in global action against climate change.  Labour go a little further than the Tories with an explicit commitment to replace the Great Repeal Bill with an EU Rights and Protections Bill which should guarantee environmental standards.   Both manifestos, however, make some bizarre claims: the Tories will create a “Shale Environmental Regulator” when the best way to manage shale gas extraction (if this were to be allowed) would be to strengthen the current regulator’s powers.  Labour’s plan to take utilities (including water) back into public ownership also makes little sense from an environmental perspective.  Indeed, by making utility bills into a political issue, it may even become harder for regulators to argue for price rises to pay for the capital investment necessary to improve water and environmental quality.  Neither manifesto addresses the important questions raised by Caroline Lucas and others (see “(In)Competent Authority?”) about who will watch the watchmen once the oversight of the European Court of Justice is removed.

What of the other parties? UKIP come straight out with a commitment to repeal the Water Framework Directive, claiming that it led to serious flooding in many parts of the country by preventing river dredging.  Curiously, given that this statement is tenuous, to say the least, their next sentence promises that UKIP will promote “evidence-based environmental schemes”.   Given the current state of Environment Agency finances, I hope that this claim is supported by plenty of cash, otherwise the evidence needed to promote strong environmental regulation will not be available.  Or maybe that’s the point?   Flick to the end of their manifesto and look at the fiscal plan: no explicit mention of the environment and only a meagre sum allocated to “provision for other policy items”.   I think we can move on …

The other two national parties give the environment more prominence.  The Liberal Democrats devote a whole chapter to “Keep our Country Green” and maintaining EU environmental standards is part of their case for fighting a hard Brexit.   Like the Tories and Labour, they commit to encouraging natural flood management; unlike the larger parties, they put a cash sum (£2 billion) against this pledge in the body of the manifesto.   Apart from this, the Lib Dems make few specific claims for how the aquatic environment will be managed; however, there is a much wider range of green measures in their manifesto than either of the two main parties can manage.

The Green Party also follow the trend for natural flood management and promise to put environmental protection at the heart of any future trade deals.  However, there is less detail in this manifesto than there was in their 2015 manifesto and, in particular, only a single reference to farming.   Even the most ardent EU supporters find it hard to mourn the end of the Common Agricultural Policy, and the Greens are not the only ones who realise that there is an opportunity here to build a greater emphasis on countryside stewardship into the system that replaces the CAP.

Despite this, the Green Party have the most visionary outlook of all the major political parties.  Labour and the Conservatives are still wedded to GDP growth, with the environment as a subsidiary issue but the Greens have a more holistic world view that challenges traditional economics.   I have just read Kate Raworth’s Doughnut Economics (Random House, 2017), which explores the possibility of alternative, more inclusive, healthier approaches to economic management.  The Greens get this, pointing out that “consumer capitalism is the problem, not the solution.”   That, however, is their greatest weakness as well as their strength: electoral success for them will depend upon those consumer capitalists being satisfied with less despite the larger parties seducing them with offers of greater prosperity.

We should probably look beyond specific references to the environment in order to divine the prospects for our water and countryside.   The Brexit negotiations will be key yet the manifestos can only capture the ambition of one half of the negotiation process.   What exactly does the Tory desire for a “deep and special partnership with the EU, which will allow free trade between the UK and the EU’s member states” mean?   It could mean a willingness to accept EU employment and environmental standards to ensure a level-playing field for trade.  But Theresa May has to keep a wary eye on the belligerent right-wing of the Tory party who will see such standards as “red tape” that should be sacrificed rather than compromise with the hateful Brussels bureaucracy.

Overall, I think that Theresa May and the Tories will bring too much ideological baggage to the negotiating table to allow a truly constructive partnership with the EU to emerge.   The narrow vote in last June’s referendum would justify a “soft” Brexit, for which the other parties will be better placed to negotiate.   The Lib Dems and the Greens both promise a second referendum to confirm the terms of UK’s leaving, which is a reasonable aspiration with the sole drawback that the electorate are already jaded and unlikely to have any enthusiasm to be dragged to the polling stations yet again.   Overall, given the lacklustre approach to the environment from both the main parties, perhaps our best hope on June 8 is another coalition?   If that includes the Green Party then maybe an awareness of the importance of sustainability and the environment will start to be embedded throughout Government policy instead of being quietly relegated to the margins.

Escape to the Howgills

Driving from my home in Durham towards the south eastern side of the Lake District or to Lancaster leads me across the A66 before I turn off and descend through the Eden Valley and Kirkby Stephen before entering the Lune valley where I join the M6, which follows the course of the Lune through the narrow gap between the high hills of the Howgills and the Winfell Ridge.  It is one of the most spectacular stretches of motorway in the country and I yearn for occasions when I do not have to rush past these hills in pursuit of deadlines.   Those chances do not come very often and, when they do, the weather is not always conducive to walking at high altitude.  However, last Friday, the gods smiled on me: the weather was perfect and I had nothing to pull me back across the Pennines and every excuse to linger.   I pulled off the M6, followed the A684 into Sedbergh and, just 15 minutes after I left the motorway, I was locking my car and following a footpath onto the fells.

There is something about the geology of the Howgills that sets them apart from the hills around them: the Lake District peaks have hard, jagged outlines whilst the Pennines reflect the tilted beds of Carboniferous limestone and sandstone.   The Howgills, however, have soft, convex outlines.  They are of Silurian sandstone though why this should give them such a different topography to the surrounding areas, I do not know.  From a distance they resemble a herd of recumbent cattle and it is no surprise that the highest peak – to which I was heading – was The Calf.

The convex form of these hills means that the first part of the walk is hard work and I had to pause at intervals to look down on s to look back at the small market town of Sedbergh below me and, beyond, the westernmost extremities of the Yorkshire Dales.   As I gained altitude, however, the slope gradually lessened and I was soon on an undulating, but gradually rising, grassy ridge heading north with just a few sheep for company.   The closely-cropped springy turf made for comfortable walking but the absence of wild flowers amidst the grass reminded me of George Monbiot’s phrase “sheep-wrecked”.   Apart from these sheep, I had the fells almost to myself, passing just half a dozen other walkers in three hours.

The summit of the calf is marked by a triangulation point, which offered the culmination of a series of outstanding views.  To the south west, I could see the northern end of Morcambe Bay glistening in the late afternoon sunlight.  Letting my eyes move northward from here, I could see the peaks of the Lake District laid out before me: Old Man of Coniston, Scafell Pike and Great Gable, Helvellyn and, in the far distance, Blencathra.  Then, continuing my panorama across the Eden Valley, I saw the sharp outline of the Pennines with, just discernible, the crenellations that marked Cross Fell, Great Dun Fell and Little Dun Fell.   Far below, I could just see the M6 snaking through the valley below, where drivers, no doubt, were gazing wistfully up at the hills just as I had done so often in the past.

Eventually, I tore myself away from the top of the Calf and followed the path back towards Sedbergh.  It was early evening as I rounded Winder, the first (or final, depending on your direction) undulation on the ridge.   Below me, I could make out activity on the fields of Sedbergh School, and could hear the distant cheers of spectators to what may have been a tug-of-war contest.   The summit of Winder is, I have been told, the turning point for the school’s cross-country run; it is a school with a ferocious reputation for sport, as I could hear.   Ironically, the town’s other claim to fame is its association with the foundation of the Quakers, the religious group whose views mostly closely align with my own.  Their founder, George Fox, preached both in the churchyard of St Andrew’s church below me, and on the nearby Firbank Fell, and the meeting house at Brigflatts, just outside the town, is the second oldest in the country.

The 12 kilometre loop took me about three hours and I was sitting in lengthening shadows outside the local fish and chip shop (the “Haddock Paddock”) sipping shandy from a can and enjoying the last of the afternoon’s sun.   And then it was back into the car for the drive across to the Eden Valley and finally onto the A66 to cross the Pennines.   It’s a tough commute.   But you shouldn’t feel too sorry for me…

What does it all mean?

Just over a quarter of a century ago, my friend and colleague Steve Juggins and a group of other palaeoecologists came up with a clever way to relate the composition of diatom samples taken from different levels of a sediment core to the environmental conditions of the lake at the time that these diatoms were alive.   At the heart of this was a set of statistical tools called “transfer functions” and the use of these has proliferated over subsequent years, spilling from diatoms to many other groups of organisms and from palaeoecological studies to contemporary investigations of man’s impact on the environment.   So pervasive have these methods become that Steve returned to the subject a few years ago and critiqued the many misuses of the method that he was seeing in the literature.

The principle behind the use of transfer functions is that each species has a characteristic response to an environmental pressure gradient (in early studies this was pH) which could be portrayed as a unimodal (approximately bell-shaped curve).   The point along the gradient where a species is most abundant represents the “optimum” condition, the level of the pressure where the species thrives best.  The average of the optima of all organisms in a sample, Steve and colleagues showed, could be then used to estimate the value of the pressure.   This unlocked the door to quantitative reconstructions of changes in acidification of lakes in the UK and Scandinavia that, in turn, ultimately shaped environmental policy. It was one of the most impressive achievements of applied ecologists in the 20th century.

A diagrammatic representation of the principle behind transfer functions: each organism has a characteristic response to the predominant pressure (nutrient/organic pollution in this case).

Part of the reason for their success in building strong predictive models was, I suspect, that the pollutant that they were focussed upon had a direct effect on the physiology of the cells which, in turn, created strong selective pressures on the community.   Another reason was that palaeoecological samples condense all the habitat variation within a lake (plankton v benthic, seasonal differences etc) into a single assemblage.   This, then, begs the question of how well we should expect transfer functions to perform when applied to assemblages which represent much narrower windows of space and time, and when the pollutants of interest exert indirect rather than direct effects on the organisms.   Or, to recast that question another way, are some of the problems we encounter interpreting diatom indices from rivers another form of the misuse of transfer functions that Steve dissects in his review?

It is easy to believe that transfer functions do work when applied to contemporary diatom assemblages from rivers.   If you evaluate datasets you will almost certainly find that the “optima” for all the species do appear to be arranged along a continuum along the pressure gradient.  The question that we need to ask is whether this represents a causal relationship or is just a statistical artefact?  I touched on this issue in “What we expect is often what we get …” but, in that post, I was mostly interested in how samples react along a gradient, not the response of individual species.  I suspect that, given the importance of alkalinity in freshwater algal ecology (see “Ecology in the Hard Rock Café”), this must influence the distribution of optima along a nutrient gradient.   This will be compounded when sample sizes are small, as the likelihood is that the sample optimum will not correspond exactly to the “true” optimum for the species in question (a question Steve has also addressed in a more recent paper – see reference list below).  Finally, this is all embedded within a larger problem: that most of the work I have discussed here involves statistical inference from datasets compiled from samples collected from a range of sites in a region, but is intended to address changes in time rather than space (so-called “space-for-time substitution – see reference by Pickett below).   There has been relatively little testing of species preferences under controlled experimental conditions.

In practice, I suspect, the physiological response of benthic algae to nutrients is less complicated than our noisy graphs suggest.   I set out a version of this in “What we expect is often what we get …”.   That post dealt primarily with communities of microalgae; this is the same basic scheme (with some slight revisions) but posed in terms of the physiological response of the organisms.  It borrows from the habitat matrix conceptual model of Barry Biggs, Jan Stevenson and Rex Lowe (which, itself, builds on earlier work on terrestrial plants by Phil Grime and colleagues).

An alternative explanation for the response of benthic algae to nutrients and organic pollution.  a., b., c. and d. are explained in the text.

  1. Low nutrients / high oxygen concentrations – the “natural state” in most cases. Biggs et al. referred to species adapted to such conditions “stress-adapted” as they can cope in situations where nutrients are scarce. Associated with TDI scores 1 and 2.  Examples: Hannaea arcus, Achnanthidium minutissimum, Tabellaria flocculosa.
  2. high nutrients / no “secondary effects” of eutrophication – these are “competitive” species in Biggs et al.’s template and can thrive when there is anthropogenic enrichment of nutrients. Ideally, this group would consist of species that have a physiological adaptation that allows them to thrive when nutrients are plentiful though, in practice, our understanding is based mostly on inference from spatial patterns. The “window” where such species can thrive is wide, and will overlap with the two states described below, in many cases.  Associated with TDI scores 3 and 4.  Examples: Amphora pediculus, Rhoicosphenia abbreviata, Cocconeis pediculus.  Cladophora glomerata would be a good example of a non-diatom that belongs to this group.
  3. high nutrients plus “secondary effects” of eutrophication – this category extends the habitat template of Biggs et al. to include organisms whose are reacting to secondary effects  of nutrient enrichment (e.g. shade and low oxygen) rather than to the elevated nutrients per se and is, consequently, difficult to differentiate from a direct response to organic pollution. Associated with TDI scores 4 and 5. Examples include several species of Nitzschia as well as Mayamaea and Fistulifera, amongst others.   Importantly, this group may co-exist with representatives from group b. – perhaps inhabiting different zones of the biofilm that typically blend together when a sample is taken.
  4. high nutrients / very low oxygen – a final category that represents extreme situations when an ability to cope with reducing conditions is beneficial, and where diatoms that are facultative heterotrophs may thrive. Associated with TDI score 5. Heterotrophic fungal and bacterial growths (“sewage fungus”) may also be abundant.  Once again, there is likely to be some overlap between this and other groups.   Technically, this group is more likely to be associated with serious organic pollution than with nutrients; however, it will be found at sites where nutrient concentrations are high and it is possible that an association with nutrients may be inferred from spatial patterns.

We are left, in other words, with a choice between deriving optima along a continuous scale based on inferences from spatial patterns within which we know that there are significant confounding variables or dividing species into a few physiologically-defined categories for which there is not very much experimental underpinning.   Neither is ideal, and some of our recent analyses suggest that, in terms of model strength, there is little to choose between them.   The former, in my view, suggests an artificially high level of precision that is unrealistic, given the current state of knowledge.   The latter, on the other hand, links the data to a conceptual model rather than simply relying upon the numbers that squirt out at the far end of a statistical process.

That does not mean that such an approach might not be appropriate for some other groups of organisms.  The reason why I urge simplicity for diatoms is largely because of the scale of the habitats that we are sampling, in relation to the wider patterns of variability.  A continuous series of optima may be appropriate in some cases too.   Macrophytes surveys, for example, encompass all visible organisms found along a 100 m stretch.   These will have a range of life history and nutrient acquisition strategies: some of these will take up nutrients from the water, some from the sediments.  Different types of sediment will vary in the supply of phosphorus and nitrogen, and so on.   There will still be issues of confounding variables and risks of inferring from correlative rather than causal relationships, but perhaps the overall patchiness experienced over the survey length will create a more complex web of interactions between nutrients and community that justifies a continuous scale.

For diatoms, however, simplicity is probably the best choice.   In the absence of definitive evidence one way or the other we apply Occam’s Razor (“entities should not be multiplied unnecessarily”) and opt for the simpler of the two hypotheses pending evidence to the contrary.   This, in turn, may address a deeper issue: that of finding robust answers to complex problems (see “Unravelling causal thickets …”).   Inference from statistical models is only as good as the conceptual models that underpin those models and, I fear, we too often are so lost in the detail of the many confounding variables that we lose sight of our goals.  Being able to understand our observations in terms of ecological process is the first step to finding robust solutions to our problems.

References

Bennion, H., Juggins, S. & Anderson, N.J. (1996).  Predicting epilimnetic phosphorus concentrations using an improved diatom-based transfer function and its application to lake eutrophication management. Environmental Science & Technology 30: 2004-2007.

Biggs, B.J.F., Stevenson, R.J. & Lowe, R.L. (1991). A habitat matrix conceptual model for stream periphyton. Archiv für Hydrobiologie 143: 21-56.

Birks, H.J.B.,  Line, J.M., Juggins, S., Stevenson, A.C. & ter Braak, C.J.F.  (1990). Lake surface-water chemistry reconstructions from palaeolimnological data. Diatoms and pH reconstruction. Philosophical Transactions of the Royal Society of London Series B 327: 263-278.

Juggins, S. (2013).  Quantitative reconstructions in palaeolimnology: new paradigm or sick science?  Quaternary Science Reviews 64: 20-32.

Kelly, M.G., King, L. & Ní Chatháin, B. (2009).  The conceptual basis of ecological status assessments using diatoms.  Biology and Environment: Proceedings of the Royal Irish Academy 109B: 175-189.

Pickett, S.T.A. (1988).  Space-for-time substitution as an alternative to long-term studies.  Pp. 110-135.   In: Long-term Studies in Ecology: Approaches and Alternatives (edited by G.E.. Likens).  Springer-Verlag, New York.

Reavie, E.D. & Juggins, S. (2011).  Exploration of sample size and diatom-based indicator performance in three North American phosphorus training sets.  Aquatic Ecology 45: 529-538.

Back to the Himalayas …

It is always nice to tie up loose ends left in earlier posts, so I was pleased to find a recent paper that put a name on a diatom that I had illustrated, but not been able to name, during my examination of material from a high altitude lake in Ladakh (see “Diatoms from Pangong Tso”).   I had assumed that this was a species of Gomphonema; however, Pat Kociolek and colleagues have placed it in a completely new genus, Gomphosinica.

Following their paper, the diatom that was abundant in the littoral of Pangong Tso is most likely Gomphosinica lacustris and this would be the first record of the genus in India.  The type location for this species is Kalakule Lake in the Kunlum Mountains of Xianjiang Province, northwest China, some 800 km north of Ladakh, and on the other side of the Tibetan Plateau.   They describe their sample as “planktonic in the lake”, whereas the populations I described formed distinct growths in the littoral zone (see “Return to Pangong Tso”).  They also have recorded it from Sichuan province, in southwest China.   Pangong Tso actually marks the Indian-Chinese border, so it should not be a great surprise to have found it here.

Altogether, Pat Kociolek and colleagues found three new species of Gomphosinica in China, and transferred a previously-described species of Gomphonema found in Nepal to the genus.  However, they also found four species in Montana, in the USA, and made one further transfer of a Gomphoneis first described from the Great Lakes.  Bear in mind, too, that Gomphosinica species are distinctive, so it is unlikely that the absence of Gomphosinica in regions other than China and the USA is an oversight on the part of diatomists.  There is clearly more to learn about the biogeography of this genus.

Having said that Gomphosinica is distinctive, it is hard to say exactly how it differs from Gomphonema based on what we can see with the light microscope alone.  The distinctive features can only be seen with scanning electron microscope, and it would be interesting to get some molecular barcodes from members of this genus to see how these compare with those from Gomphonema and relatives.  This might also shed some light on the differences between the North American and Asian species.

The same journal part also contained a paper on diatoms from the Doon Valley, near Dehra Dun in Uttarakhand, which may shed some light on the diatoms that I found nearby in the Ganges at Rishikesh (see “Diatoms from a holy river”).   I named these using the identification literature that I had to hand (mostly from Europe) and included “Gomphonema pumilum” in my list.  This new paper suggests that there may be local species which look very similar, including G. juettnerii and G. doonensis.   My population does not fit the dimensions of either of these exactly, and my inclination would still be that at least the larger of the two specimens I illustrated is G. pumilum, but there is enough in this paper to remind me that trusting a European flora when studying the diatoms of Asia is dangerous.   Whether these diatoms actually fill different niches in their respective ecosystems, or whether they are just genetically-distinct forms of what is, basically, food for relatively unfussy invertebrate larvae on both continents is a question for another day.

Note: the photograph at the top of the post is an early-evening view of a river in the Outer Himalaya Zone in the vicinity of Dehra Dun.

Reference

Karthick, B., Nautiyal, R., Kociolek, J.P. & Ramachandra, T.V. (2015).  Two new species of Gomphonema (Bacillariophyceae) from Doon Valley, Uttarakhand, India.  Nova Hedwigia, Beiheft 144: 166-174.

Kociolek, J.P., You, Q-M., Wang, Q-X. and Liu, Q. (2015).  A consideration of some interesting freshwater gomphonemoid diatoms from North America and China, and the description of Gomphosinica gen. nov..  Nova Hedwigia, Beiheft 144: 175-198.