More about Platessa oblongella and Odontidium mesodon

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

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

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

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

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

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

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

Reference

The dataset used for these analyses is that used in:

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

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

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

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

Annex 2: notes on species-environment plots

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

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

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

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

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

 

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Small details in the big picture …

I’ve written about Platessa oblongella, a small diatom common in low alkalinity environments, before (see “A tale of two diatoms …” and links therein) but my travels around west Cumbria are gradually revealing more and more about the ecology of this organism, so bear with me as I explain my latest findings.

My first graph shows how the distribution of this diatom varies in different types of water body in the Ennerdale catchment.   I have analysed 223 samples from this small area over the past few years and, within this dataset, there is a very clear distinction between situations where Platessa oblongella is abundant and situations where it is very rare.   I have very few records from Ennerdale Water itself (present in just two out of 27 samples, and never comprising more than 2.7% of all diatoms in the sample) nor from the River Ehen, which flows out of the lake (present in just 16 out of 164 samples, and always £ 1% of all diatoms).  By contrast, in Croasdale Beck and in streams that flow into the north-west corner of the lake, it is present in 28 out of 32 samples, with a maximum relative abundance of 69%.   In ten samples it forms more than 10% of all diatoms present.   Several of my samples from the small streams were collected from just a few metres above the point where they joined the lake, which makes the distinction between these streams and the lake that much more intriguing.

My theory – based on data I showed in A  tale of two diatoms  is that Platessa oblongella is a species of disturbed habitats and that the littoral zone of a lake, whilst subject to some turbulence, is less disturbed than the rough world of an unregulated stream.  The contrast between the River Ehen immediately below the dam at the outfall of the lake and the various small tributary streams also supports this idea.

Differences in percentage of Platessa oblongella (including P. saxonica) in epilithic samples from Ennerdale Water and associated streams.  Data collected between 2012 and 2018 (along with one sample from River Ehen collected in 1997).   The photograph at the top of the post shows Ennerdale Water, photographed in January 2018.

Some of the populations I looked at seemed to consist of two distinct forms, one broader than the other.   This variability is quite common in Platessa oblongella and Carlos Wetzel and colleagues recently published a paper which suggests that these are, in fact, two distinct species.   When I first started looking at diatoms, John Carter, my mentor, used the name Achnanthes saxonica, but Krammer and Lange-Bertalot, in the revised Süsswassserflora, regarded this as a synonym of Achnanthes oblongella, a species first found in Thailand.   Wetzel’s study shows, as well as the difference in valve width, differences in the fine details of the striae between the two species.   They also decided that both species belonged in the genus Platessa, rather than Achnanthes.

Platessa oblongella (top) and P. saxonica (bottom) from Croasdale Beck, October 2017.  Scale bar: 10 micrometres (= 1/100th of a millimetre).

Valve width is, however, a very useful criterion, as the histograms below show.   The left hand graph shows a distinctly bimodal distribution of widths in specimens from Croasdale Beck, whilst the right hand graph shows a much tighter, and clearly unimodal, range.   This comes from another tributary stream flowing into the Ehen about 500 metres below the lake itself.  Quite why two species can co-exist in one stream but only one is present in another is not clear.

The modes of these populations are very close to the median widths for P. saxonica (narrow, ± 4/5 – 5 mm) and P. oblongella (broader, ± 6.5 mm) respectively but, as the left hand histogram shows, there is some overlap.    You might have trouble, for example, deciding whether a valve that was 5.5 mm wide was a “fat” P. saxonica or a “thin” P. oblongella.   My standard advice in situations such as this is that we should identify populations not individuals although, in the case of Croasdale Beck, this will still leave a grey area between the “fat” and “thin” valves where a judgement call is necessary.   In this case I think I could have done it because the P. saxonica valves in this stream tended to have a greater length:breadth ratio than those of P. oblongella, though I have not actually quantified this.

Width of valves in populations of “Achnanthes oblongella” from a) Croasdale Beck, and b) an unnamed tributary stream of the River Ehen, October 2017. 

There is more to say about the ecology of these species, but I have probably written enough for now.  I will leave you, for now, to bask in the rare sensation that occurs when diatom taxonomists make a situation clearer rather than more opaque, and return to this subject in a future post.

References

Carter, J.R. (1970).   Observations of some British forms of Achnanthes saxonica Krasske.  Microscopy: Journal of the Quekett Microscopical Club 31: 313-316.

Wetzel, C.E., Lange-Bertalot, H. & Ector, L. (2017).  Type analysis of Achnanthes oblongella Østup and resurrection of Achnanthes saxonica Krasske (Bacillariophyta).  Nova Hedwigia, Beiheft 146: 209-227.

Murder on the Barcode Express …

A long time ago, Agatha Christie imagined a train coming to a halt in a snowdrift somewhere in Croatia.  By the morning, one of the passengers was dead.   Eighty years later, a group, only slightly larger than Hercule Poirot’s pool of suspects, gathered in a room in modern Zagreb to plot another fiendish murder.   The victim, this time, would be  …. traditional diatom taxonomy.

“Murder” is far too strong a term for this particular whodunit; maybe I should say “aiding and abetting” rather than actually committing the crime, but I think the outcome might be the same.  The conspirators in Zagreb are all involved in developing methods that use molecular barcoding to identify diatoms and have been busily collecting sequences of the many diatom species in order to establish the libraries that we need to link these barcodes to the appropriate Linnaean binomial.   Some years into this, we still have no more than about 15% of freshwater diatom species matched to barcodes.  We are starting to think about ways of filling in the gaps more quickly than is possible using the conventional approach of isolating a diatom, growing it in culture and then sequencing the appropriate marker genes.

The most radical of these alternatives is to by-pass Linnaean binomials altogether and classify diatoms by their barcodes alone – as “operational taxonomic units” or OTUs.   Most of us have spent most of our careers using morphology-based taxonomy and any move away seems like an act of treachery towards a fundamental tenet of our craft.  But the time has come to take a dispassionate view and ask what a species name brings to ecology.   At a very practical level, the use of Linnaean binomials makes it much easier for us to compare data with colleagues and with records in the literature.    Taxonomists would argue that their work helps us to understand the relationships between species but, unfortunately, in this particular branch of science, we make little use of these relationships, and the role of taxonomy is primarily to give us a consistent means of organising the myriad tiny pieces of silica which we find in our samples.

That business of consistent naming could, in theory, be performed for barcodes just as efficiently using digital tags as OTUs and this would also work for the 85% of species where the link between traditional morphology-based taxonomy and marker genes has not yet been established.   So what about the link that Linnaean binomials give us to established knowledge?   Here, again, we need to be brutally frank: ecological information for most freshwater diatoms is limited to information about preferences for hardness/alkalinity, inorganic nutrients, organic pollution, acidity and salinity and that information can be replicated very easily by linking files of metabarcoding and environmental data.  There are very few experimental studies that offer insights into the ecology of freshwater benthic diatoms beyond that gained from looking for associations between diatom distribution and a few common variables.

The plotters plotting …  DNAqua-net workshop in Zagreb, November 2017.  The top photograph shows Zagreb cathedral against the skyline.

The problem is not that we do not see the merits of traditional Linnaean taxonomy, it is that we cannot make a strong case for the funding necessary to collect barcodes for all species.   The final downward thrust of the dagger will, in other words, be inflicted by the bureaucrats whose budgets will not stretch to cataloguing the enormous breadth of algal diversity.   Diatoms sit in the awkward middle ground between larger organisms such as fish where any suggestion of not using traditional taxonomy would be greeted with derision and the microbial world where the idea of applying Linnaean binomials to the enormous diversity uncovered by molecular techniques is equally risible.   Diatom names mean little to the bureaucrats who manage our environmental agencies and, given the choice between a spreadsheet of incomprehensible Latin names or one of equally incomprehensible OTUs, all else being equal, they will choose the cheapest.

“All else being equal” is the key phrase.   I think that there is growing awareness now that one downside of barcoding is that it risks sidestepping the need for trained biologists at all: samples will be collected by technicians, processed in high-throughput laboratories and results churned out through black box computer programs.   The situation for diatoms is worse than for most groups of organisms used for ecological assessment because so much attention is given to the laboratory stages of producing a list of taxa and relative abundances.  We are, however, now approaching the point when DNA sequencers can produce data of equivalent sensitivity to that produced by light microscopy.   The message that barcoding has the potential to be a good friend but a poor master could be lost as our paymasters recognise the potential for reducing costs.   What we need to do now is use those “little grey cells” to ensure that good biological insight is not the victim of a heinous crime.

As if through a glass darkly …

Life used to be so easy: I stared down my microscope, named the diatoms I could see, counted them and, from these data, made an evaluation of the quality of the ecosystem that I was studying.   Along with the majority of my fellow diatomists, I conveniently ignored the fact that I was looking at dead cell walls rather than living organisms.   My work on molecular barcodes as an alternative to traditional microscopy has been revelatory as I try to reconcile these two types of data.   At one level, what I see down the microscope is a benchmark for what I should expect to see in my barcode output.  Yet, at the same time, the differences between the two types of data show up the limitations of traditional data – and the assumptions that underpin the ways that we work.

Take a look at the plate below which shows two of the most common diatoms in UK rivers: Ulnaria ulna is one of the largest that I encounter regularly whilst Achnanthidium minutissimum is often one of the most abundant in my samples, particularly when the level of human pressure is relatively low.  When we analyse samples with the light microscope, we record individuals, so both of these score “1” in my data book despite the fact that U. ulna is about 100x larger (by volume) than A. minutissimum.

Specimens of Ulnaria ulna (top) and Achnanthidium minutissimum (bottom).  Both are from cultures used for obtaining sequences for the reference library for our molecular barcoding project.   Scale bar: 10 µm.   Photographs: Shinya Sato, Royal Botanic Gardens, Edinburgh.

When we analyse a sample using Next Generation Sequencing (NGS), we count not cell walls but copies of the rbcL gene, which provides the blueprint for Rubisco, a key photosynthetic enzyme.   As I write, there is no clear understanding of how the number of rbcL copies relates to the number of individuals.  We know that each chloroplast within a cell will have at least one copy of this gene, and usually several. There is also some evidence that larger chloroplasts have more copies of the gene than smaller ones and there is also likely to be a measure of environmental control.  The key message that I try to get across in my talks is that NGS data are different to the data we are used to gathering using microscopy.  These differences do not mean that it is wrong, just that we need to leave some of our preconceptions before starting to interpret this new type of data.

However, we could also argue that counting the number of copies of the gene for an important photosynthesis enzyme should be giving us a better insight into the contribution of a species to primary productivity than counting the number of cell walls.  In other words (whisper this …), rbcL might not just be different, it might be better, especially if our purpose is to understand the contribution the various species in the biofilm make to primary productivity in stream ecosystem.  At the moment there are plenty of problems with the NGS-based method, not least the fact that we often cannot assign half the copies of the rbcL gene in a sample to a species, but the situation is improving all the time …

Some recent work pushes this a little further.   Jodi Young and colleagues at Princeton University have demonstrated large variation in the kinetics of Rubisco in diatoms, and in their carbon-concentrating mechanisms (see “Concentrating on carbon …” for more about these).  Although their work is focussed on marine phytoplankton, the variation within Rubisco and carbonic anhydrases could go some way to explaining the sensitivity of diatoms to inorganic carbon (see “Ecology in the Hard Rock Café …”).   In other words, rbcL is not an irrelevant DNA sequence, as the term “barcode” may imply (in contrast to barcodes based on the ITS region, for example), it is deeply implicated in the reasons why a species lives in particular place.

And yet, and yet, and yet …  The same could be argued for morphology, up to a point at least.   The shape of a Gomphonema or a Navicula also helps us to understand the organism’s relationship with its environment.   The problem is that modern taxonomists tend to focus on a much finer level of detail – on the arrangement and structure of the various pores on the silica frustule, for example – and offer few insights into what these minute differences mean in terms of the ecophysiology of the organisms.  Even at the whole-cell scale, information on habit, which is linked to form (Gomphonema tending to live on stalks or short mucilage pads secreted from their foot poles for at least part of their life-cycle, for example) is rarely incorporated into assessment systems.   The move from using light microscopy to using NGS, in other words, means replacing an imperfect system with which we are familiar with one that we are still learning to understand.  Both offer unique information and the gains from using one approach rather than the other, will be offset by losses of insight.

That leaves us with two big challenges over the couple of years, as UK diatom-based assessments move from light microscopy to NGS.  The first is to work harder to understand what NGS outputs are actually telling us about the environment over and above the minimalist ecological status indices that spew out of our “black box” computer programs.   The second is to maintain an understanding of the properties of whole organisms and how these interact with one another and with their environments.   I guess I should add a third challenge to this pair: persuading middle managers who have at best a sketchy understanding of diatoms and phytobenthos and already-stretched budgets that any of this matters …

References

Badger, M.R. & Price, G.D. (2003).  The role of carbonic anhydrase in photosynthesis.  Annual Review of Plant Biology 45: 369-392.

Young, J.N. & Hopkinson, B.M.M. (2017).  The potential for co-evolution of CO2-concentrating mechanisms and Rubisco in diatoms.  Journal of Experimental Botany doi: 10.1093/jxb/erx130.

Young, J.N., Heureux, A.M.C., Sharwood, R.E., Rickaby, R.E.M., Morel, F.M.M. & Whitney, S.M. (2016).  Large variations in the Rubisco kinetics of diatoms reveals diversity among their carbon-concentrating mechanisms.  Journal of Experimental Botany 67: 3445-3456.

What’s a pretty diatom like you doing in a place like this?

Whilst looking at some samples from an experiment conducted on mesocosms beside a chalk stream, Candover Brook in Hampshire for Mark Ledger and colleagues, I came across a diatom that I had not seen before and which, at first glance, was out of place.   As the images above show, it is a diatom whose cells join together to form chains which, in turn, means that they typically present their sides to the viewer rather than the valve face, which is the way that the writers of identification guides generally assume that we can see. It took some time to track down a couple of cells that were lying face-upwards so that I could try to name the species and some of the few that were lying this way were damaged (see left hand image), perhaps itself a consequence of the naturally strong links between the cells.

Naming the genus was relatively straightforward: the valve shape, fine striae and very narrow axial area (the gap along the median line of the valve face between the two rows of striae) coupled with the tendency to form chains all pointed to Fragilariforma.   However, most of the Fragilariforma that  I encounter are in soft water, often acid habitats whilst this sample was from a flume beside a chalk stream in southern England.   After scratching my head a little more, and sending images to my friend Lydia in Germany, I eventually decided that Fragilariforma nitzschioides was the most likely name for this diatom.  Searching through my records, I found only one other record for this species: from the River Itchen (into which Candover Brook drains) in the mid-1990s.  That must be more than coincidence.   Interestingly, Hoffman et al. (2011) describe the species as “rare” and say that its ecological preferences are “difficult to define”.

The limited records that we have show that this species does not behave in the same way as most other representatives of the genus.   The weighted average of pH for the genus is 6.6 (see graph below), but there are plenty of records extending into more acid waters.  By contrast, the River Itchen population was recorded at pH 8.1 and the pH in Candover Brook will be very similar.   Most of the records for the genus came from relatively soft water, in contrast to the very hard water found in a chalk stream.  The scarcity of records of a species that is well described in the literature also suggests that this might be a genuinely rare diatom (see “A “red list” of endangered British diatoms”).

One other peculiarity of this species is the name itself.   Fragilariforma was one of a number of genera split away from Fragilaria by Dave Williams and Frank Round in 1986, originally as “Neofragilaria”.  Fragilaria nitzschoides, was not formally transferred at the time, presumably because the authors did not have access to the type material.  They presented good evidence for this new genus but a few people – notably Horst Lange-Bertalot – have continued to group these species under Fragilaria.   This is the situation in Diatomeen im Süsswasser-Benthos von Mitteleuropa but, curiously, for Fragilaria nitzschoides, he created the new combination of “Fragilariforma nitzschoides” purely as a synonym (see p. 268).   The good news is that the next version of this book (see “Tales of Hoffman”) does use these new names.

The relationship between Fragilariforma spp and pH (left) and alkalinity (right) in UK rivers, based on the mid-1990s dataset described in “The challenging ecology of a freshwater diatom”.  Vertical lines show the boundaries for high (blue), good (green), moderate (orange) and poor (red) status classes based on current UK standards and the arrows show the location of the River Itchen population of Fragilariforma nitzschoides along these gradients. 

References

Hofmann, G., Werum, M. & Lange-Bertalot, H. (2011).   Diatomeen im Süßwasser-Benthos von Mitteleuropa. A.R.G. Gantner Verlag K.G., Rugell.

Williams, D.M. & Round, F.R. (1987).  Revision of the genus Fragilaria.  Diatom Research 2: 267-288.

Williams, D.M. & Round, F.R. (1988).  Fragilariforma nom nov., a new generic name for Neofragilaria Williams & Round.  Diatom Research 3: 265-267.

The way things were …

Writing the previous post led me to contemplate how much things had changed over the time that I have been working in this field.  Back in the early 1990s when I first set out to look at the response of diatoms to nutrients in streams, few in the National Rivers Authority (NRA, predecessor to the Environment Agency) regarded phosphorus as a serious pollutant in rivers, and most biologists thought about ecological quality solely in terms of organic pollution and invertebrates.   In order to investigate the effect of nutrients, I wanted to visit sites where organic pollution was not a problem.

I was helped in this task by the work done by biologists at the then Institute for Freshwater Ecology (now Centre for Ecology and Hydrology) who had just developed the early versions of RIVPACS (“River Invertebrate Prediction and Classification System”) which established the principle of expressing ecological quality as the observed quality / expected quality.  This, in turn, required an ability to predict the “expected” condition for any stream.   The work that had developed these equations started from a dataset of invertebrate and environmental data collected from a wide range of “unpolluted” running water sites which, in those far off days, was compiled by asking biologists working for the Regional Water Authorities (predecessors to the NRA) for their recommendations of sites that were of “good” or “fairly good” quality.  Nowadays, screening sites to be used for calibrating ecological methods is a much more rigorous procedure but this was the first tentative step on a long journey and “expert judgement” was as good a place to start as any.

The paper that emerged from this exercise (see reference below) analysed data from these “unpolluted” sites and classified them into eight groups.  Each of these groups consisted of sites that shared similar invertebrate assemblages which reflected similarities in the habitat, from upland, fast flowing becks to deep, wide slow-flowing rivers in the lowlands.  The authors included a useful table that listed the physical and chemical characteristics of each of these groups and I noticed that the phosphorus concentrations reported for these spanned a very wide range.   This meant that I could use these as the basis for putting together a sampling program that spanned a long gradient of nutrient pressure without the complications of organic pollution.   The outcome of that work was the first of the two papers referenced in my previous post.

Time has moved on and I thought it would be interesting to revisit these “unpolluted” sites to see how they would be classified using the UK’s current standards for phosphorus.  This highlights a striking difference between the prevailing idea of “unpolluted” in the early 1980s and the present day, as all of these groups had average concentrations that equate to substantial enrichment by modern standards; in half the groups this average concentration would be classified as “poor status” whilst the maximum concentrations in three groups equates to “bad status”.   Whatever way you look at it now, these sites were far from “unpolluted”.

Classification of TWINSPAN end-groups of unpolluted river sites in Great Britain based on Armitage et al. (1984) along with average and maximum phosphorus concentrations recorded in each group and the phosphorus status based on current environmental standards.  M = moderate status; P = poor status; B = bad status.

I am not being critical of the approach taken by Patrick Armitage and colleagues.  In many ways, I regard the work of this group as one of the most significant contributions to the science of ecological assessment in my lifetime.   I am just intrigued to see how the thinking of ecologists and regulators has moved on in the thirty years or so since this paper was published.  I know from my own early conversations with NRA biologists that inorganic nutrients were not perceived as a problem in rivers until the early 1990s.   It was probably the European Community’s Urban Wastewater Treatment Directive (UWWTD) that started to draw the attention of biologists in the UK to these problems, and which led to the development of stricter environmental standards for nutrients, though not without opposition from several quarters.

This, then is a situation where good legislation provided the impetus needed to start the process.  There were places in the UK – rivers in the Norfolk Broads, for example – where nutrients were already being regulated, but these were special circumstances and nutrient problems in most rivers were largely ignored. Indeed, as I said in my previous post, phosphorus was not even measured routinely in many rivers.   I heard via my professional grapevine that it was the Netherlands who had made the case for the clauses in the UWWTD concerning regulating nutrients, as their stretches of the lower Rhine were subject to numerous problems caused by unregulated inputs of nutrients from countries upstream.   I do not know if this is true, but it is certainly plausible.   However, once the need to control eutrophication in rivers was codified in UK law, then the debate about how to evaluate it started, one of the outcomes of which was more funding for me to develop the Trophic Diatom Index (referenced in the previous post).  And, gradually, over time, concentrations in rivers really did start to fall (see “The state of things, part 2”).   I’d like to think the TDI played a small part in this; though this might also mean that I am partially responsible for the steep increase in water charges that everyone endured in order to pay for better water quality …

Reference

Armitage, P.D., Moss, D., Wright, J.F. & Furse, M.T. (1984).  The performance of a new biological water quality score system based on macroinvertebrates over a wide range of unpolluted running-water sites.  Water Research 17: 333-347.