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).

 

Finding the balance …

Gammarus fossarum (Scale bar: 1 millimetre).  Photograph: Drew Constable.

Back in March I wrote about the challenges facing those who planned to implement Next Generation Sequencing (NGS) methods for ecological assessment (see “Ecology’s Brave New World”).  In that post I argued that the success (or otherwise) of using DNA for ecological assessment was as much down to the structure and values of the organisation implementing the method as to the method itself.   More particularly, there were likely to be problems if the process was viewed primarily as a means of gathering ecological data rather than for enhancing the capabilities of ecologists.

This is an important distinction.  Much of an ecologist’s time is spent collecting the basic data, whether in the field or laboratory, from which the condition of a particular habitat can be inferred.   But, with traditional methods, there was always a possibility that this basic data collection could be supplemented by observations and insights made by the ecologist that would inform their judgements.  These people have also added to our knowledge of the UK’s biodiversity over the years (see “A new diatom record from Sussex” for an example).   My fear is that adoption of NGS approaches in order to reduce costs will limit the potential for ecologists to make these serendipitous additions to our understanding of a habitat.

A recent paper by Rosie Blackman and colleagues from the University of Hull and Environment Agency offers a good example of how traditional and DNA-based methods can be complementary.  Rosie had looked at invertebrate assemblages in rivers in England using both approaches and discovered that some of the DNA in her samples came from a species, Gammarus fossarum, not previously recorded in the UK.  Other representatives of this genus of small crustaceans, including the extremely common G. pulex, had been abundant in her samples.  Now, however, going back to her sites with the knowledge that G. fossarum might also be present, she was on the lookout for the subtle differences in morphology that separated G. fossarum from other Gammarus species.  She found it in large numbers at 23 out of 28 sites, spread around the country, and in historical material stored at the Natural History Museum dating back to 1964, suggesting that it has been overlooked by those identifying it by traditional means.

This is a great example of biologists working in the sweet zone where traditional and molecular methods combine to give us new insights that are greater than the sum of their parts.   The shortcomings of traditional morphology-based taxonomy in the past are clear but, at the same time, this was essential for verification step once the presence of Gammarus fossarum had been detected by molecular approaches.   The obvious conclusion is that regulatory organisations should move into the future using both traditional and molecular methods in a complementary manner.   Yet, if you look at that statement from another perspective, I have just advocated increasing the cost of ecological assessment at a time when budgets for such assessments are under extreme pressure.

The likelihood is that, as molecular methods are developed (and if they are shown to be substantially cheaper), traditional approaches to ecological assessment will be dropped.  That would not be a problem were it not that the hours spent in the field and laboratory are an important pathway for graduate ecologists to deepen their understanding of organisms and habitats.   Shifting wholesale to molecular methods without retaining at least some infrastructure for traditional methods will mean first, that future discoveries such as Rosie’s will be harder to validate and, second, that the next generation of ecologists will first encounter these organisms not in a pond net but on a spreadsheet.  That link between a name and an organism with distinctive qualities, and between that organism and particular habitats or conditions, will be lost.

Equally, it is unrealistic to assume that complementary use of both approaches will be the norm.   That will place yet more pressure on already tight budgets and could only happen if everyone was happy to accept that monitoring networks could be much smaller (see “Primed for the unexpected?”).  So how do we retain this “sweet zone” between old and new?   I have not yet heard a satisfactory answer to that question so perhaps we should return to the point I made earlier about the structure and values of the organisations that take on these new methods.  Broadly speaking, the adoption of these methods purely to save money is likely to be the road to perdition, because these savings will look most impressive to the senior levels of management (who are probably not biologists) only if there is a wholesale move to the new methods with no retention of traditional infrastructure.

The tragedy is that, within a decade, molecular technology may have moved on to such an extent that it is possible for a biologist to detect invasive species and make other assessments in real time, rather than having to send samples off to remote high-throughput laboratories in order to maximise economies of scale.  Instruments such as Oxford Nanopore’s Minion are still not the finished article from the point of view of ecological end-users, but it is only a matter of time.   Unfortunately, in the here and now, the infrastructure that generates ecological data is already being dismantled in order to squeeze cost-savings from the shift to NGS.   Whether there will be anyone left to inherit this Brave New World is, I am afraid, open to debate.

Two examples of Oxford Nanopore’s Minion portable DNA analysis systems, which can be plugged into the USB port of a laptop.

Reference

Blackman, R.C., Constable, D., Hahn, C., Sheard, A.M., Durkota, J., Hänfling, B. & Lawson Handley, L. (2017).  Detection of a new non-native freshwater species by DNA metabarcoding of environmental samples – first record of Gammarus fossarum in the UK.  Aquatic Invasions 12 (in press)

The exception that proves the rule …

If you are going to understand river ecology, you need to be able to consider landscapes at several different scales simultaneously.   In the River Ehen, this means looking upstream towards Ennerdale Water and, beyond, to Great Gable and the other Lake District peaks in order to appreciate the geology that gives the catchment its bones.  But, at the same time, you need to look around at the meanders of the river and the bankside vegetation that create the immediate habitat for the organisms, and then to look even more closely at the individual stones that line the river bed.

Peering into the water last week, the pebbles, cobbles and boulders that make up the substratum of the River Ehen looked bare of filamentous algae for the most part.  There were a few clumps but, at this time of year, when grazing invertebrates are active, the algal flora is reduced to a thin film, invisible to the naked eye and apparent only as a slimy sensation when you run your fingers across the stone’s surface.   However, when I picked up a couple of cobbles, I noticed small, pale green gelatinous growths stuck on the upper surface.   Most were just a few millimetres across with the largest up to about a centimetre.

A growth of Draparnaldia glomerata on the upper surface of a cobble in the River Ehen, Cumbria, April 2017.

These growths are composed of the green alga Draparnaldia glomerata.  I have written about this alga before (see “The River Ehen in February”) but, under the microscope, it is such a beautiful organism, that I am not going to apologise for writing about it again.   The alga lives inside the gelatinous mass and consists of a relatively thick central filament from which tufts of narrower side-branches emerge.  The cells that make up these side branches gradually narrow, and the chloroplast becomes smaller until, eventually, the cells form a colourless “hair”.   These hairs are relatively short on the material illustrated below but can be much longer (some longer hairs were present but did not present nicely for photography).  The hairs are, in fact, an adaptation to help the alga acquire phosphorus, something I described in an earlier post about a relative, Stigeoclonium tenue (see “A day out in Weardale”).

Draparnaldia glomerata from the River Ehen, April 2017 showing filaments and side branches. Scale bars: a.: 50 micrometres (= 1/20th of a millimetre); b.: 20 micrometres (= 1/50th of a millimetre).

A low concentration of phosphorus is usually regarded as a Good Thing by aquatic ecologists, as this limits the amount of energy produced  by the plants at the base of the food chain.  This, in turn, means that the microbes and animals that depend on these are not using up all the oxygen in the water, or having other deleterious influences on the ecosystem.   I would usually regard the presence of an organism such as Draparnaldia as a sign of a healthy stream, as it is adapted to thrive when phosphorus is relatively scarce.

I was, however, careful to place “relatively” in front of “scarce”.   Studies by my colleagues (referenced in the earlier post) showed that the production of the phosphatase enzyme that boosts the alga’s ability to acquire phosphorus when it is scarce is determined by the ratio of nitrogen to phosphorus inside the cell itself, rather than in the water.   The physiology of nutrient limitation is all about the balance between the different “ingredients” that a cell needs.   If you have three eggs and 170g of sugar, for example, you can only make one cake, regardless of how much flour you have in your cupboard.   So it is with algae: most of the locations where I find Draparnaldia have very little nitrogen, but even less phosphorus.   There are barely enough ingredients for the algal “cake” so it is advantageous to the organism to pump out some enzyme to order to make up the shortfall.  This means that I can say with confidence that Draparnaldia is usually a good indicator of healthy streams.

Just occasionally, however, I get Draparnaldia in places where I would not usually expect it to be found.   The picture below shows a colleague standing in the Terman River, just before it flows into Lough Erne in Northern Ireland.   She is holding a skein of Cladophora glomerata in her left hand and a skein of Draparnaldia in her right hand.  I associate the former with nutrient-rich rivers where I would not usually expect to find Draparnaldia.  But both were growing prolifically at this site which defied my expectations until I started to think about the physiology of the organism.   Had I had the facilities to analyse the tissues of the algae, I expect that I would have found very high concentrations of nitrogen which, in turn, creates a demand for yet more phosphorus so that it could convert that nitrogen into the proteins it needs to grow.  However, that cannot be the whole story, because normally, under such circumstances, I would expect a competitive alga such as Cladophora to out-compete and overgrow the Draparnaldia.   Here, they were growing side-by-side.   It is, to date, the most luxuriant growth of Draparnaldia that I have seen, and also the only occasion where I have seen these two species co-existing in such abundance.

My colleague, Bernie White, holding skeins of Cladophora glomerata (left hand) and Draparnaldia glomerata (right hand) from the Terman River near Toome.  The border between the Republic of Ireland and the UK runs along the middle of this river.

I can extend my lesson from the first example to say that, to understand the ecology of any particular river you need to have perspectives obtained from many other rivers.   But, in this case, we see a potential limitation: the case of the “rare exception” that clouds an otherwise clear picture of an association between an organism and a particular set of circumstances.   The problem is particularly acute when dealing with the effect of nutrients because we are usually dealing with indirect, rather than direct effects.   Draparnaldia glomerata is usually associated with clean rivers with low concentrations of nutrients but it is not there because nutrient concentrations are low.   As for the diatom Amphora pediculus (see “The challenging ecology of a freshwater diatom?”) a more nuanced understanding of the relationship between an organism and nutrients yields more useful insights than simply assuming a cause-effect relationship.

Ecology in the Hard Rock Café …

Before I was diverted by the delights of Bukhara and Samarkand, I was writing about the struggles that aquatic plants have to undergo in order to obtain the carbon that they need for photosynthesis (see “Concentrating on carbon …”).   In this post, I want to show the scale of the effect of inorganic carbon supply on the diatoms that we find in freshwaters.

My earlier post pointed out that aquatic plants have two possible sources of carbon to use for photosynthesis: dissolved carbon dioxide or bicarbonate.   The latter is derived, ultimately, from the rocks through which the water seeps before ending up in a stream or river.   Calcium carbonate, in turn, reacts with hydrogen ions in the water to form the bicarbonate that plants can use for photosynthesis.   A rock such as limestone, which is made of calcium carbonate, for example, provides a better supply than a hard siliceous rock such as granite.

Aquatic biologists use the term “alkalinity” to refer to the relative amounts of carbon dioxide, bicarbonate and carbonate in water.   This can confuse people as, in this context, “alkalinity” has little to do with the pH of the water itself and, indeed, water that is alkaline (i.e. has pH > 7) does not have to have a high alkalinity.   For now, just accept that low alkalinity water has little bicarbonate relative to dissolved carbon dioxide, whilst high alkalinity water has mostly bicarbonate and relatively little dissolved carbon dioxide.   In practice, alkalinity is a good indicator of the geology underlying the catchment from which a sample was collected, with low values associated regions of hard rocks (such as the Ordovician granites in Ennerdale’s catchment) and high values particularly associated with limestone and chalk.

I’ve spent a quarter of a century trying to understand how diatoms react to pollution and one of the surprising by-products of those studies is the realisation that alkalinity is just as important as pollution in determining the diatoms that are found at a site.   This is the case for most groups of freshwater organisms, but the scale of the effect on diatoms is particularly strong, as the graph below indicates.

Relationship between alkalinity and the average TDI at 430 UK river sites (r2 = 0.52).   The blue line shows a regression line fitted to the 10th percentile using the “quantreg” package in R. 

This graph shows a data from 430 UK sites where at least one TDI (Trophic Diatom Index) measurement was available, with alkalinity plotted on a logarithmic scale on the x axis and the TDI on the y axis.   There is a clear relationship between the two variables with about half of all the variation in the TDI accounted for by alkalinity (i.e. geology) alone, and this is manifest, in particular, by alkalinity setting a “floor” below which the TDI is unlikely to fall at any given alkalinity value (indicated by the blue line).  The red line, then, indicates the variation in TDI due to other factors, mostly human pressures such as eutrophication.

The blue line, in other words, indicates the best that the TDI is likely to be at any given alkalinity and if we were to look at samples which plot close to this line we will see quite marked differences in the diatoms as we moved from the low end towards the high.   When alkalinity is low, we will find Tabellaria flocculosa, some Brachysira species (e.g. B. neoexilis) and maybe a few Eunotia species too.  As alkalinity increases, so the diatom assemblage will be dominated by Achnanthidium minutissimum and relatives, but we will also see Hannaea arcus and Fragilaria gracilis, amongst other species.   We will see some Achnanthidium and Fragilaria species at low alkalinity, too, but either different to those at moderate to high alkalinity or in lower numbers.

There are several possible explanations for this but Brian Moss, in a classic paper from 1972, suggested that the availability of dissolved carbon dioxide was a major factor.  The “soft water” species, in other words, were better adapted to life without bicarbonate but were out-competed in moderate and hard water where the supply of bicarbonate was greater.   Very roughly, this switch from domination by free carbon dioxide users to bicarbonate users occurs at no more than 20 mg L-1 CaCO3.   There is more going on than just the supply of inorganic carbon: low alkalinity water is more likely to have low pH, which brings a separate set of challenges to aquatic organisms, and very high alkalinity water is often associated with productive agricultural areas.  This means that effects at both ends of the scale may be hard to separate completely from human pressures.  However, the broad story that emerges is that hard rock, in ecology as in music, is not to everyone’s taste.

Reference

Moss B. (1973).  The influence of environmental factors on the distribution of freshwater algae: an experimental study. II. The role of pH and the carbon-dioxide-bicarbonate system.  Journal of Ecology 61: 157-177.

The challenging ecology of a freshwater diatom?

amp_pedi_pollybrook

Amphora pediculus from Polly Brook, Devon, December 2016. Scale bar: 10 micrometres (= 1/100th of a millimetre).

The images above show one of the commonest diatoms that I find in UK waters.  It is a tiny organism, often less than 1/100th of a millimetre long, which means that it tests the limits of the camera on my microscope.  In recent months, however, it is not just the details on Amphora pediculus’ cell wall that I am struggling to resolve: I also find myself wondering how well we really understand its ecology.

The received wisdom is that Amphora pediculus favours hard water, does not like organic pollution and is relatively tolerant of elevated concentrations of inorganic nutrients.  This made it a very useful indicator species in a period of my career when we were using diatoms to identify sewage work s where investment in nutrient-removal technology might yield ecological benefits.  There were many nutrient-rich rivers, particularly in the lowlands, where any sample scraped from the upper surface of a stone was dominated by these tiny orange-segment-shaped diatom valves.   Unfortunately, twenty years on, many of those same rivers have much lower concentrations of nutrients (see “The state of things, part 2”) but still have plenty of Amphora pediculus.   Did I get the ecology of this species wrong?

The graph below shows some data from the early- and mid- 1990s showing how the abundance of Amphora pediculus was related to phosphorus.   The vertical lines on this graph show the average position of the boundaries between phosphorus classes based on current UK standards.   Records for A. pediculus are clustered in the “moderate” and “poor” classes, supporting my initial assertion that this species is a good indicator of nutrient-enriched conditions, but there are also samples outside this range where it is also abundant, so A. pediculus is only really useful when it is one of a number of strands of evidence.

aped_v_p

The relationship between Amphora pediculus and reactive phosphorus in UK rivers, based on data collected in the early-mid 1990s.  Vertical lines show the average boundaries between high and good (blue), good and moderate (green), moderate and poor (orange) and poor and bad (red) status classes based on current UK standards and the two arrows show the optima based on this dataset (right) and data collected in the mid-2000s (left).

If we weight each phosphorus measurement in the dataset by the proportion of Amphora pediculus at the same site (i.e. so that sites where A. pediculus is abundant are given greater weight), we get an idea of the point on the phosphorus gradient where A. pediculus is most abundant.   We can then infer that this is the point at which conditions are most suitable for the species to thrive.  In ecologist’s shorthand, this is called the “optimum” and, based on these data, we can conclude that the optimum for A. pediculus is 154 ug L-1 phosphorus.  The right hand arrow indicates this point on the graph below. However, I then repeated this exercise using another, larger, dataset, collected in the mid-2000s.   This yielded an optimum of 57 ug L-1 phosphorus (the left hand arrow on the graph), less than half of that suggested by the 1990s dataset.   There are, I think, two possible explanations:

First, the 1990s phosphorus gradient was based on single phosphorus samples collected at the same time that the diatom sample was collected (mostly spring, summer and autumn) whilst the mid-2000s phosphorus gradient was based (mostly) on the average of 12 monthly samples.  As phosphorus concentrations, particularly in lowland rivers, tend to be higher in summer than at other times of the year, it is possible that part of the difference between the two arrows is a result of different approaches.  (For context, in the 1990s, when I first started looking at the effect of nutrients in rivers, phosphorus was not routinely measured in many rivers, so we had no option but to do the analyses ourselves, and certainly did not have the budget or time to collect monthly samples).

However, another possibility is that the widespread introduction of phosphorus stripping in lowland rivers in the period between the mid-1990s and mid-2000s means that the average concentration of phosphorus in the rivers where conditions favour Amphora pediculus have fallen.   In other words, A. pediculus is tolerant of high nutrient conditions but is not that bothered about the actual concentration.   My guess is that it thrives under nutrient-rich conditions so long as the water is well-oxygenated and, as biochemical oxygen demand is generally falling, and dissolved oxygen concentrations rising (see “The state of things, part 1”), this criterion, too is widely fulfilled.   I suspect that both factors probably contribute to the change in optima.

But the second point in particular raises a different challenge:  We often slip into casual use of language that implies a causal relationship between a pressure such as phosphorus and biological variables whereas, in truth, we are looking at correlations between two variables.   Causal relationships are, in any case, quite hard to establish and the effect that we call “eutrophication” is really the result of interactions between a number of factors acting on the biology.   All of these simplifications mean that it is useful, from time to time, to look back to see if assumptions made in the past still hold.   In this case, I suspect that some of our indices might need a little fine-tuning.  There is no disgrace in this: the evidence we had in the 1990s led us to both to a conclusion about the relative sensitivity of Amphora pediculus to nutrients but also fed into a large-scale “natural experiment” in which nutrient levels in UK rivers were steadily reduced.   When we evaluate the results of that natural experiment we see we need to adjust our hypotheses.  That’s the nature of science.  As the sign on the door of a friend who is a parasitologist reads: “if we knew what we were doing, it wouldn’t be research”.

References

The 1990s dataset (89 records) is mostly based on data used in:

Kelly M.G. & Whitton B.A. (1995).   A new diatom index for monitoring eutrophication in rivers.   Journal of Applied Phycology 7: 433-444.

The mid-2000s dataset (1145 records) comes from:

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.

Not so Bleak Midwinter?

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Occasionally – just occasionally – the gods smile on us when we least expect it.  And Wednesday was one of those days: fieldwork on a glorious winter day in the Lake District without a cloud in the sky and barely a breath of wind.  The pleasure of being outside on such a day was offset slightly by the necessity of plunging my arm into freezing cold water at intervals, but the views of the mountains beyond Ennerdale Water more than compensated for these temporary discomforts.

The coldness of the water, today, offers me a link to a book I am reading, about the 19th century German scientist Alexander von Humboldt, a polymath who was ahead of his time in many ways, and whose writing pre-empted ecological thinking of the twentieth century.   One of his strongly held beliefs was that scientists could not really understand nature from a laboratory: they had to be outside, experiencing nature first hand.   That seems to be a fine New Year message in a world where ecologists seem to spend more and more time staring at screens, and their managers are increasingly reluctant to let them spend time in the field.

The ecology of lakes and rivers in this area in winter continues to fascinate me.   Look at the picture below: a stream bed at the coldest time of year that is covered with lush growths of algae in a range of hues, most strikingly the pink-red of the Rhodophyta Audouinella, complemented by the green and blue-green algae around it.  The first young olive-green filaments shoots of Lemanea, another Rhodophyta, were also apparent at a couple of the sites that I visited, and there were thick brown diatom blooms smothering many of the stones too.   These are all thriving at a time of year when either most nature has shut down for the winter or most natural historians have plonked themselves onto the sofa to watch Living World II rather than challenging the first clause in this sentence.  You decide.

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A riot of colour on the stony substrata of the River Ehen, a few kilometres downstream of Ennerdale Water, Cumbria, January 2017. 

One of Humboldt’s big concerns was that scientists saw the big picture (“naturgemälde”) rather than getting bogged down with details.   He was someone whose mind had been formed by the Enlightenment, when the necessity of cataloguing and classifying the diversity of nature was a primary concern.  However, he saw that this was not enough, and that one had to understand the connections between these different life forms, and between each of these and their environment.  He saw the natural world as a web of interdependencies, and humans as potential disruptors of the delicate balances that existed.

The problem we have in the modern age is balancing the need to see the big picture in focus without losing site of important details.  Or, as Ed Tipping said during a meeting at CEH last year: “we stick to the principle of simplifying to just short of the point of naivety”.   He had his tongue in his cheek but there is an important point here: the complexity of the natural world means that its secrets will only be yielded to those scientists who can keep their natural proclivity to get lost in detail in check.   At the same time, if we forget that those details are out there we may reach erroneous conclusions.  And, I fear, microscopic benthic algae may be ecology’s Sirens, sitting on submerged rocks and luring the unsuspecting into a world of taxonomic detail that is too rarely accompanied by profound ecological insight.

William Wordsworth, born in Cockermouth, just a few miles away from Ennerdale, was one of Humboldt’s readers.  He recognised the need to be outside experiencing nature applied as much to a poet as to a scientist and reacting against the dry, dissected knowledge that the Enlightenment encouraged.  His words offer a succinct conclusion for this first post of 2017, and encapsulate my resolution to be as holistic as possible in my thinking during the year ahead:

For was it meant
That we should pore, and dwindle as we pore,
For every dimly pore on things minute,
On solitary objects, still beheld
In disconnection dead and spiritless,
And still dividing and dividing still
Break down all grandeur …

William Wordsworth, The Excursion, 1814

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