Follow this link to find a guest post that I have just written for Luca Marazzi’s blog, describing the biology of Navicula lanceolata.
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.
- 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.
- 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.
- 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.
- 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.
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.
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.
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.
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.
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)
In my first post of 2017, I wrote “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” (see “Not so bleak midwinter?”). That was a post about Ennerdale Water and the River Ehen and I return to that same remote and beautiful part of north-west England to put some flesh onto the bones of that statement.
I chose a slide Ennerdale Water for the latest “ring test” which tests the competence of the analysts involved in routine ecological assessments using diatoms. Everyone analyses the same slide and sends their results to me and I sort through and note areas of disagreement. In this case, there was quite a lot of disagreement even amongst the experienced analysts but, as if to prove the point in my opening paragraph, this did had only a small effect on the conclusions that people reached on the quality of the ecosystem that the sample represented.
One of the areas of disagreement was a population of Brachysira that some described as Brachysira brebissonii whilst others suggested it was B. intermedia. My inclination, following the illustrations in Hofmann et al’s Diatomeen im Süsswassser-Benthos von Mitteleuropa, was to call it B. intermedia as the illustrations of B. brebissonii show more broadly-rounded ends than were apparent in the Ennerdale population. However, Bryan Kennedy, a member of our scheme, has been looking into the taxonomy of this genus as part of his PhD and directed me towards a paper by Bart van der Vijver, in which he had examined the type material for B. brebissonii (as “Navicula brebissonii Brébisson). His illustrations show a population with more acute ends than Hofmann et al.’s illustrations. Its shape is, in fact, much closer to that of the examples of B. intermedia that they illustrate. Yet, we were all agreed, this taxonomic ambiguity had little effect on the interpretation that an ecologist would reach. Most Brachysira species (with the curious exception of the type species, B. aponina) are associated with circumneutral to slightly acid water and low nutrients. The species of the genus vary in their preference for alkalinity and hardness but, generally, indicate high quality conditions.
Another of the species present in the same sample was Frustulia crassinervia, which presents an interesting counterpoint to the situation I described for Brachysira brebissonii. In this case, there was another possible candidate, Frustulia saxonica and, in fact, the rhombic-lanceolate outline of the Ennerdale Water population did suggest this species. However, the clearly protracted ends and the size are more characteristic of F. crassinervia which was the name that the majority of experienced analysts opted for. Once again, however, both species have similar ecological requirements: soft, often peaty water with low levels of enrichment.
Frustulia crassinervia from the south-east end of Ennerdale Water (circa NY 127 140), July 2016, scale bar: 10 micrometres (= 1/100th of a millimetre). The top illustration shows Brachysira brebissonii from the same location. Photographs by Lydia King.
The difference between the situation for Brachysira and Frustulia is that, in the case of Frustulia, the traditional morphological taxonomy is underpinned by molecular studies whereas, in the case of Brachysira, we only have morphological evidence on which to base names. The interesting point is that the molecular studies of Frustulia suggest that there is no genetic difference between F. crassinervia and F. saxonica. This alone does not mean that they are not separate species (they did not look at the whole genome), but it does suggest that more work is needed before we have a full understanding of the limits of the species in this genus. A similar study on Brachysira (and, indeed, on any genus with a surfeit of recently described species separated purely on morphological criteria) might emphasise that differences in shape to which traditional taxonomists assign so much importance are real or it might not. That would, at least, give people such as myself who use diatoms to gain a wider perspective of ecological health a better insight into where we really need to put in time and effort to discriminate between species. In the post I mentioned at the start of this article I referred to the nineteenth century scientist Alexander von Humbolt and his concern that scientists got so bogged down in detail that they missed the big picture (“naturgemälde”). Some things never change …
Hofmann, G., Werum, M. & Lange-Bertalot, H. (2011). Diatomeen im Süßwasser-Benthos von Mitteleuropa. A.R.G. Gantner Verlag K.G., Rugell.
Lange-Bertalot, H. & Moser, G. (1994). Brachysira. Monographie der Gattung. Bibliotheca Diatomologica 29: 1-212.
Urbánková, P., Scharfen, V. & Kulichová, J. (2016). Molecular and automated identification of the diatom genus Frustulia in northern Europe. Diatom Research 31: 217-229.
Van der Vijver, B. (2014). Analysis of the type material of Navicula brachysira Brébisson with the description of Brachysira sandrae, a new raphid diatom (Bacillariophyceae) from Iles Kerguelen (TAAF, sub-Antarctica, southern Indian Ocean). Phytotaxa 184: 139-147.
Veselá, Urbánková, P.,Černá, K. & Neustupa, J. (2012). Ecological variation within traditional diatom morphospecies: diversity of Frustulia rhomboides sensu lato (Bacillariophyceae) in European freshwater habitats. Phycologia 51: 552-561.
In an earlier post about diatoms from this location (see “Reflections from Ennerdale’s far side …”) I showed some images of live cells of Stenopterobia sigmatella but added a comment to say that there was a very similar species, S. densestriata, that I could not rule out on the basis of observations of live cells alone. I now have had the opportunity to have a looked at cleaned material as well and can confirm that the population in Ennerdale Water is, indeed, S. sigmatella. S. sigmatella has less than 24 striae in 10 micrometres whilst S. densestriata, as the name suggests, has more (> 26 / 10 micrometres). S. densestriata is also shorter (< 110 micrometres) and narrows to more acutely-rounded ends.
We also found a few valve of S. delicatissima, the other member of the genus that has been recorded from the UK.
Stenopterobia spp. from Ennerdale Water, July 2016. i.: part of a cleaned valve of S. sigmatella; j. S. delicatissima. Scale bar: 10 micrometres (= 100th of a millimetre). Photographs by Lydia King.
Several of my posts on this blog allude to the “Cinderella” status of algae in the eyes of most natural historians, and I am always keen to emphasise the important role that algae play in aquatic ecosystems. Over the past few months, however, I have had some help in this endeavour, in the form of two placement students from Newcastle University’s marine science degree. We have been taking the first tentative steps towards developing some web pages that emphasise the very large contributions that algae, mostly overlooked, have made to the modern world.
I thought that it might be a good idea to “beta test” the content of these pages on my blog, before finding a permanent home for them on the web. The two students, Alexandra Jackson and Matt Walker, second year undergraduates at Newcastle, worked up a short list of major benefits of algae, based on a survey of experienced phycologists (the council of the British Phycological Society) then wrote some text to explain each of these, along with some visual aids and a list of authoritative sources for further reading. This, then, is what will be the “home page” of this series of pages, with text written by Alex and Matt and edited by myself. As we get more pages ready to go, I will insert hyperlinks to make it easier to navigate around the pages.
Alexandra Jackson and Matt Walker, the placement students from Newcastle University responsible for this series of posts. The image at the top of this post is Günter Forsterra’s winning image from the 2015 Hilda Canter-Lund photography competition.
The term “algae” encompasses an enormous and diverse group of photosynthetic organisms that can be found in marine, freshwater and terrestrial environments throughout the world. Some can be seen with the naked eye but many are so small that they can only be seen with microscopes.
Algae influence our lives in far more ways than anyone might imagine. All the plants around us, including all the crops which feed us and provide us with fuel and fabric, have evolved from algae. In addition to being, ultimately, responsible for life on earth, algae form the foundation for many ecosystems (as a food source and primary producer). Collectively, algae are responsible for over 50% of global primary production and, in the process, produce over 50% of the oxygen that we breathe. As this primary production is fuelled by carbon dioxide, algae can help tackle climate change by acting as a ‘carbon sink’. In addition to these global contributions to our livelihoods, algae also contribute in myriad other ways: they are used in waste-water treatment, agriculture and for coastal defences. Algae are, themselves, a major food resource in some parts of the world (particularly Asia). Even when algae are not consumed directly, they can provide raw materials for the food industry, including natural food colourings. Algal products are also used in some medicines, toiletries and cosmetics and the potential for algae to replace fossil fuels is being explored.
There will be more about these topics in future posts. You can also read more about algae in the latest issue of the Natural History Museum’s Evolve magazine, which has illustrations of several winning and short-listed entries from the Hilda Canter-Lund competition illustrating an article written by me describing the diversity and relevance of algae.
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.