Blind to the obvious

I’ve moved just a few kilometres from the River Liza to the location for this post: Croasdale Beck, a stream which joins the River Ehen at Ennerdale Bridge.   Croasdale Beck has featured in a number of posts in the past (see, for example, “That’s funny …” and “Croasdale Beck in February”) partly because it continues to surprise me.   Maybe that reflects a level of complacency on my part: regular visits mean that I know what to expect which, in turn, means that I am alert to things that I do not expect.   Seeing something new in a stream I have never previously visited is evidence of life’s rich pattern; noticing something that was probably there on previous occasions but which I overlooked is a more profound and, somehow, humbling experience.  This post is about one of each of these sensations.

There are, for example, a number of turquoise-coloured boulders in the beck that were certainly not there when I first started visiting in about 2015.   Most of the stones in the beck are cobbles rather than boulders, so these stand out both for their size and colour.   The colour is, if you look closely, due to a thin surface film – a Cyanobacteria which I will call Lyngbya vandenberghenii although, because it is difficult to scrape off (the filaments live in amongst the rock crystals), and lacks any really eye-catching features, it is hard to be totally certain about this.   Presumably it likes the stability that boulders confer in this very flashy little stream.    I also see it in the River Ehen nearby but there its presence is easier to explain as it is confined to chunks of limestone washed in from the foundations of a section of the Coast-to-Coast walk.

Simulium argyreatum growing on a cyanobacteria-covered boulder from Croasdale Beck, Cumbria (shown in the photo at the top of the post).  The stone is about 30 cm across.   

Today, however, I’m interested in what is growing on top of the Lyngbya rather than in the Lyngbya itself: dense patches of what looks, with the naked eye, like small tan-coloured seeds.   These are the tiny larvae of Simulidae, whose adult phases are the annoying blackflies that swarm around streams on summer evenings.   They spin a web of silk on the substrate to which they anchor themselves using a ring of hooks at their posterior.   Their mouthparts include a pair of fans (one of which can be seen in the image below) and, by extending themselves above the stone, they can trap tiny particles (including algae) drifting in the current. They produce a secretion which makes the fans sticky and also have mandibles adapted to brush the trapped particles from the fans into their mouths.   Most descriptions of the Simulidae refer to this filter-feeding life-style but I’ve also seen them bent double so that their fans can brush up the algae which grow on the stone surfaces. 

Larvae of Simulum argyreatum on boulders in Croasdale Beck.  The upper photograph was taken in situ with the macro facility on my Olympus Tough camera (each is ~0.3 – 0.5 millimetres long) from a stone without the crust of Lyngbya whilst the lower photograph shows a magnified view of the feeding fan of one larva.

At some point, the larvae cease feeding and spin slipper-shaped cocoons with the closed end facing upstream and the open end downstream.   Six white ribbon-like gills protrude from the open end, ensuring a ready supply of oxygen to the pupa inside.   The adult develops inside this cocoon, eventually emerging with a duel raison d’être of having sex and irritating humans.  “Adult” hardly seems like the appropriate word: “perpetual teenager” seems more apt. 

Whilst the adult males feed on nectar, the females need a blood meal before mating, adding a dark Gothic twist to their natural history.  This difference arises from the roles each plays in reproduction: the male only needs the spurt of energy that the sugary nectar confers whilst the female needs the proteins and minerals from the blood in order to nourish the eggs.  In the south of England, bites from the Blandford Fly, a relative of the Simulium I watched in Croasdale Beck, can cause nasty rashes whilst in large parts of Africa the bites from other species of Simulium can inject the parasite responsible for Onchocerciasis, or river blindness.   This was a common disease in the region of Nigeria where we lived in the early 1990s so I’ve seen the damage that these flies can cause.   Much as we find black flies and midges to be a nuisance in this country, at least they are not vectors for potentially deadly diseases. 

At a deeper level, knowing about the life cycle of Simulium reminds us that we are not just observers of aquatic ecosystems, we are, indirectly, part of these ecosystems too.  We may like to think of ourselves as the ultimate predator (remembering that this power brings with it great responsibility) but sometimes, as here, we can be the prey too. 

Clusters of Simulium argyreatum pupae on the Lyngbya-covered surface of a boulder in Croasdale Beck.   Each is about 3-5 millimetres long. 

Reference

www.blackfly.org.uk

And thanks to Richard Chadd for identifying the Simulium from my photographs.

Some other highlights from this week:

Wrote this whilst listening to:  The late great Toots Hibbert, remembering, in particular, Toots and the Maytals’ set on the West Holt Stage on a glorious summer evening at Glastonbury 2010

Cultural highlights:  We’re in the Lake District this week and, having recently watched part of Simon Scharma’s BBC series on the Romantic Movement, I’m reflecting on the role that the landscapes around me played in catalysing the work of Wordsworth, Turner and others. 

Currently reading:  English Pastoral by James Rebanks, a thoughtful analysis of the state of British agriculture that does not shy away from criticism either of farmers or naïve ecologists.

Culinary highlight:  James Rebank’s thesis hangs on the necessity of animal husbandry to maintain healthy soils.  With that in mind, I ate a Lakeland lamb steak at the Shepherd’s Arms hotel in Ennerdale Bridge with a clear conscience. 

The dark side of the leaf …

Whites_Level_lower_epiphytes

Having mentioned in my previous post that the epiphytes on the top and bottom surfaces of a Potamogeton polygonifolius leaf were different, I have produced a companion piece to the painting I showed in that post.   The new painting is of the lower surface, and shows a greater number of diatoms than are present on the upper surface.  In order to explain why this is the case, it is helpful to look at the structure of the Potamogeton leaves.  The first image, therefore, shows a section through a leaf. It is quite a thick section but we can see the upper epidermis, the palisade mesophyll cells below this, which have plenty of chloroplasts in order to capture the sunlight that the plant needs for photosynthesis.  Below this, we can see parenchymous tissue arranged to create some large internal air spaces which contribute to the leaves buoyancy. Finally, at the bottom, there is a single layer of epidermal cells.   All this is crammed into a thickness of about half a millimetre.

Potamogeton_polygonifolius_leaf_section

Part of a section of a leaf of Potamogeton polygonifolius.  The leaf vein is on the left, thinning to the leaf blade on the right.  The leaf blade is about half a millimetre thick.   The picture at the top of the post shows an artist’s impression of diatoms and Chamaesiphon cf. confervicolus on the lower surface of a Potamogeton polygonifolius leaf. 

 Viewed from the underside, these parenchymous tissues create polyhedronal chambers, ranging from about 100 to 200 micrometres (a tenth to a fifth of a millimetre) along the longest axis.  There are also a few stomata scattered across the leaf surfaces (see the right hand image below).

With this in mind, take a look at my impression of the epiphytes growing on the lower surface of a Potamotgen polygonifolius leaf.   There are a number of cells of Chamaesiphon cf confervicolius, as seen on the upper surface, but there are several cells of the diatom Achnanthidium minutissimum, growing on short stalks, plus a few long, thin cells of Ulnaria ulna, growing in small clusters on the leaf surface (there were a few other species present, but such low numbers that I have not included them here).    It might seem strange to think of two surfaces of a leaf having such different communities of epiphytes but that’s because we’re thinking like large land-dwelling organisms, not like algae.   The longest alga visible in the image of the leaf underside is Ulnaria ulna, at about a 10th of a millimetre in length.  Therefore, to get a realistic impression of the two images, we really need to put a distance of five of these between them, and then pack the gap with chloroplast-rich mesophyll cells inside the Potamogeton leaf.   Allowing for foreshortening, this distance is about five times the height of the image.

Pot_polygonifolius_leaf

The structure of a Potamogeton polygonifolius leaf viewed from the underside.  The left hand image (100x magnification) shows a leaf vein running diagonally across the lower right hand side along with the polyhedron-shaped chambers; the right hand image (400x magnification) shows the outline of one of these chambers superimposed behind the epidermal cells with a stomata with two guard cells visible just above the centre.   Scale bar: 20 micrometres (= 1/50th of a millimetre). 

The epiphytes on the upper surface of the leaf get first dibs at the meagre Pennine sunlight, which then has to pass through the upper layers of the Potamogeton leaf, where the mesophyll cells will continue to feast on the tastiest wavelengths, leaving relatively meagre pickings for the epiphytes that hang around on the underside of the leaf.

Chlorophyll, the molecule that makes plants green, absorbs light over a relatively narrow range of wavelengths – predominately red and blue – and this means that there are plenty of other wavelengths awaiting an organism with different pigments.   Diatoms have chlorophyll, but they also have some carotenoids (principally fucoxanthin) that grabs energy from the green part of the visible light spectrum (which is reflected, rather than absorbed by chlorophyll) and passes it to the cell’s photosynthetic engine.  Having this capability means that they can survive in relatively low light, which is why we see more diatoms on the underside of the Potamogeton leaf than on the top.

And that, best beloved, is the story of how Potamogeton got its epiphytes …

 

Some other highlights from this week:

Wrote this whilst listening to: more Bob Dylan.   I’ve got to the mid-70s, which means the live version of Like a Rolling Stone on Before the Flood plus the great Blood on the Tracks.  Also, as I was reading Ian Rankin, I listened to John Martyn’s Solid Air.

Cultural highlights:  we’re watching the BBC adaptation of Sally Rooney’s Normal People

Currently reading:  Ian Rankin’s Rather be the Devil.

Culinary highlight:   A rather fine vegetarian chilli, from Felicity Cloake’s column in The Guardian last week.   Served with corn bread, using a recipe we got from a hand-me-down American housekeeping magazine during our time in Nigeria.

 

Whatever doesn’t kill you …

Potamogeton_epiphytes_April20

The previous post focussed mostly on the higher plants that I found in the short stream that connects White’s Level with Middlehope Burn.  I mentioned the mass growths of algae that I found growing immediately below the entrance to the adit, but I did not talk about them in any detail, instead spinning off on a tangent while I mused on why the water cress had a purplish tinge.

When I did find time to examine the algal floc, I found it to consist of a mix of three different algae, the most abundant of which was Tribonema viride, but there were also populations of a thin Microspora (not illustrated) and Klebsormidium subtile.   I talked about Tribonema in the drainage from the Hadjipavlou chromite mine in Cyprus last year (see “Survival of the fittest (1)”) and both Microspora and Klebsormidium are also genera that are known to frequent these habitats.  Indeed, there is evidence that the populations that grow in these extreme habitats have physiological adaptations that help them to cope with the conditions.  Brian Whitton, my PhD mentor, led several studies on these adaptations in the streams of the northern Pennines in the 1970s, and Patricia Foster did similar studies in Cornwall at about the same time.   There is probably a mixture of physiological strategies involved, including the production of low-molecular weight proteins, which bind the toxic metals, and the production of extracellular mucilage.  Most of the populations I find in such habitats have a distinctly slimy feel due to the production of extracellular polysaccharides, and it is possible that these play a role in trapping the metal ions before they can get into the cell and cause damage.

Tribonema_viride_WhitesLevel_Apr20

Filamentous algae from the drainage channel below White’s Level, upper Weardale, April 2020.  a., b. & c.: Tribonema cf. viride, showing the characteristic H-shaped cell ends.   d.  Klebsormidium cf. subtile.  Scale bar: 10 micrometres (= 100th of a millimetre).   The picture at the top of the post shows an artist’s impression of Chamaesiphon cf. confervicolus on the upper surface of a Potamogeton polygonifolius leaf. 

I also had a look at the algae growing on the submerged leaves of Potamogeton pergonifolius in the channel between the adit and Middlehope Burn.   One easy way of examining them is to add a small amount of stream water then shake the leaves vigorously in a plastic bag.  The result is a brownish suspension of algae that can be sucked up with a Pasteur pipette and placed on a microscope slide.  When I did this, I found a community that was dominated by a short cyanobacterium, closest in form to Chamaesiphon cf. confervicolus.  The other abundant alga in the sample was Achnanthidium minutissimum, which is often common in minewaters, along with smaller numbers of a few other species.  The total number of species in the sample was just 12, which is low by the standards of streams without metal pollution, but such suppression of all but the hardiest species is another characteristic effect of heavy metal pollution.

I’ve added a “cf” (from the Latin conferre, meaning “compare to”) to my identification of Chamaesiphon confervicolus because this is the closest name, based on a comparison with images in the Freshwater Algal Flora of Britain and Ireland.  However, it is not an exact match.  Whether this is because the metals have strange effects on Chamaesiphon (as we saw for diatoms in “A twist in the tale …”) or whether our knowledge of the species within this genus is imperfect is not clear.  But discretion is the better part of valour in this instance.  Chamaesiphon species fall into two groups: those that live on stone surfaces (see “Survival of the fittest (2)”) and those that live on algae and plants, such as the one we see today (another is illustrated in “More from the River Ehen”).   They consist of a single, elongate but gently tapering cell, attached at one end to the plant and enclosed in a sheath.   The upper end of the filament forms small spherical buds (technically “exospores”).  One reason that I am wary of calling this population C. confervicolus is that most illustrations of this species show a stack of exospores in the sheath, whereas the White’s Level population all had just a single exospore.

Chamaesiphon_confervicolus

Chamaesiphon confervicolus, growing on Potamogeton polygonifolius in White’s Level outflow, April 2020.   Note the exospores at the end of the cell.  f. and g. show the sheath very clearly.  Scale bar: 10 micrometres (= 100th of a millimetre). 

The picture at the top of this post shows an artist’s impression of the Chamaesiphon cf confervicolus on the upper surface of the Potamogeton leaf.   I wanted to get some idea of the size, shape and arrangement of the epidermal and stomatal cells on the Potamogeton leaves and resorted to the tried and tested technique of painting a layer of clear nail varnish onto the leaf surface, then peeling this off when it had dried.  This had the added (and unexpected) benefit of also pulling of the epiphytes, giving some idea of their arrangement on the leaf surface at the same time.   One extra observation that this yielded was that upper surface was dominated by Chamaesiphon, growing in clusters, whilst the lower surface had greater representation of diatoms.   I’ve also tried to portray the chloroplasts in the stomata guard cells.  Plant epidermal cells generally do not contain chloroplasts, as their purpose is to protect the mesophyll cells that are the main centres of photosynthesis.  Guard cells of stomata, however, need energy to open and close the stomata so these are the exception to this rule.  I had not even been sure that I would see stomata on the upper surface of the cell, as these are mostly found on the underside of leaves; however, Potamogeton appears to have stomata on both surfaces.  As ever, there is a certain amount of evidence along with a dose of extrapolation.   Imagined, but not imaginary …

You can find a description of the terrestrial plant life of Slitt Mine and its environs in this post on Heather’s blog.

References

Foster, P.L. (1982).  Metal resistances of Chlorophyta from rivers polluted by heavy metals. Freshwater Biology 12: 41-61.

Harding, J.P.C. & Whitton, B.A. (1976).  Resistance to zinc of Stigeoclonium tenue in the field and the laboratory. British Phycological Journal 11: 417-426.

Robinson, N.J. (1989).  Algal metallothioneins: secondary metabolites and proteins.  Journal of Applied Phycology 1: 5-18.

Say, P.J., Diaz, B.M. & Whiton, B.A. (1977).  Influence of zinc on lotic plants. I. tolerance of Hormidium species to zinc.  Freshwater Biology 7: 357-376.

Sorentino, C. (1985).  Copper resistance in Hormidium fluitans (Gay) Heering (Ulotrichaceae, Chlorophyceae).  Phycologia 24: 366-368.

(Note that Hormidium is the old name for the genus Klebsormidium.  There is an orchid genus called Hormdium and, as this was described first, it takes priority.)

 

Some other highlights from this week:

Wrote this whilst listening to: Bob Dylan’s New Morning and Pat Garrett and Billy the Kid.   Also, Samuel Barber’s Prayers of Kirkegaard.

Cultural highlights:  The Netflix series Unorthodox, about a young woman fleeing a Hassidic community in New York.

Currently reading:  Agatha Christie’s A.B.C. Murders.

Culinary highlight:   Arroz con leche (Spanish rice pudding) served with peaches poached in madeira.

Blessed are you that hunger …

Mackay_et_al_2020_graphical_abstract

At the time of writing, four of my five working days are given over to ecology whilst the fifth is spent volunteering for the local Foodbank, which is gearing itself for a huge run on the stocks built up from generous donations over the Christmas period.   It occurred to me last week that I spend four days extolling hunger in an ecological context whilst spending the fifth trying to alleviate it in a human one.

“Hunger” in an ecological context is a them to which I have returned several times over the years.   We set the threshold for “good ecological status” for attached algae at a point that we thought coincided with the algal community switching from species that were adapted to “stressed” (i.e. nutrient-poor) conditions to those adapted to compete when nutrients were not in short supply (see “What does it all mean?” and references therein).   I’ve also talked, in some of my posts, about the adaptations some algae have to scavenge scarce nutrients (“A day out in Weardale”).   We’ve then gone on to try to work out what that means in terms of nutrient concentrations in UK and European rivers (references at the end of the post).

So I was pleased to see a paper appear last week that confirms some of these hunches.   Broadly speaking, Eleanor Mackay and colleagues have shown, using in situ bioassays, that as the concentration of inorganic nutrients decreases so the algae make more use of phosphorus and nitrogen bound into organic complexes.  As the algae get more “hungry”, in other words, they become more adept at scavenging for the resources that they need.

The graph at the top of this post is the graphical abstract from the paper which summarises this, whilst the one below shows the response to organic sources of phosphorus as a function of the concentration of “soluble reactive phosphorus” (the standard measure of “inorganic” phosphorus).  I’ve added an arrow to the right-hand side of this which shows roughly the current UK threshold, based on the work mentioned above.   Ellie’s graph seems to be confirming that, once that limit is exceeded, the algae are no longer “hungry”, meaning that they no longer need the nutrients bound into organic complexes.  Because organic phosphorus utilisation depends upon production of phosphatase enzymes to break down the organic complexes to releasee the phosphorus, there must be a greater energetic cost to the organism than if there was a ready supply of inorganic phosphorus that they can access.  I have, I must admit, never seen any figures that quantify this cost.

Mackay_et_al_2020_Fig5c

Fig. 5c from Mackay et al. (2020):  The relationship between “soluble reactive phosphorus” and dissolved organic phosphorus use by algae in in situ bioassays.  The “response ratio” is the natural logarithm of the ratio between the chlorophyll concentration of the treatment and the chlorophyll concentration of the corresponding control.  The arrow on the right-hand side indicates the approximate position of the regulatory threshold for phosphorus (see note at end of post).  The figure at the top of the post is the graphical abstract from Mackay et al. (2020). 

Part of me, then, is reassured that the regulatory threshold for phosphorus is roughly in the right place.  The Environment Agency’s reliance on a single measure of inorganic phosphorus, measured infrequently, is often criticised by hydrochemists but we can take some comfort from knowing that other forms of phosphorus (more difficult to analyse and quantify) only become important at concentrations lower than the current UK targets.   There is still part of me, however, that sees room for improvement.  That there are relationships between algae and other plants and phosphorus is not in doubt, and I am sure that a shift in strategies for nutrient acquisition help to define this relationship, particularly at low concentrations.  However, the relationships are not very strong and predictions about the ecological benefits of lowering phosphorus concentrations are imprecise.

Adding another strand of evidence to the current decision-making process makes scientific sense, and looking at how organisms respond to nutrients, rather than just measuring chemistry and describing community structure, seems like a sensible way of doing this.   In situ bioassays clearly have potential, as this paper shows; however, they are time consuming.   An alternative would be to measure phosphatase activity directly.  The Environment Agency did, in fact, fund research on this in the late 1990s and David Harper used these assays in a DEFRA-funded project in the early 2000s, but they have never become routine.  That’s a shame because, particularly for catchment-level investigations, they could add a useful additional insight.

The downfall of all these methods is not science, but the “more-with-less” ethos that has prevailed in the UK public sector for the past decade.  Everyone recognises that diffuse nutrient pollution offers a challenge that current monitoring and decision-making processes struggle to address.  However, most of the serious research effectively concludes with “if you spend a lot more money, you’ll discover that the problem is more complicated than you initially thought”.   That’s a difficult message to pass up through managerial hierarchies trying to keep a cash-starved regulatory agency moving forward.

Reference

Mackay, E. B., Feuchtmayr, H., De Ville, M. M., Thackeray, S. J., Callaghan, N., Marshall, M., Rhodes, G., Yates, C.A., Johnes, P.J. & Maberly, S. C. (2020). Dissolved organic nutrient uptake by riverine phytoplankton varies along a gradient of nutrient enrichment. Science of the Total Environment 722: 137837.   https://doi.org/10.1016/j.scitotenv.2020.137837

Poikane, S., Kelly, M. G., Salas Herrero, F., Pitt, J. A., Jarvie, H. P., Claussen, U., Leujak, W., Solheim, A.S., Teixeira, H. & Phillips, G. (2019). Nutrient criteria for surface waters under the European Water Framework Directive: Current state-of-the-art, challenges and future outlook. Science of the Total Environment 695: 133888. https://doi.org/10.1016/j.scitotenv.2019.133888

 

Note: regulatory threshold for inorganic phosphorus

The arrow indicating the approximate position of the regulatory threshold for phosphorus uses the current UK TAG phosphorus standard.   This is site specific, using altitude and alkalinity as predictor variables.  This means that a range of thresholds is possible and the position of the arrow reflects the average alkalinity (50 mg L-1 CaCO3) and altitude (75 m) in a database of river samples collected as part of DARES project. Note, too, that P standards are based on the Environment Agency’s standard measure, which is unfiltered molybdate reactive P.  This approximates to “soluble reactive P” or “orthophosphate-P” in most circumstances but the reagents will react with phosphorus attached to particles that would have been removed by membrane filtration.

 

Some other highlights from this week:

Wrote this whilst listening to: My lockdown project of listening to all Bob Dylan’s albums in sequence has brought me up to Bringing It All Back Home and Highway 61 Revisited.

Cultural highlights:  Bait, a low-key black and white British film from 2019.  Definitely sits in the “sub hero” genre that I much prefer to the crash, bang, wallop of most Hollywood blockbusters.

Currently reading:  About three-quarters of the way through Hilary Mantel’s The Mirror and The Light now.  Jane Seymour is gone; Anne of Cleeves coming up next.

Culinary highlight:   Grilled mackerel with sautéed potatoes, probably.  A close second was home-made tortellini filled with mushroom paté and served with garlic mustard (Alliaria petiola) butter.   Mrs K is forager-in-chief hereabouts.

grilled_mackerel

Quantifying our ignorance …

Petta_Water_May19

I am fairly sure that I am not a popular person after my latest choice of slide for the “ring test”, the regular calibration exercise that UK and Irish diatomists perform.   I had noticed a few taxa that we had not seen in previous ring tests in a sample I collected during my visit to the Shetland Islands back in May 2019 (see “Hyperepiphytes in the Shetland Islands”) but, on closer examination, the sample proved to be both highly diverse and very challenging.  The seven experienced analysts who provide the benchmark analyses for the ring test found, between them, over 150 different species: some we could name with confidence, but others we could match to no published description.  Amongst those was the species of Achnanthidium photographed below.   It might be Achnanthidium digitatum or possibly A. ertzii but, then again, it does not quite match the characteristics of either of these so, once again, we have left it unnamed (you can find the original descriptions of both these species in the reference list).

According to Algaebase there are 116 species of Achnanthidium that are currently accepted but descriptions of these are scattered through the literature so it is really hard to be confident that you have found a new species during a routine survey.  This is particularly the case when we only have light microscopical analyses with which to work, as the small size of Achnanthidium species means that you really need a scanning electron microscope to see the fine details clearly.  This, however, assumes that the pool of unnamed Achnanthidium species is finite and that the 116 species on Algaebase is a significant proportion of the total number of Achnanthidium species.  A recent study by Eveline Pinseel and colleagues based on samples from Arctic regions offers hints that there is still plenty of diversity within the genus that cannot be linked to named species

This may, however, be a naïve assumption.   My colleague Maria Kahlert, who works in Sweden, comments that she is quite happy looking at samples that I send her from polluted sites in the UK as she can name most of the species (Achnanthidium and otherwise) from her own experience.   It is the samples from pristine habitats that fox her because so many of the forms are different to anything she has encountered in Sweden.  We have, in other words, a neat reversal of the opening line of Anna Karenina (“All happy families are alike, each unhappy family is unhappy in its own way”), with very high beta and gamma diversity of diatoms (probably other microalgae too) as a characteristic of regions with low population density (see “Baffled by the benthos (2)”).  We often miss this in our enthusiasm to fit all that we see down the microscope to published descriptions, but when we take time to look hard, that diversity – and those differences between sites – start to mount up.

Achnanthidium_Petta_Water_May19

The unknown Achnanthidium species from Petta Water, Mainland, Shetland Islands (pictured at the top of the post).  Scale bar: 10 micrometres (= 1/100th of a millimetre).   Photographs: Lydia King

Let’s think of this as an ecological experiment to understand the diversity of Achnanthidium, following the capture-mark-capture approach.   Capture-mark-recapture is a technique used by ecologists to assess the size of a population.   As it is rarely possible to count all individuals, a portion of the population is collected, marked (a dab of paint on a snail’s back, for example) and released.   Some time later, the population is sampled again, and the proportion of those that bear the mark in this second sample is used as an indicator of the proportion of the population captured by the original sample.   Though devised for population biology, some have used the same principles to understand diversity in other contexts too so might it work as a means of understanding the yet-to-be discovered diversity of diatoms?

What we have in the scattered taxonomic literature is a record of all the Achnanthidium species that have been “captured” (i.e. observed) and “marked” (i.e. described) by taxonomists.   Suppose we now go some locations not previously visited by taxonomists, take some new samples and see 1) how many different forms of Achanthidium we can see and b) how many of these are “recaptured” (i.e. forms that align with previously described species).   Or, thinking about the problem in a different way, the number of named species could be compared with the number of distinct “operational taxonomic units” (“OTUs”) detected by metabarcoding.   More relevantly, how many extra OTUs are added when more lakes and streams are added to the dataset?   There are well-established methods for deriving “rarefaction curves” that might be useful in understanding regional diversity of diatoms, and modifications of “capture-mark-recapture” have been used to understand taxonomic diversity in palaeobiolgoical contexts, so why not in contemporary ecology too?

The Shetland Islands would make an ideal test ground for such a study as they are geologically-diverse habitats providing the types of conditions where Achnanthidium species thrive (low population density and agricultural intensity.   The diatoms of the region were studied about 40 years ago by my late mentor John Carter and although one of his samples yielded the type material for Achnanthidium caledonicum there have been so many developments in Achnanthidum taxonomy subsequently that this archipelago represents a tabula rasa for a modern taxonomist.   Its many remote lochs and streams offer the setting for a natural experiment which sets out, to put it bluntly, to quantify our ignorance.

Achnanthidium_caledonicum_Osgaig

Achnanthidium caledonicum from Loch Osgaig, Highland Region, Scotland.   Originally described as Achnanthes microcephala f. scotica Carter & Bailey-Watts 1981 (Scale bar: 10 micrometres (= 100th of a millimetre).  Photographs: Lydia King.

References

Carter J. R., Bailey-Watts A. E. (1981). A taxonomic study of diatoms from standing freshwaters in Shetland. Nova Hedwigia. 33: 513-630.

Pinseel, E., Vanormelingen, P., Hamilton, P. B., Vyverman, W., Van de Vijver, B., & Kopalova, K. (2017). Molecular and morphological characterization of the Achnanthidium minutissimum complex (Bacillariophyta) in Petuniabukta (Spitsbergen, High Arctic) including the description of A. digitatum sp. nov. European Journal of Phycology 52: 264-280. https://doi.org/10.1080/09670262.2017.1283540

Van der Vijver, B., Jarlman, A., Lange-Bertalot, H., Mertens, A., de Haan, M. & Ector, L. (2011).  Four new European Achnanthidium species (Bacillariophyceae).  Algological Studies 136/137: 193-210.

Liow, L.H. & Nichols, J.D. (2010). Estimating Rates and Probabilities of Origination and Extinction Using Taxonomic Occurrence Data: Capture-Mark-Recapture (CMR) Approaches.  The Paleontological Society Papers 16: 81-94).

This week’s other highlights:

Wrote this whilst listening to: Sheku Kanneh-Mason’s recording of Elgar’s Cello Concerto.   Taking me back to his performance at the proms on a warm evening last summer.

Cultural highlight: Sam Mendes’ film 1917 which, coincidentally, uses the River Tees (as featured sporadically in this blog) as one of its locations

Currently reading: I have just finished Good Economics for Hard Times by Abhijit V. Banerjee and Esther Duflo, which I mentioned a couple of weeks ago.  It left me with the feeling that, had both Boris Johnson and Jerermy Corbyn read it and taken on its messages, the election campaign and the UK political landscape might have been very different.

Culinary highlight: OK Diner on the southbound side of the A1 near Grantham.  Felt like we were walking into the opening scene from Pulp Fiction (the one where Tim Roth jumps up onto a table and attempts to rob all the customers).   Escaped with wallet intact.

 

Fit for purpose?

 

Durham_School_boathouse_Jan2020

It is sobering to think that the Water Framework Directive (WFD) will be twenty years old this year (23 October, to be precise).  The 70 pages of legalese that comprise this directive have, to a large extent, determined the course of my career over the past two decades (it is a few sentences in Annex V, to be precise, but unravelling and interpreting these has been enough).  Just before this anniversary arrives, however, the European Commission has published a “fitness check”, giving the Directive a thorough once-over before reaching a mixed verdict on its performance.

The report’s conclusion is that the WFD has provided a governance framework for water management but, overall, the condition of Europe’s water bodies has shown little significant improvement since the WFD passed into law.   The original objective – grossly optimistic in hindsight – was for all Europe’s water bodies to be at least at good status by 2015.  Instead, we are still in the situation where less than half are at good status.  There is no doubt that there have been local improvements, and the rate of deterioration may have decreased but this is not the same as a general trend towards better ecological quality in our water bodies.   I’ll offer three possible reasons for the shortcomings, based on my own experience of WFD implementation, in the hope that lessons learned from turning a well-intentioned policy instrument from theory into practice will have some broader lessons as we tackle the climate emergency.

The first lesson is that complex problems, by necessity, spawn complicated legislation.  The Water Framework Directive arose from an attempt, in the early 1990s, to produce a directive addressing the Ecological Quality of Waters.  As debates about this progressed, people realised that you cannot consider the state of the aquatic environment in isolation, without also considering broader economic issues such as water pricing and, indeed, all aspects of catchment management that respects the rights of other legitimate users.  Each of these issues requires a small army of bureaucrats to unpack and apply within the 28 Member States.   In some countries and for some aspects of the legislation, there were procedures in place that simply needed tweaking to be fit-for-purpose.  Some other aspects were, however, completely new for almost everyone.

The whole idea of using the health of an aquatic ecosystem (“ecological status”) as a measure of the long-term sustainability, for example, was something never attempted on such a scale before.  It had been advocated in the academic literature, and there were a few localised attempts to apply the system (RIVPACS in the UK, for example) but, as the sun rose on 23 October 2000, the task of working out how the fine words of Article 4 had to be translated to a practical reality that was both faithful to the intentions of the WFD and that worked within public sector budgets had to start.

rower_on_Wear

A second big issue that was relatively under-acknowledged in the fitness check is that solving environmental problems cannot be achieved without engaging other sectors as well.   A recent review, to which I contributed, highlighted this, emphasising the need, first, to integrate water policy with other sectors (such as agriculture) whilst, at the same time, emphasising the need to demonstrate tangible benefits that extend beyond the subtleties of shifts in ecological parameters.  Bring agriculture on board to achieve more sympathetic management of catchments, in other words, recognise the contributions that farmers make (“public money for public goods”) but also back this up with substantial demonstrations of reduced flood risk for urban areas downstream.   That calls for a level of joined-up thinking across sectors that has not yet been achieved in Europe and which is, perhaps, an opportunity that the UK, shortly to be freed from the leviathan that is the Common Agricultural Policy, may be in a better position to address.  We live in hope.

The third reason may be that the ambition of the WFD may be higher than many politicians and civil servants are happy with.   Article 1 sets out the objective of promoting “sustainable water use based on a long-term protection of available water resources”.  A phrase such as that could have appeared in any of the party manifestos for our recent election but when the scientists unpack this and explain that this will mean that every river in the country needs to have average phosphorus concentrations of well under 0.1 milligrams per litre, and the water planners put a price on this, alone, that runs into hundreds of millions (if not billions) of euros, then that ambition falters.   More particularly, the noisy nature of much ecological and environmental data gives ample opportunity for bureaucrats to prevaricate rather than take steps that are unlikely to play well with the media (the WFD enshrines the “polluter pays” principle and, as we all contribute to urban wastewater loading, this translates to “voter pays”).

As its 20th anniversary approaches, the WFD will have spanned four electoral cycles (assuming national parliaments have five-year terms), at each of which policy wonks will have been thinking less about long term sustainability of water resources and more about short-term swings in voting preferences.   Moreover, since 2008, much of Europe has felt the consequences of the banking crises, with public sector finances often badly affected.  Again, the scientific challenges that the WFD creates provides easy excuses for cash-strapped regulators to kick the can down the road rather than make potentially unpopular decisions.

Governance may be in place, in other words, but a willingness to push this governance to deliver may be lacking.  That, in turn, reflects a perceived unwillingness on the part of the electorate to accept the costs.  Imperfect democracies will always deliver imperfect solutions, particularly when the underlying problems are complex and the opportunity costs are high.

ducks_on_Wear

Pictures in this post are from a New Year’s Day walk around the riverbanks in Durham.  New feature for 2020 is a few notes on what else I’ve been up to during the week in which this post gestated:

Wrote this whilst listening to:  Bob Dylan’s John Wesley Harding; Bruce Springsteen’s Nebraska

Cultural highlight: Greta Gerwig’s Little Women.

Currently reading: Good Economics for Hard Times by Abhijit V. Banerjee and Esther Duflo – two Nobel Prize winners setting global problems into a broader economic framework.  Not an easy read but very stimulating.   A good follow-up to Kate Raworth’s Doughnut Economics, which I mentioned in a couple of posts last year.

Culinary highlight: followed a recipe in The Guardian which involved cramming all the leftovers from our Christmas dinner (turkey, stuffing, roast potatoes, parsnips, brussel sprouts) into a loaf tin along with some breadcrumbs and two eggs to bind.  This created a meatloaf which I froze and then produced on New Year’s Day to provide a final reminder of the festive season before the realities of 2020 intruded.  Doubly enjoyable as West Ham had their first win of the Festive Season as it was being demolished.

Reference

Carvalho, L., Mackay, E. B., Cardoso, A. C., Baattrup-Pedersen, A., Birk, S., Blackstock, K. L., Borics, G., Borja, A., Feld, C.K., Ferreira, M.T., Globevnik, L., Grizzetti, B., Hendry, S., Hering, D., Kelly, M., Langaas, S., Meissner, K., Panagopoulos, Y., Penning, E., Rouillard, J., Sabater, S., Schmedtje, U., Spears, B.M., Venohr, M., van de Bund, W. & Solheim, A. L. (2019). Protecting and restoring Europe’s waters: An analysis of the future development needs of the Water Framework Directive. Science of the Total Environment 658 1228-1238. https://doi.org/10.1016/j.scitotenv.2018.12.255

Messy bedrooms …

Sand_Loch_May19

When I was tramping around the Shetland Islands earlier this year (see “Hyperepiphytes in the Shetland Islands“), looking at the algae that live in the freshwater lochs, I noticed some meandering hieroglyphs made from fine sediment on the tops of some of the stones in the littoral zone.   I see these occasionally at other places too, and know that they are the “galleries” of caseless caddis flies.  Caddis flies are close relatives of the butterflies and are best known because many of their larvae use “found materials” (in contemporary art jargon) to construct cases to protect themselves.  Some species use fine gravel, silt and sand, some use fragments of plants, some have cases that are very neat, some have a more haphazard approach to construction.  However, a few families of caddis flies eschew cases and, instead, build these galleries.

Many caddis fly larvae, whether cased or not, are grazers, scraping the algae off the rocks on the bed of the stream or lake.   There is evidence that the cases offer some protection against predators such as trout which, by increasing survival rate, means that it is worthwhile for the caddis larvae to divert some of their hard-earned energy into building these.   Presumably, their caseless cousins gain the same advantage to building their galleries but recent research has suggested that these galleries offer a further benefit.

Think of caddis larvae as adolescent caddis flies.  Now imagine that the caddis gallery is the equivalent of an adolescent’s bedroom.   Horribly messy, in other words.   Let’s leave that image of a teenager behind (as most human teenagers know their way to the bathroom) and consider what happens to all that waste material that emerges from the far end of a caddis larva’s digestive system.   This nutrient-rich “ manure” encourages algae, meaning that our caseless caddis flies are, in fact, gardeners and are able to tap into this extra energy resource within their galleries in order to grow.   That brings us back to the analogy with teenagers, as these also frequently graze in their bedrooms (the diatom Campylodiscus is even the same shape as a Pringle, whose empty containers litter the bedroom floor of my own progeny).   I guess it is a good thing that caddis larvae don’t wear socks as, with six legs and two prolegs, the mess inside the gallery would be indescribable.

Psychomiiddae_Sand_Loch_May19

Galleries of caseless caddis flies (possibly Psychomiidae) on the top surface of a cobble from Sand Loch, Shetland Islands with (right) a close-up of a single gallery. The photograph at the top of the post shows Sand Loch in May 2019.

A recent study in the Lake District has shown that this “gardening” means that the algae which grow in the fine sediment from which the galleries are constructed are different to those found elsewhere on the rock surface, with a greater proportion of diatoms, which are considered to be more palatable to invertebrates than other types of algae.  Some caddis flies are thought to go even further, and can selectively remove and discard the algae that are least palatable (some Cyanobacteira, for example).

It is possible that up to 40% of the larva’s energy needs are met from the gallery itself.   The tube is, in fact, not a static construction: the larva pokes its head out in order to graze the algae immediately in front of the gallery, and extends the gallery as the food supply within easy (and safe) reach is exhausted.   At the same time, it is consuming the alga-rich rear part of the gallery (reminiscent of Hansel and Gretel eating the gingerbread house?).   A gallery only has a life-span of 10 days in the laboratory; whether this is the same under field conditions is not clear but that gives us some idea of the transience of these structures.   This rapid turnover means that the caddis larva is always feeding on succulent early-succession species, rather than the tougher and less digestible algae that might appear in more mature biofilms.

I also see similar galleries on the bed of the River Ehen from time to time but have been told that these are formed by non-biting midge (chironomid) larvae, rather than by caddis.  I presume that the same processes are happening in these although I have not been able to find much written in the literature.

Organisms that can significantly alter the habitat in which they live, and affect the conditions experienced by other species in the habitat are termed “ecosystem engineers”.  Beavers are good examples, as their dams can have significant effects on organisms extending for hectares.  Yet, in their own small way, caseless caddis larvae are also ecosystem engineers.  As are adolescent boys.   Which makes me wonder, having only talked until now about the algae in their galleries, whether caseless caddis larvae also have patches of mould extending up their walls.

Chironomid_galleries_Ehen_March19

Galleries made by chironomid larvae on a boulder in the River Ehen, March 2019.

References

Hart, D. D. (1985). Grazing insects mediate algal interactions in a stream benthic community. Oikos 44: 40-46. https://doi.org/10.2307/3544041

Johansson, A. (1991). Caddis larvae cases (Trichoptera, Limnephilidae) as anti-predatory devices against brown trout and sculpin. Hydrobiologia 211: 185-194. https://doi.org/10.1007/BF00008534

Ings, N. L., Hildrew, A. G., & Grey, J. (2010). Gardening by the psychomyiid caddisfly Tinodes waeneri: Evidence from stable isotopes. Oecologia 163: 127-139. https://doi.org/10.1007/s00442-009-1558-8

Ings, N. L., Grey, J., King, L., McGowan, S., & Hildrew, A. G. (2017). Modification of littoral algal assemblages by gardening caddisfly larvae. Freshwater Biology 62: 507-518. https://doi.org/10.1111/fwb.12881

Otto, C., & Johansson, A. (1995). Why do some caddis larvae in running waters construct heavy, bulky cases? Animal Behaviour 49: 473-478. https://doi.org/10.1006/anbe.1995.0061

The presence of absence in Castle Eden Dene

CED_Aug19

Some of my strongest impressions of Castle Eden Burn after last week’s visit concerned not what I found in the stream, but what was not there.  I mentioned in my previous post that I had not seen the mosses that I associated with streams in northern England in Castle Eden Burn, but there were other species, too, that I had expected to see but had not noticed.   Once I have noticed that something is absent, this absence becomes present.  I have noticed the presence of absence.  Woohoo: I’ve shoehorned Jean-Paul Sartre’s Being and Nothingness into a blog about ecology.

When I got back home I had read a chapter about the FBA’s study of the Winterbourne in Dorset, an intermittent stream flowing off the chalk downland, and noticed that they had recorded plants there that I knew from north-east English rivers, but which I could not remember seeing in Castle Eden Burn.  Was this because I had not searched the stream environs thoroughly, or is this a real difference between intermittent streams on chalk and on Magnesian limestone?

I went back this weekend to try to answer these questions, taking Heather with me, as her skills with the higher plants far exceed mine, and walked as much of the stream bed as we could, starting near the remains of a footbridge at NZ 424 389, and making our way downstream to Denemouth, where Castle Eden Burn joins the North Sea.  If my original intention was to better understand the burn by traversing space within the Dene, my first lesson concerned time: a week with some heavy rainfall separated my two visits and it was clear straight away that the Burn had been flowing during the week, with a fine layer of silt and mud spread across much of the surface, making parts of it slippery to walk upon.  There were standing pools of water at several points in the upper part of the Burn too.   Within a week the stream had come and gone, offering scant opportunities for any water-loving organism to establish.

We made our way along the Burn through the delicious silence of the forest.  The banksides were richly vegetated: masses of opposite-leaved golden saxifrage plus the mosses I described last time and many others, along with plenty of harts-tongue fern (Asplenium solopendrium).   Then, with a very clear demarcation, there was the stony stream bed with very little vegetation at all.    We looked hard for three plants, in particular, that I associated with the damp margins of streams, and which I had expected to see here: Verronica beccabunga (brooklime or water speedwell), Rorippa nasturtium-aquaticum (water cress) and Mentha aquatica (water mint).  None seemed to be present in any of the stretches we visited apart from a single sorry looking brooklime in the freshwater marsh at Denemouth .

What we did find, a little further downstream, was a pebble and gravel-dominated stretch with a straggly array of plants, all bent over in the direction of flow.   These included broad-leaved dock (Rumex obtusifolius), nettles (Urtica diocia), a few shoots of Himalayan balsam (Impatiens glandulifera) and some grasses.    Were I not standing on a dry stream bed I would have assumed that this was a bare piece of ground being colonised by typical ruderal species.  And that, I think, offers some insights into the ecology of Castle Eden Burn.   This is not a stream that occasionally dries out: it is a long-thin terrestrial habitat that is occasionally flushed through by water.   Welcome to north-east England’s premier wadi.

Rumex_obtusifolius_CED

Rumex obtusifolius and other ruderal vegetation on the stream bed of Castle Eden Burn, August 2019.

This hypothesis really needs corroboration by a hydrologist, but the graph I showed in “Out of my depth …” shows that, despite flow being generally low,  episodes of high flow are scattered throughout the year, and I suspect that these keep the substratum mobile and, more important, stop organic matter accumulating to give amphibious plants an opportunity to establish.   The water table, too, I guess, is too far below the stream bed in between the spates to make it easy for plants to stay hydrated.   This is one of the main differences between Castle Eden Burn and the southern chalk streams, which are characterised by very stable flow regimes

From the point at which Blunt’s Burn enters Castle Eden Burn (NZ 436 396) there does seem to be permanent flow down to the sea.  Still, however, there was very little in-stream vegetation.  That was in contrast to the forest around us, which was floristically-rich (Heather has written more about this on her blog) and, on this warm summer morning, positively humming with bees and aflutter with butterflies.

A large embankment takes the busy A1086 over the Dene, the Burn passing through a long culvert at this point, after which there is a viaduct taking the coastal railway line across before the dene widens out into a large area of meadow just before it reaches the sea.   The stream’s path to the sea is, however, blocked by mine waste that was dumped from the coal mines that used to line the Durham coast.  This forces the stream to turn ninety degrees south for a few hundred metres before finding a way through and, gradually, trickling and percolating through the beach. The mines have all gone now and the sea is gradually eroding this compacted mass of waste.  Before the waste arrived, apparently, there was an area of saltmarsh at the mouth of the burn.   Now, there is a freshwater marsh, dominated by reeds (Phragmites australis).  When the mine waste finally goes, maybe the saltmarsh will return.  Meanwhile, Castle Eden Burn has no grand finale: it ends on a whimper not a bang.

We climbed a narrow, steep pathway up through gorse and brambles onto the clifftops overlooking these final stages of Castle Eden Burn to get a view that was, in light of all that had passed through my mind earlier, oddly symbolic.  The stream flowed almost due east until it encountered the bar, and the gentle arc which it then describes looks just like a question mark.   How ironic, I thought, for a stream that raises more questions than answers to sign off in that way ….

Denemouth_CED_Aug19

Denemouth, at the end of Castle Eden Dene, just above the point where the stream joins the North Sea.

Out of my depth …

Castle_Eden_Dene_March19

I was about to start writing up an account of my latest visit to Castle Eden Dene, when I realised that I had forgotten to describe my previous visit, back in March.   I’ve already described a visit in January, when the stream was dry (see “Castle Eden Dene in January” and “Tales from a dry river bed”) and promised regular updates through the year.   It seems that, amidst all the travel that filled my life over the last three months, I overlooked the post that I should have written about the visit that I made in early March.

Whereas the river was dry in January, rain during February meant that, when I returned to the Dene on 11 March, some rather turbid water was flowing down the channel on its short journey to the North Sea.   There is, finally, something more like a stream habitat from which I can collect some diatoms.

Many of the diatoms that I found in March belonged to taxa that I had also seen in January; however, the proportions were quite different.   In some cases, species that were common in January were less common now (e.g. Humidophila contenta*) but there was a small Nitzschia species with a slightly sigmoid outline that was very sparse in the January sample but which was the most abundant species in the March sample.  I’ve called this “Nitzschia clausii” but the Castle Eden Dene population does not fit the description of this perfectly.   A lot can change in a couple of months, especially when dealing with fast-growing organism such as these, as my posts on the River Wear showed (see “A year in the life of the River Wear”).  Castle Eden Burn’s highly variable discharge just adds another layer of complication to this.

CED_diatoms_Mar19

Diatoms from Castle Eden Dene, March 2019:   a. – e.: Nitzschia cf clausii; f. Tabularia fasiculata; g. Tryblionella debilis; h. Luticola ventricosa; i. Luticola mutica; j. Ctenophora pulchella.  Scale bar: 10 micrometres (= 1/100thof a millimetre).   The picture at the top of the post shows Castle Eden Burn at the time that the sample was collected.   

Nitzschia clausii is described as being “frequent in brackish freshwater habitats of the coastal area and in river estuaries, as well as in inland waters with strongly increased electrolyte content”.   A couple of the other species from this sample – Ctenophora pulchella and Tabularia fasiculata (both illustrated in the diagram above) – have similar preferences.    My experience is that we do often find a smattering of individuals belonging to “brackish” species in very hard water, as we have in Castle Eden Burn.  Average conductivity (based on Environment Agency records) is 884 µS cm-1; however, values as high as 1561 µS cm-1.   The fluctuating discharge plays a role here, as any evaporation will serve to concentrate those salts that are naturally present in hard freshwater.   This should probably not be a big surprise: life in brackish waters involves adapting to fluctuating osmotic regimes so species that can cope with those conditions are also likely to be able to handle some of the consequences of desiccation.

Average values of other chemical parameters from 2011 to present, based on Environment Agency monitoring are: pH: 8.3; alkalinity: 189 mg L-1 CaCO3; reactive phosphorus: 0.082 mg L-1; nitrate-nitrogen: 1.79 mg L-1; ammonium-nitrogen: 0.044 mg L-1.   There is some farmland in the upper catchment, and the burn also drains an industrial estate on the edge of Peterlee but, overall, nutrient concentrations in this stream are not a major concern.   The Environment Agency classifies Castle Eden Burn as “moderate status” due to the condition of the invertebrates but does not offer any specific reason for this. I suspect that the naturally-challenging habitat of Castle Eden Burn may confound assessment results.

I’ve also been given some data on discharge by the Environment Agency which shows how patterns vary throughout the year.  The two sampling locations are a couple of kilometres above and below the location from which I collect my samples and both have more regular flow.  However, we can see a long period between April and September when discharge is usually very low.   The slightly higher values recorded in July are a little surprising, but are spread across a number of years.   It is also, paradoxically, most common for the burn to be dry in July too: clearly, a month of extremes.  As my own visits have shown, it is possible for the burn to be dry at almost any time of the year, depending on rainfall in the preceding period   The dots on the graph (representing ‘outliers’ – records that exceed 1.5 x interquartile range) show that it is also possible to record high discharges at almost any time during the year too.  I should also add that, as I am not a hydrologist, I am rather outside my comfort zone when trying to explain these patterns.  I would have said ‘out of my depth’ though that’s not the most appropriate phrase to use in this particular situation.

CED_discharge

Discharge in Castle Eden Burn, as measured by the Environment Agency between 2007 and present.   Measurements are from NZ 4136 2885 (‘upstream’) and NZ 45174039 (‘downstream’).  

* Note on Humidophila contenta:it is almost impossible to identify this species conclusively with the light microscope as some key diagnostic characters can only be seen with the scanning electron microscope.   However, all members of this complex of species share a preference for intermittently wet habitats so these identification issues are unlikely to lead to an erroneous ecological interpretation.  It is probably best to refer to this complex as “Humidophila contenta sensu lato” rather than “Humidophilasp.” order to distinguish them from those species within the genus that can be recognised with light microscopy.

Reference

Lange-Bertalot, H., Hofmann, G., Werum, M. & Cantonati, M. (2017).  Freshwater Benthic Diatoms of Central Europe: over 800 Common Species Used in Ecological Assessment. English edition with updated taxonomy and added species.  Edited by M. Cantonati, M.G. Kelly & H. Lange-Bertalot.  Koeltz Botanical books, Schmitten-Oberreifenberg.

The complexities of measuring mass…

Benthotorch_in_action

Once upon a time, measuring the quantity of algae growing on the beds of streams and rivers was a painstaking, slow process that invariably revealed large amounts of spatial and temporal variation that, very often, obscured the ecological signals you were looking for. That has changed in the last decade thanks to the availability of field fluorimeters such as the BenthoTorch.  This makes it much quicker and easier to measure chlorophyll concentrations, the usual proxy for algal quantity.  Thanks to devices such as this it is now much easier to discover that your ecological signal is masked by spatial and temporal variation.

We’ve generated a lot of data about the fluxes of algae in the River Ehen using a BenthoTorch over the past five years and are in a position where we can start to make some generalisations about how the quantity of algae vary over the course of a year.  In broad terms, the results I showed in “The River Ehen in January” back in 2014 have not varied greatly over subsequent years, with peak biomass in mid-winter and low biomass in the summer (due, we presume, to intense grazing by invertebrates).  Curiously, we see a much less distinctive seasonal pattern in the nearby Croasdale Beck, but that is a story for another day….

The BenthoTorch uses an algorithm to partition the fluorescence signal between three major algal groups and, though this is not without issues (see below), I thought it might be interesting to see how these groups varied with biomass trends, and consider how this links to ecological theory.  The first group I’m considering are the green algae which, in this river, are mainly filamentous forms.   The general pattern, seen in the graph below, is for a gradual increase in the proportion of green algae, which fits with the current understanding of thicker biofilms having greater structural complexity with filamentous algae out-competing attached single celled algae to create a “canopy” of algae that are more effective at capturing light and other resources.  The relationship is, however, strongly wedge-shaped so, whilst many of the thickest biofilms have a lot of green algae, there are also thick biofilms where green algae are scarce or even non-existent.  Croasdale Beck shows a similar, but less pronounced, trend.

green_algae_in_Ehen

Relationship between the proportion of green algae and the total quantity of benthic algae (expressed as chlorophyll concentration) in the River Ehen (a.) and Croasdale Beck (b.).   The blue lines show quantile regression fits at p = 0.8, 0.5 and 0.2.   The image at the top of the post shows Ben Surridge using a BenthoTorch to measure algal biomass beside Croasdale Beck in Cumbria.

The second graph shows that this pattern of a gradual increase in proportion is also the case for diatoms and, once again, there is a broad wedge of points with an upward trend.  But, once again, there are also samples where biomass is high but diatoms are present in very low numbers or are even absent.   What is going on?

The problem is clear I think, if one looks at the final image in “The only way is up …” where the very patchy nature of algal communities in the River Ehen (and, indeed, many other rivers).   There are plenty of algae on this boulder, but not organised in a homogeneous manner: some zones on the boulder are almost pure diatom whilst others are almost pure green algae (and there are also zones that are almost pure Lemanea– I’ll come to that in a future post).   We try to sample the stones as randomly as possible so you can see the potential for getting very different numbers depending on where, on a stone, we point the BenthoTorch’s sensor.

diatoms_in_the_Ehen

Relationship between the proportion of diatoms and the total quantity of benthic algae (expressed as chlorophyll concentration) in the River Ehen (c.) and Croasdale Beck (d.).   The blue lines show quantile regression fits at p = 0.8, 0.5 and 0.2.  

With experience, you can make an educated guess about the types of algae present in a biofilm.  I’ve tried to capture this with my watercolours, using washes of raw sienna for the diatoms and a grass-green for the green algae, which roughly matches the colour of their respective growths in the photo in my earlier post.   The two groups of algae a are relatively distinct on that particular boulder.   The top row roughly matches the upper “edge” of the graph showing variation in diatoms, whilst the bottom row emulates the upper “edge” of the graph showing variation in green algae.  These are the two extreme situations; however, we also often see darker brown growths in the field, which can be recreated by mixing the raw sienna and grass-green together.  When I peer through a microscope I often see green algae smothered in diatoms: genera such as Oedogoniumare particularly prone as they have less mucilage than some of the others we find in the Ehen. Their filaments often host clusters of Fragilariacells as well as Achnanthidium minutissimum, whilst stalked Gomphonemaand chains of Tabellaria flocculosaoften grow through the tangle of green filaments.   The dark brown colour is deepened yet further by the colour of the underlying rock, so my effort on white watercolour paper is a little misleading.

colour_patches

A colour chart showing how different proportions of green algae and diatoms influence the colour of biofilms.

The final graph shows how, as the average biomass increases in the River Ehen, so the variability in biomass also increases.   The River Ehen is one of the cleanest rivers I know but I suspect that this pattern in benthic algal quantity could be reproduced in just about any river in the country. What I would not expect to see in any but the purest and most natural ecosystems is quite so much variation in the types of algae present.   Once there is a little enrichment, so I would expect the algae to become more of a monoculture of a dominant filamentous alga plus associated epiphytes.  Like much that happens in the microscopic world of rivers, it is easier to describe than it is to measure.

That, however, is only part of the story but I’ll come back to explain the patterns in the other main groups of algae in the Ehen and Croasdale Beck in my next post.

mean_biomass_by_stdev

The relationship between mean chlorophyll density and the standard deviation (based on measurements from five separate stones) for samples from the River Ehen and Croasdale Beck.