Mayfly mayhem …

If my previous post, on the role viruses may play in ecological cycles was just a little too close to home, maybe this post will be an antidote.  It continues the theme of organisms interacting to create the ecosystems we see in streams, but this time dealing with the effect that invertebrates, rather than microorganisms and viruses, may have on the algae that live in our lakes and streams.  

The painting at the top of this post is the end result of picking a mayfly nymph off a stone submerged in the River Ehen and realising that I only had the sketchiest of ideas about how it went about feeding on algae.   I brought it home with me, managed to identify the genus, if not the species to which it belonged (Ecdyonurus) and took a photograph.    You can see the characteristic flattened head that identifies it as a member of the Heptageniidae, along with the two large, upward-facing eyes, a row of plate-like gills along the abdomen and three tails protruding from the final segment.  

A nymph of the mayfly genus Ecdyonurus (Heptageniidae) from the River Ehen, Cumbria.  The organism, including the tail, is about 1.5 cm long.  

What we cannot see in this image are the mouthparts, hidden below that bulbous head.  I had to search around the internet to find detailed images of the mouthparts of Ecdyonurus, eventually locating some useful scanning electron micrographs in a paper by Todd Wellnitz and J.V. Ward.  These images are quite difficult for a non-entomologist to decipher, because insects have extremely different mouthparts to vertebrates.  In brief, they have, like us, upper and lower jaws (the maxillae and labium respectively) but both of these can have jointed extensions (“palps”) that are, in effect, “arms” with built-in cutlery, each adapted to that insect’s particular diet.  Imagine that a human ancestor had wanted to do all the things that we do with our arms but didn’t want to stop walking on four legs.  Now imagine that humans evolved a short pair of arms on either side of the face.   If that doesn’t sound bizarre enough, some arthropods have these palps on both the upper and lower jaws, so there could be four separate items of cutlery descending upon an unsuspecting prey organism.   

In the case of Ecdyonurus, the labial palps are brush-like appendages that it uses to sweep the algae that live on the stones towards its mouth.  That’s what the structure on the right of the image is supposed to represent, and part of my intention is to show this in relation to the size of the algae that are typically found in the habitats where Ecdyonurusfeeds.  The individual bristles that make up this brush are about five or ten micrometres (= 1/200th to 1/100th of a millimetre) apart, whereas the smallest diatoms are about ten micrometres long.   So Ecdyonurus should be pretty effective at scraping up most of the algae that it encounters.  

By way of analogy, a 1.5 cm Ecdyonurus nymph grazing on stream diatoms of this size is roughly equivalent to a human eating peas.  And all this is done, remember, without the mayfly actually seeing what it is doing – its eyes, if you remember, are on the top of the head.   Some mayfly nymphs have been shown to have sensory receptors in their labial palps, so it is reasonable to assume that Ecdyonurus has something similar.  This doesn’t necessarily mean that it feeds with any great discrimination, rather it chomps through all the algae, just leaving behind those that are too firmly attached to be removed easily.   Whether it can separate out two algae of similar sizes mid-meal (selecting peas but leaving sweetcorn, as it were) seems unlikely.  But then, if you know anything about insect lifecycles, you’ll know that mayfly nymphs don’t have parents they can easily annoy, which takes away most of the fun of separating peas from sweetcorn.  Or, by analogy, one species of Achnanthidium from another. 

References

Wellnitz, T.A. & Ward, J.V. (1998).  Does light intensity modify the effect that mayfly grazers have on periphyton. Freshwater Biology 39: 135-149.

Gaino, E. & Reborna, M. (2004).  The sensilla on the labial and maxillary palps of Baetis rhodani (Ephemeroptera, Baetidae).   Research update on Ephemeroptera and Plecoptera (E. Gaino, editor), University of Perugia, Perugia.

Some other highlights from this week:

Wrote this whilst listening to:  Gryphon, early seventies band who blended medieval and folk music with progressive rock.  

Cultural highlights:  Rewatched the film Amélie, probably my favourite film about Paris. 

Currently reading:   continuing with Wanderlust: A History of Walking, by Rebecca Solnit.  

Culinary highlight:    Continued the French theme with hake steaks on a cassoulet made with roast fennel, followed by crêpes suzette, made with garden apples rather than oranges.

The stream eats itself …

Do you mind if I write a post about viruses?   If you’ve had enough of viruses in 2020, feel free to skip this and wait until next week when normal service will be resumed.  For those of you that are still with me, this is, I hope, a first step in rehabilitating the reputation of viruses amongst natural historians.  

I’ve been meditating on the contents of a short review on the role that fungi and viruses play in microbial mats, and wondering if there are clues in this paper to some of the questions I’ve raised in this blog over the years about the extraordinary diversity of microscopic algae (see “Baffled by the benthos (1)” and “Baffled by the benthos (2)”).   To set the scene, I’ve included a plate showing at least four different species from the same genus present in a single sample from a loch on Shetland (the same one I discussed in “Quantifying our ignorance”).   Classical theory about niches helps us understand why we may get this level of variety in a single genus in different samples but is less effective at explaining how these species can apparently co-exist in such close proximity.   It was this that stimulated G. Evelyn Hutchinson to pose the Paradox of the Plankton which, in turn, was the starting point for my own admission to being Baffled by the Benthos. 

Five species of Cavinula from a single sample collected from Petta Water, Shetland Mainland, May 2019.  a. – g.: C. jaernefeltii; h. – k.: C. pseudoscutiformis; l. – m.: C. intractata; n. C. cocconeiformis; o. – s.: Cavinulasp.   Scale bar: 10 micrometres (= 1/100th of a millimetre).  Photos: Lydia King.   The photograph at the top of the post shows mixed algal growths (mostly Spirogyra spp.) growing in the River Irt, Cumbria.   Could it be that these clumps are sustained by viruses?

There is already some evidence that tiny chytrid fungi can affect algal abundance (see “Little bugs have littler bugs upon their backs to bite ‘em …”) and also that diatom species differ in their tolerance to such infections.   Chytrids can be seen by a keen observer using a light microscope and it is reasonable to expect viruses, which we cannot see with a light microscope, to have similar effects.   How might these explain the presence of so many members of one diatom genus in a single sample?

If we assumed that all four species of Cavinula were competing for the same limiting resources (light, nutrients etc) then we might expect the one that was most efficient at acquiring the resources it needed to thrive at the expense of the others.   However, by virtue of being successful, this same species will become the most obvious target for any pathogenic organisms that are present.   Just as the very success of humans at exploiting niches right across the planet has created ideal circumstances for Covid-19 to spread, so the most successful Cavinula species in a Shetland loch is going to be most likely to succumb to viral infections.  Theoretical studies have shown that this “kill the winner” strategy can explain the coexistence of closely-related species in circumstances that can be readily transposed to habitats where we find benthic diatoms.   That means that viruses are, ironic as it may seem from our present standpoint, partly responsible for the extraordinary diversity of life on earth.

There’s a second potential benefit that viruses bring to stream ecosystems: the end result of killing the winner is the breakdown of the cell membranes, making its constituents available to other organisms.  I’ve written before about how the algae in a healthy streams are usually hungry for nutrients (see “Blessed are you that hunger …”) so any nutrients that seep into the water in the aftermath of a viral infection will be rapidly hoovered up.    I’ll go one step further and suggest that this type of pathway (a variant on the so-called “microbial loop”) may explain why we often see algae in healthy rivers growing in clumps: proximity to other organisms means proximity, too, to any leaked nutrients as well as some protection from stream currents which might whip any nutrients away before they can be reabsorbed by other nutrients.   The title of this post is an adaptation of a sentence from the start of Barbara Kingsolver’s Poisonwood Bible: “The forest eats itself and lives forever”. That’s a fine encapsulation of a general ecological principle that works as well in a cold stream or Scottish loch as it does in a forest in the Congo.  

Flicking through some of the books on my shelf, I note just a single reference to viruses in T.T. Macan and E.B. Worthington’s New Naturalist volume Life in Lakes and Rivers (1951) and one in Brian Whitton’s River Ecology(1975).  Both of these refer to the potential presence of human pathogens in polluted water. By 2015, Brian Moss’s revision of the New Naturalist book, Lakes, Loughs and Lochs, contains eight references, only one of which alludes to human pathogens (and then only to mention Macan and Worthington’s single reference).  What can we conclude?  That there is yet another variant of the New Normal, this time with viruses recognised as key players in ecological processes.

References

Carreira, C., Lønborg, C., Kühl, M., Lillebø, A.I., Sandaa, R.-A., Villanueva, L. & Cruz, S. (2020).  Fungi and viruses as important players in microbial mats.  FEMS Microbiology Ecology 96 fiaa187. https://doi.org/10.1093/femsec/fiaa187

Short, S.M. (2012).  The ecology of viruses that infect eukaryotic algae.   Environmental Microbiology 14: 2253-2271.

Thingstad, T.F. (2000).  Elements of a theory for the mechanisms controlling abundance, diversity and biogeochemical role of lytic bacterial viruses in aquatic systems.  Limnology and Oceanography 45: 1320-1328.

Some other highlights from this week:

Wrote this whilst listening to:  Jazz 625: BBC4 programme about the British jazz scene, featuring Sons of Kemit and Ezra Collective, bands we enjoyed at Green Man in 2019.   Also, humming Janet Kay’s Silly Games, after its use in Small Axe

Cultural highlights:  Small Axe: Steve McQueen’s dramas on BBC1: the first, Mangrove, was a tough watch, focussing on racism in west London in the 1960s, but the second in the season, Lover’s Rock, has a more uplifting vibe.

Currently reading:   Wanderlust: A History of Walking, by Rebecca Solnit. 

Culinary highlight:   A Persian meal, supplied by Taste of Persia, formerly a Durham takeaway, now a London-based mail order company.  

On the pleasures of untidiness …

Go, sit upon the lofty hill,
and turn your eyes around
where waving woods and waters wild,
do hymn an autumn sound.”
 Elizabeth Barrett Browning, The Autumn 1833

Naturalists are explorers, of a sort. They are not necessarily of a type that ranges far and wide in search of exotic phenomena.  That’s one sort of exploration, for sure.  Another type is to travel to the same place over and over again and watch how it changes.   That type of journey is through time rather than space, but it is no less of an adventure, so long as your senses are attuned to what is happening around you.   Indeed, watching how much one location can change both through seasons and between years offers a cautionary tale to anyone who thinks that isolated, concentrated bursts of activity at a distant location (what our archetypal “explorers” are doing) are likely to yield definitive insights.

With that thought in mind, I was back at the River Ehen last week.   I’ve been coming here for eight years now, so I think I know it quite well but one of the things I have learned is that there is always more to discover.   Sometimes, this is because the river really has changed, sometimes (quite often, I suspect) it is because I have noticed something that has been there for a while but which I have hitherto overlooked.  Sometimes, it is a mixture of the two.

That fallen tree on the right hand side of the image at the top of the post is a case in point.   When I first started coming here, eight years ago, there was a patch of Nitella flexilis tucked under the footbridge close to the right bank.  A couple of years ago it disappeared for no apparent reason. I presume that it had been dislodged in a spate and washed downstream.   Then, over the past few months, I’ve gradually seen patches of Nitella appear again, though not in the same place.  Some of the most prolific stands are in the lee of the tree, where fine sediments have gradually accumulated.   It grows there along with Myriophyllum alterniflorum (alternate water milfoil) and Callitriche brutia ssp. hamulata (intermediate water starwort), creating a hotspot of macrophyte diversity, at least until that tree is dislodged and this area is exposed to the full force of the Ehen’s current again.  

Nitella flexilis (left) and Callitriche brutia var. hamulata (right) in sediments downstream of a fallen tree in the River Ehen, November 2020.  The fallen tree can be seen just to the right of the footbridge in the photo at the top of the post.

Aquatic ecologists can get quite misty-eyed at the mention of large woody debris because of the important role it can play in supporting biodiversity.   This example in the River Ehen is a good example of how natural treefall creates patches within a stream that creates a set of conditions enabling a range of species to thrive.   I’ve focussed on one of our larger freshwater algae in this particular example, but there is evidence of similar benefits working on invertebrate and fish populations too.   One day, probably after an exceptional flood, I’ll come back and find that this fallen tree has been washed downstream and these patches of finer sediments, along with the algae and plants that they support, have been eroded away.   Patches exist in time as well as in space: you just need to think in another dimension.  

The problem is that this type of woody debris makes rivers look untidy and owners and bailiffs are prone to remove it.   Anything that holds back water can cause localised flooding, which may be a problem for the owner of adjacent land; however, slowing down the flow is, generally, beneficial for communities downstream.   So that’s another reason why we should not pull out fallen tree trunks.   Once upon a time, a fallen tree trunk was only ever seen as a nuisance, now it is a “public good” with at least two tangible benefits for streams and rivers.   “Tidiness”, however, is not one of those but, then, look at an ecologist’s study or even the boot of his or her car, and you will see that it is rarely a trait that we endorse either.   

Reference

Thompson, M.S.A., Brooks, S.J., Sayer, C.D., Woodward, G., Axmacher, J.C., Perkins, D.M. & Gray, C. (2019).  Large woody debris “rewilding” rapidly restores biodiversity in riverine food webs.  Journal of Applied Ecology 55: 895-504.  

Some other highlights from this week:

Wrote this whilst listening to:  Stevie Wonder.  Because, halfway through Lockdown 2.0, we neeed cheering up.  

Cultural highlights:  To the Ends of the Earth: a Japanese film about a team making a travel documentary in Uzbekistan.   If Lost in Translation is about westerners discombobulated by Japanese culture, then this is (Lost in Translation)2.  Many of the scenes were set around locations in Tashkent and Samarkand that we recognised from our visit in 2017. 

Currently reading:   Why I No Longer Talk to White People About Race by Reni Addo-Lodge.

Culinary highlight:   A toss-up between Bouillabaisse and a water cress and horseradish risotto. 

Worst case scenarios …

After last week’s post from the Lake District I have travelled vicariously to the headwaters of the River Wye to report on a paper looking at the effect of liming on the ecology of headwater streams.   These areas are located in mountainous areas with soft water and, in many cases, extensive afforestation.  Although acidification due to “acid rain” has decreased in recent decades, ecosystem recovery is slow, hindered by accumulated acidity in the soil, as well as by the many conifer plantations.   One way to speed up this process is to spread lime in the headwater catchments in order to provide more buffering and to raise the pH and our paper (I played only a minor role in data analysis) followed the chemical and ecological changes that ensued.

For me, the study is interesting as a road-test of our Diatom Acidification Metric (DAM), developed with funding from Environment Agency.   What we expected to see was a gradual increase in the value of DAM following liming, indicating a shift from an acidified flora to one closer to what would be expected in a stream of that type.  And, indeed, that is what happened although, as is usually the case, there was a lot of noise in the relationship.  Values of DAM for samples collected following periods of high flow, for example, tended to be lower than those collected during periods of low flow. That is because hydrogen ions are washed out of soil during rainfall and make their way into the streams, depressing the pH.    

Adding lime to acid grassland in the upper Wye valley.  The photograph at the top of the post shows coniferous plantations in the upper Irfon valley.  Both photos courtesy of Ingrid Jüttner.

These discrepancies are, however, the whole point of monitoring such as this.  If there was a wholly predictable and stable relationship between biology and pH, we would not need to measure both: simply measuring pH would give you all the information you need.  Because pH is naturally variable, a single measurement of either biology or chemistry isn’t particularly helpful.   Even if you measured pH in a stream monthly, your estimate of typical conditions would still only be based on reliable insights from just three per cent of the year.  The rationale for biological monitoring has always been that the organisms are exposed continually to stream conditions and so give better insights than isolated visits to measure chemistry.

However, when we compared the diatom results with measured pH, we found that the average result for diatoms consistently over-estimated the condition of the streams.  The UK’s standards for pH and acid neutralising capacity (ANC) were set based on knowledge of the toxicity of pH to salmon fry.   This life stage is extremely sensitive so a single pulse of acid water could be enough to kill any fry exposed to it.   We got a much better fit between the diatoms and measured pH and ANC when we used the minimum value of DAM recorded at a site, rather than the average, as this gives a better indication of the “worst case” to which the fry will be exposed.   

Relationship between mean and minimum phytobenthos EQR (Ecological Quality Ratio) and mean pH and ANC in the upper Wye and Irfon catchments.  Horizontal and vertical lines indicate thresholds for high (solid) and good (dashed) status.  The regression line fitted by locally weighted scatterplot smoothing (LOESS) is shown as a solid line and the dashed diagonal line indicates the regression line fitted to the 10th percentile of the data.  More details in Jüttner et al. (2020).  

The problem is that routine assessments in the UK are based on just two diatom samples, and you’ll need to be very lucky to capture the whole range of conditions with such a lean strategy.   When designing a case study, however, there is no excuse for not having at least three or four replicates.   The more you have, the greater the chance of the lowest of them indicating the worst-case scenario for the stream you are monitoring.

There is one other possibility, which may work better if you have a small number of samples: subtract about 0.2 from the mean value of DAM.  That is about enough to correct it from giving an accurate insight into the condition of the diatoms to giving an insight into the overall state of the stream biota.  That should give you enough insight into what is happening on the 97% of the days when no-one collected a chemical sample.  From this, you should be able to work out what is going on in a stream and, more importantly (and as our paper shows) demonstrate when genuine improvements have been made.

References

Juggins, S., Kelly, M., Allott, T., Kelly-Quinn, M. & Monteith, D. (2016).  A Water Framework Directive-compatible metric for assessing acidification in UK and Irish rivers using diatoms.  Science of the Total Environment 568: 671-678. 

Jüttner, I., Kelly, M.G., Evans, S., Probert, H., Orange, A., Ector, L. & Marsh-Smith, S. (2020).  Assessing the impact of land use and liming on stream quality, diatom assemblages and juvenile salmon in Wales, United Kingdom.  Ecological Indicators https://doi.org/10.1016/j.ecolind.2020.107057

Some other highlights from this week:

Wrote this whilst listening to:  Benamin Britten’s War Requiem, as it is Remembrance Sunday.

Cultural highlights:  About Endlessness, a strange film by Ron Andersson in which a series of apparently mundane events are elevated to high art through a series of beautifully-lit and  carefully-crafted vignettes.

Currently reading:  Walking to Jerusalem by Justin Butcher, an account of a pilgrimage from London to Jerusalem to commemorate the 100th anniversary of the Balfour Declaration.  

Culinary highlight:   Pumpkin pie.

Ever changing worlds …

The trip to the Lake District that featured in my previous post also took me to the River Liza, which I last visited in September (see “Sunbathing beside a Cumbrian stream …”).  Then, I wrote about the extensive patches of the cyanobacterium Stigonema mamillosum growing on top of boulders.   I commented in that post that the patches seemed to be more extensive than earlier in the summer, so I was curious to see what they looked like two months on.  

What I saw this time was an abundance of green algae (BulbochaeteUlothrix aequalis and Zygnema sp.) growing on the larger submerged boulders, a closer look revealing that these were growing on top of the Stigonema mats that I described previously.  My theory is that Stigonema is an organism that can thrive in those parts of streams and lakes that are intermittently exposed to the air whilst the green algae are unable to cope with extended periods out of the water.   As the summer gives way to autumn and river levels are a little higher, so the Stigonema spends more time underwater.  The tangled mat of filaments then provides an place where juvenile filaments of green algae can lodge and grow, much better protected from the current than on a bare rock face.  I’ve tried to capture this in my diorama at the top of the post.

Green algae (mostly Bulbochaete in this image) growing out of mats of Stigonema mamillosum on a submerged boulder in the River Liza, October 2020.   The image at the top of the post is a visualisation of green algae (Ulothrix aequalis and Zygnema sp.) growing in and around Stigonema mamillosum.

We could think of these cyanobacterium-green algal patches as mini-ecosystems, exhibiting temporary successions.   I wrote about this in “How to make an ecosystem” and “How to make an ecosystem (2)” – the latter based on a farmyard just a few kilometres from the boulders I was observing in the River Liza.   In all three of these cases, the cyanobacteria are the “pioneers” which stabilise the environment and create conditions into which another species can exploit.   If I was considering terrestrial ecosystems, I would write about the gradual transition from the pioneers to the “climax” vegetation and conjure visions of forests extending far above our heads.  In the River Liza, however, these humble green algae are probably the pinnacle of this particular succession, clinging to their stones amidst the rough and tumble of a Lakeland stream.   This mosaic of temporary, ever-shifting patches may be present at this site for generations, but any individual patch has only a transitory, fleeting presence before it is lost. 

That sentence captures the essence of the Lake District: look up from these mundane intricacies and you can see Red Pike, Great Gable, Pillar and other solid, immutable peaks towering over the landscape.   Let your eye travel down the slopes and – particularly at this time of year -there are many hues of vegetation – heather moorland, bracken, deciduous and coniferous woodland – changing season by season and, less perceptibly, year by year.   Then turn back to the stream and, with practice, you start to see patches such as these algae which change month by month, sometimes even week by week.   As the Greek philosopher Hereclitus once said, “change is the only constant in life”.  

Some other highlights from this week:

Wrote this whilst listening to: Mozart’s Marriage of Figaro.  My “box set” dates from the days when a boxed set really was a set of LPs in a box. 

Cultural highlights:  The film Rocks, set in East London and filmed with a young, inexperienced cast is well worth watching.  Some reviews described it as a “feel good” movie but, as we watched it straight after Johnson announced the new lockdown, it had its work cut out to raise our moods.  

Currently reading:  Still reading The Poisonwood Bible by Barbera Kingsolver.  Heart of Darkness set in the mid-20thcentury and peopled by believable characters.

Culinary highlight:   Pasta alla Carbonara made with smoked salmon rather than bacon, followed by a raspberry, blackberry and walnut friand.