Castle Eden Dene in January

castle_eden_burn_jan19

The story so far: in 2018 I made bi-monthly visits to the River Wear, my local river and tried to capture, in my posts, the changes in the algae that occurred over the course of 12 months (follow the links in “A year in the life of the River Wear” to learn more).  It was an interesting exercise, partly because last summer’s exceptional weather led to some intriguing changes over the course of the year.   Consequently, as 2019 dawned, I thought I should find a different type of stream within a short drive from my home and try again.  So, bearing in mind that Wolsingham is south and west from where I live, I turned in the opposite direction and drove due east instead, stopping on the edge of the brutal concrete housing estates of Peterlee, a most unprepossessing location for a National Nature Reserve.

My journey has brought me right across the Permian limestone that dominates the eastern Durham landscape. Its escarpment rises up close to my home, and I have written about the algae that live in the ponds at the foot of it (see “A hitchhiker’s guide to algae…”).  On the other side, however, the limestone ends in a series of cliffs overlooking the North Sea and small streams have cut into the limestone to create a series of wooded valleys, or “denes”.   I’ve come to Castle Eden Dene, the largest of these: if you want a cultural reference point, watch the film “Billy Elliott”, set just a few miles further north along the coast, or read Barry Unsworth’s The Quality of Mercy.

We made our way down the footpath into the dene on a crisp and very cold winter morning, past the old yew trees from which the name is derived, and myriad ferns.   A deer bounded across the path ahead and disappeared into some scrub, and then we turned a corner and looked into Castle Eden Burn, which runs along the bottom of the dene.   To my surprise, the stream was dry.   This is a valley that cuts through limestone, so it is common for the stream to be dry in the summer, but I had not expected it to be dry in the middle of winter.  Thinking back, however, I realised that there has not been much rain for some weeks, and this may have meant that the water table, still low, perhaps, after last summer’s dry weather, is too low for the stream to flow.

blunts_burn_jan19

Diatoms and cyanobacterial colonies in Blunt’s Burn, Castle Eden Dene, January 2019.   The top photograph shows diatom growths on bedrock; the lower image shows Phormidium retzii colonies, each about two millimetres across.   The photograph at the top of the post shows a yew tree overhanging Castle Eden Burn. 

A few hundred metres further down the dene, we finally heard the sound of running water where a small tributary stream, Blunt’s Burn, joined the main burn.  Judging from my OS map, it drains a good part of Peterlee so it might not have very high water quality.  It was, however, a stream and it did, as I could see with the naked eye, have some distinct diatom-rich growths.    These, I discovered later, were dominated by the diatoms such as Navicula tripunctataand N. lanceolata which are typical of cold weather conditions (see, for example, “The River Wear in January”).   A closer look showed that the orange-brown diatom growths were, in places, flecked with dark brown spots.  Somehow, I managed to get my cold fingers to manipulate a pair of forceps and pick up a few of these spots for closer examination.

blunts_burn_diatoms

Diatoms from Blunt’s Burn, January 2019: a. Navicula tripunctata; b. N. lanceolata; c.Gyrosigma cf. acuminatum; d. Nitzschiacf. linearis (girdle view); e. N. linearis(valve view).  Scale bar: 10 micrometres (= 1/100thof a millimetre).

I had a good idea, when I first saw these spots, that they were colonies of a filamentous cyanobacterium and, peering through my microscope a few hours later, once I had warmed myself up, I was relieved to see that I was right.  I picked out a dark patch and teased it apart before putting it onto a slide with a drop of water.  Once I had done this, I could see the tangle of filaments along with a mass of organic and inorganic particles and lots of diatoms.   The filaments themselves were simple chains of cells (a “trichome”) of Phormidium retzii, surrounded by a sheath.   There were also, however, a few cases, where I could see the sheath without the Phormidium trichome, and in some those I could also see diatom cells.

There are some diatoms that make their own mucilage tubes (see “An excuse for a crab sandwich, really …”) but Nitzschia is not one of those most often associated with tube-formation (there are a few exceptions).    On the other hand, there are some references to Nitzschiacells squatting in tubes made by other diatoms.   Some of those who have observed this refer to Nitzschia as a “symbiont” but whether there is any formal arrangement or is just a by-product of Nitzschia’s ability to glide and seek out favourable microhabitats, is not clear.  There are, as far as I can see, no references, to diatoms inhabiting the sheaths of Cyanobacteria, though Brian Whitton tells me he has occasionally seen this too.

We made our way back along the dry bed of Castle Eden Burn.  Many of the rocks here were quite slippery, suggesting that there had been some water flowing along it in the recent past.  That encouraged me to scrub at the top surface of one with my toothbrush and I managed to get a sample that certainly contains diatoms though these were mostly smaller than the ones that I found in Blunt’s Burn, and there was also a lot of mineral matter.   I’ll need to get that sample prepped and a permanent slide prepared before I can report back on just what diatoms thrive in this tough habitat.  Watch this space …

blunts_burn_phormidium

Cyanobacterial filaments from Blunt’s Burn, Co. Durham, January 2019: a. a single trichome of Phormidium retzii; b. and c. empty sheaths colonised by cells of Nitzschia; d. aPhormidiumfilament with a sheath and a trichome but also with epiphytes and adsorbed organic and inorganic matter.  Scale bar: 10 micrometres (= 1/100thof a millimetre).   

References

Carr, J.M. & Hergenrader, G.L. (2004).  Occurrence of three Nitzschia(Bacillariophyceae) taxa within colonies of tube-forming diatoms. Journal of Phycology23: 62-70.

Houpt, P.M. (1994). Marine tube-dwelling diatoms and their occurrence in the Netherlands. Netherlands Journal of Aquatic Ecology28: 77-84.

Lobban, C.S. (1984). Marine tube-dwelling diatoms of the Pacific coast of North America. I. BerkeleyaHasleaNitzschia, and Navicula sect. Microstigmaticae.  Canadian Journal of Botany63: 1779-1784.

Lobban, C.S. & Mann, D.G. (1987).  The systematics of the tube-dwelling diatom Nitzschia martiana and Nitzschia section Spathulatae. Canadian Journal of Botany.  65: 2396-2402, 

 

Advertisements

Little round green things …

Apatococcus_on_Fence

Nature does not get much more prosaic than this: my garden fence covered with a fine, powdery green coating. For most of the year this is hidden behind the foliage of our apple and willow trees but as autumn gives way to winter, so the bare green slats became visible again.  Last week, staring out of the window whilst completing my previous post, it occurred to me that, in the six years that I have been writing this blog, I have never made the short journey across the grass to look at one of the most common algae in the country.

I scraped a single-sided razor blade across the surface of one of the slats to harvest a small quantity of the damp, powdery film, and put a few specks under my microscope in order to take a closer look.   What this revealed was lots of clumps of small near-spherical green cells.   That, along with an ability to live in terrestrial habitats are about all the natural historian has to go on when trying to name this organism.  My old copy of West and Fritsch suggests Pleurococcus naegelii, adding that “there is probably no other alga about which there has been so much confusion” whilst the latest guide to British algae would call it either Apatococcus lobatus or Desmococcus olivaceum– their descriptions are very similar.   Desmococcus olivaceum has been described as “the commonest green alga in the world”, which is a bold claim.  Certainly, green powdery coatings such as these are found in shaded locations in a great many places but is the singular “alga” really appropriate?  Cells such as these offer so few visual clues that the microscopist is apt to latch onto a phrase such as “one of the commonest terrestrial algae” alongside a description that roughly matches the material, and considers it to be job done.  These groups have also been referred to as “LRGT” (“little round green things”) – the phycological equivalent of the ornithologist’s “little brown jobs”.   Recent molecular studies suggest that there is a lot of diversity within these powdery films, and this is almost certainly going to be very difficult to resolve with traditional methods.  It looks as if we going to struggle with these “LRGT” for the foreseeable future.

The Class Trebouxiphyceae seems to have a particularly large number of LRGT.   Some (such as the one I am describing in this post) are free-living and capable surviving desiccation, but this group also includes many of the algae that unite with fungi to form lichens, whilst others prefer to live in truly aquatic situations.   But it is the fence-dwelling forms that are of interest to me today, and even if I cannot put the exact name onto my powdery film, I can perhaps offer some thoughts on why it thrives where it does.

Apatococcus_lobatum_181218

Cells of Apatococcus lobatum(?) from a garden fence in County Durham.  Scale bar: 10 micrometres (= 1/100thof a millimetre).   The photograph at the top of the post shows the fence in my back garden from which it was collected.

Some of the other terrestrial (or semi-terrestrial) green algae that I’ve described in this blog are endowed with brightly coloured pigments that protect them from the damaging ultra violet rays in sunlight (see “Fake tans in the Yorkshire Dales” and “An encounter with a green alga that is red”).  Apatococcusand Desmococcus, by contrast, do not come with preloaded sunscreens.  They thrive, by contrast, in relatively shaded locations where the gradual accumulation of cells on the fence surface means that the outer cells take the primary ultra violet hit and, in the process, protect those cells underneath.   There is also evidence of Apatococcus producing lots of stress compounds.  These belong to a class of compounds call “polyols” – complicated alcohols.  A lifestyle that involves single cells sitting on a damp fence indefinitely might seem like an evolutionary dead end.   However,  they have the last laugh as, unlike us, they are genetically adapted to produce their own booze when the going gets tough.

A further adaptation that has been observed is that the cells can switch between producing their own simple sugars via photosynthesis, and absorbing sugars and other organic compounds directly, a strategy known as “mixotrophy”.   Walls and fences are challenging habitats for any organism so having the ability to mop up any spare fuel (leaking down from one of those outermost cells that took one for the team, perhaps?) might give the organism a slight competitive advantage over time.

Back when I was doing my Fine Art degree, I was using algae to explore the boundaries between abstraction and representational art.  My thesis was that an image of an alga could be either representational or abstract depending on how much prior knowledge the viewer brought to the image.   I used the Apatococcus (or is it Desmococcus) from my garden fence as subject matter for this exploration, creating a sextych (honestly, that’s the word for a painting on six panels) that juxtaposed the minimal outline of fence panels with microscopic views of the alga.  The three fence panels offer the unprepossessing view that most people will walk past for their entire life without a thought, whilst the microscopic views give an insight into the hidden world even though the arrangement of shapes and colours will not match any of the schemata lodged in the memories and experiences of most of the viewers (see “Abstracting from reality …”).

Apatococcus

Apatococcus. 2008 50 x 130 cm.  Acrylic and photomicrograph on canvas.

References

Gustavs, L., Schumann, R., Karstens, U. & Lorenz, M. (2016).  Mixotrophy in the terrestrial green alga Apatococcus lobatus(Trebouxiphyceae, Chlorophyta).  Journal of Phycology52: 311-314.

Laundon, J.R. (1985). Desmococcus olivaceus– the name of the common subaerial green alga.   Taxon 34: 671-672.

Lemieux, C., Otis, C. & Turmel, M. (2014).  Chloroplast phylogenomic analysis resolves deep-level relationships within the green algal class Trebouxiphyceae.  BMC Evolutionary Biology14: 211.

 

The River Wear in November

Wear_Wolsingham_181119

I was back at the River Wear last week for my final visit of the year.   The heatwave that dominated the summer seems like an aeon ago as I plunged my arm into the cold water to find some stones and take some photographs.  I’m curious to see what is here, though.   The river has surprised me several times already this year.  Has it reverted to type as the British climate has regained a semblance of normality, or will the changes that we saw in the summer (see “Summertime blues …” and “Talking about the weather …”) still have consequences for the algae growing on the river bed?

The river bed itself had many patches of green filamentous algae which, on closer examination, turned out to be my old friend Ulothrix zonata, an alga that is common in these parts and which has a distinct preference for early spring conditions (see “Bollihope Bavakakra” and references therein).   A closer look showed two types of filament present: the normal vegetative ones with a single chloroplast encircling the cell but also some where the cell contents have divided to produce zoospores which are released and which, if they land on a suitable surface, will produce new vegetative filaments.   The “parent” filaments, themselves, are produced as zygotes, produced back in the spring, germinate.  The zygotes are the product of sexual reproduction, triggered by lengthening days (see reference in earlier post) and are dormant through the summer, only germinating once day length shortens and temperatures start falling.

Wear_Wolsingham_bed_Nov18

The river bed of the River Wear at Wolsingham, November 2018, showing conspicuous growths of Ulothrix zonata.

Ulothrix_zonata_Nov18

Magnified views of Ulothrix zonatafilaments from the River Wear at Wolsingham.  The upper image shows a vegetative filament and the lower image shows filaments where the cell contents have divided up prior to the release of zoospores.  Scale bar: 20 micrometres (= 1/50thof a millimetre).

The areas between the patches of Ulothrix zonatawere covered with a thick film, composed primarily of diatoms, in contrast to the situation on my last two visits when non-filamentous green algae predominated.  This time, it was Achnanthidium minutissimumdominated my count (about 70% of cells) although, because they are relatively small, they comprised just under half of the total volume of algae present.   Other diatoms bumped this up to about 70 per cent of the total volume, with motile cells of Navicula and Nitzschia, which were so abundant at the start of the year, beginning to appear in numbers again.   The green cells that dominated my counts in July and September now only constitute about five per cent of the total.   The River Wear, in other words, has shaken off the effects of the summer, just as a healthy human gets over a winter cold, and is now back to its old self.

Wolsingham_181119_#1

A view down my microscope whilst examining samples from the River Wear at Wolsingham showing the predominance of Achnanthidium minutissimum with (on the right-hand side) a filament of a narrow Ulothrix (not U. zonata).  

More green algae from the River Wear

Having discussed some of the recent name changes in green algae in the previous post, I thought that I would continue this theme using some of the other taxa that I found in the samples I collected from the River Wear a couple of weeks ago.   The plate below shows some specimens that, 20 years ago, I would not have hesitated to call Scenedesmus, characterised by coenobia of either four cells or a multiple of four cells arranged in a row.   Over 200 species, and 1200 varieties and forms have been recognised although there were also concerns that many of these so-called “species” were, in fact, variants induced by environmental conditions.  A further problem is that Scenedesmus and relatives do not have any means of sexual reproduction.  This means that any mutation that occurs and which does not have strong negative effects on the organism will be propagated rather than lost through genetic processes.  Working out what differences are really meaningful is always a challenge, especially when dealing with such tiny organisms.

Scenedesmus and Desmodemus species from the River Wear, Wolsingham, September 2018.  a. and b. Scenedesmus cf ellipticus; c. Desmodesmus communis.   Scale bar: 20 micrometres (= 1/50th of a millimetre).

The onset of the molecular era shed some new light onto these problems but, in the process, recognised differences within the genus itself that necessitated it being split into three, two of which are on the plate below.  Scenedesmus, in this modern sense, has cells with obtuse (rounded) apices and mucilage surrounding the cells whilst Desmodesmus has distinct spines at the apices of marginal cells and, sometimes, shorter ones elsewhere too.   In addition to these there is Acutodesmus, which is similar to Scenedesmus (i.e. without spines) but whose cells have more pointed (“acute”) ends and which does not have any surrounding mucilage.   A further genus, Pectinodesmus, has been split away from Acutodesmus on the basis of molecular studies, although there do not seem to be any features obvious under the light microscope which can differentiate these.

The genera Ankistrodesmus and Monoraphidium present a similar situation.  In the past, these long needle- or spindle-shaped cells would all have been considered to be Ankistrodesmus.   Some formed small bundles whilst others grew singly and this, along with a difference in their reproductive behaviour, was regarded as reason enough for splitting them into two separate genera.   Both were present in the Wear this summer, but only Monoraphidium presented itself to me in a manner that could be photographed.  Two species are shown in the plate below.   Recent molecular studies seem to not just support this division but also suggest that each of these could, potentially, be divided into two new genera, so we’ll have to watch out for yet more changes to come.

Monoraphidium species from the River Wear, Wolsingham, September 2018.  a. and b.: M. griffthii; c. M. arcuatum.  Scale bar: 20 micrometres (= 1/50th of a millimetre).

The final illustration that I managed to obtain is of another common coenobium-forming alga, Coelastrum microporum.   Though the three-dimensional form makes it a little harder to see, cell numbers, as for Pediastrum, Scenedesmus and Desmodesmus, are multiples of four.  I apologise if the picture is slightly out of focus, but it is a struggle to use high magnification optics on samples such as these.  The oil that sits between the lens and the coverslip conveys the slight pressure from fine focus adjustments directly to the sample, meaning that the cells move every time I try to get a crisper view.  That means it is impossible to use my usual “stacking” software.   The answer is to use an inverted microscope so that the lens was beneath the sample.  However, I do this type of work so rarely that the investment would not be worthwhile.

That’s enough for now.   I’m off on holiday for a couple of weeks, so the next post may be from Portugal and perhaps I will also find time to sample the River Duoro as well as the products of the vineyards in it’s catchment…

Coelastrum microporum from the River Wear,Wolsingam, Septmber 2018.  Scale bar: 20 micrometres (= 1/50th of a millimetre).

References

An, S.S., Friedl, T. & Hegewald, E. (2008).  Phylogenetic relationships of Scenedesmus and Scenedesmus-like coccoid green algae as inferred from ITS-2 rDNA sequence comparisons.   Plant Biology 1: 418-428.

Hegewald, E., Wolf, M., Keller, A., Friedl, T. & Krienitz, T. (2010).  ITS2 sequence-structure phylogeny in the Scenedesmaceae with special reference to Coelastrum (Chlorophyta, Chlorophyceae), including the new genera Comasiella and Pectinodesmus.   Phycologia 49: 325-355.

Krienitz, L. & Bock, C. (2012).  Present state of the systematics of planktonic coccoid green algae of inland waters.   Hydrobiologia 698: 295-326.

Krienitz, L., Bock, C., Nozaki, H. & Wolf, M. (2011).   SSU rRNA gene phylogeny of morphospecies affiliated to the bioassay alga “Selanastrum capricornutum” recovered the polyphyletic origin of crescent-shaped Chlorophyta.  Journal of Phycology 47: 880-893.

Trainor, F.R. & Egan, P.F. (1991).  Discovering the various ecomorphs of Scenedesmus: the end of a taxonomic era.   Archiv für Protistenkunde 139: 125-132.

A hitchhiker’s guide to algae …

One of the recurring themes of this blog is the hidden delights of natural history for anyone prepared to take a closer look at unprepossessing locations, so it is appropriate that we have found some quite rich habitats within walking distance of our home in County Durham.   I’ve written before about visits to Crowtrees, a local nature reserve (see “More pleasures in my own backyard” and “Natural lenses”) and Heather is also writing a series of posts about the ever-changing flora of this small vale at the foot of the Permian limestone escarpment (see “Crowtrees LNR July 2018 part 2: gentians to grasses” for the most recent and links back to previous ones).   I visited again last week, taking Brian Whitton along for company.

His interest was the red alga Chroothece ricteriana, which I described in one of my earlier posts about Crowtrees but we did not find it on this particular visit.   Instead, my eye was drawn to soft clouds of green filaments that floated just above the bed of the pond.   When I looked closely under my microscope, I saw that these were thin filaments of Oedogonium.  Typically, these had no reproductive organs, so cannot be named (see “Love and sex in a tufa-forming stream” for a rare exception), but all showed characteristic “cap cells” (see lower illustration).

Growths of Oedogonium in Crowtrees pond, August 2018.   The frame width is about 30 centimetres.   The photograph at the top of the post shows Brian Whitton searching for algae during our visit.

The diatom Achnanthidium minutissimum was growing on small stalks attached to the Oedogonium filaments, often alone but also in pairs and stacks of four, as the diatom cells divided and re-divided.  Oedogonium is a rougher alga to the touch than filamentous genera such as Draparnaldia, Stigeoclonium and Spirogyra, and often carries epiphytes, and I presume the lack of mucilage is a factor in this.   Achnanthidium minutissimum is a diatom that is very common on the upper surface of submerged stones in both lakes and rivers, but it is not fussy and I often see it as an epiphyte if conditions are right.  In this case, I suspect that the very hard water of Crowtrees Pond is a factor: calcium carbonate is constantly being precipitated from the water to create a thin layer of “marl” (see photo in “Pleasures in my own backyard”).   This makes life difficult for a tiny diatom that cannot move, so hitch-hiking a ride on the back of a filamentous alga that floats about the lake bottom makes a lot more sense.

Oedogonium filaments with epiphytic Achnanthidium minutissimum, from Crowtrees pond, August 2018.  Scale bar: 20 micrometres (= 1/50th of a millimetre).  

Oedogonium is an adaptable genus.  It is also common in the River Ehen (soft water, low nutrients) and I also find it in lowland polluted rivers too.  Being able to name the species would, I am sure, help us to better understand the ecology but this is, as I have already mentioned, problematic (see “The perplexing case of the celibate alga”).   However, in each of the cases I’ve mentioned, the epiphytes are different (Achnanthidium minutissimum here, Tabellaria flocculosa and Fragilaria species in the Ehen, Rhoicosphenia and Cocconeis placentula in enriched lowland rivers) and I suspect that these might offer an easier way to interpret the habitat than the filaments themselves, at least until someone finds a stress-free way of naming them.

Spring comes slowly up this way …*

I took a few minutes out on my trip to Upper Teesdale to stop at Wolsingham and collect one of my regular samples from the River Wear.  Back in March, I commented on the absence of Ulothrix zonata, which is a common feature of the upper reaches of rivers such as the Wear in early Spring (see “The mystery of the alga that wasn’t there …”).   I put this down to the unusually wet and cold weather that we had been experiencing and this was, to some extent, confirmed by finding prolific growths of Ulothrix zonata in late April in Croasdale Beck (see “That’s funny …”).   Everything seems to be happening a little later than usual this year.   So I should not have been that surprised to find lush growths of green algae growing on the bed of the river when I waded out to find some stones from which to sample.

These growths, however, turned out to be Stigeoclonium tenue, not Ulothrix zonata (see “A day out in Weardale”): it is often hard to be absolutely sure about the identity of an alga in the field and, in this case, both can form conspicuous bright green growths that are slimy to the touch.   Did I miss the Ulothrix zonata bloom in the River Wear this year?   Maybe.   Looking back at my records from May 2009 I see that I recorded quite a lot of narrow Phormidium filaments then but none were apparent in this sample.   That taxon thrived throughout the summer, so perhaps, again, its absence is also a consequence of the unusual weather.

Growths of Stigeoclonium tenue on a cobble in the River Wear at Wolsingham, May 2018.  

The photograph illustrates some of the problems that ecologists face: the distribution of algae such as Ulothrix zonata and Stigeoclonium zonata is often very patchy: there is rarely a homogeneous cover and, often, these growths are most prolific on the larger, more stable stones.   I talked about this in Our Patchwork Heritage; the difference now is that the patchiness is exhibited by different groups of algae, rather than variation within a single group.   Ironically, the patchiness is easier to record with the naked eye than by our usual method of sampling attached algae using toothbrushes.   That’s partly because we tend to sample from smaller substrata (the ones that we can pick up and move!) but also because of the complications involved in getting a representative sample.   We have experimented with stratified sampling approaches – including some stones with green algae, for example, in proportion to their representation on the stream bed – but that still means that we have to make an initial survey to estimate the proportions of different types of growth.

Under the microscope, therefore, the algal community looks very different.   There are fewer green cells and more yellow-brown diatom cells, these dominated by Achnanthidium minutissimum, elegant curved cells of Hannaea arcus and some Navicula lanceolata, still hanging on from its winter peak.   The patterns I described in The mystery of the alga that wasn’t there … are still apparent although the timings are all slightly adrift.

A view of the biofilm from the River Wear, Wolsingham in May 2018.

The schematic view below tries to capture this spatial heterogeneity.  On the left hand side I have depicted the edge of one of the patches of Stigeoclonium.   Healthy populations of Stigeoclonium do no support large populations of epiphytes, probably as a result of the mucilage that this alga produces.  My diagram also speculates that the populations of Gomphonema olivaceum-type cells and Ulnaria ulna may be living in the shadow of these larger algal growths, as neither is well adapted to the fast current speeds on more exposed rock surfaces.  Finally, on the right of the image, there are cells of Achnanthidium minutissimum, small fast-growing cells that can cope with both fast currents and grazing.   I have not included all of the taxa I could see under the microscope, partly because of the space available.  There is no Hannaea arcus or Navicula lanceolata and I have also left out the chain of Diatoma cells that you can see on the right hand side of the view down the microscope.

The speckled background in the image of the view down the microscope is, by the way, a mass of tiny bacteria, all jigging around due to Brownian motion.  The sample had sat around in the warm boot of the car for a few hours after collection so I cannot be sure that these were quite as abundant at the time of collection as they were when I came to examine it.  However, some people have commented on the absence of bacteria – known to be very abundant in stream biofilms – from my pictures, so these serve as a salutary reminder of an extra dimension that really needs to be incorporated into my next images.

Schematic view of the biofilm from the River Wear at Wolsingham, May 2018.  a. Stigeoclonium tenue; b. Gomphonema olivaceum complex; c. Ulnaria ulna; d. Meridion circulare; e. Achnanthidium minutissimum.   Scale bar: 10 micrometres (= 1/100th of a millimetre).

* Samuel Taylor Coleridge, Christabel (1816)

 

A return to the River Team

Team_160124

A microscopic view of the River Team, near Causey Arch, showing Cladophora glomerata filaments with epiphytic Cocconeis spp (mostly C. euglypta), Rhoicosphenia abbreviata and the cyanobacterium Chamaesiphon incrustans. At the bottom right hand corner there is a patch of sediment inhabitated by Nitzschia palea.   This is a composite based on various visits over the past few years.   The Rhoicosphenia abbreviata cells in the foreground are approximately 20 micrometres (=1/50th of a millimetre) long.

My intention with my paintings of the submerged world was partly to convey the wonder of the microscopic world to the wider world, but also to provoke a debate with colleagues about what the data that we spent so many hours collecting, actually means.   One consequence of this is that I have to go back to some of my earliest pictures and change them, in response to the feedback I receive.   This picture is a case in point.   To be frank, my original image of the River Team (see “An Indian summer on our riverbanks …”) was one of the earliest that I had produced (back in 2009) and I have learned quite a lot about the media that I use to produce the images in the interim. I had been hunting through my collection of pictures to find one to illustrate a scientific paper that I am writing with colleagues, and ended up producing a completely new image.

These days, the bed of the River Team is usually smothered with lush growths of the green alga Cladophora glomerata although when I first arrived in the region in 1983, you never saw it in the river, as it could not tolerate the high concentrations of zinc released from a battery factory a few kilometres upstream.   That factory has long since closed, but the river still receives effluent from a sewage works, and is one of the few rivers in the north east where I still see sewage fungus.

A quick look at my records for this one short but rather polluted river puts the diversity of the microscopic world into perspective. I have records for 59 different samples on my database from 13 sites, spanning a distance of about 15 kilometres collected between 2004 and the present and these contain 175 different species of diatom. Many of these are not very common (more than half never form more than one per cent of the total and about a quarter were only ever recorded in a single sample) but it is still an impressive total.   That the average number of species per sample was only 36 further puts this number into perspective, and highlights the amount of variation that can be encountered over short distances and over time.   The three diatom species illustrated in my painting were all amongst the top ten, ranked by both frequency of records and abundance but, at the same time, a quick look in the river (see below) or at my picture (above) put this long list of diatom names into perspective.

Team_Causey_2009_squeezed

The bed of the River Team at Causey Arch, April 2009, smothered with Cladophora glomerata.

First, the passer-by’s immediate perception of the river is of the green algae smothering the bed of the river, rather than the diatoms. It was this that stimulated the development of RAPPER (see “The democratisation of stream ecology?”). But, if we want to understand diatom ecology, we cannot ignore the other algae, which create the habitat upon and around which the diatoms live.   The Cladophora filaments have a big influence on the type of diatoms that we find at a site; even if we are not sampling them directly (and it is hard not to include at least a few filaments in every sample), they are providing inocula of diatoms that can colonise other surfaces.

I should not be too dismissive about diatoms, as they have provided the bulk of my income for over twenty years, but I cannot help but howl with frustration, at times, at the lack of engagement of diatom specialists with functional ecology. Over the last twenty years, we have learnt much more about the taxonomy of diatoms, but this has not really filtered through into better approaches for ecological assessment.    Part of this is, I am sure, that the diatomist looks at samples that are so divorced from their context that a suite of analytical and statistical methods has developed which work around this problem.   That works fine for palaeoecology but is a problem when it comes to relating the lists of diatoms that we collect back to the habitats from which we collected them. I have some theories for how this situation has arisen, but these will have to wait for another day.

My next challenge is to incorporate some of the other microbial life that I see as I peer through my microscope.   As well as other algae, shoots of Cladophora are often smothered with filamentous bacteria, and I really ought to think about how to incorporate these into my illustrations, to make the point that there is a profound shift in the energy sources that underpin organically-polluted rivers.   I have tried to incorporate animals before (see “More about Very Hungry Chironomids”) but it was a struggle to understand the complex structure of their mouthparts, as my notes in that post show.   Bacteria are morphologically simpler but, even so, it would be another step outside my comfort zone.

But isn’t that the point?   In science the emphasis is always on specialisation yet, as we learn more and more about one aspect, we run the risk of losing touch with peripheral areas.   And ecology, more than almost any other discipline, needs that holistic overview.   The specialist is always at a disadvantage … though I would not dare say that in front of some of my diatomist friends …