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).


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


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.


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 …

A return to Cassop


Cassop Pond, and Cassop Vale, looking towards Durham, April 2015.

One of my earliest posts described the algae that I found in Cassop Pond, which lies at the foot of the Permian escarpment close to my house in County Durham (see “Cassop”).   I returned there a couple of days ago to grab a couple more samples and see what had changed since my last visit.   The first of these samples was a handful of submerged plant stems, which I crammed into a sample bottle and shook to dislodge the algae.   I pipetted a couple of drops of the brown suspension that this produced onto a microscope slide and put it under my microscope.   Prominent amongst the diatoms that I could see in this sample was a nice colony of Gomphonema truncatum var. capitatum and also a cell of the large diatom Cymbella lanceolata, displaying its characteristic lobed chloroplast. I also saw a few cells of another diatom, Epithemia adnata. This diatom is relatively uncommon in the UK, but it gives us some interesting insights into the ecology of Cassop Pond.


Diatoms from Cassop Pond, April 2015.   a. Gomphonema truncatum var. capitatum; b. Cymbella lanceolata; c. Epithemia adnata, valve view; d. Epithemia adnata, girdle view of two recently divided cells.   Scale bar: 25 micrometres (= 1/40th of a millimetre).

Cells of the genus Epithemia contain small cyanobacteria-type cells that are capable of nitrogen-fixation (the reality is a little more complicated – see the reference by Prechtl et al. below for more information). This means that it can thrive in situations where nitrogen is relatively scarce compared to other nutrients.   I came across it in a small stream in Northumberland, downstream of a forestry plantation. I was interested in the stream because the plantation was being fertilised with phosphorus at the time and I wanted to see what this would do to the stream.   Every time it rained (which was quite often in the Northumberland hills), some of the phosphorus was washed into the stream and, gradually, over a period of about a month after the fertilisation, the proportion of Epithemia in my samples increased from undetectable at the start to over forty percent of all the diatoms a couple of months later.   What I suspect was happening was that most of the algae in this remote stream could not use this extra phosphorus because nitrogen was naturally very scarce. However, Epithemia is one of a very small number of diatoms that can overcome nitrogen limitation and so was able to thrive. Finding it in Cassop Pond is, therefore, a clue that this pond is, periodically at least, limited by nitrogen rather than by other nutrients.

The other photograph I’ve included in this post was taken by Chris Carter and shows Epithemia growing on the surface of Chara virgata. Epithemia is a genus that does often seem to be associated with plants, although I have also seen it growing on rocks. Chris’ photograph also shows the lobed chloroplast very clearly.


Epithemia sp. growing on a stem of Chara virgata.   Photograph by Chris Carter.


DeYoe, H.R., Lowe, R.L. & Marks, J.C. (1992). The effect of nitrogen and phosphorus on the endosymbiont load of Rhopalodia gibba and Epithemia turgida (Bacillariophyceae). Journal of Phycology 23: 773-777.

Kelly, M.G. (2003). Short term dynamics of diatoms in an upland stream and implications for monitoring eutrophication.   Environmental Pollution 125: 117-122.

Prechtl, J., Kneip, C., Lockhart, P., Wenderoth, K. & Maier, U.G. (2004). Intracellular spheroid bodies of Rhopalodia gibba have nitrogen-fixing apparatus of cyanobacterial origin. Molecular Biology and Evolution 21: 1477-1481.

An Indian summer on our riverbanks …

One of my favourite places in County Durham is, paradoxically, also one of the most polluted.   When I first visited, over thirty years ago, the stretch of the River Team pictured below was heavily polluted from several sources, including sewage works, abandoned coal mines and a large battery factory, just a couple of kilometres upstream. The latter made the river an ideal open-air laboratory for work that we were doing at the time on the effect of heavy metals.   The River Team, like many other rivers in this part of County Durham, flowed through a gorge incised into the landscape. The steep slopes of the gorge made agriculture and settlement impractical so locations such as this are blessed by beautiful wooded valleys (the incised meander at Durham City is the most famous example of these – see “The River Wear in summer”).


Causey Arch crossing the River Team in County Durham, July 2014.

Causey Arch, the bridge in my picture dates from 1726 and is the oldest surviving single-arched railway bridge in the world and is a reminder of the region’s industrial heritage. George Stephenson is a local lad and the Stockton and Darlington Railway, the first public railway to use steam locomotives, is just 50 km to the south. So do not be deceived by the idyllic view of the wooded gorge around Causey Arch: the surrounding area has a high population density and a long history of water-polluting industries.

Although the battery factory that polluted the river in the 1970s and 80s has now closed, the river is still heavily polluted, particularly from a sewage works about a kilometre upstream from where this photograph was taken.   There has, however, been one other change since my first visits in the early 1980s: the banks of the river are now thick with the pink-flowered plants of Himalayan Balsam, Impatiens glandulifera.   The riverbanks do not just look different, they are also heavy with Himalayan Balsam’s sweet, pungent odour.  Ironically, these changes have taken place as the water quality itself has gradually improved.

As the name suggests, Himalayan balsam is not a native plant; it was introduced here in the 19th century for its attractive pink flowers but has escaped from gardens to become a nuisance over much of the country.   You can read more about this in Heather’s blog, written in preparation for her trip to the Himalayas later this year.   She refers back to Frank Smythe’s classic book “Valley of Flowers” in which he notes that, even in the Himalayas, this plant is often a nuisance weed.


Himalayan balsam, Impatiens glandulifera, growing at Causey Arch, County Durham, July 2014.

My own private gripe with Himalayan balsam is that the tall plants grow just far enough apart to allow nettles to thrive in the gaps. I’ve struggled to get into rivers to collect my samples several times this year already and suffered multiple stings as a result.   I have slowly realised that my chest waders, though very hot to wear on a sunny day, are much preferable to bare legs.


There is more about invasive plants in the Plant Invaders episode of Plants: from Roots to Riches on the BBC iPlayer. Curiously, Himalayan balsam is not mentioned in this programme but it is worth a listen nonetheless.

Wonders in my own backyard …

One of my many half-worked out and not-fully-proven theories is that the golden age of Victorian microscopy coincided with an era when many educated British men were heading off to the colonies and sending back reports of weird and wonderful flora and fauna that they encountered.   The microscope was, for those left behind, a similar portal into hitherto unexplored worlds; one that, furthermore, could be found without leaving your own grounds.

A case in point: here is a photograph of some moss on my driveway.  I have walked past these mosses thousands of times without giving it a second thought.   Today, however, I have a point to prove.   The second photograph is a close up of the same moss, taken with the extreme macro lens on my new Olympus TG2 compact camera.   This reveals the colonies to consist of tongue-shaped leaves, each terminating in a long hair-like projection.   My somewhat dated guide to mosses tells me that these are plants of Bryum capillare.   Even at barely a millimetre across, these leaves are enormous compared to the algae I normally write about here.


A row of bright green colonies of Bryum capillare beside my driveway in County Durham, with a lens cap (five cm across) as an indication of scale.

The next step is to strip a few of the leaves off the plants using a pair of forceps and blade and mount these in a drop of water to examine under my high power microscope.   Ironically, the lowest magnification lens I have on this microscope (10x) is too powerful and I cannot get all the leaf into a single image, but we can see the hair point as an extension of the “nerve” that extends the length of the leaf.   Just visible, too, are the long, narrow cells which form a border around the leaf edge.  The cells, themselves, are just a single cell thick, each parallelogram-shaped, about 50 micrometres long and containing a number of small chloroplasts.

I wrote about the tops of boulders being like miniature deserts last year (“Upper Teesdale In March”) and the same applies to man-made habitats such as paths and driveways.  The cushion-like growth forms contains networks of tiny spaces which turn the whole plant into a miniature sponge, soaking up and retaining water, enabling it to continue to grow long after the ground around it has dried up.   In the past, I presume, mosses such as Bryum capillare would have been rare but, with our modifications to the landscape, including building walls and driveways, we have greatly expanded the habitat available to this species.   As a result, our sedentary Victorian naturalist had just as many opportunities to explore deserts as Richard Burton, Charles Montagu Doughty and Wilfred Thesiger.


Bryum capillare.  The left hand image is taken with a macro lens; the right hand image was taken under a microscope; the scale bar is 100 micrometres (1/10th of a millimetre).   The hair is roughly double the length of the portion included in the image.