In search of the source of the Wear …

Having investigated the microscopic world at Wolsingham (see “The River Wear in January” and “The curious life of biofilms), I decided that it would be interesting to head further upstream and see how much difference there was between the algae at the two locations.  I drove up to Wearhead on a cold Saturday morning to take a look but was immediately faced with a conundrum: the River Wear is formed from the confluence of two very different streams, both with extensive catchments on the moors of the northern Pennines.   One of these is Burnhope Burn, which is fed by Burnhope Reservoir, about a kilometre above Wearhead, and the other is Killhope Burn, which drains a large area of blanket bog, forestry and, importantly, abandoned metal mines.   Burnhope Burn is on the left of the photograph above whilst Killhope Burn comes in from the right.  I thought it might be rather interesting to take a sample from each and see how they compared.

The two streams look quite different to one another.   Burnhope Burn, its flow regulated by the reservoir, is the Cain of the pair whilst Killhope Burn is the unruly turbulent Abel.   This was apparent, too, when I was collecting the samples and, again, when I peered at them through my microscope.   Burnhope Burn’s biofilm was thicker and the most conspicuous algae that I could see were green filaments of Klebsormidium.  Killhope Burn’s was thinner and dominated by diatoms.   Many of the same diatoms were found in the two samples, but Burnhope Burn had more of the motile Nitzschia species that benefit from the tangled matrix of green algal filaments that thrived there.

Views of the biofilm from Burnhope Burn (a.) and Killhope Burn (b.) just above their confluence to form the River Wear, February 2018.

I’ve tried to capture the essence of the biofilm from Burnhope Burn in the schematic diagram below.  Compare this with the diagram of the biofilm from the Wear that I showed in my earlier post.   In both cases, we have a mix of organic and inorganic elements, with the organic matter further divided into living organisms and agglomerations of particulate matter.  A few of the species are common to both but there are also some notable differences.   The biofilm in the Wear, for example, had almost no green algae (though that may change over the coming months) whilst that from Burnhope Burn has many filaments of Klebsormidium.   There were motile diatoms at both locations but the species are different: Navicula lanceolata and N. gregaria at Wolsingham and Nitzschia dissipata at Burnhope Burn.  People usually describe differences in the ecology of diatoms in terms of their chemical environment but I sometimes wonder if, in the case of motile diatoms, the nature of the matrix within which they live also plays a role in determining which thrive.

The difference between Burnhope and Killhope Burns is a variation of the theme that I discussed in “Small details in the big picture …”.  Again, regulation of a river or stream plays a role in determining which species of algae can thrive.  However, whereas I found a lot of Platessa oblongella in the unregulated streams of the Ennerdale catchment, the more base-rich environment of the Pennines means that I am much less likely to find P. oblongella in these streams.  In fact, I don’t think I have ever seen it in north-east England (see distribution maps in “Why do you look for the living amongst the dead”).

That reminds me: I was going to write more about the ecology of Platessa oblongella before I was diverted by desmids and Wearhead.   Soon …

A schematic view of the vertical structure of a submerged biofilm from Burnhope Burn, Wearhead, February 2018.   a. Klebsormidium fluitans; b.  Phormidium; c. Nitzschia dissipata (valve view); d. N. dissipata (girdle view); e. Gomphonema cf. calcifugum (valve and girdle views); f. inorganic particles; g. fine particulate organic matter.  Scale bar: 20 micrometres (= 1/40th of a millimetre).



Small details in the big picture …

I’ve written about Platessa oblongella, a small diatom common in low alkalinity environments, before (see “A tale of two diatoms …” and links therein) but my travels around west Cumbria are gradually revealing more and more about the ecology of this organism, so bear with me as I explain my latest findings.

My first graph shows how the distribution of this diatom varies in different types of water body in the Ennerdale catchment.   I have analysed 223 samples from this small area over the past few years and, within this dataset, there is a very clear distinction between situations where Platessa oblongella is abundant and situations where it is very rare.   I have very few records from Ennerdale Water itself (present in just two out of 27 samples, and never comprising more than 2.7% of all diatoms in the sample) nor from the River Ehen, which flows out of the lake (present in just 16 out of 164 samples, and always £ 1% of all diatoms).  By contrast, in Croasdale Beck and in streams that flow into the north-west corner of the lake, it is present in 28 out of 32 samples, with a maximum relative abundance of 69%.   In ten samples it forms more than 10% of all diatoms present.   Several of my samples from the small streams were collected from just a few metres above the point where they joined the lake, which makes the distinction between these streams and the lake that much more intriguing.

My theory – based on data I showed in A  tale of two diatoms  is that Platessa oblongella is a species of disturbed habitats and that the littoral zone of a lake, whilst subject to some turbulence, is less disturbed than the rough world of an unregulated stream.  The contrast between the River Ehen immediately below the dam at the outfall of the lake and the various small tributary streams also supports this idea.

Differences in percentage of Platessa oblongella (including P. saxonica) in epilithic samples from Ennerdale Water and associated streams.  Data collected between 2012 and 2018 (along with one sample from River Ehen collected in 1997).   The photograph at the top of the post shows Ennerdale Water, photographed in January 2018.

Some of the populations I looked at seemed to consist of two distinct forms, one broader than the other.   This variability is quite common in Platessa oblongella and Carlos Wetzel and colleagues recently published a paper which suggests that these are, in fact, two distinct species.   When I first started looking at diatoms, John Carter, my mentor, used the name Achnanthes saxonica, but Krammer and Lange-Bertalot, in the revised Süsswassserflora, regarded this as a synonym of Achnanthes oblongella, a species first found in Thailand.   Wetzel’s study shows, as well as the difference in valve width, differences in the fine details of the striae between the two species.   They also decided that both species belonged in the genus Platessa, rather than Achnanthes.

Platessa oblongella (top) and P. saxonica (bottom) from Croasdale Beck, October 2017.  Scale bar: 10 micrometres (= 1/100th of a millimetre).

Valve width is, however, a very useful criterion, as the histograms below show.   The left hand graph shows a distinctly bimodal distribution of widths in specimens from Croasdale Beck, whilst the right hand graph shows a much tighter, and clearly unimodal, range.   This comes from another tributary stream flowing into the Ehen about 500 metres below the lake itself.  Quite why two species can co-exist in one stream but only one is present in another is not clear.

The modes of these populations are very close to the median widths for P. saxonica (narrow, ± 4/5 – 5 mm) and P. oblongella (broader, ± 6.5 mm) respectively but, as the left hand histogram shows, there is some overlap.    You might have trouble, for example, deciding whether a valve that was 5.5 mm wide was a “fat” P. saxonica or a “thin” P. oblongella.   My standard advice in situations such as this is that we should identify populations not individuals although, in the case of Croasdale Beck, this will still leave a grey area between the “fat” and “thin” valves where a judgement call is necessary.   In this case I think I could have done it because the P. saxonica valves in this stream tended to have a greater length:breadth ratio than those of P. oblongella, though I have not actually quantified this.

Width of valves in populations of “Achnanthes oblongella” from a) Croasdale Beck, and b) an unnamed tributary stream of the River Ehen, October 2017. 

There is more to say about the ecology of these species, but I have probably written enough for now.  I will leave you, for now, to bask in the rare sensation that occurs when diatom taxonomists make a situation clearer rather than more opaque, and return to this subject in a future post.


Carter, J.R. (1970).   Observations of some British forms of Achnanthes saxonica Krasske.  Microscopy: Journal of the Quekett Microscopical Club 31: 313-316.

Wetzel, C.E., Lange-Bertalot, H. & Ector, L. (2017).  Type analysis of Achnanthes oblongella Østup and resurrection of Achnanthes saxonica Krasske (Bacillariophyta).  Nova Hedwigia, Beiheft 146: 209-227.

The curious life of biofilms …

My explorations of the microscopic world of the River Wear have now gone one step further with the transformation of the schematic representation that I presented in The River Wear in January into a three-dimensional diorama.   This shows the “biofilm” on the top of submerged stones, with a layer of Navicula lanceolata at the top (the chocolate brown layer in the photograph from the earlier post) intermingled with small Gomphonema cells on long stalks and some cyanobacterial filaments.   A large part of the biofilm, however, is inorganic particles and aggregations of organic matter.

I’m curious about why this biofilm is thickest in the winter, not just in the River Wear but in many other rivers too.   Part of the reason is that the organisms that form this film can outpace the bugs that want to eat them at this time of year but this is not the whole story.    As the image shows, the biofilm is about far more than just algae, so we need to know a little more about all that organic matter that takes up so much of the space in the picture.   Where does it come from and why does it accumulate on stone surfaces?

The story starts with the polysaccharides that algae and other microorganisms (fungi and bacteria) secrete as they grow.   These polysaccharides play several roles – they provide the stalks for diatoms such as Gomphonema, they help motile diatoms such as Navicula move and they also ensure that any enzymes that the organisms secrete stay in the proximity of the cell while they perform their functions.  However, as well as servicing the organisms that produce them, they also alter the chemical and physical environment on the stone surface.   Organic and inorganic particles, for example, can be trapped amongst the stalks of diatoms such as Gomphonema, but there are also chemical interactions.  River water contains dissolved organic matter, the end-result of the breakdown of organic matter such as leaves further upstream.   This can flocculate to form small particles which can be physically trapped, or it may be adsorbed onto the various polysaccharides in the biofilm.

If you think of a snowball rolling down a hill and growing in size as more and more snow gets stuck on the outside, you have a very rough idea of how a biofilm grows.   Simply being a biofilm is enough to help it become a bigger biofilm, as the wide range of biological, chemical and physical interactions that take place will increase the quantity of living and dead organic material, along with inorganic particles.  The supply of organic material varies through the year, and is greatest in autumn, following leaf fall (see “A very dilute compost heap …”).  The biofilm, unlike the snowball, is largely static; it is the water around it which is moving, bearing with it the raw materials to help it grow.  However, the biofilm also bears the seeds of its own destruction: all that organic matter – whether produced by algae in situ or imported from upstream – makes it a nutritious food source for the small invertebrates that inhabit the stream bed.  I often see midge larvae eating their way through both living and dead matter when I am examining samples under my microscope.   They are there throughout the year, but are busier in the warmer months when, as a consequence, the biofilms are thinner.

Curiously, despite having collected this sample from a stretch of the Wear where I could feel the strength of the current pushing against my legs, flow has relatively little effect on biofilms.   There is a thin layer just above the bed of the river where there is almost no current, due to frictional drag and the biofilms exist in this zone.   Only when the discharge becomes so strong that the stones themselves are overturned do we see major losses to the biofilm itself.   I have seen a medium-sized summer spate in the Wear lead to the opposite effect: a rapid increase in biofilm thickness, presumably because the invertebrates were more vulnerable than the smaller algae.

I will return to the same location on the River Wear in March to see how things have changed.


Blenkinsopp, S.A., & Lock, M.A. (1994).  The impact of storm-flow on river biofilm architecture.   Journal of Phycology 30: 807-818.

Liu, W., Xu, X., McGoff, N.M., Eaton J.M., Leahy, P., Foley, N. & Kiely, G.  (2014).  Spatial and seasonal variation of dissolved organic carbon (DOC) concentrations in Irish streams: importance of soil and topography characteristics.  Environmental Management 53: 959-967.

Lock, M.A., Wallace, R.R., Costerton, J.W., Ventullo, R.M. & Charlton, S.E. (1984).  River epilithon: toward a structural-functional model.  Oikos 42: 10-22.

Stevenson, R.J. (1990).  Benthic algal community dynamics in a stream during and after a spate. Journal of the North American Benthological Society 9: 277-288.