A day out in Wasdale


A few days after my trip to Weardale I found myself beside the River Irt, a few hundred metres below the point where it flows out of Wastwater, in the western part of the Lake District.   Whereas the River Wear drains a catchment underlain by Carboniferous rocks, including a high proportion of limestone (see “Co. Durham’s secret Karst landscape”), the Irt’s catchment is largely underlain by ancient volcanic rocks, resulting in much softer water.   I was curious to see how different the algae were here compared to those in the Wear.

The river bed at this point is dominated by boulders of granite, which host a patchwork of mosses, filamentous algae and discrete growths of diatoms (visible on the right-hand side of the figure below).  Between these there were areas of pebbles and gravels, suggesting good habitat for freshwater mussels.   The patches of filamentous algae (mostly no more than a couple of centimetres in length) were a mixture of Mougeotiaand Zygnema, similar to the forms that I find in the River Ehen, a 30 minute drive to the north.   These two species differ in the form of their chloroplasts (Mougeotiahas a flat plate whilst Zygnemahas two star-shaped chloroplasts, attached by thin cytoplasmic strands to resemble an animal skin stretched on a frame) but are closely-related, both belonging to the family Zygnemtaceae.


An underwater photograph of the substratum of the River Irt in November 2018 showing patches of filamentous green algae, mosses and (on the right-hand side) diatoms growing on granite boulders.


Filamentous green algae from the River Irt, November 2018.   The upper photograph shows cells from a filament of Mougeotiawhilst the lower image shows two filaments of Zygnema. Scale bar: 20 micrometres (= 1/50thof a millimetre).

In between the tufts of filamentous algae were apparently bare patches of rock (they almost certainly had a very thin biofilm that would be hard to sample in isolation from the lusher algal growths that shared their habitat) and some conspicuous orange-brown growths of colonial diatoms.  These turned out to be almost pure growths ofGomphonema hebridense, or a close relative (I can’t give a definitive answer until I have examined cleaned material), growing on long mucilaginous, sometimes branched, stalks to create a veritable “bush” of diatoms.  There were a few other species of diatom growing within this bush, most notably some cells of Achnanthidium (cf.) caledonicumthat seemed to be growing on short stalks attached to the Gomphonemastalks, but also a few cells of Gomphonema capitatum(which also grows on long stalks) and some chains of Tabellaria flocculosa.

Gomphonema hebridenseis a diatom that I have written about several times before, as it is also common in the River Ehen, and also presents a number of interesting challenges to taxonomists (see “Diatoms and dinosaurs”). Whatever future studies reveal, however, the presence of colonies of this (or these) species that are visible with the naked eye is something I associate with only the cleanest rivers in the country during the cooler times of year.  It should not have been a great surprise to me to find it flowing out of one of the most pristine lakes in England (see “The Power of Rock …”).


A close up of cells within a colony of Gomphonemacf hebridense.  Several mucilaginous stalks are also visible as well as (top left) a cell of Achnanthidiumcf caledonicum.   Scale bar: 10 micrometres (= 100th of a millimetre).

The predominance of boulders over smaller, more easily moved stones, suggests a river that has more energy than the River Ehen, one of my regular Lake District haunts.   Both flow out of lakes whose catchments include some of the wildest and most mountainous terrain in the country.   Lakes tend to act as shock absorbers in catchments, slowing down the water that pours off the fells after heavy rain.   Streams in this part of the world that have no such impediments to flow tend to have rocky, mobile beds and relatively sparse algal communities.   By contrast, the Irt and Ehen just below their respective lakes have relatively lush growths of algae.   The substrates of the two rivers, however, are very different: the Ehen having very few boulders in comparison to the Irt, due to the presence of a weir at the outfall. This allows Ennerdale Water to be used as a water supply for the towns of north west Cumbria but, at the same time, turns the lake into an even more effective hydrological shock absorber.  Yet more of the energy that should be washing smaller stones down the river is no longer available except after the most exceptional storms.

That’s my working hypothesis, then: the Irt is a river that is subject to just enough high energy events to move the rocky substrates around yet no so many that rich algal communities cannot develop between these.  The Ehen, by contrast, has fewer events, leading to fewer opportunities for the algae to be scoured away, whilst unregulated streams such as Croasdale Beck (see “What a difference a storm makes …”) have such regular scouring spates that the algal communities are usually sparse.   I might be wrong, of course and I might be back in a years time with a better hypothesis.  Until then …




The River Wear in November


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.


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


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.


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).  

The natural history of numbers

I have made a few facetious comments in this blog about the tendency for ecologists to spend more time staring at spreadsheets than engaging directly with the organisms and habitats they are trying to understand.   There is, of course, a balance that needs to be struck.   We can learn a lot from analysing big datasets that would not have occurred to a biologist who spent all his or her time in the field.  And, I have to admit, somewhat grudgingly, there is a beauty to the numerical landscapes that becomes apparent when a trained eye is brought to bear on data.

I’ve been involved in a project for the European Commission which has been trying to find good ways of converting the ecological objectives that we’ve established for the Water Framework Directive into targets for the pressures that lead to ecosystem degradation.   The key principle behind this work is summarised in the graph below: if the relationship between the biology (expressed as an Ecological Quality Ratio, EQR) and a pressure (in this case, the phosphorus concentration in a river or lake) can be expressed as a regression line then we can read off the phosphorus concentration that relates to any point on the biological scale.   (Note that there are many other ways of deriving a threshold phosphorus concentration, but this simple approach will suffice for now.)


Relationship between biology (expressed as an Ecological Quality Ratio, EQR) and phosphorus concentration for a hypothetical dataset.  The blue line indicates the least squares regression line, the horizontal green line is the position of the putative good/moderate status boundary and the vertical green line is the phosphorus concentration at this boundary position.  Coefficient of determination, r2= 0.89 (rarely achieved in real datasets!)

This is fine if you have a strong relationship between your explanatory and response variables and you are confident that there is a causal relationship between them.  Unfortunately, neither of these criteria are fulfilled in most of the datasets we’ve looked at; in particular, it is rare for the biota in rivers to be so strongly controlled by a single pressure.  This means that, when trying to establish thresholds, we also need to think about how a second pressure might interact with the factor we’re trying to control.   If this second pressure has an independent effect on the biota then we might expect some sites that would have had high EQRs if we just considered phosphorus might now be influenced by this second pressure, so the EQR at these sites will fall below the regression line we’ve just established.   When we plot the relationship between EQR and phosphorus taking this second pressure into account, our data no longer fits a neat straight line, but now has a “wedge” shape, due to the many sites where the second pressure overrules the effect of phosphorus.   If you were tempted to put a simple regression line through this new cloud of data, you would see the coefficient of determination, r2, drop from 0.89 to 0.35.  Note, too, how the change in slope means that the position of the phosphorus boundary also falls.   More worryingly, we know that, for this hypothetical dataset, the new line does not represent a causal relationship between biology and phosphorus.  That’s no good if you want to use the relationship to set phosphorus targets and, indeed, you now also need to think about how to manage this second pressure.


The same relationship as that shown in the previous graph, but this time with an interaction from a second pressure.  The blue line is the regression line established when phosphorus alone was considered, and the red line is the regression between EQR and phosphorus in the presence of this second pressure.

My purpose in this post is not to talk about the dark arts of setting targets for nutrient concentrations that will support healthy ecosystems but, rather, to talk about data landscapes.  Once we saw and started to understand the meaning of “wedge”-shaped data, we started to see similar patterns occurring in all sorts of other situations.   The previous paragraph and graph, for example, assumed that the factor that confounded the biology-phosphorus relationship was detrimental to the biology, but some factors can mitigate the effect of phosphorus, giving an inverted wedge, as in the next diagram.  Once again, the blue line shows the regression line that would have been fitted if this was a pure biology versus phosphorus relationship.


The same relationship, but this time with a second factor that mitigates against the effect of phosphorus.  Note how the original relationship now defines the lower, rather than the upper, edge of the wedge. 

Wedge-shaped data crop up in other situations as well.  The next graph shows the number of diatoms I recorded in a study of Irish streams and there is a distinct “edge” to the cloud of data points.   At low pH (acid conditions), I rarely found more than 10-15 species of diatom whereas, at circumneutral conditions, I sometimes found 10-15 species but I could find 30 or more.   Once again, we are probably looking at a situation where, although pH does exert a pressure on the diatom assemblage, lots of other factors do too, so we only see the effect of pH when its influence is strong (< pH 5).


The number of diatom species recorded across a pH gradient in Irish streams.  Unpublished data.

In this case, the practical problem is that the link between species number and pH is weak so it is hard to derive useful information from the number of species alone.   It would be dangerous to conclude, for example, that the ecology at a site was impacted by acidification on the strength of a single sample.  On the other hand, if you visited the site several times and always recorded low species numbers, then you have a pretty good indication that there was a problem (not necessarily low pH; toxic metals would have a similar effect).   Whether such a pattern would be spotted will depend on how often a site is visited and the sad reality is that sampling frequencies in the UK are now much lower than in the past.

However, this post is not supposed to be about the politics of monitoring (evidence-based policy is so much easier when you don’t collect enough uncomfortable evidence) but about the landscapes that we see in our data, and what these can tell us about the processes at work.   Just as a field biologist can look up from the stream they are sampling and gain a sense of perspective by contemplating the topography of the surrounding land, so we should also be aware of the topography of our data before blithely ploughing ahead with statistical analyses.


With Geoff Phillips and Heliana Teixaira – fellow-explorers of data landscapes in our project to encourage consistent nutrient boundaries across the European Union.

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.

Keeping the cogs turning …

A few algae lift my soul when I see them under the microscope through their beauty.   To see such intricate yet symmetrical organisation in something too small to be visible with the naked eye never ceases to amaze and delight me.   One of the genera that has that effect is the green alga Pediastrum, which forms cog-like colonies: flat plates of cells whose outer members bear horn-like projections.   One of its representatives, Pediastrum boryanum, was common in the River Wear when I visited recently (see previous post).   You can see, from the illustration above (the scale bar is 20 micrometres – 1/50th of a millimetre – long), the characteristic disc-like arrangement of cells, always in multiples of four (there are 16 in the colony above).   There are many species of Pediastrum, differing in the shape of both the inner and marginal cells, and the number and length of the horns.

I have found Pediastrum on many occasions in the Wear in the past, but never quite as abundant as it was in my most recent samples.  Pediastrum boryanum is the species I find most often, here and elsewhere, but other species occur too.  I have also found Pediastrum in some unusual places, including deep in lake sediments when I was searching for fossil pollen grains and there is evidence that the cell walls of Pediastrum contain both silica and a sporopollenin-like material (sporopollenin is the extremely tough material found in the outer walls of pollen grains (which probably explains why it had survived the fierce mix of chemicals that we used to prepare the lake sediments for pollen analysis).   I am guessing that the sporopollenin and silica both add some structural integrity to the cells.   There are references in the literature to Pediastrum being planktonic but I often find it in samples from submerged surfaces and associated with submerged macrophytes, so I suspect that it is one of those species that moves between different types of habitat.  It should not really be a surprise that a relatively large colonial alga with a payload of silica and sporopollenin in addition to the usual cellulose cell wall, is going to be common in benthic films in a river towards the end of a long, dry summer.

Pediastrum is another genus that has been shaken up in recent years as a result of molecular studies.  According to these, Pediastrum boryanum should now be called Pseudopediastrum boryanum although the Freshwater Algal Flora of the British Isles continues to use the old name.   Not everyone agrees with this split (see McManus and Lewis’ paper in the list below) but the divisions suggested by molecular data do also seem to match differences in morphological characteristics of the group (see Table below).

Pediastrum is part of the family Hydrodictyaceae and, as I was writing this, it occurred to me that I have never written about another interesting member of this family, Hydrodictyon reticulatum.  As I like to accompany my posts with my own photographs, I spent part of yesterday afternoon tramping around a location where I have found Hydrodictyon in the past.   All I got for my troubles, however, was two damp feet, so that post will have to wait for another day.


Differentiating characteristics of Pediastrum and similar genera (after Krienitz & Bock, 2011).

Genus Features
Pediastrum Flat coenobia with intercellular spaces, marginal cells with two lobes
Lacunastrum Flat coenobia with large intercellular spaces, marginal cells with two lobes
Monactinus Flat coenobia with large intercellular spaces, marginal cells with one lobes
Parapediastrum Flat coenobia with intercellular spaces, marginal cells with two lobes, each divided into two projections
Pseudopediastrum Flat coenobia without intercellular spaces, marginal cells with two lobes, each with a single projection
Sorastrum Three-dimensional coenobia, each cell with two or four projections.
Stauridium Flat coenobia without intercellular spaces, marginal cells “trapezoid”* with deep incision to create two lobes, each with a concave surface, though the lobes are not really extended into “projections”

* not all of the illustrations show marginal cells that are strictly “trapezoid” (e.g. with at least one pair of parallel sides).


Buchheim, M., Buchheim, J., Carlson, T., Braband, A., Hepperle, D., Krienitz, L., Wolf, M. & Hegewald, E. (2005).  Phylogeny of the Hydrodictyaceae (Chlorophyceae): inferences from rDNA data.  Journal of Phycology 41: 1039-104.

Good, B.H. & Chapman, R.L. (1978).  The ultrastructure of Phycopeltis (Chroolepidaceae: Chlorophyta). I. Sporopollenin in the cell walls.  American Journal of Botany 65: 27-33.

Jena, M., Bock, C., Behera, C., Adhikary, S.P. & Krienitz, L. (2014).  Strain survey on three continents confirms the polyphyly of the genus Pediastrum (Hydrodictyaceae, Chlorophyceae).  Fottea, Olomouc 14: 63-76.

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

McManus, H.A. & Lewis, L.A. (2011).  Molecular phylogenetic relationships in the freshwater family Hydrodictyaceae (Sphaaeropleales, Chlorophyceae), with an emphasis on Pediastrum duplex.   Journal of Phycology 47: 152-163.

Millington, W.F. & Gawlik, S.R. (1967).  Silica in the wall of PediastrumNature (London) 216: 68.

Talking about the weather …

September is here.  When I visited this site two months ago we were in the midst of the heatwave and the samples I collected from the Wear at Wolsingham were different to any that I have seen at this location before, dominated by small green algae (see “Summertime blues …”).   As I drove to Wolsingham this time, I could see the first signs of autumn in the trees and the temperatures are more typical of this time of year.   We have had rain, but there has not been a significant spate since April and this means that there has been nothing to scour away these unusual growths and return the river to its more typical state.

That does not mean, however, that there have been no changes in the algae on the submerged stones.  Some of these differences are apparent as soon as I pick up a stone.  Last month, there was a thin crust on the surface of the stones; that is still here but now there are short algal filaments pushing through, and the whole crust seems to be, if anything, more consolidated than in July, and I can see sand grains amidst the filaments.   Biofilms in healthy rivers at this time of year are usually thin, due to intense grazing by invertebrates, so I’m curious to know what is going on here this year.

A cobble from the River Wear at Wolsingham, showing the thick biofilm interspersed with short green filaments.   Note, too, the many sand grains embedded in the biofilm.  The bare patch at the centre was created when I pulled my finger through it to show how consolidated it had become.  The cobble is about 20 centimetres across.

Many of the organisms that I can see when I peer at a drop of my sample through my microscope are the same as those I saw back in July but there are some conspicuous differences too.   There are, for example, more desmids, some of which are, by the standards of the other algae in the sample, enormous.   We normally associate desmids with soft water, acid habitats but there are enough in this sample to suggest they are more than ephemeral visitors.   And, once I had named them, I saw that the scant ecological notes that accompanied the descriptions referred to preferences for neutral and alkaline, as well as nutrient-rich conditions.  Even if I have not seen these species here before, others have seen them in similar habitats, and that offers me some reassurance.    In addition to the desmids, there were also more coenobia of Pediastrum boryanum and Coelastrum microporum compared to the July sample.

A view of the biofilm from the River Wear at Wolsingham on 1 September 2019. 

There were also more diatoms present than in my samples from July – up from about 13 percent of the total in July to just over 40 per cent in September.   The most abundant species was Achnanthidium minutissimum, but the zig-zag chains of Diatoma vulgare were conspicuous too.  The green filaments turned out to be a species of Oedogonium, not only a different species to the one I described in my previous post but also with a different epiphyte: Cocconeis pediculus this time, rather than Achnanthidium minutissimum.   I explained the problems associated with identifying Oedogonium in the previous post but, even though I cannot name the species, I have seen this form before (robust filaments, cells 1.5 to 2 times as long as broad) and associate it with relatively nutrient-rich conditions.  That would not normally be my interpretation of the Wear at Wolsingham but this year, as I have already said, confounds our expectations.   I did not record any Cladophora in this sample but am sure that, had I mooched around for longer in the pools at the side of the main channel, I would have found some filaments of this species too.

Desmids and other green algae from the River Wear at Wolsingham, 1 September 2019.  a. Closterium cf. acerosum; b. Closteriumcf. moniliferum; c. Cosmarium cf. botrysis; d. Closterium cf. ehrenbergii; e. Coelastrum microporum; f. Pediastrum boryanum.   Scale bar: 50 micrometres (= 1/20th of a millimetre).  

It is not just the differences between months this year that I’m curious about.  I did a similar survey back in 2009 and, looking back at those data, I see that my samples from August and September in that year had a very different composition.   There was, I remember, a large spate in late July or early August, and my August sample, collected a couple of weeks later had surprised me by having a thick biofilm dominated by the small motile diatom Nitzschia archibaldii.   My hypothesis then was that the spate had washed away many of the small invertebrates that grazed on the algae, meaning that there were few left to feed on those algae that survived the storm (or which had recolonised in the aftermath)..   As the algae divided and re-divided, so they started to compete for light, handing an advantage to those that could adjust their position within the biofilm.   This dominance by motile diatoms was, in my experience of the upper Wear, as uncommon as the assemblages I’m encountering this summer, though probably for different reasons.

Other algae from the River Wear at Wolsingham, September 2018.    The upper image shows Diatoma vulgare and the lower image is Oedogonium with epiphytic Cocconeis pediculus.   Scale bar: 20 micrometres (= 1/50th of a millimetre).

I suspect that it is the combination of high temperatures and low flows (more specifically, the absence of spates that might scour away the attached algae) that is responsible for the present state of the river.  This, along with my theory behind the explosion of Nitzschia archibaldii in August 2009, both highlight the importance of weather and climate in generating some of the variability that we see in algal communities in rivers (see “How green is my river?”).   The British have a reputation for talking about the weather.   I always scan the weather forecasts in the days leading up to a field trip, mostly to plan my attire and make sure that I will, actually, be able to wade into the river.  Perhaps I also need to spend more time thinking about what this weather will be doing to the algae I’m about to sample.

Summertime blues …

My reflections on the effects of the heatwave on freshwater algae continued with the latest of my regular visits to the River Wear at Wolsingham.  A comparison of the picture above with that at the head of “Spring comes slowly up this way …” says it all: the sun was shining and the gravel berms that I usually use to enter the river were occupied by families with barbeques whilst their children splashed around in the water.   At times such as this, a grown man picking up stones and then vigorously brushing their tops with a toothbrush would have invited too many questions, so I slunk off 100 metres or so downstream and found a quieter spot to explore.

The biofilm in the main channel of the River Wear at Wolsingham, July 2018. 

The first thing I noticed was that the biofilm coating the submerged stones at the bottom of the river had a greenish tinge, rather than its usual chocolate brown appearance.  It also was more crusty and less slimy to the touch than I usually see in this river.  When I got a specimen under the microscope, I could see that the composition was completely different to that which I had observed in previous months.   Most samples from this location that I’ve looked at in the past have been dominated by diatoms, with occasional spring flourishes of filamentous green algae.  Today, however, the sample was dominated by small green algae – a group that I am not very confident at identifying.   My rough estimate is that these formed about three quarters of all the algae that I could see, with diatoms and cyanobacteria each accounting for about half of the remainder.   The most abundant greens were a tiny single-celled alga that I tentatively identified as Keratococcus bicaudatus, along with a species of Scenedesmus and Desmodesmus communis.   There were also a number of cells of Monoraphidium arcuatum and some of Ankistrodesmus sp.

Two views of biofilms from the River Wear, Wolsingham in January 2018.   Left: from the main channel; right: from pools at the edge of the channel.

Green algae from the River Wear at Wolsingham, July 2018: a. Desmodesmus communis; b. Monoraphidium arcuatum; c. Scenedesmus sp.; d. unidentified, possibly Keratococcus bicaudatus.  Scale bar: 10 micrometres (= 1/100th of a millimetre).

However, there were also pools at the side of the channel, away from the main current but not so cut off that they were isolated from the river itself.   These were dominated by dense, brown filamentous growths, very similar in appearance to the Melosira varians flocs I described in “Some like it hot …”.  The filaments, however, felt coarser to the touch and, in close-up, could be seen to be branched, even without recourse to a microscope.   Once I got these under the microscope, I could see that they were filaments of Cladophora glomerata, another green alga, but so smothered with epiphytic diatoms (mostly Cocconeis pediculus) that they appeared brown in colour.

This combination of Cladophora glomerata and Cocconeis pediculus in the backwaters were as much of a surprise as the green-algae-dominated biofilms in the main channel.   These are species usually associated with enriched rivers (see “Cladophora and friends”) and, whilst I have seen Cladophora in the upper Wear before, it is an unusual occurrence.   Just as for the prolific growths of Melosira varians described in “Some like it hot …” it is tempting to leap to the conclusion that this must be a sign that the river is nutrient-rich.  However, the same conditions will apply here as there: “nutrients” are not the only resource that can limit plant growth and a steady trickle of phosphorus combined with warm, sunny conditions is just as likely to lead to prolific growths as a more conventionally “polluted” river.

Cladophora filaments smothered by the diatom Cocconeis pediculus in a pool beside the River Wear at Wolsingham, July 2017.   The frame width of the upper image is about 1 cm; the scale bar on the lower images is 20 micrometres (= 1/50th of a millimetre).

Another way to think of these situations is that, just as even healthy people are occasionally ill, so healthy streams can go through short periods when, based on a quick examination of plants and animals present, they exhibit symptoms associated with polluted conditions or simply (as for the first sample I described) different to what we usually expect to find.   A pulse of pollution might have passed downstream or, as seems to be happening at the moment, an unusual set of conditions lad to different organisms thriving.   Just as the ability to fight off infection forms part of a doctor’s understanding of “health”, so I expect that the River Wear will, in a few weeks time, be back to its usual state.   Healthy ecosystems, just like healthy humans, show “resilience”.   The irony is that, in this case, the “symptoms” are most obvious at a time when we are enjoying a summer better than any we’ve had in recent years.