A river runs through it …


I made an journey via Paris to Orleans last weekend to wave off my wife and daughter as they walk part of the Camino de Santiago.   The part of this austere Medieval pilgrimage route that they chose to follow happens to be one that meanders along the Loire Valley, passing chateaux and wineries en route.   Apparently, blisters are providing an element of self-mortification to keep the spirit of pious ascetism alive.

Orleans has a beautiful Medieval old town, dominated by a cathedral, along with plenty of reminders that it was here that Joan of Arc whupped the English.   More importantly, for a freshwater ecologist, a river runs through it.   The mighty Loire – the longest river in France – rises in the Masif Central and then flows north until it reaches Orleans, then it swings round to flow west to join the Atlantic Ocean at Nantes, a total length of just over 1000 km.  The river at Orleans is broad and shallow, divided into two channels by a wooded island; the left-hand chnnel is braided, with many small gravel islands, some with grass and herbaceous vegetation, and the water is generally shallow.  However, the bed of the river itself was a dark green in colour.  The Loire Valley is known as the “Garden of France” and this gives a clue to the scale of nutrient enrichment that we might expect in the river.  In the backwaters, this green backdrop was enlivened by patches of red due to the aquatic fern Azolla (see “Escape to Southwold”).


Patches of Azolla floating over algae-smothered substrata in a backwater of the River Loire at Orleans.  The picture at the top of the post shows a view looking downstream from the left side of the Pont George V. 

I brought a sample of the algae from some stones that were just within reach of the shore home on the Eurostar in a Perrier Water bottle for a closer inspection and was surprised by the diversity. In particular, I noticed several clumps of a narrow cyanobacterium that proved hard to photograph (possibly Homoeothrix janthina: see “Algae from the Alto Duoro”) along with many green algae.  I also saw Cladophoraglomerata, which is one of the classic indicators of nutrient rich conditions, along with Stigeoclonium, two forms of Spirogyra, two forms of Oedogonium and myriad green unicells and coenobia.  Stigeocloniumis another good indicator of nutrient enrichment,as  the filaments narrow to long colourless “hairs” when key nutrients are scarce – these cells are physiological adaptions to scavenge phosphorus and their absence at Orleans shows that this nutrient is not in short supply (see “A day out in Weardale”).


Stigeoclonium cf. tenue (based on properties of erect filaments) from the River Loire at Orleans, September 2019.  Scale bar: 10 micrometres (1/100thof a millimetre). 

As well as green algae and cyanobacteria, there were also a lot of diatoms.  A few of these are illustrated below, and I’ll describe the diatoms in more detail in a future post.   As well as those I have photographed, I also saw long chains of a species of Fragilaria and another small araphid (possibly Staurosira) as well as Ulnaria ulna and some cells of Gomphonema and Navicula.   Note, in particular, the tube of Encyonema (possibly E. prostratum).   I’ve talked about tube-forming diatoms before (see “An excuse for a crab sandwich, really …”) but still can’t really explain what advantage this structure confers on a diatom.   What we can, perhaps, say, is that tube-dwelling is one of a several habits represented in the diatoms from Orleans – chains, erect, free-living motile, epiphytes  and more – and the mass of other algae create a rich diversity of microhabitats that the diatoms can exploit.


Some diatoms from the River Loire at Orleans: a. – d. Diatoma vulgare; e. Cocconeis pediculus; f. Encyonemasp.   Scale bar: 10 micrometres (= 1/100thof a millimetre).  The identity of the tube-dwelling form will have to wait until a cleaned sample is available. 

This abundance and diversity of green algae reminded me of some of the lush growths I had seen in UK rivers during the 2018 heatwave (see “Talking about the weather …”).  In a healthy river in the UK, I would expect to find less conspicuous growths than this, as invertebrate grazers would keep the algal biofilm shorn to a short stubble.   If, for any reason, the invertebrates cannot keep up with the algal growth, then a feedback loop is set up: the algae suck the valuable oxygen the invertebrates need from the water, the grazing reduces in intensity yet further, leading to a situation such as that I saw in the River Loire is the result.

Warm weather exacerbates the situation because water holds less oxygen at high temperatures.  In my posts about the River Wear last summer I commented that the plants in the river suggested that the river was more enriched with nutrients than was actually the case and I suspect that this was the result of these interactions.   The more southerly, more continental climate of the Loire Valley will experience these effects more often and it is possible that what I am looking at in Orleans may become the norm for UK rivers, as global warming intensifies.  Over the past decade I’ve worked on how to estimate the threshold concentrations of nutrients that a healthy river can endure.  However, nutrients rarely exert a direct effect on the plants and animals in a river but, instead, work through their effect on other factors such as oxygen. That will mean that global warming may wipe out any benefits of expensive nutrient reduction programs based on current estimates of the relationship between nutrients and river ecosystem health.  That’s a rather depressing prospect.

So I’ll end on a more cheerful note: the reason Heather and Rosie chose to start their Camino in Orleans was that they wanted to avoid a flight if possible.  At least that is how they sold it to me.   That they also chose to start their pilgrimage in a town close to the Sancerre vineyards may be pure coincidence.   Or maybe not ….


Surt, M.M., Jansen, M.A.K., Harrison, S.S.C. (2011).  Invertebrate grazing and riparian shade as controllers of nuisance algae in a eutrophic river. Freshwater Biology 56: :2580-2593

Wilco, C.E., Verbrak, P., Durance, I., Vaughn, I.P. & Ormerod, S.J. (2016).  Field and laboratory studies reveal interacting effects of stream oxygenation and warming on aquatic ectotherms.  Global Change Biology 22: 1769-1778.


Orleans cathedral, August 2019.


Hilda Canter-Lund competition winners 2019


This year’s Hilda Canter-Lund competition for the best algal-themed photograph has been won by Zoe Loffler for her image “Symphony of Seaweeds” taken on a at low tide near Apollo Bay, Victoria, Australia, while on a family camping trip.  She took the photo using a Google Nexus 5x Smartphone.  Zoe grew up diving in temperate waters near Melbourne, Australia. She completed her undergraduate degree and PhD at James Cook University in Townsville, Australia, studying the ecology of canopy-forming seaweeds (particularly Sargassum spp.) on coral reefs. She is now based in Sydney, and enjoys snorkelling and diving in temperate waters where there is such a wonderful diversity and abundance of seaweeds. The image meets Henri Cartier-Bresson’s maxim of the “decisive moment” (see “How to win the Hilda Canter-Lund prize”) and Zoe comments in her caption that the photo shows all who are unfamiliar with seaweeds that “they are not just brown and smelly!”.


Zoe Loffler: winner of the 2018 Hilda Canter-Lund prize for algal photography, for her image “Symphony of Seaweed”, shown at the top of the post.

Since 2016 we have also offered a second prize which is awarded to a photograph in a contrasting style to the overall winner.  This year, that prize goes to Damien Sirjacobs of the University of Liege in Belgium  for his image “Blue Haze”. This shows a bloom of benthic blue diatoms of the genus Haslea (H. ostrearia, H. provincialis) covering a community of macroalgae (Padina pavonica, Acetabularia acetabulum, Halopteris scoparia, Dictyota sp.) in the shallow water of Calvi Bay (Corsica, France). For scale, the circular caps on the end of the Acetabularia stalks are 5 – 10 mm in diameter.   The image was taken at a depth of four metres in May 2018 with a LUMIX TZ10 in an underwater housing, under natural light conditions, while scuba-diving along rocky shores of the Revellatta peninsula (Calvi Bay).


Damien Sirjacobs’ image: Blue haze”.  

There is a lot to interest readers of this blog in Damiens’s image; first of all, Acetabularia is another challenge to the generally-accepted view that multicellularity is the only option for large organisms.  Although the plant is quite large and, unlike Vaucheria is more elaborate than a simple tube of cytoplasm (see “The pros and cons of cell walls …”) .  The whole organism is, in fact, just one giant cell with a single nucleus.  

The diatom Haslea that grows over Acetabularia and the other macroalgae in Damien’s picture creates a blue haze due to a pigment called “marennine” which is found in vacuoles inside the cells (as you can see in the photograph below).   When marennine-containing species of Haslea are abundant around oyster beds (as is the case in parts of Brittany), then the pigment turns the gills of the oyster green and such oysters are highly sought after by gastronomes.   Whether or not these oysters really taste better is debatable but marennine certainly does have some antimicrobial properties.


Damien Sirjacobs: co-winner of the 2019 Hilda Canter-Lund prize for algal photography.


Whilst Zoe’s image has direct visual appeal, and most people will recognise it almost straightaway as depicting seaweeds, Damien’s image has a more other-worldly quality.   Unless you are familiar with the habitats and organisms, then it is difficult to interpret what is portrayed (see “Abstracting from reality …”).   One of the challenges of photographing algae is that we are dealing with the real yet little understood aspects of biodiversity, creating a multi-layered problem: first, of capturing an impression of the organism(s) but, also,  of interpreting the image to a lay-audience.   In the case of Blue Haze we have that intriguing combination of beauty, mystery and economic relevance.   That is what makes phycology such a fascinating subject.


Gastineau, R., Prasetiya, F.S., Falaise, C., Cognie, B., Decottignies, P.,  Morançais, M., Méléder, V., Davidovich, N., Turcotte, F., Tremblay, R., Pasetto, P., Dittmer, J., Bardeau, J.-F., Pouvreau, J.-B. & Mouget, J.-B. (2018). Marennine-like pigments: blue diatom or green oyster cult?   pp. 529-551.  In: Blue Biotechnology: Production and Use of Marine Molecules (edited by Stéphane La Barre and Stephen S. Bates).  Wiley VCH Verlag GmbH & Co. KGaA

Out of my depth …


I was about to start writing up an account of my latest visit to Castle Eden Dene, when I realised that I had forgotten to describe my previous visit, back in March.   I’ve already described a visit in January, when the stream was dry (see “Castle Eden Dene in January” and “Tales from a dry river bed”) and promised regular updates through the year.   It seems that, amidst all the travel that filled my life over the last three months, I overlooked the post that I should have written about the visit that I made in early March.

Whereas the river was dry in January, rain during February meant that, when I returned to the Dene on 11 March, some rather turbid water was flowing down the channel on its short journey to the North Sea.   There is, finally, something more like a stream habitat from which I can collect some diatoms.

Many of the diatoms that I found in March belonged to taxa that I had also seen in January; however, the proportions were quite different.   In some cases, species that were common in January were less common now (e.g. Humidophila contenta*) but there was a small Nitzschia species with a slightly sigmoid outline that was very sparse in the January sample but which was the most abundant species in the March sample.  I’ve called this “Nitzschia clausii” but the Castle Eden Dene population does not fit the description of this perfectly.   A lot can change in a couple of months, especially when dealing with fast-growing organism such as these, as my posts on the River Wear showed (see “A year in the life of the River Wear”).  Castle Eden Burn’s highly variable discharge just adds another layer of complication to this.


Diatoms from Castle Eden Dene, March 2019:   a. – e.: Nitzschia cf clausii; f. Tabularia fasiculata; g. Tryblionella debilis; h. Luticola ventricosa; i. Luticola mutica; j. Ctenophora pulchella.  Scale bar: 10 micrometres (= 1/100thof a millimetre).   The picture at the top of the post shows Castle Eden Burn at the time that the sample was collected.   

Nitzschia clausii is described as being “frequent in brackish freshwater habitats of the coastal area and in river estuaries, as well as in inland waters with strongly increased electrolyte content”.   A couple of the other species from this sample – Ctenophora pulchella and Tabularia fasiculata (both illustrated in the diagram above) – have similar preferences.    My experience is that we do often find a smattering of individuals belonging to “brackish” species in very hard water, as we have in Castle Eden Burn.  Average conductivity (based on Environment Agency records) is 884 µS cm-1; however, values as high as 1561 µS cm-1.   The fluctuating discharge plays a role here, as any evaporation will serve to concentrate those salts that are naturally present in hard freshwater.   This should probably not be a big surprise: life in brackish waters involves adapting to fluctuating osmotic regimes so species that can cope with those conditions are also likely to be able to handle some of the consequences of desiccation.

Average values of other chemical parameters from 2011 to present, based on Environment Agency monitoring are: pH: 8.3; alkalinity: 189 mg L-1 CaCO3; reactive phosphorus: 0.082 mg L-1; nitrate-nitrogen: 1.79 mg L-1; ammonium-nitrogen: 0.044 mg L-1.   There is some farmland in the upper catchment, and the burn also drains an industrial estate on the edge of Peterlee but, overall, nutrient concentrations in this stream are not a major concern.   The Environment Agency classifies Castle Eden Burn as “moderate status” due to the condition of the invertebrates but does not offer any specific reason for this. I suspect that the naturally-challenging habitat of Castle Eden Burn may confound assessment results.

I’ve also been given some data on discharge by the Environment Agency which shows how patterns vary throughout the year.  The two sampling locations are a couple of kilometres above and below the location from which I collect my samples and both have more regular flow.  However, we can see a long period between April and September when discharge is usually very low.   The slightly higher values recorded in July are a little surprising, but are spread across a number of years.   It is also, paradoxically, most common for the burn to be dry in July too: clearly, a month of extremes.  As my own visits have shown, it is possible for the burn to be dry at almost any time of the year, depending on rainfall in the preceding period   The dots on the graph (representing ‘outliers’ – records that exceed 1.5 x interquartile range) show that it is also possible to record high discharges at almost any time during the year too.  I should also add that, as I am not a hydrologist, I am rather outside my comfort zone when trying to explain these patterns.  I would have said ‘out of my depth’ though that’s not the most appropriate phrase to use in this particular situation.


Discharge in Castle Eden Burn, as measured by the Environment Agency between 2007 and present.   Measurements are from NZ 4136 2885 (‘upstream’) and NZ 45174039 (‘downstream’).  

* Note on Humidophila contenta:it is almost impossible to identify this species conclusively with the light microscope as some key diagnostic characters can only be seen with the scanning electron microscope.   However, all members of this complex of species share a preference for intermittently wet habitats so these identification issues are unlikely to lead to an erroneous ecological interpretation.  It is probably best to refer to this complex as “Humidophila contenta sensu lato” rather than “Humidophilasp.” order to distinguish them from those species within the genus that can be recognised with light microscopy.


Lange-Bertalot, H., Hofmann, G., Werum, M. & Cantonati, M. (2017).  Freshwater Benthic Diatoms of Central Europe: over 800 Common Species Used in Ecological Assessment. English edition with updated taxonomy and added species.  Edited by M. Cantonati, M.G. Kelly & H. Lange-Bertalot.  Koeltz Botanical books, Schmitten-Oberreifenberg.

Hyperepiphytes in the Shetland Islands


I was lucky enough to spend a couple of days in the Shetland Islands during last week’s spell of warm weather and spent one of my mornings there hiking in shirtsleeves across moorland to a remote loch.   Good infrastructure is a legacy of the Shetland Islands’ association with the oil industry, and this includes a strong mobile network, meaning that I managed to find this particular loch using the Ordnance Survey maps on my smartphone. I would not normally rely upon a mobile signal to navigate across such remote terrain but in Shetland it is often possible.  I would, nonetheless, recommend keeping a paper map and a GPS in your kit just in case, as I did lose the signal on a few occasions during my stay.

Most of the lochs in the northern part of mainland Shetland are shallow, peaty water bodies, with soft water and relatively sparse assemblages of aquatic plants.   Parts of the littoral zone of this particular loch, however, had extensive growths of submerged mosses.  It is a long time since I was proficient at identifying aquatic mosses but these clumps look likeWarnstofia fluitans to me, though I am willing to be proved wrong.  I did try to remove some leaves and have a proper look but that task was complicated by tufts of attached filamentous algae.   In their submerged state, these formed distinct clusters at intervals along the straggly stems of the moss but, once removed, the filaments collapsed to smother the leaves and confound my attempts to run a scalpel blade along the stem.


Submerged colonies ofWarnstorfia fluitans(?) smothered byOedogoniumfilaments in Gossa Water, north Mainland, Shetland (HU 4354 6047). Gossa Water (one of five that share this name in the Shetland Islands!) is illustrated in the photograph at the top of this post.

The filamentous alga proved easier to unmask: the unbranched filaments, reticulate (net-like) chloroplasts and distinctive ‘cap cells’ all identifying it as the green alga Oedogonium.  As is often the case, however, the populations lacked any sexual organs so it was impossible to know which species (see “The perplexing case of the celibate alga“ and, for a rare case of a sexually-mature filament, “Love and sex in a tufa-forming stream”).   Abundant epiphytes can be another feature of Oedogonium: unlike several other filamentous green algae it produces little mucilage which makes it easier for diatoms, in particular, to colonise.  As well as colonies of needle-shaped cells of Fragilaria gracilis there were also several Achnanthidium cells and, entangled around the filaments and the moss, chains of Tabellaria flocculosa.   Given that the Oedogonium was, itself, an epiphyte, these diatoms are ‘hyperepiphytes’, a term that attracts remarkably few Google hits, almost all associated with lower plants.

The ‘cap cells’ are one of the most distinctive features of Oedogonium and results from a distinctive mode of cell division that leaves rings of scar tissue at the point where the two cells split.   That we see four or more of these scars on a few cells whilst the great majority have none suggests that we are looking at a primitive form of specialisation, with a few cells in a filament being responsible for all the cell division.  What is more, these cap cells are also often the ones that form oogonia (see “Love and sex in a tufa-forming stream” for an illustration of this) and asexual zoospores, so there must be something slightly different in the biochemistry within these cells that drives these processes.   However, at this point the formal scientific literature goes strangely silent apart from a single paper published in 1962.  Curiously, the evolution of multicellularity is one of those big questions that attract a lot of top academics (see the reference to a recent paper in Nature Scientific Reports below)  whilst a genus of algae that seem to show some faltering first steps towards specialisation of some cells are largely ignored.  Another case of the “trailing edge” of science?


Oedogonium filaments growing on Warnstofia fluitans in the littoral zone of Gossa Water, north Mainland, Shetland, May 2019.   The arrow on the top image shows the “cap cells”.   Note also the cluster of Fragilaria gracilis(plus a few cells of Achnanthidium) on the lowermost filament and, in the middle image, two of the many cells of Tabellaria flocculosa that were entangled with the Oedogonium filaments and moss stems.  Scale bar: 20 micrometres (= 1/50thof a millimetre). 


A zoospore being released from a filament of Oedogonium.  This series of photographs was taken by me in about 1993 and I have no details of the location from which it came.  The filament is about 40 micrometres (= 1/25thof a millimetre) in diameter.


Herron, M.D., Borin, J.M., Boswell, J.C., Walker, J., Chen, I-C. K., Knows, C.A., Boyd, M., Rosenzweig, F. & Ratcliff, W.C. (2019).  De novo origins of multicellularity in response to predation.  Nature Scientific Reports 9, Article number: 2328

Rawitscher-Kunkel, E. & Machlis, L. (1962).  The hormonal integration of sexual reproduction in Oedogonium.   American Journal of Botany 49: 177-183.


Sightseeing in Shetland: the tombolo (sandy isthmus) linking St Ninian’s Isle with Mainland in the Shetland Islands, May 2019.

Beyond the Tower of Babel …


A week after I return from China, I was off on my travels again; this time to Vienna for a workshop between molecular ecology specialists and ECOSTAT, the committee of Member State representatives who oversee ecological aspects of Water Framework Directive implementation.   As ever, I found some time to visit some art galleries around the meeting and, as Vienna has one of the most impressive collections of paintings by Pieter Brueghel, I could not resist spending some time in front of his “Tower of Babel”.  A few years ago I cheerfully included this picture in a talk on EU ecological assessment methods, as we tried to make sense of the myriad national approaches.   Three years after the Brexit vote, however, it seems to better reflect UK domestic politics where, ironically, language is one of the few things that all protagonists do have in common.

The River Danube seems to encapsulate the reasons why Europe needs collaborative thinking on the state of the environment.  It is the second longest river in Europe, after the Volga, and flows through ten countries, with tributaries extending into nine more.   Eight of the nine countries through which the river flows are members of the EU (the ninth, Serbia, is in the process of joining) so the river represents a case study, of sorts, on whether EU environmental policies actually work.   This is not just an academic question: ecologists are generally in favour of integrated management of entire catchments whilst the EU operates on a principle of “subsidiarity”, which means that decision-making is devolved to the lowest competent authority (individual Member States in the case of the environment).   Finding the right balance between these principles takes a lot of patient discussion and is one reason why EU decision-making can appear to be agonisingly slow.


Pieter Bruegel’s “Tower of Babel” in the Kunsthistorisches Museum in Vienna.

And there are more problems: the Water Framework Directive evaluates the sustainability of water bodies by their naturalness yet very large rivers such as the Danube have been very heavily modified by human use for centuries.   The river has been broadened, deepened and impounded, and its banks have been straightened and strengthened in order to make it navigable, and there is a huge human population, with associated industry, living on its banks.  The stretch of the Danube along which I walked on my last morning in Vienna was also lined with embankments to protect the surrounding land from flooding but these, at the same time, cut the river off from the ecological benefits of the floodplain.

What hope for a large river such as the Danube in the face of all these challenges?   First of all, when dealing with rivers such as these we need to adjust our expectations, recognising that they are so central to the economic life of the regions through which they flow that there are limits to their capacity to ever resemble truly natural rivers.   Once we have done this, we can start to unpick the challenges that can be addressed by individual Member States.  In the case of water quality, in particular, the story for the Danube is encouraging and European environmental legislation has played its role in this process.  By the time the Danube reaches the borders with Romania, for example, nutrient concentrations are low enough for many of the benthic algal-communities to meet criteria for “good ecological status”.

You can see this in the graph below, from a paper that we’ve published recently.   The Romanian sites are largely clustered at the top left hand side of the graph, relative to data from other countries – indicating low phosphorus concentrations and good ecology (expressed as “ecological quality ratios”, EQRs).   Thanks to an extensive exercise that took place a few years before I started grappling with the Romanian data, we already had a consensus view of the EQR boundaries for high and good status, and most of the Romanian data fits into the band representing “good status”.  That’s encouraging and whilst these communities are just one element of a much more complex ecosystem, but it is a clear step in the right direction.


The relationship between dissolved phosphorus and ecological status of the phytobenthos (expressed as the Ecological Quality Ratio, EQR, based on the intercalibration common metric (which gives a harmonised view of status between Member States).   Horizontal lines show the average position of “high” (blue) and “good” (green) status boundaries.   RO = Romanian data; XGIG = data from other Member States.   See Kelly et al. (2018) for more details.  

Romania is, of course, a long way downstream from where I was standing in Vienna.  Before the Danube gets there it has to cross Slovakia, Hungary and Serbia.  The river also forms the boundary between Romania and Bulgaria for about 300 kilometres, so it is important that there is joined-up thinking between those responsible for water quality on the two opposite banks.  That’s why the EU is so important for the environment on a pan-European scale.  It is easy for those of us crammed onto our insignificant archipelago in the north-west corner of the continent to overlook this, but the Danube is really a great success stories for European environmental collaboration and, indeed, a reason for staying with this ambitious project into the future.   Too late, I know, but it needs to be said.


Kelly, M.G., Chiriac, G., Soare-Minea, A., Hamchevici, C. & Birk, S. (2018).  Defining ecological status of phytobenthos in very large rivers: a case study of practical implementation of the Water Framework Directive in Romania.  Hydrobiologia 828: 353-367.


Sightseeing in Vienna: Stefansdom, the historic cathedral in the city centre and the Ferris wheel at the Prater amusement park, which played a starring role in Graham Greene’s The Third Man.

The complexities of measuring mass…


Once upon a time, measuring the quantity of algae growing on the beds of streams and rivers was a painstaking, slow process that invariably revealed large amounts of spatial and temporal variation that, very often, obscured the ecological signals you were looking for. That has changed in the last decade thanks to the availability of field fluorimeters such as the BenthoTorch.  This makes it much quicker and easier to measure chlorophyll concentrations, the usual proxy for algal quantity.  Thanks to devices such as this it is now much easier to discover that your ecological signal is masked by spatial and temporal variation.

We’ve generated a lot of data about the fluxes of algae in the River Ehen using a BenthoTorch over the past five years and are in a position where we can start to make some generalisations about how the quantity of algae vary over the course of a year.  In broad terms, the results I showed in “The River Ehen in January” back in 2014 have not varied greatly over subsequent years, with peak biomass in mid-winter and low biomass in the summer (due, we presume, to intense grazing by invertebrates).  Curiously, we see a much less distinctive seasonal pattern in the nearby Croasdale Beck, but that is a story for another day….

The BenthoTorch uses an algorithm to partition the fluorescence signal between three major algal groups and, though this is not without issues (see below), I thought it might be interesting to see how these groups varied with biomass trends, and consider how this links to ecological theory.  The first group I’m considering are the green algae which, in this river, are mainly filamentous forms.   The general pattern, seen in the graph below, is for a gradual increase in the proportion of green algae, which fits with the current understanding of thicker biofilms having greater structural complexity with filamentous algae out-competing attached single celled algae to create a “canopy” of algae that are more effective at capturing light and other resources.  The relationship is, however, strongly wedge-shaped so, whilst many of the thickest biofilms have a lot of green algae, there are also thick biofilms where green algae are scarce or even non-existent.  Croasdale Beck shows a similar, but less pronounced, trend.


Relationship between the proportion of green algae and the total quantity of benthic algae (expressed as chlorophyll concentration) in the River Ehen (a.) and Croasdale Beck (b.).   The blue lines show quantile regression fits at p = 0.8, 0.5 and 0.2.   The image at the top of the post shows Ben Surridge using a BenthoTorch to measure algal biomass beside Croasdale Beck in Cumbria.

The second graph shows that this pattern of a gradual increase in proportion is also the case for diatoms and, once again, there is a broad wedge of points with an upward trend.  But, once again, there are also samples where biomass is high but diatoms are present in very low numbers or are even absent.   What is going on?

The problem is clear I think, if one looks at the final image in “The only way is up …” where the very patchy nature of algal communities in the River Ehen (and, indeed, many other rivers).   There are plenty of algae on this boulder, but not organised in a homogeneous manner: some zones on the boulder are almost pure diatom whilst others are almost pure green algae (and there are also zones that are almost pure Lemanea– I’ll come to that in a future post).   We try to sample the stones as randomly as possible so you can see the potential for getting very different numbers depending on where, on a stone, we point the BenthoTorch’s sensor.


Relationship between the proportion of diatoms and the total quantity of benthic algae (expressed as chlorophyll concentration) in the River Ehen (c.) and Croasdale Beck (d.).   The blue lines show quantile regression fits at p = 0.8, 0.5 and 0.2.  

With experience, you can make an educated guess about the types of algae present in a biofilm.  I’ve tried to capture this with my watercolours, using washes of raw sienna for the diatoms and a grass-green for the green algae, which roughly matches the colour of their respective growths in the photo in my earlier post.   The two groups of algae a are relatively distinct on that particular boulder.   The top row roughly matches the upper “edge” of the graph showing variation in diatoms, whilst the bottom row emulates the upper “edge” of the graph showing variation in green algae.  These are the two extreme situations; however, we also often see darker brown growths in the field, which can be recreated by mixing the raw sienna and grass-green together.  When I peer through a microscope I often see green algae smothered in diatoms: genera such as Oedogoniumare particularly prone as they have less mucilage than some of the others we find in the Ehen. Their filaments often host clusters of Fragilariacells as well as Achnanthidium minutissimum, whilst stalked Gomphonemaand chains of Tabellaria flocculosaoften grow through the tangle of green filaments.   The dark brown colour is deepened yet further by the colour of the underlying rock, so my effort on white watercolour paper is a little misleading.


A colour chart showing how different proportions of green algae and diatoms influence the colour of biofilms.

The final graph shows how, as the average biomass increases in the River Ehen, so the variability in biomass also increases.   The River Ehen is one of the cleanest rivers I know but I suspect that this pattern in benthic algal quantity could be reproduced in just about any river in the country. What I would not expect to see in any but the purest and most natural ecosystems is quite so much variation in the types of algae present.   Once there is a little enrichment, so I would expect the algae to become more of a monoculture of a dominant filamentous alga plus associated epiphytes.  Like much that happens in the microscopic world of rivers, it is easier to describe than it is to measure.

That, however, is only part of the story but I’ll come back to explain the patterns in the other main groups of algae in the Ehen and Croasdale Beck in my next post.


The relationship between mean chlorophyll density and the standard deviation (based on measurements from five separate stones) for samples from the River Ehen and Croasdale Beck. 


Croasdale Beck in February


My latest trip to the west Cumbria coincided with the period of freakily warm weather that marked the end of February (in marked contrast to a year previously when we were in the midst of the “Beast from the East”).   It felt like spring had come early although the skeletal outlines of leafless trees were incongruous against the backdrop of blue skies and, despite feeling the warmth of the sun on our faces as we worked, the water still had a wintery chill when the time came to plunge in my arm.

There were thick growths of algae on the bed of Croasdale Beck: a quick check with my microscope later showed this to be mostly Odontidium mesodonand Gomphonema parvulumand this piqued my curiosity to see how different species responded to the fluctuations in biomass that we observe in the streams in this region. I’ve talked about this before (see “A tale of two diatoms …”), suggesting that Platessa oblongellatended to dominate when biofilms were thin whilst Odontidium mesodon preferred thicker biofilms.  That was almost two years ago and I now have more data with which to test that hypothesis, and also to see if any other common taxa had an equally strong preference for particular states.


A cobble from the bed of Croasdale Beck in February 2019 showing a brown biofilm (approx. 1.7 micrograms per square centimetre) dominated by Gomphonema parvulumand Odontidium mesodon.   The photograph at the top of the post shows Ennerdale Water photographed on the same day.

I should also be clear that, in Croasdale Beck especially, diatoms are the main algal component of the biofilm, so they are not so much responding to a particular state of the biofilm as actively contributing biomass to create that state.  The other photosynthetic organism that is obvious to the naked eye in this part of Croasdale Beck is the cyanobacterium Chamaesiphon fuscus (see “A bigger splash …”) but this forms crusts on stone surfaces rather than contributing to the superstructure of the biofilm itself. We do find other filamentous algae, but intermittently and in smaller quantities.

We’ll look at Platessa oblongellafirst, bearing in mind that this was shown to be a mixture of two species about halfway through our study (see “Small details in the big picture …”).   The graph below, therefore, does not differentiate between these two species although, from my own observations, I have no reason to believe that they behave differently.   What I have done in these graphs is to divide the biomass measurements and the percent representation of these taxa in each sample into three categories: low, middle and high.   In each case, “low” represents the bottom 25 per cent of measurements, “high” represents the top 25 per cent of measurements and “middle” represents all the rest. The left-hand graph shows biomass (as chlorophyll a concentration) as a function of the relative abundance of the diatom whilst the right-hand graph shows the opposite: the relative abundance of the diatom as a function of the biomass.  These graphs bear out what I suggested in my earlier post: that Platessa oblongella(and P. saxonica) are species whose highest relative abundances occur when the biofilm is thin.  So far, so good.


Relationship between relative abundance of Platessa oblongella (including P. saxonica) and biomass in Croasdale Beck, Cumbria.  a. shows biomass (as chlorophyll a) as a function of the relative abundance of the two species (Kruskal-Wallis test, p = 0.047) whilst b. shows the relative abundance as a function of biomass (p = 0.057).

My second prediction in my earlier post was that Odontidium mesodonpreferred moderate or thick biofilms; however, whilst there is a clear trend in the data, differences between low, middle and high values of neither biomass nor relative abundance are significant.   The explanation may lay in the strong seasonality that O. mesodondisplays, thriving in spring but less common at other times of year (see “More about Platessa oblongella and Odontidium mesodon”).  However, there are no strong seasonal patterns in biomass in Croasdale Beck, and this disjunction introduces enough noise into the relationship to render it not significant.


Relationship between relative abundance of Odontidium mesodon and biomass in Croasdale Beck, Cumbria.  a. shows biomass (as chlorophyll a) as a function of the relative abundance of O. mesodon (Kruskal-Wallis test, p = 0.568) whilst b. shows the relative abundance as a function of biomass (p = 0.060).

I then tried looking at the relationship between relative abundance and biomass for a few other common taxa but with mixed results.   None of Achnanthidium minutissimum, Gomphonema parvulum complex or Fragilaria pectinalis showed any clear relationship; however, when I looked at Fragilaria gracilis, a different pattern emerged, with a significant relationship between the quantity of biomass and the proportion of this species in the sample.  That, too, is not a great surprise as I often see clusters of Fragilaria gracilis cells growing epiphytically on filamentous algae within the biofilm.  Whilst Platessa oblongella, which sits flat on the stone surface, seems to be a species that thrives when the biofilm is thin, so Fragilaira gracilisis favoured by a more complex three-dimensional structure, where it can piggy-back on other algae to exploit the light.   I suspect, however, that in a stream such as Croasdale Beck, where the substratum is very mobile, Fragilaira gracilis will also be one of the first casualties of a scouring spate which will, in turn, open up the canopy allowing Platessa oblongella back.   Even though my results for Odontidium mesodonare not significant, I still think it plays a part in this sequence, occupying the intermediate condition when some biomass has accumulated.  It looks to me as if it also likes cooler conditions which then complicates interpretation of my results.

Indeed, I am being rather selective in the results that I have included here.  Three of the six species I investigated showed no response and one of the three that I did include showed a trend rather than a statistically-convincing effect.  I suspect that the situation will rarely be as simple as I have shown for Platessa oblongella and Fragilaira gracilis.  Nonetheless, there is enough here to make me want to scratch a little more and try to understand this topic better.


Relationship between relative abundance of Fragilaria gracilis and biomass in Croasdale Beck, Cumbria.  a. shows biomass (as chlorophyll a) as a function of the relative abundance of F.gracilis (Kruskal-Wallis test, p = 0.010) whilst b. shows the relative abundance as a function of biomass (p = 0.036).


Croasdale Beck, photographed in February 2019.