Blind to the obvious

I’ve moved just a few kilometres from the River Liza to the location for this post: Croasdale Beck, a stream which joins the River Ehen at Ennerdale Bridge.   Croasdale Beck has featured in a number of posts in the past (see, for example, “That’s funny …” and “Croasdale Beck in February”) partly because it continues to surprise me.   Maybe that reflects a level of complacency on my part: regular visits mean that I know what to expect which, in turn, means that I am alert to things that I do not expect.   Seeing something new in a stream I have never previously visited is evidence of life’s rich pattern; noticing something that was probably there on previous occasions but which I overlooked is a more profound and, somehow, humbling experience.  This post is about one of each of these sensations.

There are, for example, a number of turquoise-coloured boulders in the beck that were certainly not there when I first started visiting in about 2015.   Most of the stones in the beck are cobbles rather than boulders, so these stand out both for their size and colour.   The colour is, if you look closely, due to a thin surface film – a Cyanobacteria which I will call Lyngbya vandenberghenii although, because it is difficult to scrape off (the filaments live in amongst the rock crystals), and lacks any really eye-catching features, it is hard to be totally certain about this.   Presumably it likes the stability that boulders confer in this very flashy little stream.    I also see it in the River Ehen nearby but there its presence is easier to explain as it is confined to chunks of limestone washed in from the foundations of a section of the Coast-to-Coast walk.

Simulium argyreatum growing on a cyanobacteria-covered boulder from Croasdale Beck, Cumbria (shown in the photo at the top of the post).  The stone is about 30 cm across.   

Today, however, I’m interested in what is growing on top of the Lyngbya rather than in the Lyngbya itself: dense patches of what looks, with the naked eye, like small tan-coloured seeds.   These are the tiny larvae of Simulidae, whose adult phases are the annoying blackflies that swarm around streams on summer evenings.   They spin a web of silk on the substrate to which they anchor themselves using a ring of hooks at their posterior.   Their mouthparts include a pair of fans (one of which can be seen in the image below) and, by extending themselves above the stone, they can trap tiny particles (including algae) drifting in the current. They produce a secretion which makes the fans sticky and also have mandibles adapted to brush the trapped particles from the fans into their mouths.   Most descriptions of the Simulidae refer to this filter-feeding life-style but I’ve also seen them bent double so that their fans can brush up the algae which grow on the stone surfaces. 

Larvae of Simulum argyreatum on boulders in Croasdale Beck.  The upper photograph was taken in situ with the macro facility on my Olympus Tough camera (each is ~0.3 – 0.5 millimetres long) from a stone without the crust of Lyngbya whilst the lower photograph shows a magnified view of the feeding fan of one larva.

At some point, the larvae cease feeding and spin slipper-shaped cocoons with the closed end facing upstream and the open end downstream.   Six white ribbon-like gills protrude from the open end, ensuring a ready supply of oxygen to the pupa inside.   The adult develops inside this cocoon, eventually emerging with a duel raison d’être of having sex and irritating humans.  “Adult” hardly seems like the appropriate word: “perpetual teenager” seems more apt. 

Whilst the adult males feed on nectar, the females need a blood meal before mating, adding a dark Gothic twist to their natural history.  This difference arises from the roles each plays in reproduction: the male only needs the spurt of energy that the sugary nectar confers whilst the female needs the proteins and minerals from the blood in order to nourish the eggs.  In the south of England, bites from the Blandford Fly, a relative of the Simulium I watched in Croasdale Beck, can cause nasty rashes whilst in large parts of Africa the bites from other species of Simulium can inject the parasite responsible for Onchocerciasis, or river blindness.   This was a common disease in the region of Nigeria where we lived in the early 1990s so I’ve seen the damage that these flies can cause.   Much as we find black flies and midges to be a nuisance in this country, at least they are not vectors for potentially deadly diseases. 

At a deeper level, knowing about the life cycle of Simulium reminds us that we are not just observers of aquatic ecosystems, we are, indirectly, part of these ecosystems too.  We may like to think of ourselves as the ultimate predator (remembering that this power brings with it great responsibility) but sometimes, as here, we can be the prey too. 

Clusters of Simulium argyreatum pupae on the Lyngbya-covered surface of a boulder in Croasdale Beck.   Each is about 3-5 millimetres long. 


And thanks to Richard Chadd for identifying the Simulium from my photographs.

Some other highlights from this week:

Wrote this whilst listening to:  The late great Toots Hibbert, remembering, in particular, Toots and the Maytals’ set on the West Holt Stage on a glorious summer evening at Glastonbury 2010

Cultural highlights:  We’re in the Lake District this week and, having recently watched part of Simon Scharma’s BBC series on the Romantic Movement, I’m reflecting on the role that the landscapes around me played in catalysing the work of Wordsworth, Turner and others. 

Currently reading:  English Pastoral by James Rebanks, a thoughtful analysis of the state of British agriculture that does not shy away from criticism either of farmers or naïve ecologists.

Culinary highlight:  James Rebank’s thesis hangs on the necessity of animal husbandry to maintain healthy soils.  With that in mind, I ate a Lakeland lamb steak at the Shepherd’s Arms hotel in Ennerdale Bridge with a clear conscience. 

Messy bedrooms …


When I was tramping around the Shetland Islands earlier this year (see “Hyperepiphytes in the Shetland Islands“), looking at the algae that live in the freshwater lochs, I noticed some meandering hieroglyphs made from fine sediment on the tops of some of the stones in the littoral zone.   I see these occasionally at other places too, and know that they are the “galleries” of caseless caddis flies.  Caddis flies are close relatives of the butterflies and are best known because many of their larvae use “found materials” (in contemporary art jargon) to construct cases to protect themselves.  Some species use fine gravel, silt and sand, some use fragments of plants, some have cases that are very neat, some have a more haphazard approach to construction.  However, a few families of caddis flies eschew cases and, instead, build these galleries.

Many caddis fly larvae, whether cased or not, are grazers, scraping the algae off the rocks on the bed of the stream or lake.   There is evidence that the cases offer some protection against predators such as trout which, by increasing survival rate, means that it is worthwhile for the caddis larvae to divert some of their hard-earned energy into building these.   Presumably, their caseless cousins gain the same advantage to building their galleries but recent research has suggested that these galleries offer a further benefit.

Think of caddis larvae as adolescent caddis flies.  Now imagine that the caddis gallery is the equivalent of an adolescent’s bedroom.   Horribly messy, in other words.   Let’s leave that image of a teenager behind (as most human teenagers know their way to the bathroom) and consider what happens to all that waste material that emerges from the far end of a caddis larva’s digestive system.   This nutrient-rich “ manure” encourages algae, meaning that our caseless caddis flies are, in fact, gardeners and are able to tap into this extra energy resource within their galleries in order to grow.   That brings us back to the analogy with teenagers, as these also frequently graze in their bedrooms (the diatom Campylodiscus is even the same shape as a Pringle, whose empty containers litter the bedroom floor of my own progeny).   I guess it is a good thing that caddis larvae don’t wear socks as, with six legs and two prolegs, the mess inside the gallery would be indescribable.


Galleries of caseless caddis flies (possibly Psychomiidae) on the top surface of a cobble from Sand Loch, Shetland Islands with (right) a close-up of a single gallery. The photograph at the top of the post shows Sand Loch in May 2019.

A recent study in the Lake District has shown that this “gardening” means that the algae which grow in the fine sediment from which the galleries are constructed are different to those found elsewhere on the rock surface, with a greater proportion of diatoms, which are considered to be more palatable to invertebrates than other types of algae.  Some caddis flies are thought to go even further, and can selectively remove and discard the algae that are least palatable (some Cyanobacteira, for example).

It is possible that up to 40% of the larva’s energy needs are met from the gallery itself.   The tube is, in fact, not a static construction: the larva pokes its head out in order to graze the algae immediately in front of the gallery, and extends the gallery as the food supply within easy (and safe) reach is exhausted.   At the same time, it is consuming the alga-rich rear part of the gallery (reminiscent of Hansel and Gretel eating the gingerbread house?).   A gallery only has a life-span of 10 days in the laboratory; whether this is the same under field conditions is not clear but that gives us some idea of the transience of these structures.   This rapid turnover means that the caddis larva is always feeding on succulent early-succession species, rather than the tougher and less digestible algae that might appear in more mature biofilms.

I also see similar galleries on the bed of the River Ehen from time to time but have been told that these are formed by non-biting midge (chironomid) larvae, rather than by caddis.  I presume that the same processes are happening in these although I have not been able to find much written in the literature.

Organisms that can significantly alter the habitat in which they live, and affect the conditions experienced by other species in the habitat are termed “ecosystem engineers”.  Beavers are good examples, as their dams can have significant effects on organisms extending for hectares.  Yet, in their own small way, caseless caddis larvae are also ecosystem engineers.  As are adolescent boys.   Which makes me wonder, having only talked until now about the algae in their galleries, whether caseless caddis larvae also have patches of mould extending up their walls.


Galleries made by chironomid larvae on a boulder in the River Ehen, March 2019.


Hart, D. D. (1985). Grazing insects mediate algal interactions in a stream benthic community. Oikos 44: 40-46.

Johansson, A. (1991). Caddis larvae cases (Trichoptera, Limnephilidae) as anti-predatory devices against brown trout and sculpin. Hydrobiologia 211: 185-194.

Ings, N. L., Hildrew, A. G., & Grey, J. (2010). Gardening by the psychomyiid caddisfly Tinodes waeneri: Evidence from stable isotopes. Oecologia 163: 127-139.

Ings, N. L., Grey, J., King, L., McGowan, S., & Hildrew, A. G. (2017). Modification of littoral algal assemblages by gardening caddisfly larvae. Freshwater Biology 62: 507-518.

Otto, C., & Johansson, A. (1995). Why do some caddis larvae in running waters construct heavy, bulky cases? Animal Behaviour 49: 473-478.

The presence of absence in Castle Eden Dene


Some of my strongest impressions of Castle Eden Burn after last week’s visit concerned not what I found in the stream, but what was not there.  I mentioned in my previous post that I had not seen the mosses that I associated with streams in northern England in Castle Eden Burn, but there were other species, too, that I had expected to see but had not noticed.   Once I have noticed that something is absent, this absence becomes present.  I have noticed the presence of absence.  Woohoo: I’ve shoehorned Jean-Paul Sartre’s Being and Nothingness into a blog about ecology.

When I got back home I had read a chapter about the FBA’s study of the Winterbourne in Dorset, an intermittent stream flowing off the chalk downland, and noticed that they had recorded plants there that I knew from north-east English rivers, but which I could not remember seeing in Castle Eden Burn.  Was this because I had not searched the stream environs thoroughly, or is this a real difference between intermittent streams on chalk and on Magnesian limestone?

I went back this weekend to try to answer these questions, taking Heather with me, as her skills with the higher plants far exceed mine, and walked as much of the stream bed as we could, starting near the remains of a footbridge at NZ 424 389, and making our way downstream to Denemouth, where Castle Eden Burn joins the North Sea.  If my original intention was to better understand the burn by traversing space within the Dene, my first lesson concerned time: a week with some heavy rainfall separated my two visits and it was clear straight away that the Burn had been flowing during the week, with a fine layer of silt and mud spread across much of the surface, making parts of it slippery to walk upon.  There were standing pools of water at several points in the upper part of the Burn too.   Within a week the stream had come and gone, offering scant opportunities for any water-loving organism to establish.

We made our way along the Burn through the delicious silence of the forest.  The banksides were richly vegetated: masses of opposite-leaved golden saxifrage plus the mosses I described last time and many others, along with plenty of harts-tongue fern (Asplenium solopendrium).   Then, with a very clear demarcation, there was the stony stream bed with very little vegetation at all.    We looked hard for three plants, in particular, that I associated with the damp margins of streams, and which I had expected to see here: Verronica beccabunga (brooklime or water speedwell), Rorippa nasturtium-aquaticum (water cress) and Mentha aquatica (water mint).  None seemed to be present in any of the stretches we visited apart from a single sorry looking brooklime in the freshwater marsh at Denemouth .

What we did find, a little further downstream, was a pebble and gravel-dominated stretch with a straggly array of plants, all bent over in the direction of flow.   These included broad-leaved dock (Rumex obtusifolius), nettles (Urtica diocia), a few shoots of Himalayan balsam (Impatiens glandulifera) and some grasses.    Were I not standing on a dry stream bed I would have assumed that this was a bare piece of ground being colonised by typical ruderal species.  And that, I think, offers some insights into the ecology of Castle Eden Burn.   This is not a stream that occasionally dries out: it is a long-thin terrestrial habitat that is occasionally flushed through by water.   Welcome to north-east England’s premier wadi.


Rumex obtusifolius and other ruderal vegetation on the stream bed of Castle Eden Burn, August 2019.

This hypothesis really needs corroboration by a hydrologist, but the graph I showed in “Out of my depth …” shows that, despite flow being generally low,  episodes of high flow are scattered throughout the year, and I suspect that these keep the substratum mobile and, more important, stop organic matter accumulating to give amphibious plants an opportunity to establish.   The water table, too, I guess, is too far below the stream bed in between the spates to make it easy for plants to stay hydrated.   This is one of the main differences between Castle Eden Burn and the southern chalk streams, which are characterised by very stable flow regimes

From the point at which Blunt’s Burn enters Castle Eden Burn (NZ 436 396) there does seem to be permanent flow down to the sea.  Still, however, there was very little in-stream vegetation.  That was in contrast to the forest around us, which was floristically-rich (Heather has written more about this on her blog) and, on this warm summer morning, positively humming with bees and aflutter with butterflies.

A large embankment takes the busy A1086 over the Dene, the Burn passing through a long culvert at this point, after which there is a viaduct taking the coastal railway line across before the dene widens out into a large area of meadow just before it reaches the sea.   The stream’s path to the sea is, however, blocked by mine waste that was dumped from the coal mines that used to line the Durham coast.  This forces the stream to turn ninety degrees south for a few hundred metres before finding a way through and, gradually, trickling and percolating through the beach. The mines have all gone now and the sea is gradually eroding this compacted mass of waste.  Before the waste arrived, apparently, there was an area of saltmarsh at the mouth of the burn.   Now, there is a freshwater marsh, dominated by reeds (Phragmites australis).  When the mine waste finally goes, maybe the saltmarsh will return.  Meanwhile, Castle Eden Burn has no grand finale: it ends on a whimper not a bang.

We climbed a narrow, steep pathway up through gorse and brambles onto the clifftops overlooking these final stages of Castle Eden Burn to get a view that was, in light of all that had passed through my mind earlier, oddly symbolic.  The stream flowed almost due east until it encountered the bar, and the gentle arc which it then describes looks just like a question mark.   How ironic, I thought, for a stream that raises more questions than answers to sign off in that way ….


Denemouth, at the end of Castle Eden Dene, just above the point where the stream joins the North Sea.

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.

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. 

Some like it hot …

My reflections on algae that thrive in hot weather continued recently when I visited a river in another part of the country.  As this is the subject of an ongoing investigation, I’ll have to be rather vague about where in the country this river flows; suffice it to say it is in one of those parts of the country where the sun was shining and your correspondent returned from a day in the field with browner (okay, redder) arms than when he started.   Does that narrow it down?

A feature of some of the tributaries, in particular, was brown, filamentous growths which, in close up, could be seen to be speckled with bubbles of oxygen: a sure sign that they were busy photosynthesising.  These were most abundant in well-lit situations at the edges of streams, away from the main flow.   Under the microscope, I could see that these were dominated by the diatom Melosira varians, but there were also several filaments of the cyanobacterium Oscillatoria limosa, chains of the diatom Fragilaria cf capucina and several other green algae and diatoms present.

Melosira varians is relatively unusual as it is a diatom that can be recognised with the naked eye – the fragile filaments are very characteristic as is its habitat – well lit, low-flow conditions seem to suit it well.   It does seem to prefer nutrient-rich conditions (see “Fertile speculations …”) but it can crop up when nutrient concentrations are quite low, so long as the other habitat requirements are right for it.  The long chains of Melosira (and some other diatoms such as Fragilaria capucina and Diatoma vulgare) help the cells to become entangled with the other algae.   I could see this at some sites where the Melosira seemed to grow around a green alga that had been completely smothered by diatoms and was, I presume, withering and dying.  In other cases, the Melosira filaments are much finer and seem to attach directly to the rocks.   Neither arrangement is robust enough for Melosira to resist any more than a gentle current which is why it is often most obvious at the edges of streams and in backwaters.   As is the case for Ulva flexuosa, described in the previous post, I suspect that the first decent rainfall will flush most of this growth downstream.   Another parallel with Ulva is that, despite this apparent lack of adaptation to the harsh running water environment, Melosira varians is more common in rivers and streams than it is in lakes.

Melosira varians-dominated filaments at the margins of a stream.  Top photograph shows the filaments smothering cobbles and pebbles in the stream margins (frame width: approximately one metre); bottom photograph shows a close-up (taken underwater) of filaments with oxygen bubbles (frame width: approximately one centimetre).

Algae from the filaments illustrated above: a. and b.: Melosira varians; c. Fragilaria cf capucina; d. Oscillatoria limosa.  Scale bar: 20 micrometres (= 1/50th of a millimetre).  

The graphs below support my comments about Melosira varians preferring nutrient rich conditions to some extent.  Many of our records are from locations that have relatively high nutrient concentrations; however, there are also a number of samples where M. varians is abundant despite lower nutrient concentrations.   How do we explain this?   About twenty years ago, Barry Biggs, Jan Stevenson and Rex Lowe envisaged the niche of freshwater algae in terms of two primary factors: disturbance and resources.   “Resources” encompasses everything that the organism needs to grow, particularly nutrients and light, whilst “disturbance” covers the factors such as grazing and scour that can remove biomass.   They used this framework to describe successions of algae, from the first cells colonising a bare stone through to a thick biofilm.   As the biofilm gets thicker, so the cells on the stone get denser and, gradually, they start to compete with each other for light, leading to shifts in composition favouring species adapted to growing above their rivals (see “Change is the only constant …”).

The relationship between Melosira varians and nitrate-nitrogen (left: “NO3-N”) and dissolved phosphorus (right: “PO4-P”).   The vertical lines show the average positions of concentrations likely to support high (red), good (green), moderate (orange) and poor (red) ecological status (see note at end of post for a more detailed explanation).

They suggested that filamentous green algae were one group well adapted to the later stages of these successions but these, in turn, create additional opportunities for diatoms such as M. varians which can become entangled amongst these filaments and access more light whilst being less likely to being washed away.   If there is a period without disturbance then the Melosira can overwhelm these green algal filaments.   Nutrients, in this particular case, do play a role but, in this case, are probably secondary to other factors such as low disturbance and high light.  Using the terminology I set out in “What does it all mean?”, I would place M. varians in the very broad group “b”, with the caveat that the actual nutrient threshold below which Melosira cannot survive in streams is probably relatively low.   Remember that phosphorus, the nutrient that usually limits growth in freshwater, comprises well under one per cent of total biomass, so a milligram of phosphorus could easily be converted to 100 milligrams of biomass in a warm, stable, well-lit backwater.

Schematic diagram showing the approximate position of Melosira varians on Biggs et al.’s conceptual habitat matrix.

The final graph shows samples in my dataset where Melosira varians was particularly abundant and this broadly supports all that has gone before: Melosira is strongly associated with late summer and early autumn, when the weather provides warm, well-lit conditions with relatively few spates.

The case of Meloisra varians is probably a good example of the problem I outlined in “Eutrophic or euphytic?”  I have seen similar growths of diatoms in other rivers recently, due to the prolonged period of warm, dry conditions.  It is easy to jump to the conclusion that these rivers have a nutrient problem.  They might have, but we also need to consider other possibilities.   Like Ulva flexuosa in the previous post, Melosira varians is an alga that is enjoying the heatwave.

Distribution of Melosira varians by season.   The line represents sampling effort (percent of all samples in the dataset) and vertical bars represent samples where M. varians forms >7% of all diatoms (90th percentile of samples, ranked by relative abundance). 


Biggs, B.J.F., Stevenson, R.J. & Lowe, R.L. (1991). A habitat matrix conceptual model for stream periphyton. Archiv für Hydrobiologie 143: 21-56.

Notes on species-environment plots

These are based on interrogation of a database of 6500 river samples collected as part of DARES project.  Phosphorus standards are based on the Environment Agency’s standard measure, which is unfiltered molybdate reactive phosphorus.  This approximates to “soluble reactive phosphorus” or “orthophosphate-phosphorus” in most circumstances but the reagents will react with phosphorus attached to particles that would have been removed by membrane filtration. The current UK phosphorus standards for rivers that are used here are site specific, using altitude and alkalinity as predictors.  This means that a range of thresholds applies, depending upon the geological preferences of the species in question.  The plots here show boundaries based on the average alkalinity (50 mg L-1 CaCO3) and altitude (75 m) in the whole dataset.

There are no UK standards for nitrate-nitrogen in rivers; thresholds in this report are based on values derived using the same principles as those used to derive the phosphrus standards and give an indication of the tolerance of the species to elevated nitrogen concentrations.  However, they have no regulatory significance.



So what?


And so to Trento, and the Use of Algae for Monitoring Rivers symposium.   I approached with mild trepidation (see comments in “The human ecosystem of environmental management …”).   The last time I attended, Diatom Jihad was just getting started, and hordes of fundamentalists were swarming over the plains of scientific reason, eliminating any apostates who dared suggest alternative methods of evaluating the condition of freshwaters.   To be honest, I was probably part of that horde, certainly in the early days, though I think that the True Believers always doubted my ultimate loyalty to the Holy Books produced by Lange-Bertalot, Krammer, Witkowski and their acolytes.

I have, indeed, had my own vision on the Road to Damascus (forgive me for mixing my religious metaphors).   It was, in reality, the culmination of several conversations and much thinking but it can be encapsulated by a comment made by a biologist from Wessex Water, one of the utility companies who operate sewage works in the UK. I had been part of a team working on the River Wylye in Wiltshire and we were discussing our results.   She looked up and said (I am paraphrasing now): “I’m not disagreeing with what you are telling us [i.e. that the river had higher concentrations of nutrients than was ideal for the ecology]. But we need to justify the price rises that would result from improved effluent treatment, and the public don’t know what diatoms are”.   She had put her finger on a very major issue: that many of us involved in applied ecology are so focused on the fine details of the ecosystems that we study, that we lose sight of the bigger picture.

I tried to emphasise this in my talk and the cartoon above follows Billy Wilder’s maxim: “If you’re going to tell people the truth, be funny or they’ll kill you”.   And, to be fair, I was not the only person who made this point.   Jan Stevenson from the University of Michigan gave us a good overview of the current situation in the USA and also emphasised the need to relate the changes in diatom assemblages to ecosystem services and, if possible, to determine thresholds in responses that help to develop consensus amongst stakeholders. So perhaps the winds of change really are now blowing on both sides of the Atlantic?

Towards the end of my talk, I used the phrase “healthy streams are slippery streams”, used by Emma Rosi-Marshall of the Cary Institute for Ecosystem Health in New York.   She is using this phrase as part of a campaign to raise awareness of the role that benthic algae play in ecosystem health. Phil Harding, the co-author of my talk, saw this written on one of my slides and commented that one of the sampling locations that his team visit regularly in the English Midlands is called “Slippery Stones” – a beautiful site on the edge of the Peak District. Is it too fanciful to suggest that this place actually has a name that reflects the quality of the freshwater ecosystem?


“Slippery Stones”, the actual place name of a site on the upper River Derwent in Derbyshire, UK.

The perplexing case of the celibate alga …

Oedogonium presents a real challenge to an ecologist.   As I mentioned in my previous post, there are many species and these are found in a wide variety of conditions. In order to identify the species we need the reproductive organs but, as is the case for several filamentous freshwater algae, these are rarely seen in the wild. I did consult two colleagues on whether it was possible to induce Oedogonium filaments to grow these in the laboratory, but both told me that this was difficult. The theory is that you are more likely to find reproductive organs in situations where the alga has been allowed to dry slowly.   This is a useful survival strategy as the spores are usually very resistant to desiccation and can survive long periods out of water. However, converting theory to practice is not straightforward.

But how, I wondered, was the section on Oedogonium in the Freshwater Algal Flora produced?   ‘From secondary sources’, came back the reply. In other words, the author of this part of the Flora had relied descriptions and illustrations in earlier publications. As the most thorough work on Oedogonium in the UK was performed by the Wests, father and son, in the late nineteenth and early 20th centuries, this means that there has been no thorough overview of Oedogonium here for over 100 years.   I searched the database Web of Science and found just 14 papers that reported studies on the taxonomy of Oedogonium in the intervening years.   Just two of these were from European laboratories: one in 1991 in Czechoslovakia and a Polish study from 1979.   That’s not very much, considering the large number of species and their very broad distribution.

Just as we can identify some flowering plants from their vegetative characteristics alone, so some people have tried to identify Oedogonium using just the properties of the filaments. However, there is not very much to go on, apart from the length and width of the cells. The best attempt is that by my colleague Susi Schneider in Norway (see “A brief excursion to Norway”).   She differentiated eight types of Oedogonium in Norwegian rivers based on cell dimensions and noted a significant relationship between these types and phosphorus concentrations in the rivers where they grew.   Interestingly, the narrow forms were associated with low nutrients whilst the broader ones were found in more nutrient rich conditions. The population I found in Stockerley Burn was relatively broad which suggests, using Susi’s criteria from Norway, that this is a nutrient-rich stream. I am, however, reluctant to import Susi’s categories directly to the UK because our rivers are very different from those in Norway. However, I think it would be interesting to see whether the broad principles could be used here, even if we needed a slightly different calibration.

These struggles with Oedogonium also suggest that this is a genus that would benefit from a molecular genetic study, which would be a much more powerful means of differentiating between forms of Oedogonium although, unless we cracked the secret of either finding or culturing fertile Oedogonium it will be difficult to reconcile the DNA results with classical taxonomy. Until then, I fear, Oedogonium, represents yet another case of the “trailing edge” of science, where we may be in danger of forgetting faster than we learn.


Schneider, S.C. & Lindstrøm, E.-A. (2011). The periphyton index of trophic status PIT: a new eutrophication metric based on non-diatomaceous benthic algae in Nordic rivers. Hydrobiologia 665: 143-155.

A case of mistaken identity?

Imagine, just for a moment, that someone makes a list of the plants growing in a river or any other aquatic habitat and includes a category called “unidentified dicotyledon”.   Most botanists would throw up their hands in horror. Yet they probably have a category on their field record sheets for “filamentous green algae” which they use on a regular basis. It takes time, after all, to take a specimen back to the laboratory to check under the microscope and, let’s face it, the identification guides that are available are not very user-friendly and are full of unfamiliar terminology.

A slight variant on this particular sin is to record all the filamentous green algae that you encounter as Cladophora glomerata.  You are on a fairly safe bet here because a) it is a very common alga; and, b) no-one is likely to check.   However, there are a few algae that can be easily mistaken for Cladophora, especially if you are not paying close attention.

Last week, I did a survey of some streams draining into the River Browney, a tributary of the River Wear in County Durham.   Most showed evidence of enrichment which was not surprising as there were small sewage works, arable cultivation and a fish farm within these catchments.   And, as a result, it was no surprise to find that Cladophora dominated the stream beds at several sites.   One site, however, had thick wefts of filaments which looked and felt like Cladophora but, when viewed under the microscope, were quite different.


Stockerley Burn at Bogle Hole,   About half the river bed is covered by thick wefts of filamentous green algae up to about 30 cm in length.

I found three different species of green alga entangled in these wefts. There was some Cladophora glomerata but the most abundant of the three was a species of Oedogonium (see “The River Wear in summer”), characterised by unbranched filaments and cap cells.   66 species of Oedogonium have been recorded from Britain and Ireland but we know little of their ecology. Whilst some forms are common in lowland, nutrient rich rivers and streams such as this one, I have also found Oedogonium in remote, low-nutrient environments such as the River Ehen in Cumbria.   It pays to be careful, in other words, and to make sure that your “Cladophora” really is Cladophora. The easiest way to do this is to check for branching using a hand-lens. If you can’t see branching in the field, take a specimen back to the laboratory and check it under a microscope. Some populations of Cladophora are much more sparsely branched than others, so you may simply confirm your original suspicions. Oedogonium is, in fact, a very distant relation of Cladophora, despite their similarity when viewed with the naked eye. Mistaking Oedogonium and Cladophora is equivalent to  confusing your best friend with a sea squirt (see “Who do you think you are?”).

More about Oedogonium in the next post.


Filamentous algae in Stockerley Burn: main picture shows the wefts of (mostly) Oedogonium; inset shows cells from a single filament with the cap cells arrowed (scale bar: 10 micrometres, 1/100th of a millimetre).

About crackers, peanut butter and marmite …

I’m a sucker for good metaphors and analogies when I’m teaching. These are great for linking the ideas that I am trying to communicate with things with which the students are already familiar. One of my favourite analogies for stream ecology comes from a 1974 review paper by the US ecologist Kenneth Cummins. He was describing the process by which leaves which fall into streams at this time of year are broken down by the organisms that live in the stream in order to release their energy. There are a number of aquatic invertebrates, termed “shredders”, whose mouthpieces are specially adapted to tearing apart these leaves. They gain their nutrition from the leaves, so the theory goes, with the partially-digested leaf material emerging from their intestines, in due course, as “fine particulate organic matter”. That itself is a euphemism. Go figure.
But leaves alone do not make a particularly nutritious diet. In fact, the shredders are not living solely on these leaves. As soon as a leaf falls from the tree it is vulnerable to attack from bacteria and fungi. Like the invertebrates (like humans eating spinach, too), they can gain nutrition from this leaf, and the enzymes they produce help to soften up the tissues making it easier for the shredders to tear apart. Once in the water, the dead leaf will also be colonised by algae whose photosynthesis will produce oxygen which will replace that used by the various bugs as they break the leaf down. The combination of fungi, bacteria and algae also add to the nutritional content of the leaf. Cummin’s great analogy was that the leaf was akin to a ‘cracker’ whilst the microbial life was akin to ‘peanut butter’. A single cracker, as you know, is not itself greatly nutritious, but we tend to use crackers as ‘carriers’ for protein- and energy-rich foods such as cheese or, in Cummin’s example, peanut butter. An even better analogy for a UK reader is a cracker spread with Marmite which really is microbial-based nutrition.
Metaphor and analogies have their limitations, of course. But in an age where science is increasingly quantitative, the importance of having strong mental images of systems before you start taking them apart and counting and measuring the various components must be emphasised. It is a tradition that goes back at least as far as Leonardo da Vinci, and possibly further.


Cummins, K.W. (1974). Structure and function of stream ecosystems. Bioscience 24: 631-641.