When the going gets tough …


Two months after the visit I described in the previous post I was back at Castle Eden Dene.    The trees were now in leaf and the floor of the forest was carpeted with wild garlic.   The stream, however, had disappeared below the surface and, once again, I could walk along the channel without getting my feet damp.

Having found a rich crop of diatoms on my last visit when the stream was dry I was intrigued to see what was growing on the stones this time, so I used a toothbrush and some water that I had brought along to scrub a few and collected the dislodged material in my white tray.   I was intrigued to see that the suspension that collected in my tray had a distinct green tinge and, when I got a drop of it under my microscope, found it to be dominated by small green cells.  These were superficially similar to the cells of Desmococcus and Apatococcus that I found on the fence in my garden (see “Little Round Green Things …”) but this is a difficult group with not many clear morphological features with which to distinguish genera so I sent a sample off to Dave John for his opinion.

His view is also that groups such as this are almost impossible to identify unless you grow them in the laboratory or have access to DNA sequencing facilities.   He commented that Desmococcus and Apatococcus both have distinctive 2- or 4-celled packets of cells, which were not common in the Castle Eden Dene sample.  Likely candidates are the generaPleurastrumand Pseudopleurococcus, both of which are subaerial or terrestrial.   Perhaps “Little Round Green Things” is as close as we need to go in this particular instance?


A distinctly-green suspension of the biofilm on stones at Castle Eden Dene in May 2019 (left) along with a magnified view showing some of the green cells which dominated the sample (right).  Scale bar: 20 micrometres (1/5thof a millimetre).   

A short distance further on I found some mats of entwined filaments on the tops of stones which also piqued my curiosity.   Under the microscope, and with the addition of a drop of water to rehydrate them, these filaments revealed themselves to belong to Vaucheria (see “Who do you think you are?”).   Technically speaking, Vaucheria is not filamentous but “siphonous”, meaning that there are no cross walls but, instead, the organism consists of branching tubes containing many separate nuclei and chloroplasts.  The cell walls of Vaucheria, however, rupture easily releasing the chloroplasts and giving the appearance of an empty sausage skin. In this case, there are still quite a few chloroplasts but a healthy Vaucheria filament has a uniformly dense green appearance that none of those that I saw in Castle Eden Burn possessed.

There was more than just vegetative filaments of Vaucheria here: scattered amongst them were some larger, spheroid or jar-shaped cells, which are part of Vaucheria’s sexual reproduction apparatus.   I’ve talked before in this blog about how sexual reproduction is relatively rare in the filamentous algae that we find in lakes and streams (see “The perplexing case of the celibate alga …”) and Vaucheria is another case in point.   Put simply, many algae do not bother with sexual reproduction when conditions are favourable and they can grow through simple cell division.   If you subjected a Vaucheria filament to Freudian analysis, it would probably tell you that one outcome of sexual reproduction was a 50% dilution of its unique genotype. So why bother if you don’t have to?  On the other hand, sexual reproduction in these organisms usually results in a zygote with a thick wall that is capable of resisting tough conditions.   The complete absence of water in Castle Eden Burn would be one such circumstance.   To put it another way, when the going gets tough, the algae get frisky.


Mats of Vaucheria growing on a small boulder in Castle Eden Dene in May 2019.  The picture frame in the left hand image is approximately 30 centimetres. 


Cell walls of Vaucheria, with a few chloroplasts still present, from Castle Eden Burn, May 2019.  Scale bar: 20 micrometres (= 1/50thof a millimetre). 

Sexual reproduction involves an antheridium (the male organ) producing a large number of spermatozoids, each with a pair of flagella.  If a spermatozoid encounters the oogonium (the female sexual organ), then it will fertilise the single egg that this contains.   The zygote will then surround itself with a thick wall and undergoes a period of dormancy before geminating into a new vegetative filament.   As I was taking the photographs below, a cloud of tiny spermatozoids was released, prompting me to call out “come quickly if you want to see an alga ejaculating” before remembering that we had visitors in the house who might think this a little weird.   I even took a video.  I’ll upload it to the Dark Web at some point.  There must be a site for algae-themed pornography out there, if only I took the time to look…


An oogonium and an antheridium on Vaucheria filaments from Castle Eden Burn, May 2019.  The one on the right was releasing spermatozoids (arrowed) at the time the photograph was taken.   Scale bar: 20 micrometres (= 1/50thof a millimetre).  


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.

Tales from a dry river bed …

Two weeks ago I stood in a dry stream bed at Castle Eden Dene, wondering at the absence of water yet also conscious that many of the stones that littered the surface had a slipperiness that suggested not only that they had been wet relatively recently, but also that the surface biofilms (which impart this slipperiness) might still be intact.   A first look at a portion of this film under my microscope suggested that this might well be the case: I could certainly see some diatoms, and some green algae cells, but most were very small and that there was also a lot of particles, both inorganic and organic, that made viewing these algae quite difficult.   Since then, I’ve prepared a permanent slide from this material, so I can now take a closer look and get a better idea of what diatoms thrive in a dry stream bed in mid-winter in northern England.

A quick analysis of the sample found 34 species, of which four were abundant (comprising over 60% of the total) and the remainder were relatively infrequent.   The most abundant species was Amphora pediculus, which I’ve written about before, and which was not a surprise, as it is a species that thrives in the hard water that I would have expected in a stream draining a limestone catchment.  The other three common species wereHumidophila contenta, Luticola muticaand Simonsenia delognei, all of which are known to survive in habitats that are not permanently submerged.   These are relatively uncommon in the typical samples that I encounter but when they do occur in large numbers, they are often found together.   It is another facet of the “London Bus” paradigm that I described in the previous post, except this time it is a characteristic assemblage of species from different genera, rather than from a single genus or family.


Some of the diatoms from Castle Eden Burn, January 2019: a. Nitzschia nana; b. – g. Luticola mutica; h. – k. Humidophila contenta.   Scale bar: 10 micrometres (= 1/100thof a millimetre). 

Diatoms in the genus Humidophilahas changed names twice over the course of my career.   Back in the 1980s, species from this genus, as well as Luticolawere considered to be part of the Navicula which was regarded as a “dump for all bilaterally symmetrical [e.g. boat-shaped] raphid diatoms lacking particularly distinctive features” according to Frank Round, Dick Crawford and David Mann.    They split several groups of species away from Naviculato create new genera, one of which was Luticola.  In other cases, to resurrect old genera that had been subsumed into Naviculain the first half of the 20thcentury.  One of these resurrected genera was Diadesmiswhich differed from “true” Naviculain several respects, not least of which was a tendency to form ribbon-like colonies.   A more recent study suggested that Diadesmis, itself, needed to be split, with several species being moved to yet another new genus, Humidophila.   Unfortunately, the criteria on which this was based are not easily seen with the light microscope.  However, one by-product of this split was that all the species within the genera that are associated with damp, rather than fully-submerged habitats, ended up in the new genus rather than in Diadesmis.   That lends weight to the split, suggesting that there is more to the separation than just minor differences in the details of the cell wall.

The final species that was common in Castle Eden Burn was Simonsenia delognei.   This is another small diatom and, as I could not get good photographs from this sample, I have included photographs from another site to show what it looks like.  It is a very delicate diatom, easily overlooked when scanning a slide, particularly as it usually only occurs in small numbers.  That, again, might be because I usually look at samples from fully-submerged habitats.   Here, it formed about 12 per cent of the total number of valves, which is four times as many as I have previously found.


Simonsenia delogneifrom Ballyfinboy River, Co. Tipperary, August 2014.   Scale bar: 10 micrometres (= 1/100thof a millimetre).  Photographs: Lydia King.

I’m quite intrigued, now, to see how the algal communities change over the course of the year. Are these diatoms that can tolerate drying ever-presents or will their proportions fluctuate over the course of the year as the stream comes and goes?   And what is it that makes some diatoms cope with these dry periods?   The ability to live out of water is associated with a few genera in particular, so what is it about their genetic make-up that lets them thrive.  What about Amphora pediculusand the other diatoms that I associate with submerged habitats? Am I looking at dormant but viable cells (I did not see many healthy chloroplasts when I made my initial observations) or are these diatom carcasses strewn across an arid desert?    At the risk of sliding into metaphor-overload, does this mean that Humidophila, Luticolaand Simonseniaare the cacti of the diatom world?


Lowe, R.L., Kociolek, P., Johannsen, J.R., van de Vijver, B., Lange-Bertalot, H. & Kopalová, K. (2014).  Humidophilagen. nov., a new genus for a group of diatoms (Bacillariophyta) formerly within the genus Diadesmis: species from Hawai’I, including one new species.  Diatom Research29: 351-360.

Round, F.E., Crowford, R.M. & Mann, D.G. (1990).  The Diatoms: Biology and Morphology of the Genera.   Cambridge University Press, Cambridge.

A twist in the tale …


After my sojourn in East Durham, described in the previous post, I have travelled back to the Pennines for this one, crossing the River Wear at Wolsingham before driving up onto the fells and finally dropping down to the woodlands that are Hamsterley Forest.  This is a large man-made plantation, dating from the 1930s and popular for recreation. In January, however, the forest is quiet, and I only have a few mountain bikers and a lone dog walker for company as I peer into the peaty waters of Euden Beck.   This stream rises on the open fells of Hamsterley Common, between Weardale and Teesdale, before flowing through the forest and joining Spurl’s Wood Beck just downstream from where I am standing, to become Hamsterley Beck.  This then joins the Wear a few kilometres downstream from Wolsingham.


Euden Beck, just above the forest drive in Hamsterley Forest, January 2019.  The photograph at the top of the post shows a view towards Hamsterley Forest. 

There is a mixture of diatoms growing on the stones here but I am most interested in the genus Fragilaria today.   One of the curiosities of this genus is that we often find several representatives growing at the same site at the same time, reminiscent of the old adage about London buses (“you wait ages, and then three come along at once”).   I’ve written about this before (see “Baffled by the benthos (2)” and “When is a diatom like a London bus?”) and Euden Beck is another good example of this conundrum in practice.

Today, I could see quite a few cells of Fragilaria teneraand smaller numbers ofF. gracilisplus a newly-described species that I will talk more about later in the post.  Fragilaria teneraforms long, needle-like cells, often clustering together to form sea urchin-like masses growing out from either a filamentous alga or particle to which they are attached (see “Food for thought in the River Ehen” for an illustration).  Most of the ones that I saw in my samples from Euden Beck were either single cells or pairs of cells, presumably following a recent division. Note how the second cell from the left in the figure below is not as straight as the others.   This is something that I often see with Fragilaria populations in streams in the northern Pennines, and indicates that there may be heavy metal pollution in the water.  There are a lot of abandoned lead mines in the northern Pennines and, sure enough, when I looked at a large scale map, I found one that I had not previously noticed in the upper part of Euden Beck’s catchment.


Live cells of Fragilaria tenera(a. – d.) and F. heatherae from Euden Beck, January 2019.   a., b. and e. are valve views; c. and d. are girdle views.  Scale bar: 10 micrometres (= 1/100thof a millimetre). 

The next image shows these valve abnormalities even more clearly, with almost all of the cells showing aberrations in their outline.   These images are from an older sample; the curiosity here is that whilst most of the Fragilaria tenera valves were twisted, fewer of the valves of Fragilaria gracilisare twisted, whilst few of the valves of the third Fragilaria species show any abnomality in their outline at all.   This species is very common in northern Pennine streams, and I have often seen distorted valves of this species in streams polluted by mine discharges.  This makes the discrepancy between the outlines of this and Fragilaria tenera in Euden Beck particularly intriguing.


Fragilaria tenera from a sample collected from Euden Beck in June 2012.  Scale bar: 10 micrometres (= 1/100thof a millimetre).   Photographs: Lydia King.

I say “Fragilaria gracilis” with a modicum of trepidation as a recent study in which I have been involved, suggests that there may well be at least two species.  These are, as far as we can tell, indistinguishable using characteristics that can be seen with the light microscope though we know that they are genetically quite distinct, and both are widespread, turning up not just in the UK but in other parts of Europe too.

The third species, to the best of our knowledge, does not match the description of any other Fragilaria species, and we are in the process of publishing it as a new species, Fragilaria heatherae.   We have found it a number of samples, not just from the UK but also from sites elsewhere in Europe.   These, by comparison with the other two species, show very little distortion at all.   Whilst several authors have noted this phenomenon in the past, the physiological cause is still not understood. My guess is that the metal ions are displacing a metal co-factor in an enzyme that is involved in the process of laying down the silica cell wall.   Fragilaria seems to be particularly susceptible, but this may be because their long needle-like cells show the distortions more clearly than in some genera but, based on the evidence from Euden Beck, there are clearly differences in susceptibility between species.

Once again, I seem to be ending a post having asked more questions than I have answered. That is always frustrating but another way of looking at this is to realise that the frontiers of ecology are only ever a short drive away from where you are now.  It is very nice to cross oceans to visit rain forests and coral reefs, but there are adventures to be had closer to your doorstep.


Fragilaria gracilis from a sample collected from Euden Beck in June 2012.  Scale bar: 10 micrometres (= 1/100thof a millimetre).   Photographs: Lydia King.


Fragilaria heatherae” from a sample collected in Euden Beck in June 2012.  Scale bar: 10 micrometres (= 1/100thof a millimetre).   Photographs: Lydia King


Duong, T.T., Morin, S., Herlory, O. & Feurtet-Mazel, A. (2008). Seasonal effects of cadmium accumulation in periphytic diatom communities of freshwater biofilms.  Aquatic Toxicology90: 19-28.

Falasco, E., Bona, F., Ginepro, M., Hlúbiková, D., Hoffmann, L. & Ector, L. (2009). Morphological abnormalities of diatom silica walls in relation to heavy metal contamination and artificial growth conditions.  Water SA35: 595-606.

McFarland, B.H., Hill, B.H. & Willingham, W.T. (1996). Abnormal Fragilaria spp. (Bacillariophyceae) in Streams Impacted by Mine Drainage. Journal of Freshwater Ecology 12: 141-149.

Castle Eden Dene in January


The story so far: in 2018 I made bi-monthly visits to the River Wear, my local river and tried to capture, in my posts, the changes in the algae that occurred over the course of 12 months (follow the links in “A year in the life of the River Wear” to learn more).  It was an interesting exercise, partly because last summer’s exceptional weather led to some intriguing changes over the course of the year.   Consequently, as 2019 dawned, I thought I should find a different type of stream within a short drive from my home and try again.  So, bearing in mind that Wolsingham is south and west from where I live, I turned in the opposite direction and drove due east instead, stopping on the edge of the brutal concrete housing estates of Peterlee, a most unprepossessing location for a National Nature Reserve.

My journey has brought me right across the Permian limestone that dominates the eastern Durham landscape. Its escarpment rises up close to my home, and I have written about the algae that live in the ponds at the foot of it (see “A hitchhiker’s guide to algae…”).  On the other side, however, the limestone ends in a series of cliffs overlooking the North Sea and small streams have cut into the limestone to create a series of wooded valleys, or “denes”.   I’ve come to Castle Eden Dene, the largest of these: if you want a cultural reference point, watch the film “Billy Elliott”, set just a few miles further north along the coast, or read Barry Unsworth’s The Quality of Mercy.

We made our way down the footpath into the dene on a crisp and very cold winter morning, past the old yew trees from which the name is derived, and myriad ferns.   A deer bounded across the path ahead and disappeared into some scrub, and then we turned a corner and looked into Castle Eden Burn, which runs along the bottom of the dene.   To my surprise, the stream was dry.   This is a valley that cuts through limestone, so it is common for the stream to be dry in the summer, but I had not expected it to be dry in the middle of winter.  Thinking back, however, I realised that there has not been much rain for some weeks, and this may have meant that the water table, still low, perhaps, after last summer’s dry weather, is too low for the stream to flow.


Diatoms and cyanobacterial colonies in Blunt’s Burn, Castle Eden Dene, January 2019.   The top photograph shows diatom growths on bedrock; the lower image shows Phormidium retzii colonies, each about two millimetres across.   The photograph at the top of the post shows a yew tree overhanging Castle Eden Burn. 

A few hundred metres further down the dene, we finally heard the sound of running water where a small tributary stream, Blunt’s Burn, joined the main burn.  Judging from my OS map, it drains a good part of Peterlee so it might not have very high water quality.  It was, however, a stream and it did, as I could see with the naked eye, have some distinct diatom-rich growths.    These, I discovered later, were dominated by the diatoms such as Navicula tripunctataand N. lanceolata which are typical of cold weather conditions (see, for example, “The River Wear in January”).   A closer look showed that the orange-brown diatom growths were, in places, flecked with dark brown spots.  Somehow, I managed to get my cold fingers to manipulate a pair of forceps and pick up a few of these spots for closer examination.


Diatoms from Blunt’s Burn, January 2019: a. Navicula tripunctata; b. N. lanceolata; c.Gyrosigma cf. acuminatum; d. Nitzschiacf. linearis (girdle view); e. N. linearis(valve view).  Scale bar: 10 micrometres (= 1/100thof a millimetre).

I had a good idea, when I first saw these spots, that they were colonies of a filamentous cyanobacterium and, peering through my microscope a few hours later, once I had warmed myself up, I was relieved to see that I was right.  I picked out a dark patch and teased it apart before putting it onto a slide with a drop of water.  Once I had done this, I could see the tangle of filaments along with a mass of organic and inorganic particles and lots of diatoms.   The filaments themselves were simple chains of cells (a “trichome”) of Phormidium retzii, surrounded by a sheath.   There were also, however, a few cases, where I could see the sheath without the Phormidium trichome, and in some those I could also see diatom cells.

There are some diatoms that make their own mucilage tubes (see “An excuse for a crab sandwich, really …”) but Nitzschia is not one of those most often associated with tube-formation (there are a few exceptions).    On the other hand, there are some references to Nitzschiacells squatting in tubes made by other diatoms.   Some of those who have observed this refer to Nitzschia as a “symbiont” but whether there is any formal arrangement or is just a by-product of Nitzschia’s ability to glide and seek out favourable microhabitats, is not clear.  There are, as far as I can see, no references, to diatoms inhabiting the sheaths of Cyanobacteria, though Brian Whitton tells me he has occasionally seen this too.

We made our way back along the dry bed of Castle Eden Burn.  Many of the rocks here were quite slippery, suggesting that there had been some water flowing along it in the recent past.  That encouraged me to scrub at the top surface of one with my toothbrush and I managed to get a sample that certainly contains diatoms though these were mostly smaller than the ones that I found in Blunt’s Burn, and there was also a lot of mineral matter.   I’ll need to get that sample prepped and a permanent slide prepared before I can report back on just what diatoms thrive in this tough habitat.  Watch this space …


Cyanobacterial filaments from Blunt’s Burn, Co. Durham, January 2019: a. a single trichome of Phormidium retzii; b. and c. empty sheaths colonised by cells of Nitzschia; d. aPhormidiumfilament with a sheath and a trichome but also with epiphytes and adsorbed organic and inorganic matter.  Scale bar: 10 micrometres (= 1/100thof a millimetre).   


Carr, J.M. & Hergenrader, G.L. (2004).  Occurrence of three Nitzschia(Bacillariophyceae) taxa within colonies of tube-forming diatoms. Journal of Phycology23: 62-70.

Houpt, P.M. (1994). Marine tube-dwelling diatoms and their occurrence in the Netherlands. Netherlands Journal of Aquatic Ecology28: 77-84.

Lobban, C.S. (1984). Marine tube-dwelling diatoms of the Pacific coast of North America. I. BerkeleyaHasleaNitzschia, and Navicula sect. Microstigmaticae.  Canadian Journal of Botany63: 1779-1784.

Lobban, C.S. & Mann, D.G. (1987).  The systematics of the tube-dwelling diatom Nitzschia martiana and Nitzschia section Spathulatae. Canadian Journal of Botany.  65: 2396-2402,