Reflections from Castle Eden Burn

As 2019 draws to a close, I have looked back at all the data I have collected from Castle Eden Burn over the past twelve months.   I chose this location precisely because it was different to my usual haunts and, despite having visited this Dene and others along the Durham coast for over thirty years, I realised that I had never had a look at the algae.  Dry river beds are not the most obvious hunting grounds for aquatic biologists, after all.   This year, I put that right over the course of a number of visits between January and November and in this post I am summarising what I found.

I found a total of 77 different diatoms in the six samples that I collected, not to mention green and yellow-green algae (see “When the going gets tough …”) and mosses (see “A thousand little mosses …”).   Of these diatoms, 48 were rare and infrequent, only found in one or two samples, and never forming more than one percent of the total number of diatoms present.   Of the remainder, only two were found in every sample (Humidophila contenta-type and Achnanthidium minutissimum) whilst another eight formed at least ten percent of the total on one occasion.  Numbers of each species waxed and waned over the year: Humidophila contenta-type was abundant in the sample from my first visit in January 2019 but relatively scarce thereafter.  In comparison, Luticola frequentissima was very abundant on two occasions (more than 80% of individuals), quite abundant on three other occasions but absent from the sample from my final visit in November.

Some of these differences are due to the variable flow regime: the stream was dry on three occasions, ponded on one and flowing on just two occasions.  Those occasions when there was no running water were those when the proportions of diatoms that are tolerant to desiccation (see “Life out of water …”) were most abundant, forming from 20 to 97 percent of all individuals.  When there was running water, it was motile Nitzschia  species that dominated.    In fact, there was a strong negative correlation between proportions of desiccation-tolerant and motile taxa in the samples, indicating that the diatoms responded rapidly to the changing pressures experienced in the stream.  There was also a relationship between the proportions of desiccation-tolerant diatoms and the number of taxa recorded – the latter is a good measure of the level of physiological stress experienced in a stream.

What of the diatoms themselves?  Humidophila contenta-type was one of the two ever-presents.  It is, however, very small (few of those in our samples were more than a 100th of a millimetre long), making it difficult to photograph and, indeed, to discern many of the features of the valve.   This species sometimes forms short chains though I did not see any in the Castle Eden Burn samples.  It is strange to think that, when I first started to identify diatoms, this was considered to be part of the genus Navicula.   Since then, it has moved into the genus Diadesmis before finally being transferred to the new genus Humidophila by Rex Lowe and colleagues in 2014.    Some recently-described Humidophila species cannot be differentiated from H. contenta without a scanning electron microscope, so I have referred to this as “Humidophila contenta-type”. Humidophila_contenta

Humidophila contenta ag. from Castle Eden Burn, Co. Durham, January 2019.  Scale bar: 10 micrometres.   Photograph: Lydia King. 

The most abundant diatom in samples collected during the dry periods was Luticola frequentissima.  I started the year referring to this as “Luticola mutica” but was gently corrected by colleagues more au fait with recent literature than me.   Luticola mutica is larger (length: 11-28 µm; breadth: 6-9.5 µm) and has more widely-spaced striae (16-18 / 10 µm) than L. frequentissima (length: 7 – 13.8 µm breadth: 4.8 – 6.8 µm; striae: 20 -24 / 10 µm).  The specimens in the plate below all fit the description for L. frequentissima.  Some of the large specimens have size ranges that overlap with L. mutica (though even the largest specimen as a striae density consistent with L. frequentissima).   L. mutica is associated with more brackish habitats whilst L. frequentissima prefers freshwaters.

Luticola_frequentissima

Luticola frequentissima from Castle Eden Burn, Co. Durham, January 2019. Scale bar: 10 micrometres (= 1/100th of a. millimetre).  Photographs: Lydia King.

Simonsenia delognei is another characteristic species of habitats that dry out periodically.   This species, which is in the same family as Nitzschia, is quite small and only lightly silicified so easily overlooked.  It was common early in the year, but rare thereafter.  Whether this is a real characteristic of the species or an artefact of the conditions in Castle Eden Burn this year is difficult to tell as it is not a particularly common species so there are few other records against which this trend can be compared.

Simonsenia_delognei_CEB

Simonsenia delognei from Castle Eden Burn, Co. Durham, January 2019.  Scale bar: 10 micrometres (= 1/100thof a millimetre). Photographs: Lydia King.

Two other species of Nitzschia were common: I illustrated N. clausii in “Out of my depth …” and have included photographs of N. sigma here.   I’m intrigued that two of the most conspicuous Nitzschia in this sample are sigmoid in outline.  I’ve visited the question of sigmoid diatoms before, and still don’t have any good explanation why a few diatoms have this outline (see “Nitzschia and a friend …”).  Note, too, that Nitzschia species can be sigmoid in valve view (i.e. looking down from above) or girdle view (i.e. looking from the side), although the great majority of species are straight in both planes.

Nitzschia_sigma

Nitzschia sigma from Castle Eden Burn, Co. Durham, January 2019.  Scale bar: 10 micrometres (= 1/100th of a millimetre).   Photographs: Lydia King.

Finally, one more relative of Nitzschia that was found in a couple of samples, but never in large numbers, was Tryblionella debilis.  The genus Tryblionella was treated as part of Nitzschia for much of the 20th century.   As it appears to form a natural group with some distinctive characteristics, it is now generally treated as a distinct genus, although the molecular evidence indicates a complicated evolutionary history.   The principle characteristic of the genus is a longitudinal undulation on the valve face that is most clearly manifest on those species in the genus which have visible striae.   T. debilis is a small species with striae that are not resolvable with the light microscope; however, the undulations are just apparent as faint longitudinal lines running along the valve face.

Tryblionella_debilis

Tryblionella debilis from Castle Eden Burn, Co. Durham, January 2019.  Scale bar: 10 micrometres (= 1/100th of a. millimetre).  Photographs: Lydia King.

That’s a lot of diatoms from a stream that is not always a stream.   I am sure that someone with interests in other groups of algae could probably make similarly long lists for some of those, and a more thorough exploration of habitats within the stream could add to the number of diatoms.  That’s before suggesting a molecular study, which might well reveal cryptic diversity (i.e. significant taxonomic variation that is impossible to discern with a light microscope) within the species I have already described.   The greater our capacity to unravel the mysteries of the microscopic world, the more, it seems, we discover we don’t know.

Reference

Lowe, R.L., Kociolek, P., Johansen, 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 Research 29: 351-360.

Castle Eden Dene in November

Castle_Eden_Burn_Nov19

For the first time this year, I heard Castle Eden Burn before I saw it.  Walking down from the car park, the distant roar of water was apparent almost as soon as the canopy of largely leafless branches closed over me.  A few trees still held their leaves – spectacularly golden on beech and birch, in particular, and the Dene’s famous yews were still green, of course – but the forest was dressed for winter now, much as it was on my first visit this year, back in January (see “Castle Eden Dene in January”).  Then, I was surprised that there was no water in the Burn.  On this trip, however, I wore my chest waders.  Back in August, I had compared Castle Eden Burn to a wadi (see “The presence of absence in Castle Eden Dene”) so the heavy rain of the previous few weeks had led me to suspect that today would be different.

The water surging through the Dene was very turbid, so collecting stones to examine involved feeling around on the river bed with my hand until I located one that was not sufficiently bedded into the substratum to remove.   That’s not ideal, but needs must and I got the five cobbles I needed, each with a distinct biofilm, slimy to the touch.  This is the first time, after eleven months, that Castle Eden Burn’s substratum has looked and felt remotely like the substratum from most of the other rivers I know in this part of the world.

Under the microscope, I see lots of particulate matter but also plenty of algae.   Apart from a few filaments of the cyanobacterium Phormidium, these were mostly diatoms.   The green algae I described in “When the going gets tough …” back in May were not obvious.  The diatoms were mostly largely motile cells of Navicula, with a few sigmoid cells of Nitzschia clausii and some smaller cells whose identity I will need to confirm once I have cleaned the sample and prepared a permanent slide.  The Navicula species, in particular, are typical inhabitants of local rivers during winter and early spring, all tolerant to a wide range of conditions.   I suspect that the rainfall has washed a lot of fine particulate debris from the industrial estates in the upper catchment into the river, and these diatoms will have the resilience to cope with such types of pollution.  A large storm sewer overflow also empties into the burn about a kilometre upstream of where I was standing and this, I suspect, has been flowing over the past month or two.

I also saw a few cells of Achnanthidium minutissimum, which I generally associate with cleaner conditions.  I suspect, however, that numbers will be relatively low compared to its more pollution-tolerant brethren.   Again, I can give a more authoritative answer once I have cleaned the sample and performed a full analysis.

CEB_diatoms_Nov19

Diatoms from Castle Eden Burn, November 2019.  a., b.: Navicula trpunctata; c. – e.: Navicula lanceolata; f., g.: Rhoicosphenia abbreviata; h., i.: Nitzschia clausii; j., k.: Navicula gregaria; l. Achnanthidium minutissimum.   Scale bar: 10 micrometres (= 1/100thof a millimetre).   The photograph at the top of the post shows Castle Eden Burn just downstream from the point I sampled.

I originally set out to visit Castle Eden Burn six times during 2019 and this was the last of those. I’ve written about most of these visits already but not about my September visit.  There was, on that occasion, little new information to justify a separate post but I will include the sample I collected in my final overview of the algae of Castle Eden Burn, just as soon as I get this final sample cleaned and analysed.   Before then, I have one more post to write about the diatoms, based on some more detailed observations of a few of the species, and then it will be time to think about where to focus my observations during 2020.

The presence of absence in Castle Eden Dene

CED_Aug19

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_CED

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_CED_Aug19

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

A thousand little mosses …

Castle_Eden_Burn_July19

Nature doth thus kindly heal every wound. By the mediation of a thousand little mosses and fungi, the most unsightly objects become radiant of beauty. There seem to be two sides of this world, presented us at different times, as we see things in growth or dissolution, in life or death. And seen with the eye of the poet, as God sees them, all things are alive and beautiful.
Henry David Thoreau (journal entry, March 13, 1842)

I was back in Castle Eden Dene earlier this week for my regular visit and, once again, encountered a dry stream bed.  This was no great surprise but, having written about the algae of dry river beds in earlier posts from Castle Eden Dene (see “When the going gets tough“ for the most recent instalment), I thought that I would focus on some of the other vegetation that I could see in and around the stream and, in particular, the bryophytes.   I asked Gaynor Mitchell, who wrote her MSc thesis on the bryophytes of the Dene, to come along and help me with these as my skills never really extended beyond those mosses and liverworts that live permanently submerged in streams and, as we have seen, there is rarely enough water in the burn here for such species to thrive.

There is a rich carpet of mosses on the woodland floor in much of Castle Eden Dene but, in the stream bed and its immediate environs, it is thalloid liverworts that are the most conspicuous bryophytes. Two species, in particular, stand out: the first is Conocephalum conicum, which has broad ribbon-shaped branches and an upper surface covered with pores – which just visible as light coloured dots to the naked eye.   The other is Pellia epiphylla, which was particularly noticeable on the top surface of boulders that are, I suspect, rarely covered, even when the burn is very full.   P. epiphylla had smaller thalli than C. conicum and, importantly, lacked the distinct pores on the upper surface.

Conocephalum_conicum_CED

Conocephalum conicum from Castle Eden Dene, July 2019. The pores are clearly visible on the thallus in the lower image.

Pellia_epiphylla_CED

Pellia epiphylla from the top of a boulder in Castle Eden Burn, July 2019

Alongside Pellia epiphylla on the boulder tops were shoots of the moss Thamnobryum alopercum.  The populations on top of the stones were rather non-descript to the naked eye, being stems growing horizontally across the rock surface. However, amidst these, we found a few of the upright stems which have a distinctly tree-like appearance.   We found more characteristic growths on the woodland floor nearby and my now-dated copy of Watson does, in fact, comment that this species has these two distinct habitats and also that it is a good indicator of calcareous conditions (for anyone who had not noticed the towering limestone cliffs in Castle Eden Dene, I presume?).   Lower down (and, thus, more frequently submerged), we saw Rhynchostegium confertum though this, too, is a species more often associated with terrestrial rather than aquatic habitats.  More significantly, the mosses I associate with streams in north-east England – Rhynchostegium riparioides, Fontinalis antipyretica and Leptodictyon riparium – are all missing from Castle Eden Burn.

Thamnobryum_allopercum_CED

Tree-like shoots of Thamnobryum alopercum from the forest floor in Castle Eden Dene in July 2019.  Growths on rocks in Castle Eden Burn were smaller but there were enough upright stems for it to be recognisable with the naked eye. 

Gaynor’s sharp eye spotted many other mosses and liverworts, though more in the woodland around the stream than in the stream bed itself.  As well as mosses and liverworts, the stream’s vegetation also consisted of a number of grasses and patches of Chrysoplenium alterniflorum, opposite-leaved golden saxifrage.

The story that the vegetation is telling is, I would venture, that Castle Eden Burn is a shaded terrestrial habitat that is occasionally wet, rather than an aquatic habitat that is often dry.  I dug out an old account of the Winterbourne Stream, an intermittent stream in the chalk downlands of southern England for comparison, and found little overlap in the species recorded.   Care is needed for this comparison as the focus of the surveys is different (the Winterbourne account, for example, includes no bryophytes and spans perennial as well as intermittent sections) but there was a mix of genuinely aquatic and amphibious species, including Callitriche sp. and aquatic Ranunculus, which I did not see in Castle Eden Burn.    I suspect that Castle Eden Burn spends longer as a dry stream bed than the upper parts of the Winterbourne.  However, we also must remember that the Winterbourne data are now almost 50 years old, so that stream, too, may have changed much in the interim.

All this adds to my opinion that Castle Eden Burn – and the streams flowing through the other coastal denes in County Durham – are a unique and understudied habitat.  And that’s before I start thinking about the animal life here…

Chrysoplenium_alterniflorum_CED

A patch of Chrysoplenium alterniflorum, opposite-leaved golden saxifrage, on the bed of Castle Eden Burn, July 2019.

Reference

Berrie, A.D. & Wright, J.F. (1984).  The Winterbourne Stream.   pp.179-206.  In: Ecology of European Rivers (edited by B.A. Whitton).  Blackwell, Oxford.

Mitchell, G. (2015).  Bryophytes: changes in diversity and habitat in Castle Eden Dene (1975-2011).   Northumbrian Naturalist: Transactions of the Natural History Society of Northumbria 79: 39-66.

Watson, E.V. (1981).  British Mosses and Liverworts. Third Edition.  Cambridge University Press, Cambridge.

When the going gets tough …

Castle_Eden_Dene_May19

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?

CED_biofilm_May19

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.

CED_Vaucheria_mat_May19

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. 

CED_Vaucheria_May19

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

However, the structures did not really match any pictures that I could find of oogonia or antheridia in Vaucheria.  I passed my images around some friends, and Gordon Beakes suggested that we might be looking at sporangia of chytrids, a group of fungi that have a string of previous convictions for infecting algae (see “Little bugs have littler bugs upon their backs to bite ‘em ….”).  As I was taking the photographs below, a cloud of tiny spores 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 (and not just because I had not yet realised that they were, in fact, fungi).   I even took a video.  I’ll upload it to the Dark Web at some point.  There must be a site for fungal-themed pornography out there, if only I took the time to look…

CED_Vaucheria_oospores

Sporangia of chytrids on Vaucheria filaments from Castle Eden Burn, May 2019.  The one on the right was releasing spores (arrowed) at the time the photograph was taken.   Scale bar: 20 micrometres (= 1/50thof a millimetre).  

Out of my depth …

Castle_Eden_Dene_March19

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.

CED_diatoms_Mar19

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.

CED_discharge

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.

Reference

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.

Castle_Eden_diatoms_Jan19

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_delognei.jpg

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?

References

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.