Close to the edge in Wastwater …

Wastwater_190610

I’m back in the Lake District for this post, standing beside Wastwater, the most remote and least disturbed of England’s lakes and, especially obvious on a sunny day in June, the most spectacularly-situated.  I stood on the western shore looking across to the screes and, beyond to the mass of Scafell Pike, England’s highest peak, looming up in the distance.

When I was done admiring the scenery I adjusted my focus to the biology of the lake’s littoral zone and some dark brown – almost black – marks on the boulders in the littoral zone.  In contrast to the grand vista stretching away to the north, these were beyond unprepossessing and my attempts to photograph them yielded nothing worth including in this post. However, I had seen similar looking marks in Ennerdale Water and there is a photograph in “Tales from the splash zone …” that should give you some idea of what I was seeing.

Under the microscope, my expectations were confirmed.  As in Ennerdale Water, these patches were composed of Cyanobacteria – gradually tapering trichomes of Calothrix fusca and more robust trichomes of Scytonema calcareum, both encased in thick, brown sheaths which, when viewed against the granite boulders on which they lived, resulted in the dark appearance of the growths.  To the untrained eye, these barely look like lifeforms, let alone plants yet they offer an important lesson about the health of Wastwater.

Calothrix_fusca_Wastwater_June19

Calothrix cf fusca from the littoral zone of Wastwater, June 2019. Scale bar: 20 micrometres (= 1/50thof a millimetre)

Though hard to see amidst the tangle of filaments in these population, both Calothrix and Scytonema have specialised cells called “heterocysts” that are capable of capturing atmospheric nitrogen (you can see these in the photographs of Nostoc commune in “How to make an ecosystem (2)”.   Nitrogen fixation is a troublesome business for cells as they need a lot of energy to break down the strong bonds that bind the atoms in atmospheric nitrogen together.   That means that plants only invest this energy in nitrogen fixation when absolutely necessary – when the lack of nitrogen is inhibiting an opportunity to grow, for example.   The presence of these Cyanobacteria in Wastwater is, therefore, telling us that nitrogen is scarce in this lake.

The dogma until recently was that phosphorus was the nutrient that was in shortest supply in lakes, so attention has largely focussed on reducing phosphorus concentrations in order to improve lake health.   Over the last ten years, however, evidence has gradually accumulated to show that nitrogen can also be limiting under some conditions.   That, in turn, means that those responsible for the health of our freshwaters should be looking at the nitrogen, as well as the phosphorus, concentration and, I’m pleased to say, UK’s environmental regulators have now proposed nitrogen standards for lakes.   That marks an important shift in attitude as, a few years ago, DEFRA were quite hostile to any suggestion that nitrogen concentrations in freshwaters should be managed.   In this respect, the UK is definitely out step with the rest of Europe, most of whom have nitrogen as well as phosphorus standards for freshwaters.

Scytonema_crustaceum_Wastwater_June16

Scytonema cf calcareum from the littoral zone of Wastwater, June 2019. Note the single and double false branches.   Scale bar: 20 micrometres (= 1/50thof a millimetre)

Wastwater flows into the River Irt and, a few kilometres down from the outflow, I found another nitrogen-fixing Cyanobacterium, Tolypothrix tenuis.  Once again, I could not get a good photograph, but you can see images of this in an earlier post from the River Ehen in “River Ehen … again”.   Nitrogen fixing organisms, in other words, are not confined to the lakes in this region, which raises the question why the UK does not have nitrogen standards for these as well (see “This is not a nitrate standard …”).   In rivers such as the Irt and Ehen that are already in good condition, it might only take a small increase in nitrogen concentration for the ecology to change.   Whether the loss of these nitrogen-fixing organisms will be noticed is another question.

For now, I am just happy to see that nitrogen in lakes has finally made it to the regulatory agenda.  It has taken about 15 years for the science to percolate through the many layers of bureaucracy that are an inevitable part of environmental management.  Give it another decade and maybe we’ll get nitrogen standards for rivers too.

References

Maberly, S. C., King, L., Dent, M. M., Jones, R. I., & Gibson, C. E. (2002). Nutrient limitation of phytoplankton and periphyton growth in upland lakes. Freshwater Biology. https://doi.org/10.1046/j.1365-2427.2002.00962.x

Moss, B., Jeppesen, E., Søndergaard, M., Lauridsen, T. L., & Liu, Z. (2013). Nitrogen, macrophytes, shallow lakes and nutrient limitation: Resolution of a current controversy? Hydrobiologia. https://doi.org/10.1007/s10750-012-1033-0

P.S. any guesses as to which 1970s prog rock group I was listening to over the weekend?  The clue is in the title.

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Blooms from above

Dianchi_lake_cyano_bloom

Saturday’s excursion saw us travelling to the southern end of the Kunming metro and joining a procession of locals trekking up the wooded slopes of the Xī Shān hills to the settlement of Lóng Mén (‘Dragon’s Gate’), which gave us some spectacular views over Diān Chí (Dian Lake) stretching away into the distance, After a lunch of fried noodles from one of the many takeaway stalls at Lóng Mén, we travelled back down to lake level by cable car, which gave us our second panoramic view of Cyanobacteria in three days.   The lake, China’s eighth largest, had a very conspicuous Cyanobacterial bloom that serves as the ‘yin’ to the Green Lake’s ‘yang’.

The environmental problems of Diān Chí are well known with an article in Newsweek describing it as the ‘ground zero of China’s toxic algae problem’.  The problems starts with Diān Chí’s location on a high plateau (1886 m above sea level) in Yunnan, which means that it has a relatively small catchment area relative to its size (40 km long, about 300 square kilometres area and with an average depth of 4.4 metres).   The city of Kunming sits at the north end of this lake and now has a population of over six million people.   For a long time, their untreated sewage was pumped directly into the lake, leading to high concentrations of phosphorus which, in turn, fertilised the lake water, allowing blooms of Microcystis aeruginosa to develop.   Many genera of Cyanobacteria, including Microcystis, produce potent toxins that attack the liver or nervous system, and which can cause skin rashes.

Unfortunately, the city of Kunming depended upon Diān Chí for its water supply in its past but now, due to this contamination, it has to rely upon reservoirs upstream of the city.  It has, according to the Newsweek article, invested $660 million dollars to reduce industrial pollutants, building sewage treatment works, intercepting polluted water and banning detergents containing phosphorus but that, as my photograph from the cable car shows, has had little effect.   There are two reasons for this.  The first of these is a reluctance to control fertiliser use in the productive agricultural areas to the west of the lake (China is not unique in this respect; a similar tardiness can be found in the West, where agriculture is a potent political lobby).  The second is that much of the phosphorus that was pumped into the lake in the past is still there, sitting in the sediments and being constantly recycled by the algae.  In small lakes it might be possible, albeit expensive, to dredge out this sediment but on a lake the size of Diān Chí this is an unimaginable prospect.

Another paper that I found online demonstrated a dramatic loss of higher plants and fish from Diān Chí. Since the 1950s, over half of all native higher plant species have been lost, along with 84 per cent of native fish.  Diān Chí also had a number of unique species, which evolved in this remote habitat, but 90 per cent of these, too, have been lost since the 1950s.  That is a catastrophe in biodiversity terms, but the collapse of the lake ecosystem also led to the loss of valuable commercial fisheries.  In the past, some of the fish and shellfish that we ate in local restaurants might have been bought from fishermen who worked the lake; now they have to be imported.

Dianchi_reaeration_equipment

A view from the cable car over Diān Chí, with yellow rafts bearing reaeration apparatus visible on the lake surface.  The picture at the top of the lake shows one edge of the Cyanobacteria bloom, with clearer water along a channel flushed by inflow from a lagoon.

We can see, in other words, another interesting case study in competing ecosystem services emerging. We might imagine a time in the far past when there was a balance between the use of the lake as a supply of resources (drinking water, fish and shellfish, irrigation water) was not compromised by the use of the lake’s natural biogeochemical cycles to break down any waste products that flowed in from the catchment.   More likely, human and animal wastes would have been recycled more directly as manure for local agriculture so, again, some sort of equilibrium would have pertained.   Now, we see the ‘provisioning’ services compromised due to the overuse of the ‘regulating’ services and, at the same time, opportunities for ‘cultural’ services such as recreation are also much reduced.

Thinking more widely, what about the ecosystem services lost due to the construction of the new water supply reservoirs around Kunming?   But then, rather than end on an overly sanctimonious tone, to what extent have we in the West, ‘solved’ some of our own environmental problems in recent decades through the contraction of our own manufacturing industries in the face of competition from countries such as China?  \

view_along_Dian_Chi

A view south along Diān Chí with the far shore, 40 km away, just visible in the distance.

References

Liu, J., Luo, X., Zhang, N. & Wu, Y. (2016).  Phosphorus released from sediment of Dianchi Lake and its effect on growth of Microcystis aeruginosaEnvironmental Science and Pollution Research23: 16321-16328.

Wang, S., Wang, J., Li, M., Du, F., Yang, Y., Lassoie, J.P. & Hassan, M.Z. (2013).  Six decades of changes in vascular hydrophyte and fish species in three plateau lakes in Yunnan, China.  Biodiversity and Conservation222: 3197-3221.

Zhu, L., Wu, Y., Song, L. & Gan, N. (2014).  Ecological dynamics of toxic Microcystis spp. and microcystin-degrading bacteria in Dianchi Lake, China.  Applied and Environmental Microbiology80: 1874-1881.

Notes:many authors, Western and Chinese, refer to ‘Dianchi Lake’.  However, as ‘chí’ means ‘lake’, I have just referred to ‘Diān Chí’ throughout.  See “Lake lakelake lake” for more about this. “La Grande Assiette de Lac Léman”  describes a similar conflict between ecosystem services in Lake Geneva, albeit with more positive outcomes.

 

Algae in a stone forest

Shilin_panorama_#1

A change in location from the previous post: this one is being written on the roof terrace of a guest house in Kunming, in Yunnan Province, China, whilst sipping a cup of the local pu’er tea.   I’m in China with my family visiting my eldest son who works in Chengdu, a sprawling metropolis of 11 million people in Sichuan Province, and have escaped to the warmer climate and more sedate environs of Kunming (only 6 million people) for a few days.  We’ll then move on to Dali (a mere village, by comparison, with less than half a million people) before returning to Chengdu.

From Kunming we travelled about 120 km southeast to Shílín, the site of a strange Karst phenomenon known as the “stone forest”, a collection of upright pillars of limestone often with other limestone blocks perched precariously on top.   In geomorphological terms, we are looking at a limestone pavement on a huge scale, but with substantial erosion of the “grykes” (the gaps between the “clints”).   Geologically, it is a little more complicated than that, with the Permian limestone being later overlain by basalt which was subsequently eroded away to leave a red soil.  However, that is enough to give you some context for what follows.

The photograph at the top of the post gives you some idea of what the stone forest looks like, and also some idea of the crowds to be expected at mainstream tourist attractions in China.  At times, the mass of people and, in particular, the overlapping amplified commentaries from tour guides, dressed in the costumes of the local Sani ethnic minority, made the experience almost unbearable.  But then, as is often the case, you turn a corner, the hubbub dies away, and you are able to enjoy the ethereal landscapes almost undisturbed.  In our case, however, we turned a corner too many, found ourselves outside the officially-sanctioned tourist beat and were unceremoniously ejected by an officious security guard.

Once we had talked our way back into the park through the main entrance, using Ed’s Mandarin skills, the park was noticeably quieter.   Most of the organised tours squeeze the stone forest and a local cave network into the same trip so the morning crowds had been hustled back onto their coaches, and the whole experience in the park was much more pleasant.   Walking through the Major Stone Forest gives you an ants-eye experience of living in a limestone pavement habitat, with the clints towering above you and only occasional glimpses of sunshine.   The park authorities have provided a concrete path and steps to lead you through but it is, at times, an arduous trek with some narrow and low gaps through which to squeeze.   This, in turn, lets you get up close to the limestone and, in my case, gets the phycological antennae twitching …

Shilin_panorama_#2

The Major Stone Forest at Shílín from the inside.   The photo at the top of the post shows the Major Stone Forest from the main public viewing area.

The limestone from which the stone forest is made is largely slate-grey in colour, rather than the creamy beige that I normally associate with this rock.   Only after reading one of the interpretation boards in the park did the penny drop, and I realised that I, and thousands of other tourists, had each spent 130 RNB to stare at algae.   After my brush with officialdom in the morning I was not in the mood to scrape at the rocks to collect a sample but am guessing we are looking at the Cyanobacterium Gloeocapsa alpina or something similar (see “The mysteries of Clapham Junction …”).   We were visiting the park close to the end of the long dry season but for the next few months the climate here will be much damper, creating a more conducive environment for these microorganisms to grow.

A few of the rock faces, particularly those associated with seepages, had multicolour streaks, with the grey supplemented with pinks and greens.  The former may well be other Cyanobacteria (possibly Schizothrix) and the greens could be Apatococcus, Desmococcus or a relative (see “Little round green things …”).  There were also a few orange-red patches of Trentepohlia (see “Fake tans in the Yorkshire Dales”).   All of these are forms are familiar to me from the UK and, whilst it would be rash to assume that the species were identical to those I find back home, the genera are generally cosmopolitan, so some extrapolation can be permitted.

Shilin_algae_Apr19

Algal crusts on rocks in the Major Stone Forest at Shílín, April 2019. The left hand image shows a mixture of Cyanobacteria and (possibly) green algae on a vertical surface associated with a seepage; the right hand image shows Heather photographing a growth of Trentepohlia nearby.

Trentepohlia_Shilin_Apr19

Trentepohlia growths inin the Major Stone Forest at Shílín, April 2019.  The picture frame spans about 10 centimetres. Photograph: Heather Kelly.

I did hunt around for some verification for these names but it is not possible to access Google Scholar in China without a VPN.  I am limited to whatever Bing throws up, and have not yet been able to find any papers on the algae of Shílín.  What I did find, during these searches, however, was an article about the world’s largest Haematococus farm, which is very close to here.   I’ve described Haematococcus in earlier posts (see “An encounter with a green alga that is red”) and mentioned that it was the source of the food colouring astaxanthin.   The combination of the limestone geology, warm weather and the huge market for food additives in China makes this possible.  Travelling in China with two vegetarians makes me realise that, even in this enormous, technocratic country, the market for natural products is growing.

Shilin_panorama_#3

 

 

How to make an ecosystem (2)

Ennerdale_Apr19

My most recent visit to Ennerdale and the River Ehen almost did not happen: unexpected overnight snowfall led to my wheels spinning on the Whinlatter Pass before I retraced my steps to Braithwaite and followed roads at lower altitudes around the outskirts of the fells.   Fieldwork in the morning took place amidst intermittent snow showers but, by the afternoon, it was dry if not quite as balmy as the visit I described in “Croasdale Beck in February”.   “Unseasonable”, I was reminded, is a two-edged term.

There was little incentive to linger with my arm in the agonisingly cold water, so this post is about some algae growing on dry land that caught my eye.   Amidst the gravel in a farmyard in Ennerdale Bridge I saw some dark brown leathery growths that I recognised straight away as the Cyanobacterium Nostoc commune (see “Nosing around for blue-green algae …”).  It looks rather nondescript, even slightly unsavoury, with the naked eye but, under the microscope, the rosary-like structure of the filaments suspended within a jelly-like matrix is revealed.  The slightly larger cells with thicker walls and lighter contents are the heterocysts, responsible for fixing nitrogen from the atmosphere (fulfilling the same function as the nodules on the roots of legumes).

Nostoc_commune_Ennerdale_Apr19_#1

A patch of Nostoc communein a farmyard in Ennerdale Bridge in April 2019.   The picture frame covers about 30 centimetres. 

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Nostoc communefrom Ennerdale Bridge under the microscope.  Scale bar: 10 micrometres (= 100thof a millimetre). 

This type of coarse, well-drained gravel is a good habitat for Nostoc and, once you know what you are looking for, it is a common sight on gravel driveways, usually to the annoyance of the owners.   If there is only a small amount, the best way to control it is simply to pick up the colonies and toss them onto the compost heap.  However, once it is established, this can be a big undertaking and many people are quite happy to tolerate some of this brown gunk on their driveways.   On the other hand, it can sometimes get out of hand and the consequences of not doing anything are well illustrated by the photograph below.  The Nostoc colonies have spread but these, in turn, have created a habitat into which first mosses and later grasses can establish.

This small farmyard on the edge of the Lake District contains, in short, the first stage of an ecological succession.  We could think of a gravel driveway as a mini-desert, as the copious Cumbrian rainfall will not be retained in the surface layers, making it hard for plants to survive.   However, if a tough organism such as Nostoc is able to establish itself, then this, in turn, will trap water and make the driveway more amenable to slightly more fussy organisms such as mosses.   As the moss and Nostoc grow together so, eventually, grasses are able to establish too.  Were there to be no interruption to this process then, eventually, decades later, we might even see trees growing on this driveway.

It is hard to imagine, but just about every type of terrestrial habitat started out, aeons ago, as a bare rock surface.  Various forms of physical weathering start the process of breaking this up allowing, over time, organisms such as Nostocto get a foothold and convert the virgin surface into a mature ecosystem (you can read about another example in “How to make an ecosystem”).   It may take centuries for this to happen in the natural world, so it is particularly fortuitous to see this human-assisted succession so well developed.  At some stage, I suspect, the owner will decide that enough is enough, and rake the gravel.  Meanwhile, however, we have a rare opportunity to reflect on the role that primitive micro-organisms play in shaping even the grandest of our natural habitats.

Nostoc_commune_Ennerdale_Apr19_#2

A lawn of Nostoc, moss and grass growing on a gravel driveway in Ennerdale Bridge, April 2019.  

Reference

Miles, J. & Walton, D.W.H. (1993).  Primary Succession on Land.  Special Publication of the British Ecological Society 12, Blackwell Scientific Publications, Oxford.

Survival of the fittest (2) …

As well as the bright green flocs of Tribonema, the stream draining the Hadjipavlou chromite mine also had bright orange-red growths on some of the pebbles on its bed.  These seemed to be composed primarily of the Cyanobacterium Chamaesiphonthough I am still not sure what species.   Using the limited literature I have, from the UK and Germany, I would opt for either Chamaesiphon polymorphusor C. polonicus.   This particular alga was very easy to remove from stones, compared to other epilithic Chamaesiphon species (see “A bigger splash …”).  This is a feature of C. polymorphus, though the colour is more typical of C. polonicus*.  On the other hand, that bright colour could be the response to high solar radiation, so maybe my north European guides are not that reliable.  It could be something else altogether.

Chamaesiphon_polonicus_Troodos

Chamaesiphon growths on pebbles in the stream draining Hadjipavlou chromite mine in the Troodos mountains, Cyprus, March 2019.

Chamaesiphon_Troodos_Mar19

Colonies of Chamaesiphon from Hadjipavlou chromite mine under the microscope.   Scale bar: 10 micrometres (= 1/100thof a millimetre). 

In addition to the Chamaesiphon, there were a few diatoms, mostly Achnanthidium minutissimumand Meridion circulare.   These are typical species of metal-rich streams, as is the general lack of diversity that was evident.   There were also a few filaments of the cyanobacterium Phormidium, along with quite a few Paramecium and Vorticella.  As these are both heterotrophs that feed on organic matter, their abundance is probably at least partly a reflection of the long time that the sample spent in my suitcase between collection and analysis.  The latter is a fascinating organism to watch: it is a goblet-shaped cell with a fringe of cilia around the lip (or “peristome”).  These beat in unison to create water currents that draw small particles towards the cell.   These particles mostly at least an order of magnitude smaller than the algae)  are then collected in food vacuoles where they are digested.   A few of these vacuoles can be seen in the image of Vorticella below.

Vorticella is attached to its substrate by a stalk which contains contractile filaments, giving it spring-like qualities.  Watching a Vorticella is a beguiling experience, with the undulating rows of cilia drawing food into the vestibule (as the opening is known).  At intervals, the whole cell lurched across the field of view as the “spring” in the stalk suddenly contracted, shortening the stalk.  After this, the stalk would gradually extend again, the cilia not having missed a beat meanwhile.   This process may simply be a device that enables the Vorticellato exploit its locality to the full, as well as creating some additional turbulence to keep a steady flow of particles towards the peristome.  To be honest, I haven’t seen a more convincing explanation but, even if we don’t know why it does what it does, Vorticella is a fascinating organism to watch, whether or not I understand what is going on.

I’ll be coming back to talk more about the diatoms in a future post, and writing these posts has also reminded me that I’ve never written about the interesting mine sites almost on my own doorstep.  I cut my ecological teeth looking at these habitats back in the 1980s and they are striking examples of natural selection in action.   So, plenty of potential for more left-field natural history …

Hadjipavlou_organisims_Mar19

Other organisms present in the Hadjipavlou chromite mine. a. – d.: Meridion circulare; e. Phormidiumsp.; f. Vorticellasp.   Scale bar: 10 micrometres (= 1/100thof a millimetre). 

* Note: after I had written this post Brian Whitton confirmed that it was, most likely, Chamaesiphon polonicus.

More about measuring biomass …

The previous post showed how the proportions of green algae and diatoms changed as the total quantity of algae in the River Ehen waxed and waned over the course of a year.   The BenthoTorch, however, also measures “blue-green algae” and so let’s look at how this group changes in order to complete the picture.

Before starting, though, we need to consider one of the major flaws of the BenthoTorch: its algorithms purport to evaluate the quantities of three major groups of algae yet, in my posts about the River Ehen I have also talked about a fourth group, the red algae, or Rhodophyta (most recently in “The only way is up …”).  Having pointed a BenthoTorch at numerous stones with thick growths of Audouinella,we can report that Rhodophyta seem to be bundled in with the blue-green alga signal, which is no great surprise given the similarity in their pigments.  It is, however, one of a number of examples of the need to interpret any BenthoTorch results with your brain fully engaged, and not just to treat outputs at face value. Similar questions need to be asked of the Xanthophyta and Chrysophyta, though the latter tend not to be common in UK streams.

cyanos_in_Ehen

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

In contrast to the green algae and diatoms, the Cyanobacteria/Rhodophyta signal shows a strong negative relationship as biomass increases though, again, there is enough scatter in this relationship to make it necessary to approach this graph with caution.  I suspect, for example, that the data points on the upper right side of the data cloud represents samples rich in Audouinella, which tends to occur in winter when biomass, generally, is much greater.   On the other hand, Croasdale Beck, in particular, has a lot of encrusting Chamaesiphon fuscus colonies which are pretty much perennial (see “a bigger splash …”) but whose relative importance in the BenthoTorch output will be greatest when the other two groups of algae are sparse.   I suspect that encrusting members of this genus are favoured by conditions that do not allow a high biomass of other algae to develop, as these will reduce the amount of light that the Chamaesiphonreceives.

Thicker biofilms in the River Ehen often have some narrow Phormidium-type filaments as well as small bundles of nitrogen-fixing Calothrix, but the overall proportion is generally low relative to the mass of diatoms and green algae that predominate.    But that is not really telling us the whole story.  I finished my previous post with a graph showing how the variation in biomass increases as the biomass increases.  The heterogeneity of stream algal communities, however, cannot be captured fully at the spatial scale at which the BenthoTorch works: there is a patchiness that is apparent to the naked eye: one of our sites has distinct mats of Phormidium autumnale towards one margin, and dense Lemaneagrowths in the fastest-flowing sections, largely attached to unmovable boulders, which makes biomass measurement very difficult. I’ve also written about distinct growths of Tolypothrix and its epiphytes (see “River Ehen … again”), another alga which forms discrete colonies at a few locations. I try to collect a random sample of stones from a site but there are constraints, including accessibility, especially when the river rises above base flow.   In the River Ehen we also have to take care not to disturb any mussels whilst removing stones.

Whilst our sampling cannot really be described as “random” I do think that there is sufficient consistency in the patterns we see for the results to be meaningful. We could spend a lot more time finessing the sampling design yet for little extra scientific gain.   I prefer to think of these measurements as one part of a complex jigsaw that is slowly revealing the interactions between the constituents of the dynamic ecosystem of the River Ehen.   The important thing is to not place too much faith in any single strand of evidence, and to have enough awareness of the broader biology of the stream to read beyond the face value indications.

Castle Eden Dene in January

castle_eden_burn_jan19

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.

blunts_burn_jan19

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.

blunts_burn_diatoms

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 …

blunts_burn_phormidium

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).   

References

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,