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.

 

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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

 

 

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, 

 

Algae from the Alto Duoro …

From the highlands of Serra da Estrela w headed north-west towards the vineyards of the Duoro Valley from which the grapes that make port are picked.  I’m supposed to be on holiday but, as the narrow road twists and turns down a steep hillside, with vineyards on both sides, I see a case study in how humans alter rivers and their catchments to suit their needs.  I wonder if the passengers on the cruise ships that move sedately through this beautiful landscape have any idea of just how difficult this same journey would have been just fifty years ago.   Now there are 51 large dams within the watershed, regulating the flow and, at the same time, generating much-needed hydroelectricity.   Before these were in place, the only way to get the port from the quintas in the Alto Duoro to Porto was to load the barrels onto a “barco rabelo”, and then to plot a perilous path through the rapids before using a combination of sail, oars and oxen to make the slow journey back upstream (you can see videos of these journeys on YouTube).

A replica of a barco rabelo moored in the Rio Duoro at Porto, September 2018.

The Rio Douro is a type of river that is rare in the UK but very common throughout the rest of Europe in that it crosses (and, for part of its course, forms) national boundaries.  There are a few rivers in Ireland which straddle borders (the Foyle is one, and some of the headwaters of the Shannon can be found in County Fermanagh) but, mostly, this is a complication that our river managers do not have to face.  By contrast, eighty per cent of the Rio Douro’s catchment lies in Spain (where it is called the Duero) and it is actually the largest watershed on the Iberian Peninsula.   The whole European project, and its environmental policy in particular, makes so much more sense when you are looking at a well-travelled river.

Our immediate objective was the Quinta do Bomfin at Pinhão, which produces grapes for Cockburns’, Dow’s and Taylor’s ports.  However, after a morning walking through the vineyards and following a tour of the winery (the robot that has replaced human grape treaders has, we learned, been carefully calibrated to match the pressure that a human foot exerts, lest the grape seeds are crushed, imparting bitterness to the resulting wine) plus some port tasting, the lure of the river was too strong.

A view across the Douro Valley from Quinta do Bomfin at Pinhão.   This, and the previous two photographs, were taken by Heather Kelly.

The river bank at Pinhão is lined with rip rap (loose stones) enclosed in mesh cages to protect it from erosion from the waves created by the many cruise ships that make their way up the river with tourists.   This, along with the floating jetties at which they embark and disembark, meant that it was not easy to get access to the river; however, I eventually found a small slipway close to the point where a small tributary joins.  There were a few loose stones with a green film in shallow water that I could just reach, plus some algal mats coating the concrete of the slipway at water level.   I managed to get small samples of each to bring back for closer examination, attracting the usual curious stares from passers-by in the process.

The mats on the slipway were composed of an alga (technically, a cyanobacterium) that has featured in this blog on several occasions in the past: Phormidium autumnale (see “In which the spirit of Jeremy Clarkson is evoked”).   This is the time of year when the Douro is at its lowest so living at this point on the slipway means that it spends a small part of the year exposed to the air, but most of it submerged.

Phormidium cf autumnale on a slipway beside the Rio Douro at Pinhão, September 2018.  The left hand image shows the mats on the lower part of the slipway; the right hand image shows individual filaments.  Scale bar: 20 micrometres (= 1/50th of a millimetre).

The stones beside the slipway had a thick greenish film which, when I looked at it under a microscope, turned out to consist largely of bundles of thin cyanobacterial filaments belonging to a relative of Phormidium: Homoeothrix janthina (kindly identified for me by Brian Whitton).   Homoeothrix differs from Phormidium in that the filament are often slightly tapered, rather than straight-sided and usually aggregated into colonies, often growing vertically towards the light rather than intertwined to form mats.   It is a genus that I see in the UK (including, sometimes, in the River Wear) but which I have not previously written about on this blog.   The photos below show tufts of filaments but it would be quite easy to imagine several of these clumps joined together to form a hemispherical colony, before I disrupted them with my vigorous sampling technique.

Left: the rip rap at the edge of the Douro at Pinhão from which I sampled algae in September 2018; right: the stone after vigorous brushing with a toothbrush.

Bundles of filaments of Homoethrix janthina from the River Douro at Pinhão. Scale bar: 20 micrometres (= 1/50th of a millimetre).

Many of my posts try to make the link between the algae that I find in lakes and rivers and physical and human factors in those water bodies and their surroundings.  That is not an easy task in a large river basin such as that of the Douro as there is so much more of a hinterland including large towns in Spain such as Valladolid.   The river, to some extent, integrates all of these influences and, whereas the vines around Pinhão have their roots in nutrient-poor granite and schist soils, the river’s journey to this point has covered a range of different rock types, including chalky clay soils in the Spanish part of the catchment and the water reflect this.   This cocktail of physical alteration and pollution, shaken up with a dash of international relations, recurs in the largest rivers throughout Europe and is either a fascinating challenge for an ecologist or a complete pain in the backside, depending on your point of view.

I’ll come back to the Douro in a few weeks, once I’ve had a chance to have a closer look at the diatoms.  Meanwhile, I have one more stop on my travels along the Rio Douro, at the port lodges of Vila Nova de Gaia to try some vintage port …

Reference

Bordalo, A.A., Teixeira, R. & Wiebe, W.J. (2006).  A water quality index applied to an international shared river basin: the case of the Douro River.  Environmental Management 38: 910-920.

The end of the journey: port maturing in barrels at Cockburn’s lodge in Vila Nova de Gaia.

 

Transitory phenomena …

Fieldwork in the River Ehen has been an unusually pleasurable experience over the past few months, even to the extent of abandoning waders altogether and wearing just a thin pair of neoprene beach shoes and shorts as I worked.   Curiously, there were few obvious signs of the prolonged period of low flow here, but that is partly due to the pumps installed by United Utilities to keep the river running whilst the lake was drawn down (see “Life in the deep zone …”).   I did, however, find some intriguing green patches on fine sediments at the margins.

Most of the bed in this part of the river consists of much coarser sediments than these which are, I suspect, silt and sand deposited on the occasions when Ben Gill (which joins the Ehen immediately below Ennerdale Water) is flowing.   Current velocity is lower at the edges of the river, allowing fine sediments to settle out and create temporary sandbanks.   One decent spate will be all that is needed, I suspect, to wash much of this downstream.  However, there has not been a period of prolonged high flow for several months and there is, as a result, a thin green mat of algae growing on the upper surface of this sediment.

Mats of Oscillatoria on fine sediments beside the River Ehen just downstream from Ennerdale Water, August 2018.   The total length of the mats in the left hand photograph is about one metre. 

I scraped up a small sample to examine under my microscope.  I was expecting to see the broad filaments of the cyanobacterium Phormidium autumnale which I often find at a site about five kilometres downstream (see “’Signal’ or ‘noise’?”) but what I saw was much narrower filaments, some of which were slowly gliding forwards and backwards.   These belong to a species of Oscillatoria, a relative of Phormidium that is common in the plankton.  A few species, however, do live on surfaces and can, as I could see in the Ehen, form mats.  I have, in fact, described a different mat-forming species of Oscillatoria (O. limosa) from the River Wear close to my home (see “More from the River Wear”) and this, too, had been favoured by a long period of warm weather and low flow.   The filaments in the River Ehen were much narrower – just a couple of micrometres wide – and had relatively long cells (two or three times longer than wide) but, in other respects, they clearly belonged to the same genus.

Microscopic views of Oscillatoria filaments from the River Ehen, August 2018.   The upper photograph was taken at medium magnification (400x) and the lower image was taken at 1000x.  The constant motion of the filaments means that it is not possible to use stacking software to obtain a crisp image.  Scale bar: 10 micrometres (= 1/100th of a millimetre). 

The motion that I could see is thought to be due to a layer of tiny fibres (“microfibrils”) which wind around the inner layer of the cell wall in tight spirals.   Movement is caused by waves that are propagated along these fibres, meaning that the filament actually rotates as it moves (though this is almost impossible to see with a light microscope).   The filaments can move either towards or away from light, depending on the intensity, at a speed of up to 11 micrometres per second (that’s about a millimetre a day or, for any petrolheads who are reading, 0.00004 kilometres per hour).  This allows the filaments can adjust their position so that they are neither in the dark nor exposed to so much light that they are likely to do damage to their photosynthetic apparatus (see “Good vibrations under the Suffolk sun” for more about this).   The result is that filaments will tend to converge, Goldilocks-style, at the point where light conditions are “just right”.  You can see some sediment particles settling on the top of the mat in one of the images and we can expect the filaments to gradually adjust their positions, incorporating these particles, over time.

Last year, I wrote about Microcoleus, a relative of Oscillatoria, which formed mats on saltmarshes and explained how this could be the first stage of colonisation of damp habitats by plants (see “How to make an ecosystem”).   We are seeing the same processes happening here, but the life expectancy of these mats is much lower.  They may well be gone next time I visit, depending on how the Cumbrian climate behaves over the next couple of weeks.   They are transitory phenomena, here today and gone tomorrow but, like the subjects of some of my other recent posts, particularly favoured by the long period of settled weather that we have enjoyed over recent weeks.

Reference

Halfen, L.F. & Castenholz, R.W. (1971).  Gliding motility in the blue-green alga Oscillatoria princeps.  Journal of Phycology 7: 133-145.

Note: you can read more about how the heatwave has affected fresh water in the Lake District in Ellie’s MacKay’s recent post on Freshwaterblog

How to make an ecosystem

I visited Scotland vicariously last week (meaning that I did not actually cross the border but a little bit of Scotland made its way south to me).   In this case, my wife had been reconnoitring potential sites for a field course along the Fife coast and had visited the sand dunes at Tentsmuir National Nature Reserve to see if the plant succession there would make a suitable exercise for her students.  Behind the sand dunes there was a low lying area of saltmarsh and, within that, large areas of algal mats.  I’m guessing that, having brought me a bottle of Highland Park whisky as a reward for not killing her houseplants during her previous trip to Scotland, she thought that my liver deserved a break.

Tentsmuir is a dynamic ecosystem, with sand dunes on the coastal side and, in places, a complete succession from colonising grasses on the seaward side to mature forest on the land.   However, there are also slacks behind the dunes which are periodically inundated by seawater, leading to the development of saltmarshes.   The periodic wetting and drying of saltmarshes is ideal for filamentous algae and these, in turn, create a mesh of interlocking filaments that binds the sand grains and traps organic matter.   Over the course of many tidal cycles, conditions become suitable for higher plants such as glasswort and sea asters.   I have a soft spot for saltmarshes and sand dunes as these are the habitats where I made some of my first forays as an ecologist (see “How to be an ecologist #4”); however, I have never looked in detail at the algal mats before.   So, I poured myself a glass of Highland Park, turned on my microscope and teased out a few of the filaments from my present.

Algal mats from saltmarsh at Tentsmuir National Nature Reserve.  The left hand picture shows a plant of Salicornia europaea agg. (Common Glasswort, or “samphire”) surrounded by algal mats (photo: Heather Kelly).  The right hand picture demonstrates how the mat retains its integrity after being removed from the saltmarsh.

The mat, in this case, seemed to be made up predominately of two types of alga: the yellow-green alga Vaucheria and the Cyanobacterium Microcoleus chtonoplastes.   I often see Vaucheria in freshwaters so it was the Microcoleus that attracted my attention.   It belongs to the same family as the Phormidium that we met in Mallerstang (see “The stresses of summertime …”) and we can see several of the same features: rows of almost identical cells and, in particular, no “heterocysts”, specialised cells that are responsible for nitrogen fixation.   Technically, the chain of cyanobacterial cells is referred to as a “trichome” and these are enclosed in a “sheath” (seen most clearly in genera such as Scytonema: see “Tales from the splash zone …”).  In Phormidium there is a single trichome per sheath but each sheath of Microcoleus contains several, often twisted around each other to form rope-like bundles.

Microcoleus cf chthonoplastes from the saltmarsh at Tentsmuir National Nature Reserve, August 2017.  Scale bar: 10 micrometres (= 100th of a millimetre).

Although I mentioned that Microcoleus lacked heterocysts, this does not mean that it is not capable of nitrogen fixation.   The reason that cyanobacterial cells need heterocysts is that the nitrogenase enzyme only works in anerobic conditions.  The oxygen that is produced as a result of photosynthesis is, therefore, a toxin that needs to be kept away from nitrogenase. Heterocysts have thick cell walls and less chlorophyll as means of keeping the nitrogenase in an oxygen-free environment.  However, some non-heterocystous cyanobacteria, including Microcoleus, are able to fix nitrogen at night (when the photosynthetic apparatus is not pumping out oxygen) .   As there seems to be no protection for the enzyme inside the cells, it is possible that the daily destruction of enzyme is offset by renewed synthesis when light levels fall and there is no more oxygen being produced internally.   Nitrogen-fixation is already an expensive process for cells, requiring a large amount of energy, and this will increase the cost further.  However, in the case of the saltmarsh at Tentsmuir, there is a large amount of habitat available and few other organisms capable of exploiting it, so perhaps this is an investment worth making?

The benefits of that investment then “trickle down” (or up, depending on your point of view) through the ecosystem.   The cyanobacteria “fix” carbon and nitrogen and, in effect, create the soil within which other organisms thrive.  Janet Sprent, of the University of Dundee, calculated that, assuming nitrogen to be the limiting nutrient, then the fixation by Microcoleus and other cyanobacteria in such habitats could probably support the biomass of higher plants that is usually observed.  They are, in other words, a self-perpetuating “green manure” that creates a habitat within which other organisms can thrive.  In turn, by binding sand, they help to stabilise coastal features and, in turn, protect other coastal habitats and the communities that live amongst these.

References

Malin, G. & Pearson, H.W. (1988).  Aerobic nitrogen fixation in aggregate-forming cultures of the nonheterocystous Cyanobacterium Microcoleus chthonoplustes. Journal of General Microbiology 134: 1755-1763.

Omoregie, E.O., Crumbliss, L.L., Bebout, B.M. & Zehr, J.P. (2004).  Determination of nitrogen-fixing phylotypes in Lyngbya sp. and Microcoleus chthonoplastes cyanobacterial mats from Guerrero Negro, Baja California, Mexico.  Applied and Environmental Microbiology 70: 2119-2128.

Sprent, J.I. (1993).  The role of nitrogen fixation in primary succession on land.  pp. 209-219.  In: Primary Succession on Land (edited by J. Miles & D.W.H. Walton), Blackwell Scientific Publications, Oxford.

Sroga, G.E. (1997).  Regulation of nitrogen fixation by different nitrogen sources in the filamentous non-heterocystous cyanobacterium Microcoleus sp.  FEMS Microbiology Letters 153: 11-15.