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. 

Nostoc_commune_x1000_Apr19_#1

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.

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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.

Both sides now …

Ennerdale_looking_NW_July16

I diverted from my usual haunts in the upper River Ehen in Cumbria recently in order to explore Ennerdale Water in greater detail.   I am used to see it from the western end as we do our fieldwork, but the length of the journey to and from the River Ehen means that we rarely have time to linger.  Finally, however, we found a July day when we could circumnavigate the lake.  “July day”,”Lake District” and “fieldwork” sounds like an intoxicating combination.  However, the photograph above shows it was not quite as idyllic as it might have been (or, even, as it was on the day before).  Hence the title of this post, borrowed from a beautiful Joni Mitchell song which includes the line “But clouds got in my way”.

In the far past, the lakes of the Lake District were thought to have “evolved” at different speeds following their formation at the end of the last Ice Age.  Ennerdale Water and Wastwater, surrounded by hard volcanic rocks which erode very slowly, were regarded as the two most “primitive” lakes, whilst Windermere and Esthwaite Water were thought to be the two most “evolved”.   That is now known not to be the case: the geology is very important in determining the type of lake, not just because erosion is the source of the inorganic salts that give the water a particular chemical character, but also because this influences how man uses the lake.   In the case of Ennerdale Water, only about five per cent of the catchment is cultivatable, and this, in turn, influences the amount of inorganic fertiliser that is added to the meagre supply of salts provided by the underlying rocks.   Ennerdale is, as a result, one of the least chemically-disturbed of all English lakes.

At the far south east end of the lake, close to where the River Liza enters the lake, I was intrigued to see some very dark spots on the rocks.  They looked like they might be cyanobacterial colonies, so I picked a few off with my forceps and put them into a bottle for later investigation.  When I was able to look at them, the following day, I saw clumps with brown filaments radiating out, and each gradually narrowing towards the tip.   Closer examination showed that the cells that made up each filament had a blue-green colour, but were each enclosed in a brown pigmented sheath.   The filaments showed a characteristic form of “false” branching, in which the daughter filament breaks off from the mother, but is contained within the same sheath.   At the base of many of the filaments, I could see a modified cell (slightly lighter and less granular than the others) called a “heterocyst”, which was responsible for nitrogen-fixation.   These are all characteristics of the genus Rivularia, which is a good indicator of very high status water.

Ennerdale_cobble_with_Rivul

A cobble (about 15 cm long) from the littoral zone of Ennerdale Water, SW end.  The dark cyanobacterial colonies are about 3-4 mm across.

Rivularia_Ennerdale_July16

Rivularia biasolettiana from the littoral zone at the south east end of Ennerdale Water, Cumbria, UK.  a. low power (x100) image showing radiating filaments gradually narrowing in width; b. filaments showing false branching (a heterocyst is present, but hard to see); c. false branching in a filament of Rivularia with heterocyst arrowed.   Scale bar (b. and c.): 10 micrometres (= 1/100th of a millimetre).

The presence of Rivularia here was interesting to me for several reasons.   First, it continues a series of observations that suggest that this genus is not confined to hard water habitats in Britain and Ireland, as once thought (see “more about Rivularia” and links) although earlier posts have also referred to its presence in soft water habitats in Norway.   I’m also fairly sure that the organism that I collected from this trip to Ennerdale is different to the one that I find in the River Ehen and, indeed, at other locations around the Ennerdale perimeter, but that is a subject for another day.

The second comment to make is that the presence of cyanobacteria (blue-green algae) is usually a sign of an unhealthy, enriched habitat, not the very high quality habitat that Ennerdale, in fact, represents.  Blue-green algae in the phytoplankton is, very often, a bad sign, suggesting enrichment by inorganic nutrients.  Several species combine the ability to fix nitrogen with positive buoyancy, which means that they have two distinct advantages over other algae as they compete to exploit limited light and nutrients.  Some of these blue-green algae are also toxic, which has implications for how the lake and its water is used.

Those blue-green algae that live attached to surfaces in streams and in the littoral zone of lakes play by a different set of rules, however.  As they cannot use positive buoyancy to compete for light, they are more likely to be overgrown by faster-growing algae in the scrabble to capture available light.   This means that investment in expensive nitrogen fixation machinery is only an advantage when other algae, too, are very short of nutrients.  And a shortage of nutrients is the natural state for most freshwater ecosystems.

I chose the title of this piece, originally, because I felt that I had looked at Ennerdale Water from both sides now.  However, the same lyric could just as well apply to blue-green algae.   The water manager sees them as a problem; in some situations, however, they can be a positive sign.   So we can, in fact, look at blue-green algae from both sides too … from give and take and still somehow … I really don’t know life (microscopic life especially) at all ….

Poking around amongst sheep’s droppings …

A couple of kilometres away from the stream featured in the previous post is an old quarry that we visit each year as part of this course (see “Nosing around for blue-green algae …”).   In a damp flush at the edge of the quarry floor, we found some patches of what looked, from a distance, like sheep droppings.   A useful strategy, shared by most of the human race, is to assume that anything that looks like a sheep’s dropping probably is a sheep’s dropping, and not to go prodding at this with a finger.   However, a curious soul in the distant past with a sense of adventure ignored this precept and discovered that a few of these were, in fact, growths of Cyanobacteria.   Most natural historians, wisely, focus their attention on more spectacular aspects of life on earth; however, a few of us have retained this childish instinct to poke at anything that looks like sheep’s droppings.

Scytonema_Whitbarrow_May15

Scytonema sp. from a flush at Whitbarrow Quarry, Cumbria, May 2015

A small part of one of these growths, teased out and mounted on a cover slip, reveals itself to have the characteristics of the genus Scytonema although, today and despite a long hunt, I could not find any filaments that presented themselves in a suitable position to photograph. The illustration below, therefore, is of a growth of Scytonema from another calcareous site in Cumbria. The Cyanobacteria is, you may remember, the modern name for the “blue-green algae” which is often confusing as many Cyanobacteria are not blue-green in colour.   What we can see here is a chain of cells (a “trichome”) which are surrounded by a thick sheath (“trichome” plus “sheath” equals “filament”, in Cyanobacteriological lore).   The sheath is a yellow-brown colour, due to a pigment called “scytonemin” which acts as a sunscreen, absorbing ultra-violet radiation and, in the process, obscuring the blue-green colour of the trichome within.

Scytonema_Sunbiggin_Tarn

Scytonema sp. from a calcareous flush at Sunbiggin Tarn, May 2005.   Scale bar: 10 micrometres (= 1/100th of a millimetre).

Two other characteristics of Scytonema are visible in the photograph.   Both the left and right hand pictures shows “false branches”: if the trichome breaks for any reason, either or both of the broken ends keep growing and break out of the filament. The left hand illustration is a single false branch and, just to the left of the branch you can see the distinct “heterocyst”, a cell where nitrogen fixation takes place.   The wall of the heterocyst is thicker than that of other cells, as nitrogen fixation can only occur in the absence of oxygen.

Walking back from the quarry towards the minibus, Allan pointed up at dark patches on the cliffs looming over us.   It was Gloeocapsa alpina, the same species that we met a short while ago in a cave on Malta (see “The mysteries of Clapham Junction …”).   The cliffs at Whitbarrow are, in effect, a vertical “desert” from the point of view of any organism that aspires to live there. These Cyanobacteria, with their ability to “re-boot” on those occasions when conditions are favourable for growth, have an advantage here.   One guesses that the damp climate of north-west England is slightly more forgiving than that of a Mediterranean hillside but it is still a tough habitat in which to survive.

Gloeocapsa_alpina_Whitbarro

Growths of Gloeocapsa alpina (arrowed) on the cliffs at Whitbarrow Quarry, May 2015.

The origins of life …

I have been reading Richard Fortey’s book “Survivors” (see “”They don’t do much, do they?””) which is a rare and precious thing in the world of natural history writing as he devotes two whole chapters to the algae.   The first of these describes his encounters with living stromatolites in Shark Bay, in Western Australia.   The point he is making as he writes is that he did not just make an extraordinarily long journey to get there (first, get to Perth, then travel a further 800 km north …) but that he is also making a similarly long journey back in time, as stromatolites are survivors from the Precambrian era and, indeed, may have played a vital role in creating the oxygen-rich atmosphere that we take for granted.

I have a polished specimen of a stromatolite, purchased from a fossil shop in Durham, which shows the characteristic fine laminations. Each of these represents a layer of Cyanobacteria (blue-green algae) filaments which have, in turn, trapped sediment particles.   The laminations are, in turn, formed into dome-like structures, reflecting the vertical growth of the filaments in search of sunlight.   This stromatolite is from the Ordovician era (I think), which dates it to between 443 and 485 million years ago.

stromatolite_argentina

A stromatolite from Argentina, possibly from the Ordovician era. The specimen is 10 cm long.

Not only were stromatolites extremely abundant in the Precambrian (approx. 4.6 billion to 540 million years ago) but the organisms from which they were formed appear to be very similar to Cyanobacteria that can still be found today.   They are extremely delicate structures that thrived, at least in part, because there were few other organisms in the Precambrian that could compete with them or graze them and, consequently, had shallow marine habitats to themselves for an extremely long time. During this period, they were busily photosynthesising away, taking carbon dioxide and water and converting it to simple sugars (which they needed to grow) and oxygen (which was, as far as the Cyanobacteria was concerned, a waste product). This oxygen was released as tiny bubbles (see “Ecological yin and yang…”) which, ever so slowly, accumulated in the atmosphere.   Maybe it is no surprise that the Precambrian, the era before fossils of multicellular organisms are common, lasts for about four fifths of the entire lifespan of the earth: it took this long for all those tiny bubbles to add up to enough oxygen to allow more complicated organisms to survive.

And what did those multicellular organisms feed upon?   That’s right: the Cyanobacteria had sown the seeds of their own destruction.   There is evidence not just of a gradual decline of stromatolites through the later Precambrian and into the Cambrian and Ordovician eras, but also of a resurgence of stromatolites after the mass extinction at the end of the Ordovician (which would have removed the multicellular grazers and left our tough little Cyanobacteria behind).   Stromatolites are found sporadically throughout the fossil record and in a small number of locations in the present day, but their heyday lies far in the past.

References

Bengston, S. (2002). The early worm catches the – what?   Pp. 289-317. In: The Fossil Record of Predation (edited by Kowalewski, M., and Kelley, P.H.). The Paleontological Society Papers 8, The Paleontological Society, Boulder, Colorado.

Fortey, R. (2011).   Survivors: the Animals and Plants that Time Has Left Behind.   Harper Press, London.

Sheehan, P.M. & Harris, M.T. (2004). Microbialite resurgence after the Late Ordovician extinction. Nature (London) 430: 75-78.

“They don’t do much, do they?”

I spent the early part of yesterday evening listening to Richard Fortey talk on “Survivors: the animals and plants that time has left behind” at Van Mildert College in Durham.   Fortey is a palaeontologist, best known for his work on trilobites, but who has also presented popular television series on natural history topics.   In this lecture, he talked about the small number of organisms that were well represented in the fossil record (some pre-dating dinosaurs) yet which had survived the various mass extinctions and which could still be found today. Did these organisms have anything in common, he wondered.

One group of organisms that he talked about were the blue-green algae (Cyanobacteria) responsible for forming “stromatolites” which are found both in Precambrian rocks over a billion years old and in a few locations today.   I’ve talked about blue-green algae in many of my posts (e.g. “More reflections from the dawn of time …”) but what amused me this evening was Richard Fortey’s anecdote about a discussion with a BBC producer as they devised a television series based on his book:

Fortey: “These are the most important organisms in the history of the earth” (commenting on their role in creating the oxygen-rich atmosphere that every organism since has relied upon)

BBC producer: “they don’t do much, do they?”

It is a subject that I have addressed several times in this blog (see “The sum of things …”, “Every second breath …”): how do we address the imbalance in natural history broadcasting between the charismatic “few” and the unfashionable “many”, bearing in mind, of course, that these are, in many cases, the hard-working, unglamorous “back office staff” who keep our planet running.   Television natural history programmers are in the entertainment business first and I’m not going to pretend that a stromatolite made of blue-green algae is, on its own, a recipe for compelling television.   But I also feel that there are possibilities that could be explored, and that we may be held back by a lack of imagination on the part of broadcasting creatives and commissioning editors.   They, too, are children of the television age, brought up on a style of broadcasting from Zoo Quest through Life On Earth and onwards that is interested only in the televisual aspects of natural history. Whoa .. hold it there … I’m coming dangerously close to criticising David Attenborough … Saint David … that’s close to heresy.

Reference

Fortey, R. (2011). Survivors: the animals and plants that time has left behind.   Harper Collins, London.

Rewriting history at Talkin Tarn …

One of the joys of modern technology is the ability to right obvious wrongs.   So, having watched my wife’s crew struggle in a regatta at Talkin Tarn, near Brampton, last weekend, having been entered in the wrong class and the wrong age category and then racing in a men’s rather than a woman’s boat, I had no option but to turn to Photoshop for justice.   Here, in a rewritten version of the Talkin Tarn regatta, you can see the DARC novice women’s coxed four half a length head of their opposition with about 400 metres to go.   The backdrop is the Pennine fells, looking north towards Hadrian’s Wall.

Talkin_tarn_rowing

Talkin Tarn regatta 2014: DARC novice women’s coxed four half a length ahead of the opposition.  

I have to confess that the entertainment value of watching one crew busting a gut to go slightly faster than another crew pales after the first hour, so I wandered off to poke around the lake’s margins.   The water here contained some just discernable green specks, all between about half a millimetre and a millimetre across.   Under the microscope, these resolve into spherical colonies each composed of filaments which gradually taper from the base.   These colonies belong to a cyanobacterium (blue-green alga) called Gloeotrichia echinulata. I’ve written about relatives of Gloeotrichia in previous posts (“Blue skies and blue flowers in Upper Teesdale”, “”Looking” is not the same as “seeing” …”, “More about Rivularia”) and have commented that the large light coloured cells, called “heterocysts” are responsible for nitrogen fixation, which enables the alga to grow even when nitrogen is relatively scarce. In the case of Talkin Tarn, the scarcity is probably not absolute but relative to other nutrients such as phosphorus. It means that the Gloeotrichia can grow even when the supply of this important nutrient has run out.

Gloeotrichia_echinulata_mac

Colonies of Gloeotrichia echinulata in a drop of water from Talkin Tarn, July 2014.   The drop is 12 millimetres across.

One other difference between the filaments of Gloeotrichia and those of Rivularia is the dark specks inside the cells.   These are gas vacuoles which make the cells and colonies of Gloeotrichia more buoyant than other algae. Leave a sample from Talkin Tarn to stand for a while and the colonies of Gloeotrichia will rise to the surface. This gives cyanobacteria a competitive advantage over other algae in lakes as they now have an adaptation that keeps them closer to the sunlight which powers their photosynthesis as well as one that helps them to overcome nitrogen limitation and an adaptation.   It is no surprise that cyanobacteria are so prolific in lakes around the world.

Gloeotrichia_echinulata_tal

Microscopic views of a Gloeotrichia echinulata colony from Talkin Tarn.   The left hand image shows an entire colony, about half a millimetre across.   The right hand image shows the same colony at higher magnification.   Note the large, round light-coloured heterocysts at the base of each filament. Scale bar: 10 micrometres (= 1/100th of a millimetre).

Gloeotrichia also has the ability to produce compounds called “microcystins” which are powerful toxins.   There are many records of livestock, dogs and humans suffering liver damage through ingesting contaminated water.   Although the rowers at Talkin Tarn were warned to wash their hands after racing, the risk is only high if a large amount of Gloeotrichia-containing water is swallowed. That is unlikely in a sport such as rowing, where the idea is to stay in the boat, but swimming in Talkin Tarn would not be a good idea. Rowing, though, is not without such dangers: the Durham women’s novice crew’s previous opponents suffered an “ejector crab” at the start which dumped one of them into the River Wear. On that occasion, cyanobacteria in the River Wear were the least of their worries and the Durham crew were able to cruise serenely down the river to a glorious victory.   Photoshop not required.