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

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Life in the colonies …

Another outcome of my visit to Ennerdale Water a couple of weeks ago in July (see “Life in the Deep Zone”) was some tiny green spheres in the sample I collected from one of the small streams flowing into the lake’s north-west corner.   The stream was very short, little more than a seepage arising from a wet rush-dominated area of a field just twenty metres or so from the lake margin and, at the point which I sampled, there was a tangle of filamentous algae (Stigeoclonium, Mirsrospora and Mougeotia) as well as a distinct diatom-dominated film on exposed stones.    The colonies looked like tiny peas in my sample tray but I suspect that they were attached to rocks or aquatic vegetation before I disrupted them. Under the microscope, these turned out to be colonies of the green alga Chaetophora pisiformis, a relative of Draparnaldia and Stigeoclonium, both of which I have written about before (see “The exception that proves the rule …” and “A day out in Weardale …”.  Like those, Chaetophora has branched filaments but they differ in forming well-defined colonies that are visible to the naked eye.

The pictures below show the form of colonies very clearly.  Chaetophora colonies are firm to the touch and cannot easily be squashed under a coverslip.   I overcame this by using a cavity slide, and taking one of the smallest colonies that I could find in order to photograph it with as little damage as possible.  Note how there is a very clear edge to the colony, whereas Draparnaldia and Stigeoclonium have a mass of filaments and mucilage but no obvious border between the “colony” and the surrounding environment.  Draparnaldia sometimes forms discrete colonies (see “The exception that proves the rule …”) but these are much softer and more easily squashed onto a slide.

Top: colonies of Chaetophora pisiformis from a small stream flowing into Ennerdale Water, with a one cent coin for scale; bottom left: lower power (x40) view of a colony.  The picture frame is about two millimetres across; bottom right: medium power (x100) view of the same colony.

Viewed at higher magnifications, the branches of the filaments are clear. They tend to be clustered towards the tops of the filaments and, in this case at least, end abruptly, rather than tapering to fine hairs.  I explained in the posts mentioned above how these fine hairs are used by the algae as means of capturing the nutrients that they need.  Chaetophora can form these hairs, but it does so less often, in my experience, than Draparnaldia and Stigeoclonium.   There will be dead and decaying vegetation in the rush-dominated swamp from which the stream originates, and the enzymes that these algae produce will be able to harvest any phosphorus from organic particles that result from this decay.  That’s the theory for Stigeoclonium at least, but I suspect that the colonies of Chaetophora are also highly efficient recycling units: the filaments are embedded in a firm mucilage that is far more than an inert polysaccharide gunk.   Any phosphorus that is released from a filament will be far more likely to be hoovered up by another filament than to drift downstream whilst the phosphatase enzymes will also be on hand at the colony surface to savenge any stray nutrients from the seepage.  These tight colonial forms are, in other words, fortresses of plenty in an otherwise inhospitable landscape: well adapted to nutrient-stressed situations and, as a paucity of nutrients is the natural condition of streams, the presence of these colonies is a good sign that this stream is in good condition.

Filaments of Chaetophora pisiformis from a small stream flowing into Ennerdale Water, July 2018.  Scale bar: 20 micrometres (= 1/50th of a millimetre). 

Reference

Whitton, B.A. (1988).  Hairs in eukaryotic algae.   pp. 446-480.  In: Algae and the Aquatic Environment (edited by F.E. Round).  Biopress, Bristol.

Life in the deep zone …

The view above – looking along Ennerdale Water from the western end – is one that I’ve used before in this blog.  The difference today is that there is about twenty metres of foreshore exposed.  Normally, water covers all the area in the foreground.   Not today: Ennerdale Water is one more victim of our present drought conditions.  During the winter, we often see water splashing over the weir at the outfall; today, the weir head is a metre above the lake level and flow in the River Ehen is maintained only by pumps installed by United Utilities.

When W.H. Pearsall visited Ennerdale Water in the 1920s, he considered it to be one of the most primitive of the Lake District’s lakes (see “The power of rock …”).   However, this supposedly wild lake had been tamed by a weir since the middle of the 19th century in order to provide drinking water for Whitehaven and Workington and surrounding areas.   That, in turn, has consequences for the river downstream, especially at times such as this when, unless augmented by pumps, there would be no water in the River Ehen below the outfall.   At some point in the next decade, a new water infrastructure project will pipe water to west Cumbria from Thirlmere, after which the weir can be removed and fluctuations in water level in both lake and river will be more natural.

The weir at the outfall of Ennerdale Water, with the fish pass at the far end. 

Meanwhile, however, I was able to explore areas of the lake littoral zone that would normally be hidden from me.  My notebook, for example, records my observations that this part of the lake shore has a stony bottom yet, as can be seen from the picture above, these form a belt about 20 metres wide, after which there is firm sand.   Normally, this would be close to the limit of safe wading but, today, I could walk out with just a pair of thin neoprene shoes.   Looking down, I could see a number of tufts of the alga Nitella flexilis growing in this sand.   I’ve written about this species before (see “Finding the missing link in plant evolution …”) and have seen it in the lake before, but not in this particular location. Standing with the lake water lapping against my shins I could bend down and take some photographs of these with my underwater camera that give this usually chilly location a semi-tropical feel.

It is a useful reminder to those of us who dabble in lake littoral zones and think that we understand their ecology that a lot happens beyond the depth in which we can safely wade.   Marco Cantonati and colleagues, for example, have found big changes in the composition of the algal flora of Alpine lakes when they used Scuba diving to explore the depths of their littoral zones.  No doubt, we would see similar changes if we were to try the same in the Lake District.

References

Cantonati, M., Scola, S., Angeli, N., Guella, G. & Frassanito, R. (2009).  Environmental controls of epilithic diatom depth-distribution in an oligotrophic lake characterized by marked water-level fluctuations.   European Journal of Phycology 44: 15-29.

Cantonati, M. & Lowe, R.L. (2014).  Lake benthic algae: toward an understanding of their ecology.  Freshwater Science 33: 475-486.

 

Desmid diversity …

Back in September, I wrote about a joint British Phycological Society and Quekett Microscopical Club field weekend looking at desmids in the Lake District (see “Desmid Masterclass”, “Lessons from School Knott Tarn” and “Different tarn, different desmids …”).  Dave John sent some of the samples that we collected to David Williamson, the UK’s leading expert on desmids but, at 92, too frail to join us, and he has now sent back some fine drawings illustrating the range of desmids that he encountered.

Two of the tarns (Long Moss Tarn, Kelly Hall Tarns) are already recognised as Internationally Important Plant Areas (IPAs) for desmids because of their desmid diversity and containing internationally very rare desmids (based largely on David Williamson’s records) so their diversity is not a complete surprise to us.  Nonetheless, David found a total of 129 desmid taxa in the three tarns, whilst another desmid specialist, Marien van Westen, identified almost 160 desmids in another set of samples from the same tarns.

The drawings are arranged in three plates, one for each tarn.   Desmids identified by David Williamson from the three tarns are illustrated.  The desmids have been numbered and the captions prepared by David John who is analysing the findings and comparing them with surveys dating back to the 1970s.   David Williamson has drawn the taxa at different scales to roughly balance the arrangement on the collage, and adjusted the sizes so important details are visible.   No details of the chloroplasts are given since all samples had been preserved in formalin.  A few of the desmids, particularly those that are very long, have not been included in the plates.

Desmids from Long Moss Tarn (SD 292 936), September 2017.   Long Moss Tarn is shown in the photograph at the top of this post.

Desmids from Kelly Hall Tarn (SD 289 933), September 2017.

Desmids from School Knott Tarn (SD 427 973), September 2017.

Key

1-Actinotaenium diplosporum; 2-Actinotaenium turgidum;  3-Bambusina borreri;  4-Closterium acerosum var. borgei; 5-Closterium angustatum;  6-Closterium archerianum var. pseudocynthia;  7-Closterium archerianum; 8-Closterium attenuatum;  9-Closterium baillyanum var. alpinum; 10-Closterium baillyanum; 11-Closterium closterioides; 12-Closterium costatum; 13-Closterium dianae var. arcuatum; 14-Closterium dianae var. minus;  15-Closterium didymotocum; 16-Closterium incurvum; 17-Closterium intermedium; 18-Closterium kuetzingii;  19-Closterium lunula; 20-Closterium navicula;  21- Closterium setaceum; 22-Closterium striolatum; 23-Cosmarium amoenum; 24-Cosmarium anceps; 25-Cosmarium binum; 26-Cosmarium brebissonii; 27-Cosmarium contractum;  28-Cosmarium davidsonii; 29-Cosmarium debaryi;  30-Cosmarium depressum; 31-Cosmarium formosulum; 32-Cosmarium hostensiense; 33-Cosmarium incrassatum var. schmidlei; 34-Cosmarium margaritatum; 35-Cosmarium margaritiferum; 36-Cosmarium monomazum var. polymazum;  37-Cosmarium obtusatum;  38-Cosmarium ornatum; 39-Cosmarium ovale;  40-Cosmarium pachydermum; 41-Cosmarium pachydermum var. aethiopicum; 42-Cosmarium perforatum var. skujae; 43-Cosmarium portianum; 44-Cosmarium punctulatum;  45-Cosmarium quadratum; 46-Cosmarium quadrum; 47-Cosmarium subochthodes var. majus; 48-Cosmarium subtumidum var. groenbladii;  49-Cosmarium subundulatum; 50-Cosmarium tetragonum var. ornatum ; 51-Cosmarium tetraophthalmum; 52-Cosmarium variolatum;  53-Cosmocladium tuberculatum; 54-Desmidium aptogonum; 55-Desmidium swartzii; 56-Docidium baculum; 57-Euastrum ampullaceum; 58-Euastrum ansatum;  59-Euastrum bidentatum var. speciosum; 60-Euastrum gemmatum; 61-Euastrum luetkemulleri; 62-Euastrum oblongum; 63-Euastrum pectinatum; 64-Euastrum pulchellum; 65-Euastrum verrucosum; 66-Gonatozygon aculeatum; 67-Gonatozygon brebissonii; 68-Groenbladia undulata; 69-Haplotaenium minutum;  70-Hyalotheca dissiliens;  71- Micrasterias americana var. boldtii; 72-Micrasterias compereana; 73-Micrasterias crux-melitensis; 74-Micrasterias denticulata; 75-Micrasterias furcata; 76-Micrasterias pinnatifida;  77-Micrasterias radiosa; 78-Micrasterias rotata; 79-Micrasterias thomasiana; 80-Micrasterias truncata; 81-Netrium digitus; 82-Netrium digitus var. latum; 83-Netrium interruptum;  84-Penium exiguum; 85-Penium margaritaceum; 86-Pleurotaenium coronatum var. robustum;  87-Pleurotaenium ehrenbergii; 88-Pleurotaenium truncatum; 89-Sphaerozosma filiforme; 90-Staurastrum arachne;  91-Staurastrum arctiscon; 92-Staurastrum bieneanum; 93-Staurastrum boreale var. robustum; 94-Staurastrum cristatum; 95-Staurastrum dilatatum; 96-Staurastrum inconspicuum; 97-Staurastrum kouwetsii; 98-Staurastrum lapponicum; 99-Staurastrum maamense; 100-Staurastrum polytrichum; 101-Staurastrum productum; 102-Staurastrum quadrangulare; 103-Staurastrum striolatum; 104-Staurastrum teliferum; 105-Staurastrum tetracerum; 106-Staurodesmus convergens; 107-Staurodesmus convergens var. wollei; 108-Staurodesmus cuspidatus var. curvatus; 109-Staurodesmus megacanthus; 110- Xanthidium antilopaeum; 111-Xanthidium antilopaeum var. laeve; 112-Xanthidium antilopaeum var. polymazum; 113-Xanthidium cristatum.

Small details in the big picture …

I’ve written about Platessa oblongella, a small diatom common in low alkalinity environments, before (see “A tale of two diatoms …” and links therein) but my travels around west Cumbria are gradually revealing more and more about the ecology of this organism, so bear with me as I explain my latest findings.

My first graph shows how the distribution of this diatom varies in different types of water body in the Ennerdale catchment.   I have analysed 223 samples from this small area over the past few years and, within this dataset, there is a very clear distinction between situations where Platessa oblongella is abundant and situations where it is very rare.   I have very few records from Ennerdale Water itself (present in just two out of 27 samples, and never comprising more than 2.7% of all diatoms in the sample) nor from the River Ehen, which flows out of the lake (present in just 16 out of 164 samples, and always £ 1% of all diatoms).  By contrast, in Croasdale Beck and in streams that flow into the north-west corner of the lake, it is present in 28 out of 32 samples, with a maximum relative abundance of 69%.   In ten samples it forms more than 10% of all diatoms present.   Several of my samples from the small streams were collected from just a few metres above the point where they joined the lake, which makes the distinction between these streams and the lake that much more intriguing.

My theory – based on data I showed in A  tale of two diatoms  is that Platessa oblongella is a species of disturbed habitats and that the littoral zone of a lake, whilst subject to some turbulence, is less disturbed than the rough world of an unregulated stream.  The contrast between the River Ehen immediately below the dam at the outfall of the lake and the various small tributary streams also supports this idea.

Differences in percentage of Platessa oblongella (including P. saxonica) in epilithic samples from Ennerdale Water and associated streams.  Data collected between 2012 and 2018 (along with one sample from River Ehen collected in 1997).   The photograph at the top of the post shows Ennerdale Water, photographed in January 2018.

Some of the populations I looked at seemed to consist of two distinct forms, one broader than the other.   This variability is quite common in Platessa oblongella and Carlos Wetzel and colleagues recently published a paper which suggests that these are, in fact, two distinct species.   When I first started looking at diatoms, John Carter, my mentor, used the name Achnanthes saxonica, but Krammer and Lange-Bertalot, in the revised Süsswassserflora, regarded this as a synonym of Achnanthes oblongella, a species first found in Thailand.   Wetzel’s study shows, as well as the difference in valve width, differences in the fine details of the striae between the two species.   They also decided that both species belonged in the genus Platessa, rather than Achnanthes.

Platessa oblongella (top) and P. saxonica (bottom) from Croasdale Beck, October 2017.  Scale bar: 10 micrometres (= 1/100th of a millimetre).

Valve width is, however, a very useful criterion, as the histograms below show.   The left hand graph shows a distinctly bimodal distribution of widths in specimens from Croasdale Beck, whilst the right hand graph shows a much tighter, and clearly unimodal, range.   This comes from another tributary stream flowing into the Ehen about 500 metres below the lake itself.  Quite why two species can co-exist in one stream but only one is present in another is not clear.

The modes of these populations are very close to the median widths for P. saxonica (narrow, ± 4/5 – 5 mm) and P. oblongella (broader, ± 6.5 mm) respectively but, as the left hand histogram shows, there is some overlap.    You might have trouble, for example, deciding whether a valve that was 5.5 mm wide was a “fat” P. saxonica or a “thin” P. oblongella.   My standard advice in situations such as this is that we should identify populations not individuals although, in the case of Croasdale Beck, this will still leave a grey area between the “fat” and “thin” valves where a judgement call is necessary.   In this case I think I could have done it because the P. saxonica valves in this stream tended to have a greater length:breadth ratio than those of P. oblongella, though I have not actually quantified this.

Width of valves in populations of “Achnanthes oblongella” from a) Croasdale Beck, and b) an unnamed tributary stream of the River Ehen, October 2017. 

There is more to say about the ecology of these species, but I have probably written enough for now.  I will leave you, for now, to bask in the rare sensation that occurs when diatom taxonomists make a situation clearer rather than more opaque, and return to this subject in a future post.

References

Carter, J.R. (1970).   Observations of some British forms of Achnanthes saxonica Krasske.  Microscopy: Journal of the Quekett Microscopical Club 31: 313-316.

Wetzel, C.E., Lange-Bertalot, H. & Ector, L. (2017).  Type analysis of Achnanthes oblongella Østup and resurrection of Achnanthes saxonica Krasske (Bacillariophyta).  Nova Hedwigia, Beiheft 146: 209-227.

Change is the only constant …

The diatoms I saw in my sample from the littoral of Lake Popovo (described in the previous post) reminded me of an assemblage that I had seen in another lake which, apart from its location, has much in common with Popovo. This lake is Wastwater, in the western part of the English Lake District (see “The Power of Rock …”).  Like Popovo, it is situated in a remote a region of hard volcanic rocks and, as such, has very soft water and is subject to few of the pressures to which most of our freshwaters are subject.  The photograph above shows me sampling Wastwater in about 2006 (more about this photograph, by the way, in “A cautionary tale …”).

I wrote about Wastwater when I was writing my book Of Microscopes and Monsters, the precursor of this blog.   I focussed, in particular, on an experiment that my friend Lydia King had performed as part of the research towards her PhD.  Her previous work had established that there were relationships between the types of algae that she found in lakes in the Lake District and the amount of nutrients that they contained.  She also saw that the types of algae she found depended upon how acid or alkaline the water was.  But the water chemistry only explained a part of the variation in the algae and now she wanted to find out about the variation that was not explained by this.   In particular, she wanted to know how much of the variation was due to the way that the algae interacted with each other.

Lydia’s experiment involved putting clay pots into the shallows at the edge of Wastwater and then watched how the algal communities changed over the course of six weeks.  She also examined small parts of the pots at extremely high magnifications using a scanning electron microscope.   These micrographs, and subsequent conversations with her, had inspired some of my early paintings and I returned to this subject several times, finally producing a series of three pictures that showed changes in the algae over time.

The microbial world of the littoral zone of Wastwater after two weeks of colonisation showing unidentified small unicellular blue-green alga,  unidentified small unicellular green alga; thin filaments of Phormidium,  Achnanthidium minutissimum and Gomphonema parvulum.

The first of these shows the surface of the plant pot after being submerged in Wastwater for two weeks.   You could think of this as a patch of waste ground that was, at the start of the experiment, bare of vegetation.   If we watched this patch over a number of weeks, we would notice some plants appearing: scattered stalks of grass, perhaps some rosebay willow herb, dock or plantains. A gardener might dismiss these as “weeds”, although this term has no ecological meaning but ecologists prefer to think of these as “pioneers”: plants adapted to colonising new habitats, growing quickly (which might mean producing lots of seeds in a short space of time or producing rhizomes or runners) and covering the ground.  This same process has taken place on Lydia’s plant pot in Wastwater: the “weeds” in this case are scattered thin filaments of the blue-green alga Phormidium, the diatoms Achnanthidium minutissimum and Gomphonema parvulum plus a number of spherical green and blue-green cells that she couldn’t identify.   Such is the scale that we are working at that this open landscape still contains about 92000 cells per square centimetre.

The microbial world of the littoral zone of Wastwater after three weeks of colonisation.   The composition is similar to that in the previous figure but the density of cells is greater.

When she came back a week later, much of the empty space had been infilled; there were now about 300,000 cells per square centimetre.  These mostly belonged to the same species that she had found the week before.  The difference is that they are now rubbing up against each other and this has some important consequences.  All plants need light and nutrients to grow and algae are no exceptions.   Sunlight provides the energy for photosynthesis but now, at week three, the density of algae is such that there is a chance that some of the light will be intercepted by a neighbouring cell.   The total amount of sunlight that filters through the water to the pot surface is already much lower than that available at the lake surface; now it has to be shared out between many more cells.   At this point, properties such as fast growth rates that helped our pioneers to colonise the plant pot become less relevant, and it is algae that are better adapted to capturing the limited light that will survive.

So when Lydia came back to Wastwater after six weeks, she saw a very different community of algae on her pots.   There was still a lot of Achnanthidium minutissimum, but rising above these was the elegant art deco shape of Gomphonema acuminatum (also found in Lake Popovo) which, importantly for our story, grows on a long stalk.  There are also cells of “Cymbella affinis” (the correct name at the time that Lydia was working but see comments in the previous post about the nomenclatural history of this species).   This, too, grows on a long-stalk, the better to grow above the Achnanthidium and other pioneers.   If we continue to use the analogy of a patch of wasteland, then it has now reached the point where it has been invaded by shrubs such as hawthorn and blackthorn.   However, in a terrestrial habitat this would happen two or three years after the first pioneers had arrived, not six weeks as Lydia had observed for the algae.   She also found the diatom called Tabellaria flocculosa which forms filaments.  These often start out loosely-attached to the substratum but more often break free and become entangled around the other algae.   In our “wasteland” analogy, these would be the brambles.

The microbial world of the littoral zone of Wastwater after five weeks of colonisation.  Gomphonema acuminatum, “Cymbella affinis” and Tabellaria flocculosa have now joined the assemblage seen in the two earlier dioramas.

The experiment finished shortly after this, terminated when the apparatus was overturned.  Whether by a wave or by vandalism, Lydia will never know but this event is, itself, a metaphor for the harsh world in which benthic algae have to survive.  In real life, the many cobbles in the littoral zone will be rolled by wave action or, as we have seen in other posts, invertebrate grazers could have removed much of the “shrubbery”, leaving a “pasture” composed of the tough, fast-growing species such as Achnanthidium minutissimum to dominate samples.   The “successions” we see in the microscopic world not only take place much more quickly than those in the macro world, but they also rarely have a stable “climax”: just a brief pause before the next onslaught from the physical, chemical and biological processes that shape their existence.

References

King, L., Barker, P. & Jones, R.I. (2000). Epilithic algal communities and their relationship to environmental variables in lakes of the English Lake District. Freshwater Biology 45: 425-442.

King, L., Jones, R.I. & Barker, P. (2002). Seasonal variation in the epilithic algal communities from four lakes of different trophic state. Archiv für Hydrobiologie 154: 177-198.

What a difference a storm makes …

I was back at Croasdale Beck last week and noticed a rather dramatic change to the meander just upstream from our regular sampling spot.   If you look at the photograph that heads the post “A tale of two diatoms …”, you’ll see the stream flowing around this meander.  Now, however, it has cut a new, shorter channel that bypasses the meander altogether.   We visited the stream just a few days after Storm Ophelia had passed through although, judging by the grass growing on the gravel of the abandoned meander, it was not necessarily this particular event that reshaped the stream.

Croasdale Beck is an unruly tributary of the River Ehen, rising on the fells above Ennerdale Water and tumbling down across rough grazing land and some semi-improved pasture (as in the picture above) before joining the Ehen in Ennerdale Bridge.   This is not the first time that we have seen conspicuous changes in the channel after a storm.  The magnitude of the flood is illustrated by the hydrograph below, which went off-scale for a period, as the discharge exceeded 3000 mega litres per day (300 MLD is the approximate limit for safe wading, in my experience).   I noticed that there was much less green algae present than we usually record at this time of year, although the diatom film was still quite thick.   Some of the stones that I picked up to sample had the slimy biofilm on the underside, suggesting that they had been recently rolled by the flooded river.   Croasdale Beck has no lake to buffer the rise and fall of the floodwaters and a huge amount of energy is carried down in a short period of time as the water surges downstream.

By the time we had arrived, the floodwaters had subsided and the sheep were contentedly grazing the surrounding land.  The stream itself was almost back to base flow (in contrast to the River Ehen which was still only just wadable).  Only the meander looked different …

The hydrograph for the River Ehen, as the aftereffects of Storm Ophelia make their way downstream.