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.


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.


The underwater world of Ennerdale Water …

I’ve tried to capture the world of microscopic benthic algae many times but never, until now, attempted the same effect with plankton.   The picture below illustrates the problem that I face: whereas the benthic flora are organised with, for the most part, a clear three-dimensional structure and known dependencies amongst organisms (species A, for example, being epiphytic on species B), plankton are randomly distributed in a very dilute solution.   My picture  below, which is based on four phytoplankton samples collected by the Environment Agency in the summers of 2014 and 2016.

A representation of the phytoplankton of Ennerdale Water with cells of Rhodomonas and Kephyrion depicted at a realistic density (c. 1000 – 2000 cells per millilitre).

I had to address two issues in producing this image, which is based on four phytoplankton samples collected by the Environment Agency in the summers of 2014 and 2016: depicting the phytoplankton cells at approximately the correct density and making sense of the list of names that appeared on the list.  Ennerdale Water is a very nutrient-poor lake and cell concentrations during the summer are in the order of 1000 to 2000 per millilitre.  That sounds a large number until you consider the scale at which we are working.   For simplicity, I assumed spherical cells of about 20 micrometres diameter (= 1/50th of a millimetre) at a density of 1000 cells/ml.    That equates to one cell per micrometre which is 1 mm x 1 mm x 1 mm.   Using these assumptions, each cell is 50 diameters distant from its nearest neighbour, which means the foreground of a picture should contain only two small cells and a lot of blue paint.

Next, I need to know what algae to paint and the problem here is that 85 per cent of the cells in the Environment Agency phytoplankton analyses were described as “picoplankton < 2 micrometres diameter” or “nanoplankton 2-20 micrometres diameter” (the latter divided into flagellates and non-flagellates).  There are, apparently, big difficulties in naming many of the cells found as preservation with Lugol’s Iodine coupled with the long time in storage before analysis can lead to loss of useful diagnostic features.   Cells in the nanoplankton category can, in theory, belong to any one of a number of groups of algae but If I focussed just on those organisms that could be named, I see that the Cryptophyta Rhodomonas lacustris var nannoplanctica (formerly R. minuta var. nannoplanctica) predominates, followed by Chrysophytes, of which Kephyrion is the most abundant.   So these are the two cells that I have put in the foreground.

I subsequently turned up a paper from 1912 by the father and son team of William and George West who looked at the phytoplankton of Ennerdale Water and a number of other lakes in the Lake District and Scotland.  The range of taxa that they found was quite different to that recorded in these recent surveys with samples dominated by desmids and almost no Chrysophytes or Cryptophytes recorded at all. That may, in part, be due to differences in methods – they collected samples using a “silken tow net”, which would probably have missed the very small Chrysophyta and Cryptophyta (an earlier paper by them tells us of the size of the nets but not the mesh itself) .  Some desmids that they found were found in the recent surveys but in much smaller quantities and it is possible that this was partly an artefact of the differences in sampling technique.  The idea of comparing count data from old papers with modern records is appealing but, in most cases, separating genuine changes in composition from differences introduced by sampling and analytical methods is always difficult.

Excuse these ramblings … there is, as you can see, not a lot of pictorial interest in the underwater world of an oligotrophic lake.   If you want excitement, tune into Blue Planet II, David Attenborough’s latest series for the BBC You will find sex and violence galore there.  The underwater world of Ennerdale Water is a quieter, more serene and certainly less televisual place.  Maybe that’s not such a bad thing …


Lund, J.W.G. (1948) A rarely recorded but very common British alga, Rhodomonas minuta Skuja. British Phycological Bulletin, 2:3, 133-139.

West, W. & West, G.S. (1909). The British freshwater phytoplankton, with special reference to the desmid-plankton and the distribution of British desmids.   Proceedings of the Royal Society of London Series B 81: 165-206.

West, W. & West, G.S. (1912).  On the periodicity of the phytoplankton of some British lakes.  Journal of the Linnaean Society, Botany 40: 395-432.

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.

Buffers for duffers …

In Ecology in the hard rock café I wrote about the challenges of living in an aquatic world where carbon – one of the raw materials for photosynthesis – was in short supply.   What I did not write about in that post is that this carbon also gives freshwater some useful additional properties.   In brief, rainwater is not pure water, but absorbs carbon dioxide from the atmosphere.  This, in turn, makes rainwater slightly acidic and, when it falls onto rocks, this weak acid dissolves the minerals from which the rock is made.  This adds two other forms of carbon to the water – bicarbonate and carbonate (the latter, particularly, from limestone).

Each of these three types of carbon in freshwater can convert to either of the other two types, with the speed of the reaction depending on the balance between the forms (the “law of mass actions”).  In essence, the reactions proceed until equilibrium is obtained, and this equilibrium, in turn, depends upon the pH of the solution.  These processes are summarised in the diagram below.

Relationship between pH and the proportion of inorganic carbon as free carbon dioxide (or carbonic acid, H2CO3 – orange line), bicarbonate (HCO3 – green line) and carbonate (CO32- – blue line).

The chemistry behind this is not easy to explain but a consequence is that any attempt to shift the pH (e.g. by adding acid) causes an automatic adjustment in the balance between the different forms of carbon.  Some of the hydrogen ions that could make the water acid are, instead , bound up as bicarbonate, and the pH, as a result, does not change.  The greater the quantity of inorganic carbon in the sample, in other words, the greater the capacity of the water to resist changes in pH.   The carbonate, bicarbonate and free carbon dioxide together act as a “buffer”, a chemical shock absorber.   Think of it as equivalent to the responsible use of a credit card or savings account to defer the cost of an unexpected bill (a car repair, for example) so that your current account does not go overdrawn.

Because life largely evolved in well-buffered marine systems, the enzymes that run our cells generally work best within a narrow range of pH (approximately 6-9).   Cells – unicellular life forms in particular – get stressed if pH strays outside this range, so the greater the buffering capacity, the easier it is for cells (life at high pH can bring additional complications, but we don’t have time to go into those here).  “Alkalinity”, as I mentioned in the earlier post, is the measure that ecologists use to assess the strength of the buffer system in a lake or river.  The principle of the measurement is straightforward: we add a dilute acid very slowly and watch what happens to the pH.   At first, nothing happens but, as soon as the water’s natural buffering capacity has been exceeded, pH drops rapidly.

I have a small portable alkalinity titration kit which involves adding drops of bromophenol blue indicator to a sample of stream or lake water.  This gives the water a blue colour when the pH is greater than 4.6.  As the pH falls, the solution becomes colourless and, eventually, turns yellow.   If you look at the graph above you will see that, at pH 4.6 most of the bicarbonate (HCO3) has been converted to carbon dioxide so the buffering capacity is pretty much non-existent.  This means that I can use the quantity of acid that is needed to make the bromophenol blue change colour as a measure of the buffering capacity of the water.

Alkalinity titrations beside Ennerdale Water (see top photograph) using a Hanna HI 3811 alkalinity test kit.  The right hand image shows acid being added to the water sample with a 1 ml pipette.  The blue colour shows that pH has not yet dropped below 4.6.

All this talk of chemical equilibria seems to be a long way from the natural history that is the core business of this blog.  Yet, at the same time, these reactions describe natural phenomena every bit as real as the plants and animals that attract the interest of naturalists.   Geology and chemistry ultimately create the context within which biology flourishes, but it is rare to meet a chemist who can talk with a naturalist’s passion.  I think that this is partly because chemistry tends not to describe tangible features of the landscape but, instead, quickly gets lost in abstract equations.  However, it is also a matter of culture: chemists need clinical separation from the mud and filth to maximise precision, whilst ecologists feel the lure of the field.  There is, nonetheless, a very basic and necessary link between the chemistry and ecology of aquatic systems.   Geology may shape a landscape but chemistry is one of the key mediators that determines the types of plants that cloak the hills and vales.  We ignore it at our peril.

Ecology in the Hard Rock Café …

Before I was diverted by the delights of Bukhara and Samarkand, I was writing about the struggles that aquatic plants have to undergo in order to obtain the carbon that they need for photosynthesis (see “Concentrating on carbon …”).   In this post, I want to show the scale of the effect of inorganic carbon supply on the diatoms that we find in freshwaters.

My earlier post pointed out that aquatic plants have two possible sources of carbon to use for photosynthesis: dissolved carbon dioxide or bicarbonate.   The latter is derived, ultimately, from the rocks through which the water seeps before ending up in a stream or river.   Calcium carbonate, in turn, reacts with hydrogen ions in the water to form the bicarbonate that plants can use for photosynthesis.   A rock such as limestone, which is made of calcium carbonate, for example, provides a better supply than a hard siliceous rock such as granite.

Aquatic biologists use the term “alkalinity” to refer to the relative amounts of carbon dioxide, bicarbonate and carbonate in water.   This can confuse people as, in this context, “alkalinity” has little to do with the pH of the water itself and, indeed, water that is alkaline (i.e. has pH > 7) does not have to have a high alkalinity.   For now, just accept that low alkalinity water has little bicarbonate relative to dissolved carbon dioxide, whilst high alkalinity water has mostly bicarbonate and relatively little dissolved carbon dioxide.   In practice, alkalinity is a good indicator of the geology underlying the catchment from which a sample was collected, with low values associated regions of hard rocks (such as the Ordovician granites in Ennerdale’s catchment) and high values particularly associated with limestone and chalk.

I’ve spent a quarter of a century trying to understand how diatoms react to pollution and one of the surprising by-products of those studies is the realisation that alkalinity is just as important as pollution in determining the diatoms that are found at a site.   This is the case for most groups of freshwater organisms, but the scale of the effect on diatoms is particularly strong, as the graph below indicates.

Relationship between alkalinity and the average TDI at 430 UK river sites (r2 = 0.52).   The blue line shows a regression line fitted to the 10th percentile using the “quantreg” package in R. 

This graph shows a data from 430 UK sites where at least one TDI (Trophic Diatom Index) measurement was available, with alkalinity plotted on a logarithmic scale on the x axis and the TDI on the y axis.   There is a clear relationship between the two variables with about half of all the variation in the TDI accounted for by alkalinity (i.e. geology) alone, and this is manifest, in particular, by alkalinity setting a “floor” below which the TDI is unlikely to fall at any given alkalinity value (indicated by the blue line).  The red line, then, indicates the variation in TDI due to other factors, mostly human pressures such as eutrophication.

The blue line, in other words, indicates the best that the TDI is likely to be at any given alkalinity and if we were to look at samples which plot close to this line we will see quite marked differences in the diatoms as we moved from the low end towards the high.   When alkalinity is low, we will find Tabellaria flocculosa, some Brachysira species (e.g. B. neoexilis) and maybe a few Eunotia species too.  As alkalinity increases, so the diatom assemblage will be dominated by Achnanthidium minutissimum and relatives, but we will also see Hannaea arcus and Fragilaria gracilis, amongst other species.   We will see some Achnanthidium and Fragilaria species at low alkalinity, too, but either different to those at moderate to high alkalinity or in lower numbers.

There are several possible explanations for this but Brian Moss, in a classic paper from 1972, suggested that the availability of dissolved carbon dioxide was a major factor.  The “soft water” species, in other words, were better adapted to life without bicarbonate but were out-competed in moderate and hard water where the supply of bicarbonate was greater.   Very roughly, this switch from domination by free carbon dioxide users to bicarbonate users occurs at no more than 20 mg L-1 CaCO3.   There is more going on than just the supply of inorganic carbon: low alkalinity water is more likely to have low pH, which brings a separate set of challenges to aquatic organisms, and very high alkalinity water is often associated with productive agricultural areas.  This means that effects at both ends of the scale may be hard to separate completely from human pressures.  However, the broad story that emerges is that hard rock, in ecology as in music, is not to everyone’s taste.


Moss B. (1973).  The influence of environmental factors on the distribution of freshwater algae: an experimental study. II. The role of pH and the carbon-dioxide-bicarbonate system.  Journal of Ecology 61: 157-177.

Concentrating on carbon …

On the other side of Ennerdale Water I could see plenty more submerged stones, all covered with green filaments but these belonged to different genera to those that I wrote about in my previous post.   Both are genera that we have met previously – Mougeotia, which has flat, plate-like chloroplasts which rotate around a central axis in order to control its rate of photosynthesis – and Spirogyra.  When light levels are low, Mougeotia’s flat chloroplast is perpendicular to the light in order to capture as much energy as possible, but in bright light it rotates so that the plate is parallel to the direction of the light, in order to slow the photosynthesis mechanism down and prevent internal damage (see “Good vibrations under the Suffolk sun” for another approach to this problem).

However, too much sunlight is the least of an alga’s problems in the Lake District.   This post looks at a different challenge facing freshwater algae and our starting point is the spherical nodules, “pyrenoids”, that you should be able to see on the chloroplasts of both Mougeotia and Spirogyra in the images below.   Photosynthesis involves a reaction between water and carbon dioxide to make simple sugars (turning fizzy mineral water into “pop”, in other words).   A submerged alga does not have a problem obtaining the water it needs, but what about carbon dioxide?   Gases are not very soluble in water, so this presents a much bigger problem to the algae.   Explaining why also presents a big problem to a blogger who conscientiously avoided physics and chemistry from age 16 onwards.  Here goes …

Mougeotia from the littoral zone of Ennerdale Water, April 2017.  Scale bar: 20 micrometres (= 50th of a millimetre).

The concentration of a gas in a liquid depends upon the concentration of that gas in the surrounding atmosphere.   As far as we know (and this is still an area of contention amongst geologists), concentrations of carbon dioxide in the deep past were much higher than they are today, in part because there were no land plants to suck it out of the atmosphere for their own photosynthesis.  So the earliest photosynthetic bacteria and, subsequently, algae, lived in water that also had higher concentrations of carbon dioxide.   As land plants spread, so the carbon dioxide concentration in the atmosphere dropped as they used it to fuel their own growth.  As a result, carbon dioxide concentrations in the water also dropped, thus depriving the algae of an essential raw material for photosynthesis.

However, carbon dioxide is not the only source of carbon available to aquatic organisms.   There is also carbon in many rocks, limestone in particular, and this can mineralise to carbonate and bicarbonate ions dissolved in the water.  Aquatic plants can get hold of this alternative carbon supply via an enzyme called carbonic anhydrase.   By concentrating the carbonic anhydrase activity in a small area of the chloroplast, the algal cell can boost the activity of the Rubisco enzyme (which evolved to function at a higher concentration of carbon dioxide).   This whole process is one of a number of forms of “carbon concentrating mechanism” that plants use to turbocharge their photosynthetic engines (see “CAM, CAM, CAM …” on my wife’s blog for more about a terrestrial version of this).

A two-chloroplast form of Spirogyra from the littoral zone of Ennerdale Water, April 2017.  Scale bar: 20 micrometres (= 50th of a millimetre).

Pyrenoids are widespread amongst algae, though a few groups (notably red algae and most chrysophytes) lack them.   Cyanobacteria (blue-green algae) use an organelle called a “carboxysome” for a similar purpose.   The only group of land plants with pyrenoids are the hornworts, relatives of mosses and liverworts.   About half of all hornworts have pyrenoids and a recent study has suggested that the ability to form pyrenoids has evolved up to five times in this group during their evolution.   The appearance of pyrenoids in distinct evolutionary lineages of algae also suggests that there may have been several evolutionary events that precipitated their formation.  And, it is important to stress, some algae which lack pyrenoids have alternative methods of concentrating carbon to enhance Rubisco activity.

So let us end where we started: in the littoral zone of Ennerdale Water on an April morning, gazing at a fine “fur” of filamentous algae clinging to the submerged rocks.   Back in October last year, I talked about how Ennerdale fitted into a pattern of increasing productivity of Cumbrian lakes first noticed by Pearsall in the early part of the 20th century (see “The power of rock …”).   Now we can start to understand that pattern in terms of basic biochemical processes: getting enough carbon from a combination of atmospheric carbon dioxide and the surrounding rocks for Rubisco and the other photosynthetic enzymes to convert to sugars.   In Ennerdale Water, one of the least productive of the Cumbrian lakes, we can see these algae during the winter and spring because the amount of biomass that those biochemical reactions produces is still just ahead of the amount that grazing invertebrates such as midge larvae can remove.  In a month or so, the grazers will have caught up and the rock surfaces will be, to the naked eye at least, bare.

Rubisco is the enzyme whose gene, rbcL, we use for molecular barcoding, subject of many recent posts (see “When a picture is worth a thousand base pairs …”).  My early desire to avoid physics and chemistry at school translated into as little biochemistry as possible whilst an undergraduate and, over the past few-years, I’ve developed a frantic urge to catch-up on all that I missed.   Just wish that those lectures explaining the Calvin cycle had been a little less … tedious …


Giordano, M., Beardall, J. & Raven, J.A. (2005).  CO2 concentrating mechanisms in algae: mechanisms, environmental modulation, and evolution.   Annual Review of Plant Biology 56: 99-131.

Villareal, J.C. & Renner, S.S. (2012).  Hornwort pyrenoids, carbon-concentrating structures, evolved and were lost at least five times during the last 100 million years.  Proceedings of the National Academy  of Science of the USA 109: 18873-18878.


Spring in Ennerdale …

My latest trip to Ennerdale Water, in the Lake District, has yielded its usual crop of spectacular views and intriguing questions (see “Reflections from Ennerdale’s far side”).   This time, my curiosity was piqued by lush growths of green algae at several locations around the lake shore.  The knee-jerk reaction to such growths is that they indicate nutrient enrichment but I am always sceptical of this explanation, as lush green growth are a common sight in spring (see “The intricate ecology of green slime …”) and these often disappear within a month or two of appearing.

Two points of interest: first, the lake seems to be lagging behind the River Ehen, which flows out of Ennerdale Water.   We often see these lush growths of algae on the river bed in winter but by this time of year the mass of algae there is lower than we saw in the lake littoral.   Second, the lake bed looks far worse (see photograph below, from the north-west corner of the lake) than the actual biomass suggests.

Filamentous algae (Ulothrix aequalis) smothering cobble-sized stones in the littoral zone of Ennerdale Water, April 2017.

Under the microscope, this revealed itself to be unbranched filaments of a green algae, whose cells each contained a single band-shaped chloroplast lapping around most of the perimeter.   This is Ulothrix aequalis, a relative of Ulothrix zonata, which I wrote about a few times last year (see link above).   Like U. zonata, this species is very slimy to the touch and, I suspect, the payload of mucilage adds to the buoyancy of the organism and means that we look down on a fine mesh of filaments which trap light and add to the unsightly appearance of the lake bed at this point.   That this part of the lake shore is close to a tributary stream draining some improved pasture triggers some suspicions of agricultural run-off fuelling the algal growths but, looking back at my notebook, I see that the lake bed was almost clear of green algae when we visited this location in July last year.  I suspect that a return visit this summer would also show a clean river bed.  Appearances can often be misleading (see “The camera never lies?”).

Ulothrix aequalis from the littoral zone of Ennerdale Water, April 2017.   Scale bar: 10 micrometres (= 1/100th of a millimetre).

This was not the only site that we visited that had conspicuous growths of green algae, though the mass of algae was greatest here.   All of the sites at the western end had these growths (see “A lake of two halves” for an explanation of geological differences within the lake) but, curiously, the genus of alga that we found differed from site to site.   In addition to Ulothrix aequalis in this corner of the lake, we found Mougeotia on the south side and Spirogyra close to the outfall.  This diversity of forms is, itself, intriguing, and I have never read a convincing explanation of what environmental conditions favours each of these genera.   I see both spatial and temporal patterns of green algae in the River Ehen too and, again, there is no satisfactory explanation for why the species I find can differ along short distances of the river and between monthly visits.

The Mougeotia and Spirogyra both have another story to tell, but that will have to wait for the next post …