The exception that proves the rule …

If you are going to understand river ecology, you need to be able to consider landscapes at several different scales simultaneously.   In the River Ehen, this means looking upstream towards Ennerdale Water and, beyond, to Great Gable and the other Lake District peaks in order to appreciate the geology that gives the catchment its bones.  But, at the same time, you need to look around at the meanders of the river and the bankside vegetation that create the immediate habitat for the organisms, and then to look even more closely at the individual stones that line the river bed.

Peering into the water last week, the pebbles, cobbles and boulders that make up the substratum of the River Ehen looked bare of filamentous algae for the most part.  There were a few clumps but, at this time of year, when grazing invertebrates are active, the algal flora is reduced to a thin film, invisible to the naked eye and apparent only as a slimy sensation when you run your fingers across the stone’s surface.   However, when I picked up a couple of cobbles, I noticed small, pale green gelatinous growths stuck on the upper surface.   Most were just a few millimetres across with the largest up to about a centimetre.

A growth of Draparnaldia glomerata on the upper surface of a cobble in the River Ehen, Cumbria, April 2017.

These growths are composed of the green alga Draparnaldia glomerata.  I have written about this alga before (see “The River Ehen in February”) but, under the microscope, it is such a beautiful organism, that I am not going to apologise for writing about it again.   The alga lives inside the gelatinous mass and consists of a relatively thick central filament from which tufts of narrower side-branches emerge.  The cells that make up these side branches gradually narrow, and the chloroplast becomes smaller until, eventually, the cells form a colourless “hair”.   These hairs are relatively short on the material illustrated below but can be much longer (some longer hairs were present but did not present nicely for photography).  The hairs are, in fact, an adaptation to help the alga acquire phosphorus, something I described in an earlier post about a relative, Stigeoclonium tenue (see “A day out in Weardale”).

Draparnaldia glomerata from the River Ehen, April 2017 showing filaments and side branches. Scale bars: a.: 50 micrometres (= 1/20th of a millimetre); b.: 20 micrometres (= 1/50th of a millimetre).

A low concentration of phosphorus is usually regarded as a Good Thing by aquatic ecologists, as this limits the amount of energy produced  by the plants at the base of the food chain.  This, in turn, means that the microbes and animals that depend on these are not using up all the oxygen in the water, or having other deleterious influences on the ecosystem.   I would usually regard the presence of an organism such as Draparnaldia as a sign of a healthy stream, as it is adapted to thrive when phosphorus is relatively scarce.

I was, however, careful to place “relatively” in front of “scarce”.   Studies by my colleagues (referenced in the earlier post) showed that the production of the phosphatase enzyme that boosts the alga’s ability to acquire phosphorus when it is scarce is determined by the ratio of nitrogen to phosphorus inside the cell itself, rather than in the water.   The physiology of nutrient limitation is all about the balance between the different “ingredients” that a cell needs.   If you have three eggs and 170g of sugar, for example, you can only make one cake, regardless of how much flour you have in your cupboard.   So it is with algae: most of the locations where I find Draparnaldia have very little nitrogen, but even less phosphorus.   There are barely enough ingredients for the algal “cake” so it is advantageous to the organism to pump out some enzyme to order to make up the shortfall.  This means that I can say with confidence that Draparnaldia is usually a good indicator of healthy streams.

Just occasionally, however, I get Draparnaldia in places where I would not usually expect it to be found.   The picture below shows a colleague standing in the Terman River, just before it flows into Lough Erne in Northern Ireland.   She is holding a skein of Cladophora glomerata in her left hand and a skein of Draparnaldia in her right hand.  I associate the former with nutrient-rich rivers where I would not usually expect to find Draparnaldia.  But both were growing prolifically at this site which defied my expectations until I started to think about the physiology of the organism.   Had I had the facilities to analyse the tissues of the algae, I expect that I would have found very high concentrations of nitrogen which, in turn, creates a demand for yet more phosphorus so that it could convert that nitrogen into the proteins it needs to grow.  However, that cannot be the whole story, because normally, under such circumstances, I would expect a competitive alga such as Cladophora to out-compete and overgrow the Draparnaldia.   Here, they were growing side-by-side.   It is, to date, the most luxuriant growth of Draparnaldia that I have seen, and also the only occasion where I have seen these two species co-existing in such abundance.

My colleague, Bernie White, holding skeins of Cladophora glomerata (left hand) and Draparnaldia glomerata (right hand) from the Terman River near Toome.  The border between the Republic of Ireland and the UK runs along the middle of this river.

I can extend my lesson from the first example to say that, to understand the ecology of any particular river you need to have perspectives obtained from many other rivers.   But, in this case, we see a potential limitation: the case of the “rare exception” that clouds an otherwise clear picture of an association between an organism and a particular set of circumstances.   The problem is particularly acute when dealing with the effect of nutrients because we are usually dealing with indirect, rather than direct effects.   Draparnaldia glomerata is usually associated with clean rivers with low concentrations of nutrients but it is not there because nutrient concentrations are low.   As for the diatom Amphora pediculus (see “The challenging ecology of a freshwater diatom?”) a more nuanced understanding of the relationship between an organism and nutrients yields more useful insights than simply assuming a cause-effect relationship.

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.

Reference

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.

Daniel and his den of diatoms …

After contemplating astronomy-without-optics at Ulug Beg’s observatory (see previous post) we walked ten minutes down the road to another of Samarkand’s sights, the mausoleum of Daniyar (the Old Testament prophet Daniel, also venerated by Muslims).  This was a much plainer structure than the polychromatic wonders we had seen elsewhere in the city and, no doubt as a result, fewer visitors.  Daniel’s sarcophagus runs the entire length of the building: the legend is that his severed leg has continued to grow, necessitating an eighteen metre tomb.  The presence of a group of pilgrims, praying with an Iman, reminded me that our word “holiday” is a concatenation of “holy day”, and that pilgrims were the original tourists, both in this part of the world and in Europe.

Our guidebook mentioned an ancient spring on the site, offering me my first opportunity to mix business and pleasure. Unfortunately, the spring was dry but, following the valley up a steep hillside, we reached a graveyard, beside which there was a water trough whose bottom, when I peered inside, was covered with dark brown circular patches, up to about half a centimetre across.  I had not really come prepared for diatom sampling but managed to commandeer an empty water bottle into which I scraped some of these colonies using a piece of plastic that I found lying on the ground nearby.   The water bottle was then stuffed into my rucksack as we continued our explorations, cutting across country towards another set of monuments that we could see on the horizon (Colin Thurbron describes the same journey in reverse in his excellent book The Lost Heart of Central Asia).   Once we were back at our hotel, I let the sample settle overnight, poured off the supernatant and then added an equal volume of local vodka to the remaining suspension.  As in India last year, this is the quickest and least hazardous way of keeping diatoms in reasonable condition when on the road (see “Diatoms from the Valley of Flowers”).

Left: the water trough near Daniyar’s mausoleum, Samerkand from which my diatom sample was collected and, right, the circular colonies of diatoms on the bottom of the water trough.  The largest of these colonies is about half a centimetre across.

Several freshwater diatoms form conspicuous colonies but what intrigued me about these particular growths was that the colonies were disc-shaped, reflecting horizontal growth with little vertical development.  Once back from my travels, I had a look under the microscope and was surprised to see that they were composed of almost-pure growths of either Achnanthidium minutissimum or a very close relative (my observations were on the vodka-preserved specimens and I have not yet had a chance to look at cleaned valves).   This is an extremely common constituent of biofilms all over the world but I have never seen it forming discrete colonies in this way.  I suspect that, given time, all of these colonies would merge to form a continuous biofilm and that, in a natural ecosystem (rather than a water trough), grazing by invertebrates would then control the biomass so that they formed a subaquatic and microscopic “turf”.   Maybe what I am seeing is the early stage of colonisation in a situation where there are, as yet, no grazers?   It is very hard to tell an ecological story from a single, brief visit to any habitat but that would be my opening gambit.

Microscopic views of Achnanthidium minutissimum colonies from the water trough near Daniyar’s mausoleum, Samarkand, April 2017.  The left hand image was taken at x100 magnification and shows a colony (or fragment) that is about 650 micrometres across.  The right hand image was taken at x1000 magnification.   Scale bar: 10 micrometres (= 1/100th of a millimetre).

Achnanthidium minutissimum was not the only familiar plant (in the broadest sense) that we saw on our travels.   The grazed grassland between Daniyar’s mausoleum and Bibi Khanym mosque, our next objective, contained several flowers familiar from home (see Heather’s blog) and our trip to the Tien’shan mountains a few days later revealed many more, including a steep-sided valley full of hawthorn bushes.   It is a good reminder that, however far from home we are, and however exotic our surroundings, we are still in the broad temperate belt around the Eurasian continent that allows a measure of biogeographical continuity across this area.  Some of the plants we saw in the wild in Uzbekistan are garden plants in north-west Europe (the tulip is a good example) but several others thrive in the wild in both places.

Botanising in the grassland between Daniyar’s mausoleum and Bibi Khanym mosque, Samerkand, April 2017.

Decoration on mausoleums at the Shar-i-Zinda complex in Samerkand, near the Bibi Khanym mosque and Daniyar’s mausoleum.  The photograph at the top of this post shows the exterior of Daniyar’s mausoleum.

Reaching for the stars …

One of the more intriguing characters I discovered during our time in Uzbekistan was Ulug Beg (1394 – 1449), grandson of Timur and inheritor of his empire.   Timur consolidated and expanded the conquests of Ghengis Khan and established Samarkand as the capital for an empire that extended from Turkey to northern India.   Ulug Beg, by contrast, was a more peaceful and intellectual character, more interested in science than statesmanship.   He endowed three Madrassas and stipulated a curriculum that extended beyond studying the Koran and Islamic law to include mathematics and astronomy too.   This is reflected in the decoration of the Ulug Beg Madrassas in both Samarkand and Bukhara which include depictions of the night sky and inscriptions extorting the Muslim faithful to aspire to knowledge.

On a hill on the outskirts of Samarkand, the distant snow-clad peaks of the Pamirs serve as a backdrop to Ulug Beg’s observatory, although all that remains now is part of a huge quadrant arc now largely buried.   Using this, Ulug Beg’s astronomers were able to determine the position of celestial bodies with great accuracy, to work out the tilt of the earth’s axis (23.52 degrees) and calculate the length of a year to within a minute of the best modern estimates.   This was accomplished two centuries before Galileo first pointed his telescope to the heavens and, indeed, without any optics at all.

The quadrant arc at Ulug Beg’s observatory in Samarkand.

His is not the only observatory from the Medieval Islamic world that has survived and I was lucky enough to visit the Mughal observatory, the Jantar Mantar (c. 1724) in Delhi last year.  Unlike the Samarkand observatory this one is above ground and has been restored, giving the visitor a much better sense of the scale of the undertaking.  The observatory at Jaipur is also in good condition and was the subject of a fascinating episode of In Our Time, for those readers who are able to access the BBC’s archives.   These were the most sophisticated scientific instruments of their day; now they stand as monuments before which modern scientists should reflect that Western hegemony of science is a very recent phenomenon.   Ulug Beg’s observatory stands as a reminder of science’s deep foundations in ancient wisdom preserved or discovered on the plains of Central Asia.

The Misra Yantra, part of the Jantar Mantar complex of astronomical instruments in Delhi.

Western astronomy’s paradigm shift occurred when people who were trying to understand the cosmos came into contact with with people with practical skills such as glassmaking (a pre-requisite for optics) and metalworking.   The individual elements were all present in the medieval Arab world but the one-off spark of genius that brought them all together just did not happen.  Glass is the least conspicuous of these components in Bukhara and Samarkand until we looked closely at the tiles which decorate their ancient buildings.  The glazes which gives these tiles their lustrous appearance are made from natural pigments mixed with ground glass (“faïence”); however, there was little evidence of extensive working of glass, so an astronomer would have been highly unlikely to have that serendipitous encounter that would have opened new opportunities. Progress is, we have to remember, often a stuttering series of serendipitous events, very few of which have any lasting impact.

What became of Ulug Beg?  He was murdered by his own son and his observatory was destroyed by religious fanatics.   He was, by all accounts, a better scientist than statesman.   The attention to detail that a scientist needs means taking your eye off the big picture.   Reading between the lines, this was Ulug Beg’s undoing and within a generation Timur’s empire had collapsed into a number of independent Khanates.    Over time, Ulug Beg’s discoveries were assimilated into European astronomical thinking and he and his fellow Central Asian polymaths were gradually written out of Western histories of science.   I’m grateful to a new generation of historians such as Peter Francopan (“The Silk Roads: A New History of the World“, Bloomsbury, 2015) who are giving us a less Euro-centric account of progress.  Compare his approach to that of Kenneth Clark in Civilisation (1969), for whom the term “barbarian” seems almost synonymous with non-European, and then look up at the intricate decoration on Samerkand’s madrassas.   Barbarians?  I don’t think so.

A view of the courtyard of the Ulug Beg Madrassa in Bukhara.   The photograph at the top of the article shows the exterior of the Ulug Beg Madrassa in the Registan in Samarkand.

Synchronicity in Samarkand …

I had intended my next post to continue the story of inorganic carbon in freshwater but a holiday has intervened. However, as is often the way with my travel, I have found some unexpected associations with my professional life.

I had wanted to show, using a graph, how much influence inorganic carbon supply (which freshwater biologists refer to, confusingly, as “alkalinity”) had on the types of diatom that are found in rivers. But the simple act of plotting a graph with Excel had, I realised, some unexpected resonances with my current location in Central Asia. I am in Samarkand, in Uzbekistan, a city with a very long history and which numbers Omar Khayyam (1048 – 1141) amongst its previous inhabitants. Omar Khayyam is best known in the West as a poet but was also a noted mathematician and astronomer. Khiva, another ancient city in Uzbekistan, was the birthplace of Muhammad ibn Musa al-Khwarizmi (c.780 – 850) regarded as one of the founders of algebra. Both, in other words, laid the groundwork for the equation y = mx + c, the equation for a straight line that allows me to express the relationship between the diatom assemblage and alkalinity in quantitative terms.

The relationship between the Trophic Diatom Index and alkalinity in a dataset drawn from UK rivers. More about this will follow in a future post but, for now, it is presented as an example of how biological data often fit y = mx + c, the equation for a straight line (indicated by the red line on the graph)

The point of algebra is that you can work out general principles that apply to a situation regardless of the quantities involved. An equation is simply a means of replacing these quantities with letters or symbols so that you can work out the value of something that you don’t know in terms of things that you do know. One of these ancient mathematicians – we don’t know who, but I am giving Uzbekistan the benefit of the doubt – decided to use the Arabic word “shay” (which means “thing”) to represent the unknown in his equations. When the early algebraic treatises were translated to Spanish in medieval times, “shay” became “xay”, which eventually was shortened to “x”. That, at least, is the legend, and no-one seems to have a better explanation. Whenever we use “x” in an equation, in other words, we should reflect that we are part of a tradition that extends back over 1000 years to the plains of Central Asia.

The straight line equation, however, bucks this neat theory to some extent as, in this realm of algebra, “x” represents the known rather than the unknown entity. The unknown, by convention, is indicated by “y”. “Why “y”?” you might ask and, I am afraid, I cannot help. It may be that there is no sensible explanation (“quarks” are, after all, named after a nonsense word in Finnigan’s Wake) but the etymology of “x” is, you have to admit, too good to waste. Especially when writing from Samarkand.

Timurlane’s tomb (Gur-i-Amir) in Samarkand. The photograph at the top of the post shows part of the Registan madrasah complex.

And, finally, I could not resist including this image of decoration on the Sher Dor Madrassa at the Registan: evidence that Medieval Islamic scholars knew about centric diatoms?

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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 …

References

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 …