Damp days in search of desmids …

Seatoller, in Borrowdale, is the wettest place in England, so we should not have been surprised by the persistent drizzle that accompanied us as we set off hunting for desmids last week.  The combination of Borrowdale’s hard volcanic rocks and a damp climate combine to create ideal habitats for bog-loving desmids and I had intelligence that Dock Tarn, on the fells above Borrowdale, was a hot spot of desmid diversity.   Getting there, however, was no easy task.  Though just a couple of kilometres from Stonethwaite on the map, there were an awful lot of contour lines awfully close together between the beginning and end of our walk.   The footpath zig-zagged through ancient woodland clinging to a steep hillside until we emerged onto the moorland above.  We then made our way across a plateau covered with heather moorland until we saw the tarn stretching away into the mist in front of us.

You know you are in good desmid habitat when there is water percolating into your body from both ends: rain dripping down from the hood of your cagoule and dampness seeping in through your shoes.  They are organisms that love marshy, boggy conditions, especially in areas where the water is as soft as it is here.   The alternative to damp feet would be to either climb up from Borrowdale in Wellingtons or waders or carry them up that steep hillside in a rucksack.   However, I suspect that the mud at the bottom of the tarn was too soft and deep for Wellington boots and lugging waders up that hillside would have been hard work so damp feet was the price I had to pay.   I leaned out as far as I could from the shore to grab some of the sedge stems which had a visible coating of attached algae, and also squeezed the peaty water from a few handfuls of Sphagnum that I pulled from a boggy pool.  That would have to do on this particular morning as the rain was now soaking through my trousers and, in any case, there were places I needed to be later that morning.   I shoved the bottles containing my samples into my rucksack and followed the path back down the hillside.

Epiphytic algae growing around a sedge stem in the outflow of Dock Tarn, Cumbria, July 2017.   The width of the stem plus epiphytes is about half a centimetre.

Dock Tarn is one of a number of sites identified as an “Important Plant Area” (IPA) on the basis of the rich desmid flora, largely due to work over the years by David Williamson.   It qualifies as an IPA on four criteria: the presence of threatened species, high diversity, a long history of study and because it represents a “threatened habitat”.   David Williamson has recorded over 50 species from this location, 13 of which are candidates for a “potential Red Data List”.   A few of these are illustrated in the figures below.   One of the species in the first image, Haplotaenium minutum, belongs to a genus only recently separated from Pleurotaenium, which looks very similar to the untrained eye (the difference lies in the structure of the ridges on the chloroplast).  Looking at these long cylindrical cells serves to emphasise just how much dexterity Chris Carter needed to produce his Hilda Canter-Lund prize winning image.  Images in the second plate include two more species of the genus Xanthidium, which we met in “Desmids on the defensive …”.

Dock tarn desmids: a. Netrium digitus var. latum; b. Tetmemorus brebissonii; c. Haplotaenium minutum.  Scale bar: 25 micrometres ( = 1/40th of a millimetre). 

The desmids in the lower plate, in particular, show one of their key characteristics very clearly: their cells are divided into two distinct lobes (“semicells”) joined by an isthmus (the word desmid comes from the Greek desmos, meaning “bond”).  The image of Staurastrum manfeldtii var. productum also shows a number of bacteria growing on the cell: these are probably growing within the mucilage that desmids secrete around themselves whilst there are distinct pyrenoids in the two Xanthidium species.  Their predilection for soft water means that they need the carbon-concentrating mechanisms that these contain if they are to thrive.   Not all desmids live in water as soft as this, and some are able to use inorganic bicarbonate to fuel their photosynthetic engine, but there will be little or no bicarbonatae in a habitat such as Dock Tarn.   I wrote about these carbon concentrating mechanisms in algae from Ennerdale Water (see “Concentrating on carbon …”) and the two filamentous algae that featured in that post, Mougeotia and Spirogyra, both belong to the same class within the green algae as the desmids (Conjugatophyceae or Zygnemtetophyceae).

There will be more about desmids on this blog over the next few months in preparation for a the weekend of 15-17 September when I am organising a joint meeting of the British Phycological Society and Quekett Microscopical Club in Windermere.  We’ll be visiting some other Lake District tarns known to be rich in desmids during this weekend and have Dave Johns and Allan Pentecost on hand, amongst others, to offer expert advice on what we find.  There are still a few places left, so hurry up to book your place.  I haven’t done a great job of selling the Cumbrian climate in this post but we have the use of the Freshwater Biological Association facilities, including a laboratory and the library, so no one need get damper than they want.   See you there…

More desmids from Dock Tarn: d. Euastrum cuneatum; e. Xanthidium cristatum var. uncinatum; f. Xanthidium antilopaeum; g. Staurastrum manfeldtii var. productum.   Scale bar: 25 micrometres
( = 1/40th of a millimetre). 

References

Coesel, P.F.M. (1994). On the ecological significance of a cellular mucilaginous envelope in planktic desmids. Algological Studies 73: 65-74.

Kiemle, S.N., Domozych, D.S. & Gretz, M.R. (2007). The extracellular polymeric substances of desmids (Conjugatophyceae, Streptophyta): chemistry, structural analyses and implications in wetland biofilms. Phycologia 46: 617-627.

Spijkerman, E., Maberly, S.C. & Coesel, P.F.M. (2005).  Carbon acquisition mechanisms by planktonicdesmids and their link to ecological distribution. Canadian Journal of Botany 83: 850–858.

 

A tale of two diatoms …

I’ve been writing about the River Ehen in Cumbria since I started this blog, sharing my delight in the diversity of the microscopic world in this small river along with my frustrations in trying to understand what it is that gives this river its character.   We know that the presence of a weir at the outfall of Ennerdale Water has a big influence so, in 2015, we started to look at a nearby stream, Croasdale Beck (photographed above), which is similar in many respects but lacks the regulating influence of a lake and weir.  Maybe, we reasoned, the differences we observed would give us a better understanding of how the regulation of flow in the River Ehen influenced the ecology.

Broadly speaking, any kind of impoundment – whether a natural lake or an artificial reservoir – removes a lot of the energy from a stream that might otherwise roll stones, move sediment downstream and, in the process, dislodge the organisms that live there.   We noticed quite early in our studies, for example, that Croasdale Beck generally had less algae growing on the stones than in the nearby River Ehen, and also that the algal flora here was less diverse.

There were also some quite big differences in the algae between the two streams.  I wrote about one of the Cyanobacteria that are found in Croasdale Beck in “A bigger splash …” but there are also differences in the types of diatoms found in the two streams.  Most diatomists think about ecology primarily in terms of the chemical environment within which the diatoms live but I think that some of the differences that I see between the diatoms in the River Ehen and Croasdale Beck are a result of the different hydrological regimes in the two streams.

Several diatom species are common to both streams but two, in particular, stand out as being common in Croasdale Beck but rare in the River Ehen.  These are Achnanthes oblongella (illustrated in “Why do you look for the living amongst the dead?”) and Odontidium mesodon.  However, a closer look at the data showed that, whilst both were common in Croasdale Beck, they were rarely both common in the same sample.   If Achnanthes oblongella was abundant, then Odontidium mesodon was rare and vice versa, as the left hand graph below shows.   There were also a few situations when neither was abundant.

Odontidium mesodon from Croasdale Beck, Cumbria, July 2015.  Photographs by Lydia King.

The story got more interesting when I plotted the relative proportions of these two taxa against the amount of chlorophyll that we measured on the stones at the time of sample collection (see right hand graph below).   This gives us an idea of the total biomass of algae present at the site (which, in this particular case, are dominated by diatoms).   Achnanthes oblongella was most abundant when the biomass was very low, whilst Odontidium mesodon peaked at a slightly higher biomass, but proportions of both dropped off when the biomass was high.   I should point out that “high” in the context of Croasdale Beck is relatively low by the standards of other streams that we have examined and this adds another layer of complexity to the story.

When the biomass exceeds two micrograms per square centimetre, both Odontidium mesodon and Achnanthes oblongella are uncommon in the biomass, and the most abundant diatoms are Achnanthidum minutissimum, Fragilaria gracilis or, on one occasion, Cocconeis placentula.   A. minutissimum and F. gracilis are both common in the nearby River Ehen but C. placentula is very rarely found there.

The difference between River Ehen and Croasdale Beck is probably largely a result of the very difernt hydrological regimes, though this is an aspect of the ecology of diatoms that has been studied relatively rarely.   The differences within my Croasdale Beck samples is probably also a result of the hydrology, but reflects changes over time.   I suspect that Achnanthes oblongella is the natural “pioneer” species of soft-water, hydrologically-dynamic streams, and that Diatoma mesodon is able to over-grow A. oblongella when the biomass on stones increases due to prolonged periods of relative stability in the stream bed.  That still does not explain what happens when biomass is high and neither are abundant: the dataset is still small and we need to collect some more data to try to understand this. But the point of the post is mostly to remind everyone of the dangers of trying to interpret the ecology of attached stream algae solely in terms of their chemical environment.   And to make the point that a little more understanding of a natural system often fuels, rather than removes, the sense of mystery that is always present in nature.

a. The relationship between representation of Achnanthes oblongella and Odontidium mesodon in samples from Croasdale Beck between May 2015 and January 2017. Both axes are presented on square-root-transformed scales; b. relationship between representation of Achnanthes oblongella and Odontium mesodon and total epilithic biomass (as chlorophyll a). Lines show a locally-weighted polynomial (LOESS) regression fitted to the data.

Taxonomic note

Odontidium mesodon is the correct name for Diatoma mesodon (see “Diatoms from the Valley of Flowers”).   The name Odontidium had fallen out of popular usage, but Ingrid Jüttner and colleagues made the case to resurrect this genus for a few species that would hitherto have been classified in Diatoma.

Achnanthes oblongella, by contrast, is definitely not the correct name for this organism.  Three other names have been proposed: Karayevia oblongella, Psammothidium oblongella and Platessa oblongella.  The first two are not convincing and I have not yet been able to see the paper describing the third.  It will be interesting to see what a combined morphological and genetic study of this species (or, more likely, complex) reveals.

Reference

Jüttner, I., Williams, D.M., Levkov, Z., Falasco, E., Battegazzore, M., Cantonati, M., Van de Vijver, B., Angele, C. & Ector, L. (2015).  Reinvestigation of the type material for Odontidium hyemale (Roth) Kützing and related species, with description of four new species in the genus Odontidium (Fragilariaceae, Bacillariophyta).  Phytotaxa 234: 1-36.

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

 

Escape to the Howgills

Driving from my home in Durham towards the south eastern side of the Lake District or to Lancaster leads me across the A66 before I turn off and descend through the Eden Valley and Kirkby Stephen before entering the Lune valley where I join the M6, which follows the course of the Lune through the narrow gap between the high hills of the Howgills and the Winfell Ridge.  It is one of the most spectacular stretches of motorway in the country and I yearn for occasions when I do not have to rush past these hills in pursuit of deadlines.   Those chances do not come very often and, when they do, the weather is not always conducive to walking at high altitude.  However, last Friday, the gods smiled on me: the weather was perfect and I had nothing to pull me back across the Pennines and every excuse to linger.   I pulled off the M6, followed the A684 into Sedbergh and, just 15 minutes after I left the motorway, I was locking my car and following a footpath onto the fells.

There is something about the geology of the Howgills that sets them apart from the hills around them: the Lake District peaks have hard, jagged outlines whilst the Pennines reflect the tilted beds of Carboniferous limestone and sandstone.   The Howgills, however, have soft, convex outlines.  They are of Silurian sandstone though why this should give them such a different topography to the surrounding areas, I do not know.  From a distance they resemble a herd of recumbent cattle and it is no surprise that the highest peak – to which I was heading – was The Calf.

The convex form of these hills means that the first part of the walk is hard work and I had to pause at intervals to look down on s to look back at the small market town of Sedbergh below me and, beyond, the westernmost extremities of the Yorkshire Dales.   As I gained altitude, however, the slope gradually lessened and I was soon on an undulating, but gradually rising, grassy ridge heading north with just a few sheep for company.   The closely-cropped springy turf made for comfortable walking but the absence of wild flowers amidst the grass reminded me of George Monbiot’s phrase “sheep-wrecked”.   Apart from these sheep, I had the fells almost to myself, passing just half a dozen other walkers in three hours.

The summit of the calf is marked by a triangulation point, which offered the culmination of a series of outstanding views.  To the south west, I could see the northern end of Morcambe Bay glistening in the late afternoon sunlight.  Letting my eyes move northward from here, I could see the peaks of the Lake District laid out before me: Old Man of Coniston, Scafell Pike and Great Gable, Helvellyn and, in the far distance, Blencathra.  Then, continuing my panorama across the Eden Valley, I saw the sharp outline of the Pennines with, just discernible, the crenellations that marked Cross Fell, Great Dun Fell and Little Dun Fell.   Far below, I could just see the M6 snaking through the valley below, where drivers, no doubt, were gazing wistfully up at the hills just as I had done so often in the past.

Eventually, I tore myself away from the top of the Calf and followed the path back towards Sedbergh.  It was early evening as I rounded Winder, the first (or final, depending on your direction) undulation on the ridge.   Below me, I could make out activity on the fields of Sedbergh School, and could hear the distant cheers of spectators to what may have been a tug-of-war contest.   The summit of Winder is, I have been told, the turning point for the school’s cross-country run; it is a school with a ferocious reputation for sport, as I could hear.   Ironically, the town’s other claim to fame is its association with the foundation of the Quakers, the religious group whose views mostly closely align with my own.  Their founder, George Fox, preached both in the churchyard of St Andrew’s church below me, and on the nearby Firbank Fell, and the meeting house at Brigflatts, just outside the town, is the second oldest in the country.

The 12 kilometre loop took me about three hours and I was sitting in lengthening shadows outside the local fish and chip shop (the “Haddock Paddock”) sipping shandy from a can and enjoying the last of the afternoon’s sun.   And then it was back into the car for the drive across to the Eden Valley and finally onto the A66 to cross the Pennines.   It’s a tough commute.   But you shouldn’t feel too sorry for me…

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.

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 …

Desmids on the defensive …

ennerdale_bog_pool_jan17

I made a short diversion back to the car after sampling at Ennerdale’s south-eastern end (see “Reflections from Ennerdale’s Far Side …”) crossing the boggy land behind the gravel spit and dipping into one of the pools to pull out a handful of submerged Sphagnum in the hope of finding some desmids, a group of algae that I have not looked at for some time (see “Swimming with desmids …” for my most recent post on this group).

Squeezing the water from a handful of Sphagnum from a bog pool into a vial and allowing the contents of this water to settle is usually a reliable way of collecting desmids; however, on this occasion the haul was rather meagre.  There were plenty of diatoms, but desmids were sparse and limited to a few Pleurotaenium and Euastrum species and some rather impressive cells of Xanthidium armatum.

The distinctive feature of the genus Xanthidium is the bristling armoury of spines around the margins.  The arrangement of spines varies between species and X. armatum has one of the most impressive collections, with bundles of three or four short spines at each angle.   The photograph below does not really capture the depth of the cell, and it is also not possible to see that there are two “decks” of marginal spines, but also bundles of spines on the top surfaces as well as at the margins.   This is truly a man-of-war amongst desmids.

xanthidium_armatum_ennerdal

Xanthidium armatum from a boggy pool at the south east end of Ennerdale Water, January 2017.  Scale bar: 10 micrometres (= 1/100th of a millimetre).  The photographs at the top of this post show the pool from which the sample was collected.

I’m intrigued by desmids but do not claim great competence with the group, so this is a good place to advertise a field meeting organised jointly by the British Phycological Society and the Quekett Microscopical Society.   We will be using the Freshwater Biological Association beside Windermere as our base but heading out to various desmid-rich locations in the Lake District over the course of the weekend.  There will be opportunities to look at other groups of algae too, but desmids will be the main focus of our weekend.  David John of the Natural History Museum will be helping with this group, but there will be experts on other groups available too.  If you are interested in coming, let me know and I will keep you informed as the programme evolves.