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.

 

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.

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.

The power of rock …

In my recent post on Ennerdale Water I referred to the interaction between geology and man in shaping the characteristics of a lake (see “A lake of two halves …”).   As I was writing, I had in mind some famous early work on this topic by Harold (“W.H.”) Pearsall, a botanist who made some of the first tentative steps towards linking patterns and processes in lake ecosystems, whilst working at the universities of Leeds and Sheffield.   He had visited many of the lakes since boyhood and co-opted his father as a field assistant to cycle around the Lake District performing the surveys that formed the basis of this paper.

Pearsall had noted differences in the types of plants growing in the various lakes in the region, and attributed these differences to the geology of the surrounding land.   He took this idea one step further by also suggesting that the lakes became modified as they increased in age, illustrating this by arranging the English Lakes into an “evolutionary sequence”, with Wastwater and Ennerdale Water representing the least evolved, and Windermere and Esthwaite Water representing the most advanced.   His first proposition is now well-established amongst those who study lakes; the second is also generally accepted (I remember writing an essay entitled “Lakes are temporary features of the landscape” as part of my A-level Geography course), although his use of the English Lakes to illustrate this is not.

lake_district_after_pearsal

The lakes of the English Lake District, arranged in the evolutionary sequence proposed by Pearsall: 1: Wastwater; 2: Ennerdale Water; 3: Buttermere; 4: Crummock Water; 5: Hawes Water; 6: Derwent Water; 7: Ullswater; 8: Bassenthwaite Lake; 9: Coniston Water; 10: Windermere; 11: Esthwaite Water.

The graph below makes Pearsall’s case, using his own data (note that his records for Hawes Water refer to the small natural lake that was submerged to form the current Haweswater Reservoir).   The left hand axis shows the proportion of land in the catchment of each lake which was under cultivation (at the time of his study) steadily increasing as we move through his evolutionary sequence.   The right hand axis shows how proportion of the shoreline of each lake that was rocky (down to a depth of 30 feet – 9.2 metres) steadily decreases through the sequence.  He pointed out that both the amount of cultivatable land and the character of the shoreline depended largely on the character of the surrounding country.

pearsall_evolutionary_seque

A graphical representation of Table 1 in Pearsall (1921): “Effects of erosion”.  Lakes are arranged in order of Pearsall’s “evolutionary sequence”.

The next graph shows the same sequence of lakes (excluding Hawes Water) but with the average values of the Lake Trophic Diatom Index (TDI) plotted on the Y axis, and with lakes sub-divided into those with low alkalinity (deriving most of their runoff from the Borrowdale Volcanics and associated hard rocks, including the Ordovician granite discussed in the post about Ennerdale) and those with moderate alkalinity (associated with softer rocks to the north and south of the Borrowdale Volcanics).   This confirms the primary role of geology, with Pearsall’s “primitive” lakes underlain by the Borrowdale Volcanics whilst the more “evolved” are associated with the softer rocks.  Within each category there is an upward trend, rather more pronounced in the moderate alkalinity lakes, as we move through Pearsall’s sequence.  I suspect that this represents the interaction between geology and man, with higher TDI values associated with lakes where there is more agriculture and greater population density.   These factors may, in turn, combine to affect the physical factors within the lake over time, but the implication that a “primitive” lake such as Ennerdale Water might one day “evolve” to have characters similar to those of Windermere is no longer accepted.   On the other hand, he did set up some testable hypotheses that kept freshwater ecologists occupied for a long time subsequently.  As Lao Tzu reminded us: “a journey of a thousand miles begins with a single step”…

pearsall_versus_ltdi

Average lake TDI values (using data from Bennion et al., 2014) for Lake District water bodies, arranged by Pearsall’s evolutionary sequence (no data for Hawes Water).   Open circles are low alkalinity lakes; closed circles are moderate alkalinity lakes.

References

Bennion, H., Kelly, M.G., Juggins, S., Yallop, M.L., Burgess, A., Jamieson, J. & Krokowski, J. (2014).  Assessment of ecological status in UK lakes using benthic diatoms.  Freshwater Science 33: 639-654.

Clapham, A.R. (1971).  William Harold Pearsall.  1891-1964.  Biographical Memoirs of Fellows of the Royal Society 17: 511-540.

Pearsall, W.H. (1921).  The development of vegetation in the English Lakes, considered in relation to the general evolution of glacial lakes and rock basins.  Proceedings of the Royal Society of London Series B 92: 259-285.

A lake of two halves …

ennerdale_161005_3

I have started this post in the same way that I started the previous two posts: with one of a series of pictures that I took from Kirkland whilst driving away after fieldwork in Ennerdale Water and the River Ehen earlier in March and noticing the rather spectacular view up the valley. This post, like those, will focus on the microscopic life of the lake but it pays to pause for a moment – as I did on my drive away from Ennerdale – to look at the landscape, and contemplate how the features that are apparent in this panorama shape the properties of a lake that are less obvious to the casual observer.

The picture shows a view across Ennerdale Water towards some of the highest peaks of the Lake District, with Great Gable prominent in the background. What we can also see is a transition: the foreground consists of softer features and more gentle slopes; the background is rugged, steep scree-covered fells. Somewhere, approximately at the point where the hills in the centre left of the picture fall into the lake, the rock type changes. In the foreground, the underlying rock is Ordovician mudstones and sandstones; beyond this, the rocks are formed from a granite intrusion resulting from volcanic activity. This activity also took place in the Ordovician period, but the rock is much harder than the sandstones and mudstones that underlie the foreground.

Most of the features that I have written about in Ennerdale Water are from the zone underlain by the granite but I also visited the north-western end of the lake, where the mudstones and sandstones predominate and the algae that I found attached to the rocks here were conspicuously different. Many of the submerged stones were covered with green filaments which, in turn, were overgrown by diatoms – mostly Tabellaria flocculosa and species of Fragilaria. The green filaments, in turn, had trapped a lot of fine sediments, presumably deriving originally from the sandstones in the catchment. Under the microscope, the green filaments resolved into a mass of Spirogyra filaments, with their distinctive helical chloroplasts, along with Bulbochaete and a few strands of other genera. The algae in this corner of the lake reminded me, in fact, of the algae that I am used to seeing in the River Ehen, just downstream from the lake outfall.

ennerdale_spirgyra_oct16

A submerged cobble in the littoral zone of Ennerdale’s north-western corner (left) with (right) two filaments of Spirogyra at high magnification, each with two ribbon-shaped chloroplasts arranged in helices. Scale bar: 20 micrometres (= 1/50th of a millimetre).

Those of us who study freshwaters know that geology has a big influence on the types of plants and animals that grow in a water body – it is probably the strongest natural factor excluding situations where there is a saline influence. The interesting point about Ennerdale is that geology not only has an effect on the lake as a whole (most of the water deriving from the granitic fells that make up the catchment), but it also has subtle effects around the margin, particularly on those algae that are growing directly on rock surfaces.

But it is not quite as simple as that. Look at the photograph at the top of this post. The foreground – the land underlain by Ordovician mudstones and sandstones – is improved pasture. The topography is such that a farmer can get a tractor onto the fields and spread some manure or fertiliser a couple of times a year which, in turn, means the land can carry more livestock. A little of those nutrients may find their way into the small streams that drain into the lake and this, too, may be having an effect on the algae. On the fells beyond, only rough grazing is possible. In other words, however hard we try to separate the effect of man from natural factors, we also have to remember that the landscape, itself, shapes the way that man uses the land. And that, in turn, influences the ecology of the lake.

I should emphasise that the algae in the north-west corner of Ennerdale Water do not suggest any malign effects from those parts of the catchment that drain into the lake here. My point is just that they are different and that the change in geology along the lake may be one factor driving this difference. It is quite subtle, the water that flows into the lake is soft and it is only very slightly less soft near the outfall. But it is enough to have an influence on the ecology of the organisms that live around the edge of the lake. The story of the lakes of the Lake District has told in terms of the rocks that form each of their catchments. What is interesting in Ennerdale Water is that we can see some of those effects of geology within a single water body.

 

Tales from the splash zone …

ennerdale_161005_2

Mougeotia was not the only alga that intrigued me in Ennerdale Water during my recent visit (see “Fifty shades of green …”).   Alongside the green tufts, and also just at water level, there were dark spots and patches on the rock that yielded to a gentle scrape with my finger nail.   The colour suggested Cyanobacteria, so I popped a little into a sample bottle to examine later.

stigonema_ennerdale_oct16

Patches of Stigonema mamillosum and Scytonema cf crustaceum growing at water level on granite boulders on the southern shore of Ennerdale Water, October 2016.   The scale bar is approximately one centimetre.

The surprise, when I looked down my microscope, was not that it was cyanobacteria, but that there were at least three genera mixed together.   The first of these was Scytonema cf crustaceum, characterised by a thick brown sheath and the presence of double “false branches”, formed when both ends of a broken filament continue to grow and, eventually, burst out of the sheath (see “Poking around amongst sheep droppings”).   In the image below you can see the narrow blue-green filament of cells within the much broader sheath.

Also present was Stigonema mamillosum, a representative of a genus with a more advanced morphology than other Cyanobacteria, with branched filaments that can be several cells thick (see “More from the River Atma”), and Calothrix sp., which has tapering filaments in a much thinner sheath.   All three genera have the capability to fix atmospheric nitrogen, so thrive in nutrient-poor habitats such as Ennerdale (see also “Both sides now …”).   Calothrix, in addition, is able to scavenge phosphorus from the water, releasing enzymes from the long colourless hairs (just about visible to the right of my photograph).

scytonema_crustaceum_oct16_

Scytonema cf crustaceum from the littoral zone of Ennerdale Water, October 2016.   Scale bar: 20 micrometres (= 1/50th of a millimetre).

stigonema_mamillosum_oct16_

Stigonema mamillosum and Calothrix sp from the littoral zone of Ennerdale Water, October 2016.   Scale bar: 20 micrometres (= 1/50th of a millimetre).

I found superficially-similar growths on rocks on the north east side of the lake, but it was clear, even from the appearance in my sample bottle, that this was something different.  The tangles of filaments from the southern shore of the lake, where I had started, had no other form when suspended in water, than an amorphous blob.  However, the material from the north-east side formed distinct “tufts”.   The superficial similarities continued when I peered down the microscope: once again the chains of blue-green cells were enclosed within a thick brown sheath and, once again, there were false branches.  This time, however, the false branches were single, not double, and formed acute angles with the “parent” filament, rather than the near perpendicular double false-branches that we saw in Scytonema.   These features are characteristic of Tolypothrix (Brian Whitton suggests T. distorta) and it is these acute branches that impart the “bushy” appearance to the colony.   Like the cyanobacteria that I found on the southern shore, Tolypothrix is capable of nitrogen fixation so, its presence here is confirmation of the nutrient poor status of the lake.

tolypothrix_ennerdale_oct16

Tolypothrix distorta (var. penicillata?) from the littoral zone of Ennerdale Water, October 2016.  a: low power view of a tuft of filaments (approximately 5 mm in length); b: filaments showing single false branching (x100 magnification); c: medium power (x400) view of false branch.   Scale bar: 20 micrometres (= 1/50th of a millimetre).

Nitrogen-fixation involves busting apart the strong bonds of atmospheric nitrogen in order that the cell can use the nitrogen to build the proteins that it needs to function.  This requires a lot of energy and, as a result, the investment is only worthwhile if other sources of nitrogen are very scarce.   That energy could, otherwise, be diverted to more useful purposes.  The presence of so many different types of nitrogen-fixing organism around Ennerdale is sending out a clear sign that this is a nitrogen-poor habitat.  Algae such as Mougeotia cannot fix nitrogen, and they presumably have to make other sacrifices (a slower growth rate, perhaps?) in order live alongside these Cyanobacteria.   As far as I know, the energy costs of scavenging phosphorus from organic compounds in the water has not been calculated but the same principle must apply: the cell has to create more of the phosphatase enzymes than normal, in order to produce a surplus that can leak through the cell membrane and react with organic molecules in the vicinity.   Again, that all requires energy that can be used for other purposes.  In contrast to nitrogen fixation, this is an ability that Cyanobacteria share with some other algae including, possibly, Mougeotia.

Finding these algae in a one of the most remote lakes in the country, where the impact of humans is very low, I start to wonder how many of our other lakes would have had such an assemblage of organisms before agricultural intensification and the rise in population numbers.   Nature is, naturally, parsimonious in the way it distributes the inorganic nutrients plants need.   Necessity, we are told, is the mother of invention and the diversity we see in near-pristine habitats such as Ennerdale Water is as much the result of plants and algae finding their own individual solutions to grabbing their share of the scant resources available.   There’s enough here for a BBC natural history documentary … apart from an anthropomorphic mammal or bird.  Which is another way of saying … no chance …