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 …

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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.

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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 …

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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 …

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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).

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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 …

Fifty shades of green …

ennerdale_161005

Last week took me back at Ennerdale Water in the Lake District to see how the algae in the littoral zone had changed since my previous visit (see “Both sides now …”).   Back in July, we had found very few algae visible with the naked eye at most of the sites around the margin that we visited; three months on, the situation is very different, with obvious growths at many locations.  As Ennerdale is a remote lake with few human influences, any changes we see are likely to be the result of natural processes rather than “pollution”, so that makes the rapid increase in quantity of algae very intriguing.

One location was particularly intriguing: it was on the south west shore, where the steep scree-laden slope of Crag Fell enters the lake.  The littoral zone has some large stable boulders washed by waves blown down the lake from the high fells to the east.   The boulders had a covering of mosses on their upper surface and this moss, in turn, had been colonised by green algae.

Under the microscope, these growths were revealed to be the filamentous green alga Mougeotia, a relative of Spirogyra, which I have written about in a number of previous posts (it is often common in the River Ehen, for example, which flows out of Ennerdale: see “The River Ehen in February”).   The curious aspect of this particular population was that there were signs of sexual reproduction.   Mougeotia, along with Spirogyra and many other filamentous green algae, is usually observed in the vegetative state (see “The River Ehen in March” and “The perplexing case of the celibate alga”).

mougeotia_ennerdale_oct16_

Boulders in the splash zone of Ennerdale with growths of Mougeotia over mosses (left) and growing directly on the rock surface (right).   The top photograph shows a view from Kirkland across Ennerdale with Great Gable in the background.

mougeotia_conjugating_1

Filaments of the green alga Mougeotia in an early stage of conjugation, with papilla growing from the lower filament towards the upper one.   Scale bar: 20 micrometres (= 1/50th of a millimetre).  

Conjugation involves cells in two adjacent filaments developing outgrowths (“papilla”) that meet and fuse, creating a copulation canal between the two cells.   The cell contents (“protoplasts”) of both cells contract and then they both moves, amoeba-like, into the canal where they fuse  to form a zygote.

The image above suggests that the upper filament may be playing hard to get, rebuffing the amorous advances of the lower filament.   I don’t know enough about conjugation of these algae to know whether the enthusiasm for sex differs between filaments, but it is also possible that what I photographed is an artefact of filaments that may well have been establishing cosy relationships with neighbours before being dragged first from the lake and then onto a slide for my voyeuristic pleasure.   What may have been, in Ennerdale, a patchwork of stable relationships between filaments becomes, amidst the chaos of sampling and slide preparation, a picture of phycological bacchanal.

The lower picture shows a later stage of conjugation, with a zygote forming in the copulation canal.  The process takes place in three dimensions and it was difficult to obtain a crisp image, even using Helicon Focus stacking software but it gives an idea of what is taking place.  The zygote will, eventually, form a tough exterior wall and sink to the bottom of the lake where they will survive until conditions become favourable again.

mougeotia_conjugating_2

Filaments of Mougeotia at a later stage of conjugation: the cell contents are in the process of fusing to form a zygote.   Scale bar: 20 micrometres (= 1/50th of a millimetre).  

The question I have been asking myself is why this particular population has chosen to conjugate at this particular time and place.   I have visited the River Ehen regularly since 2012 and have found Mougeotia or relatives on almost every visit, yet this is the first time that I have seen conjugation.   There are various theories: low nitrogen concentrations have been suggested as something that promotes conjugation in Mougeotia’s relative Spirogyra, but this is unlikely to be a factor in a nutrient-poor lake such as Ennerdale.  A more likely explanation may be found in the graph below, which shows lake levels in Ennerdale over the past year.

ennerdale_lake_levels

Lake levels in Ennerdale Water (from www.riverlevels.uk, measured at NY 088 153, near the outflow to the River Ehen) for the year preceding our visit in October 2016.  

The alga had been growing, remember, in the splash zone.  If you look at the graph, you will see that the lake had recently been almost 30 centimetres higher than it was now and, indeed, had fluctuated quite a lot over the past month or two.   My suspicion is that falling lake levels, and the accompanying risk of drying out, may also have been a factor for initiating conjugation.  Another possibility is that this is a seasonal occurrence that I was fortunate enough to stumble upon, and there is some evidence that dormancy is related to temperature, possibly allowing the zygotes to overwinter in the bottom muds before the increased solar radiation in the spring initiates germination, followed by meiosis (reduction division) to produce the germlings from which next season’s filaments will grow.

Ellerbeck and Ellerbeckia

In the post I wrote just after John Lund’s death had been announced (see: “John Walter Guerrier Lund (1912-2015)”), I mentioned that there was a diatom genus named after his house in Ambleside. As I was in the area, I thought I would pay a quick visit so that I could put a picture of Ellerbeck, the house, alongside images of Ellerbeckia, the genus.   I walked down off Loughrigg Fell, through Ambleside and onto the road that leads out towards Kirkstone Pass. A left turn onto, Sweden Bridge Lane followed by a right onto Ellerigg Road brought me, a couple of minutes later, to Ellerbeck, the last of a row of stone cottages right at the edge of the village.

Set on a hillside and surrounded by garden plants, Ellerbeck was not an easy house to photograph, so forgive the odd perspective in the picture below.   The gardens around Ellerbeck are, I imagine, quite wonderful in the summer, though today was not a day to linger.

Ellerbeck_Ambleside

Ellerbeck: the home of John and Hilda Canter-Lund in Ambleside, Cumbria, photographed May 2015.

The next pictures show Ellerbeckia arenaria, the only representative of the genus found in the UK.   First there is a colony of live cells; after this, I have included some views of cleaned valves.   It is, as you can see, a large, heavily silicified valve with a distinctive cross-hatched pattern on the mantle.  The cells are joined together to form long chains, which often stay together even after the cells have been cleaned with oxidising agents.   One interesting feature of Ellerbeckia that is not easy to see with the photographs here is that the two valves that make up the cell wall are different from one another. One has a convex face, whilst the other has a concave face.   The radial markings on the valve face also differ, so that the “ridges” on one knit with the “grooves” on the next.   This may explain why the colonies are so resilient compared to, for example, Melosira varians (see “Fertile speculations”).

Ellerbeckia_arenaria_CFC

Ellerbeckia arenaria, photographed by Chris Carter.

There is an irony to Ellerbeckia, the genus, being named after a house surrounded by the soft waters of the Lake District in northern England. Looking at my database, I noticed that most of my records were from hard waters in the south, including several chalk streams. I have found it in Cassop Pond, near my house, which is at the foot of the Permian limestone escarpment, but I would not expect to find it in the softer waters of the Lake District.   On the other hand, my old copy of West and Fritsch (1927) says it “occurs on wet rocks, sometimes forming crisp mat-like masses on dripping sandstone, and is common on the Brit[ish] Carboniferous sandstone.”   Maybe I’m just not looking in the right places.

Ellerbeckia_arenaria_112045

Cleaned valves of Ellerbeckia arenaria, from the Great Stour (Kent), Ripper’s Cross, May 2011.

Reference

Crawford, R.M. (1988) A reconsideration of Melosira arenaria and M. teres, resulting in a proposed new genus. pp. 413-433. In: Algae and the Aquatic Environment, edited by F.E. Round. Biopress, Bristol.

West, G.S. & Fritsch, F.E. (1927). A Treatise on the British Freshwater Algae.   Cambridge University Press, Cambridge.