Lost in detail?

In my first post of 2017, I wrote “I fear, microscopic benthic algae may be ecology’s Sirens, sitting on submerged rocks and luring the unsuspecting into a world of taxonomic detail that is too rarely accompanied by profound ecological insight” (see “Not so bleak midwinter?”).   That was a post about Ennerdale Water and the River Ehen and I return to that same remote and beautiful part of north-west England to put some flesh onto the bones of that statement.

I chose a slide Ennerdale Water for the latest “ring test” which tests the competence of the analysts involved in routine ecological assessments using diatoms.   Everyone analyses the same slide and sends their results to me and I sort through and note areas of disagreement.   In this case, there was quite a lot of disagreement even amongst the experienced analysts but, as if to prove the point in my opening paragraph, this did had only a small effect on the conclusions that people reached on the quality of the ecosystem that the sample represented.

One of the areas of disagreement was a population of Brachysira that some described as Brachysira brebissonii whilst others suggested it was B. intermedia.   My inclination, following the illustrations in Hofmann et al’s Diatomeen im Süsswassser-Benthos von Mitteleuropa, was to call it B. intermedia as the illustrations of B. brebissonii show more broadly-rounded ends than were apparent in the Ennerdale population.  However, Bryan Kennedy, a member of our scheme, has been looking into the taxonomy of this genus as part of his PhD and directed me towards a paper by Bart van der Vijver, in which he had examined the type material for B. brebissonii (as “Navicula brebissonii Brébisson).  His illustrations show a population with more acute ends than Hofmann et al.’s illustrations.  Its shape is, in fact, much closer to that of the examples of B. intermedia that they illustrate.  Yet, we were all agreed, this taxonomic ambiguity had little effect on the interpretation that an ecologist would reach.  Most Brachysira species (with the curious exception of the type species, B. aponina) are associated with circumneutral to slightly acid water and low nutrients.   The species of the genus vary in their preference for alkalinity and hardness but, generally, indicate high quality conditions.

Another of the species present in the same sample was Frustulia crassinervia, which presents an interesting counterpoint to the situation I described for Brachysira brebissonii.   In this case, there was another possible candidate, Frustulia saxonica and, in fact, the rhombic-lanceolate outline of the Ennerdale Water population did suggest this species.  However, the clearly protracted ends and the size are more characteristic of F. crassinervia which was the name that the majority of experienced analysts opted for.   Once again, however, both species have similar ecological requirements: soft, often peaty water with low levels of enrichment.

Frustulia crassinervia from the south-east end of Ennerdale Water (circa NY 127 140), July 2016, scale bar: 10 micrometres (= 1/100th of a millimetre).  The top illustration shows Brachysira brebissonii from the same location.   Photographs by Lydia King.

The difference between the situation for Brachysira and Frustulia is that, in the case of Frustulia, the traditional morphological taxonomy is underpinned by molecular studies whereas, in the case of Brachysira, we only have morphological evidence on which to base names.   The interesting point is that the molecular studies of Frustulia suggest that there is no genetic difference between F. crassinervia and F. saxonica.   This alone does not mean that they are not separate species (they did not look at the whole genome), but it does suggest that more work is needed before we have a full understanding of the limits of the species in this genus.   A similar study on Brachysira (and, indeed, on any genus with a surfeit of recently described species separated purely on morphological criteria) might emphasise that differences in shape to which traditional taxonomists assign so much importance are real or it might not.   That would, at least, give people such as myself who use diatoms to gain a wider perspective of ecological health a better insight into where we really need to put in time and effort to discriminate between species.   In the post I mentioned at the start of this article I referred to the nineteenth century scientist Alexander von Humbolt and his concern that scientists got so bogged down in detail that they missed the big picture (“naturgemälde”). Some things never change …

Reference

Hofmann, G., Werum, M. & Lange-Bertalot, H. (2011).  Diatomeen im Süßwasser-Benthos von Mitteleuropa.  A.R.G. Gantner Verlag K.G., Rugell.

Lange-Bertalot, H. & Moser, G. (1994).  Brachysira.  Monographie der Gattung.  Bibliotheca Diatomologica 29: 1-212.

Urbánková, P., Scharfen, V. & Kulichová, J. (2016).  Molecular and automated identification of the diatom genus Frustulia in northern Europe.  Diatom Research 31: 217-229.

Van der Vijver, B. (2014).  Analysis of the type material of Navicula brachysira Brébisson with the description of Brachysira sandrae, a new raphid diatom (Bacillariophyceae) from Iles Kerguelen (TAAF, sub-Antarctica, southern Indian Ocean).  Phytotaxa 184: 139-147.

Veselá, Urbánková, P.,Černá, K. & Neustupa, J. (2012). Ecological variation within traditional diatom morphospecies: diversity of Frustulia rhomboides sensu lato (Bacillariophyceae) in European freshwater habitats.  Phycologia 51: 552-561.

Note:

In an earlier post about diatoms from this location (see “Reflections from Ennerdale’s far side …”) I showed some images of live cells of Stenopterobia sigmatella but added a comment to say that there was a very similar species, S. densestriata, that I could not rule out on the basis of observations of live cells alone.   I now have had the opportunity to have a looked at cleaned material as well and can confirm that the population in Ennerdale Water is, indeed, S. sigmatella.  S. sigmatella has less than 24 striae in 10 micrometres whilst S. densestriata, as the name suggests, has more (> 26 / 10 micrometres).   S. densestriata is also shorter (< 110 micrometres) and narrows to more acutely-rounded ends.

We also found a few valve of S. delicatissima, the other member of the genus that has been recorded from the UK.

Stenopterobia spp. from Ennerdale Water, July 2016.  i.: part of a cleaned valve of S. sigmatella; j. S. delicatissima.   Scale bar: 10 micrometres (= 100th of a millimetre).  Photographs by Lydia King.

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.

Reflections from Ennerdale’s Far Side …

ennerdale_far_end_170105

Ennerdale Water is, as I have described in earlier posts, is a lake of two halves, with a south eastern end influenced by granite and the north western end by softer mudstones and sandstones.  That has a big effect on the algae that we find in the littoral zone, with Cyanobacteria (blue-green algae) abundant in the south-east end and Chlorophyta (green algae) more conspicuous at the other end.   Diatoms are conspicuous in the littoral zone all around the lake, although there are some differences in the types of species encountered.  That is a story for another day, but I did find one species in some of the samples I collected from the south-eastern end that point to one other influence on the ecology of Ennerdale’s littoral zone.

Look at the photograph at the start of this post.  It was taken as I walked up to the south-eastern end (circa NY 127 140) and shows the view up the lake, with Angler’s Crag visible on the left hand shore in the distance.   The River Liza enters the lake on the right hand side (just out of the frame) and the low lying area between the River Liza and the raised ground where I was standing is an area of wet heath with a range of Sphagnum species and several boggy pools.   The shoreline of the lake itself is formed by a shingle spit which acts as a barrier between the wet heath and the lake itself.

ennerdale_gravel_bar_170105

The shingle spit separating the wet heath at the south-east end of Ennerdale from the lake itself.   Photographed in January 2017.

Several of the diatoms that I found at this end of the lake were species that I associate with acid conditions although, curiously, the limited chemical data that we have does not show a lower pH here than elsewhere in the lake.   I suspect that the proximity to the acid Sphagnum heath may lead to occasional pulses of acid water entering this area and exerting a subtle effect on the attached algae before being diluted by the water of the lake as a whole.   Of the species that I found, the most intriguing was Stenopterobia sigmatella, a long, sigmoid diatom with a single plate-like chloroplast.

The genus Stenopterobia fulfils most of my criteria for a genuinely rare diatom (see “A “red list” of endangered British diatoms”).   I only have 11 records in my dataset of 6500 samples, and in only one case did Stenopterobia constitute more than one percent of the diatoms in the sample.   These samples are all from acid habitats (mean pH: 6.1), with low nutrient concentrations (never more than 2 mg L-1 reactive phosphorus).  Those for which we have location information are plotted below.   The record in East Anglia needs further investigation (meaning: “I don’t believe it … but I haven’t had a chance to track down the slide for a closer look”). If we ignore this, the distribution is confined to mountainous regions of western Britain, and these Ennerdale samples also fit this trend, although the lake has soft water and is circumneutral rather than acid.

Stenopterobia sigmatella is another diatom with a sigmoid outline, and this brings me back to a question that I have posed before (see “Nitzschia and a friend …”): what advantages does a sigmoid outline confer on a diatom?  I cannot think of any other genera of algae that has species with a sigmoid outline, which only adds to the mystery. All of the diatoms that are sigmoid are motile, so I guess that the explanation may be linked to movement, but I don’t know for sure what the reason may be.   For all of the rich diversity that we see in diatoms, there is still, to pick up on a phrase from my biography of Humboldt, a “poverty of meaning” …

stenopterobia_sigmatella_en

Stenopterobia cf sigmatella from Ennerdale Water, October 2016 and January 2017.  Scale bar: 10 micrometres (= 1/100th of a millimetre).

stenopterobia_distribution_

A distribution map of records of Stenopterobia in Great Britain.   S. curvula is a synonym for S. sigmatella (see taxonomic note below).  Map prepared by Susannah Collings (see “Why do you look for the living among the dead?” for more details of how this was done)

stenopterobia_densestriata_

A valve of Stenopterobia densestriata.  Photograph from the ADIAC database (photographer: Micha Bayer).  Scale bar: 10 micrometres (= 1/100th of a millimetre).

Taxonomic note

I have used the name “Stenopterobia sigmatella” in this post, but this still needs confirmation as there is a closely-related species, S. densestriata (Hustedt) Krammer 1987 (see image above).  S. sigmatella has < 24 striae in 10 micrometres whilst S. densestriata has > 26 striae in 10 micrometres.  S. densestriata also has slightly smaller overall dimensions.

David Mann made the following comment about Stenopterobia sigmatella on the website Common Freshwater Diatoms of Britain and Ireland (predecessor to the new Diatom Flora of Britain and Ireland: “A nomenclatural mess. For most of the 20th century, this species was referred to (wrongly) as S. intermedia. Ross (in Hartley, 1986) stated that there is an earlier name, sigmatella, that could be applied to this species and made a new combination S. sigmatella. Unfortunately, this was wholly ignored by Krammer (in Lange-Bertalot & Krammer, 1987; and see Krammer & Lange-Bertalot, 1988) who made the new combination S. curvula. However, Nitzschia curvula of W. Smith is preceded by N. sigmatella of Gregory (1856, 1854, respectively).”   The references can all be found on the Common Freshwater Diatoms website.

 

Not so Bleak Midwinter?

ennerdale_170104

Occasionally – just occasionally – the gods smile on us when we least expect it.  And Wednesday was one of those days: fieldwork on a glorious winter day in the Lake District without a cloud in the sky and barely a breath of wind.  The pleasure of being outside on such a day was offset slightly by the necessity of plunging my arm into freezing cold water at intervals, but the views of the mountains beyond Ennerdale Water more than compensated for these temporary discomforts.

The coldness of the water, today, offers me a link to a book I am reading, about the 19th century German scientist Alexander von Humboldt, a polymath who was ahead of his time in many ways, and whose writing pre-empted ecological thinking of the twentieth century.   One of his strongly held beliefs was that scientists could not really understand nature from a laboratory: they had to be outside, experiencing nature first hand.   That seems to be a fine New Year message in a world where ecologists seem to spend more and more time staring at screens, and their managers are increasingly reluctant to let them spend time in the field.

The ecology of lakes and rivers in this area in winter continues to fascinate me.   Look at the picture below: a stream bed at the coldest time of year that is covered with lush growths of algae in a range of hues, most strikingly the pink-red of the Rhodophyta Audouinella, complemented by the green and blue-green algae around it.  The first young olive-green filaments shoots of Lemanea, another Rhodophyta, were also apparent at a couple of the sites that I visited, and there were thick brown diatom blooms smothering many of the stones too.   These are all thriving at a time of year when either most nature has shut down for the winter or most natural historians have plonked themselves onto the sofa to watch Living World II rather than challenging the first clause in this sentence.  You decide.

ehen_ob_170104

A riot of colour on the stony substrata of the River Ehen, a few kilometres downstream of Ennerdale Water, Cumbria, January 2017. 

One of Humboldt’s big concerns was that scientists saw the big picture (“naturgemälde”) rather than getting bogged down with details.   He was someone whose mind had been formed by the Enlightenment, when the necessity of cataloguing and classifying the diversity of nature was a primary concern.  However, he saw that this was not enough, and that one had to understand the connections between these different life forms, and between each of these and their environment.  He saw the natural world as a web of interdependencies, and humans as potential disruptors of the delicate balances that existed.

The problem we have in the modern age is balancing the need to see the big picture in focus without losing site of important details.  Or, as Ed Tipping said during a meeting at CEH last year: “we stick to the principle of simplifying to just short of the point of naivety”.   He had his tongue in his cheek but there is an important point here: the complexity of the natural world means that its secrets will only be yielded to those scientists who can keep their natural proclivity to get lost in detail in check.   At the same time, if we forget that those details are out there we may reach erroneous conclusions.  And, I fear, microscopic benthic algae may be ecology’s Sirens, sitting on submerged rocks and luring the unsuspecting into a world of taxonomic detail that is too rarely accompanied by profound ecological insight.

William Wordsworth, born in Cockermouth, just a few miles away from Ennerdale, was one of Humboldt’s readers.  He recognised the need to be outside experiencing nature applied as much to a poet as to a scientist and reacting against the dry, dissected knowledge that the Enlightenment encouraged.  His words offer a succinct conclusion for this first post of 2017, and encapsulate my resolution to be as holistic as possible in my thinking during the year ahead:

For was it meant
That we should pore, and dwindle as we pore,
For every dimly pore on things minute,
On solitary objects, still beheld
In disconnection dead and spiritless,
And still dividing and dividing still
Break down all grandeur …

William Wordsworth, The Excursion, 1814

ennerdale_170104_2

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.