Close to the edge in Wastwater …

Wastwater_190610

I’m back in the Lake District for this post, standing beside Wastwater, the most remote and least disturbed of England’s lakes and, especially obvious on a sunny day in June, the most spectacularly-situated.  I stood on the western shore looking across to the screes and, beyond to the mass of Scafell Pike, England’s highest peak, looming up in the distance.

When I was done admiring the scenery I adjusted my focus to the biology of the lake’s littoral zone and some dark brown – almost black – marks on the boulders in the littoral zone.  In contrast to the grand vista stretching away to the north, these were beyond unprepossessing and my attempts to photograph them yielded nothing worth including in this post. However, I had seen similar looking marks in Ennerdale Water and there is a photograph in “Tales from the splash zone …” that should give you some idea of what I was seeing.

Under the microscope, my expectations were confirmed.  As in Ennerdale Water, these patches were composed of Cyanobacteria – gradually tapering trichomes of Calothrix fusca and more robust trichomes of Scytonema calcareum, both encased in thick, brown sheaths which, when viewed against the granite boulders on which they lived, resulted in the dark appearance of the growths.  To the untrained eye, these barely look like lifeforms, let alone plants yet they offer an important lesson about the health of Wastwater.

Calothrix_fusca_Wastwater_June19

Calothrix cf fusca from the littoral zone of Wastwater, June 2019. Scale bar: 20 micrometres (= 1/50thof a millimetre)

Though hard to see amidst the tangle of filaments in these population, both Calothrix and Scytonema have specialised cells called “heterocysts” that are capable of capturing atmospheric nitrogen (you can see these in the photographs of Nostoc commune in “How to make an ecosystem (2)”.   Nitrogen fixation is a troublesome business for cells as they need a lot of energy to break down the strong bonds that bind the atoms in atmospheric nitrogen together.   That means that plants only invest this energy in nitrogen fixation when absolutely necessary – when the lack of nitrogen is inhibiting an opportunity to grow, for example.   The presence of these Cyanobacteria in Wastwater is, therefore, telling us that nitrogen is scarce in this lake.

The dogma until recently was that phosphorus was the nutrient that was in shortest supply in lakes, so attention has largely focussed on reducing phosphorus concentrations in order to improve lake health.   Over the last ten years, however, evidence has gradually accumulated to show that nitrogen can also be limiting under some conditions.   That, in turn, means that those responsible for the health of our freshwaters should be looking at the nitrogen, as well as the phosphorus, concentration and, I’m pleased to say, UK’s environmental regulators have now proposed nitrogen standards for lakes.   That marks an important shift in attitude as, a few years ago, DEFRA were quite hostile to any suggestion that nitrogen concentrations in freshwaters should be managed.   In this respect, the UK is definitely out step with the rest of Europe, most of whom have nitrogen as well as phosphorus standards for freshwaters.

Scytonema_crustaceum_Wastwater_June16

Scytonema cf calcareum from the littoral zone of Wastwater, June 2019. Note the single and double false branches.   Scale bar: 20 micrometres (= 1/50thof a millimetre)

Wastwater flows into the River Irt and, a few kilometres down from the outflow, I found another nitrogen-fixing Cyanobacterium, Tolypothrix tenuis.  Once again, I could not get a good photograph, but you can see images of this in an earlier post from the River Ehen in “River Ehen … again”.   Nitrogen fixing organisms, in other words, are not confined to the lakes in this region, which raises the question why the UK does not have nitrogen standards for these as well (see “This is not a nitrate standard …”).   In rivers such as the Irt and Ehen that are already in good condition, it might only take a small increase in nitrogen concentration for the ecology to change.   Whether the loss of these nitrogen-fixing organisms will be noticed is another question.

For now, I am just happy to see that nitrogen in lakes has finally made it to the regulatory agenda.  It has taken about 15 years for the science to percolate through the many layers of bureaucracy that are an inevitable part of environmental management.  Give it another decade and maybe we’ll get nitrogen standards for rivers too.

References

Maberly, S. C., King, L., Dent, M. M., Jones, R. I., & Gibson, C. E. (2002). Nutrient limitation of phytoplankton and periphyton growth in upland lakes. Freshwater Biology. https://doi.org/10.1046/j.1365-2427.2002.00962.x

Moss, B., Jeppesen, E., Søndergaard, M., Lauridsen, T. L., & Liu, Z. (2013). Nitrogen, macrophytes, shallow lakes and nutrient limitation: Resolution of a current controversy? Hydrobiologia. https://doi.org/10.1007/s10750-012-1033-0

P.S. any guesses as to which 1970s prog rock group I was listening to over the weekend?  The clue is in the title.

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How to make an ecosystem (2)

Ennerdale_Apr19

My most recent visit to Ennerdale and the River Ehen almost did not happen: unexpected overnight snowfall led to my wheels spinning on the Whinlatter Pass before I retraced my steps to Braithwaite and followed roads at lower altitudes around the outskirts of the fells.   Fieldwork in the morning took place amidst intermittent snow showers but, by the afternoon, it was dry if not quite as balmy as the visit I described in “Croasdale Beck in February”.   “Unseasonable”, I was reminded, is a two-edged term.

There was little incentive to linger with my arm in the agonisingly cold water, so this post is about some algae growing on dry land that caught my eye.   Amidst the gravel in a farmyard in Ennerdale Bridge I saw some dark brown leathery growths that I recognised straight away as the Cyanobacterium Nostoc commune (see “Nosing around for blue-green algae …”).  It looks rather nondescript, even slightly unsavoury, with the naked eye but, under the microscope, the rosary-like structure of the filaments suspended within a jelly-like matrix is revealed.  The slightly larger cells with thicker walls and lighter contents are the heterocysts, responsible for fixing nitrogen from the atmosphere (fulfilling the same function as the nodules on the roots of legumes).

Nostoc_commune_Ennerdale_Apr19_#1

A patch of Nostoc communein a farmyard in Ennerdale Bridge in April 2019.   The picture frame covers about 30 centimetres. 

Nostoc_commune_x1000_Apr19_#1

Nostoc communefrom Ennerdale Bridge under the microscope.  Scale bar: 10 micrometres (= 100thof a millimetre). 

This type of coarse, well-drained gravel is a good habitat for Nostoc and, once you know what you are looking for, it is a common sight on gravel driveways, usually to the annoyance of the owners.   If there is only a small amount, the best way to control it is simply to pick up the colonies and toss them onto the compost heap.  However, once it is established, this can be a big undertaking and many people are quite happy to tolerate some of this brown gunk on their driveways.   On the other hand, it can sometimes get out of hand and the consequences of not doing anything are well illustrated by the photograph below.  The Nostoc colonies have spread but these, in turn, have created a habitat into which first mosses and later grasses can establish.

This small farmyard on the edge of the Lake District contains, in short, the first stage of an ecological succession.  We could think of a gravel driveway as a mini-desert, as the copious Cumbrian rainfall will not be retained in the surface layers, making it hard for plants to survive.   However, if a tough organism such as Nostoc is able to establish itself, then this, in turn, will trap water and make the driveway more amenable to slightly more fussy organisms such as mosses.   As the moss and Nostoc grow together so, eventually, grasses are able to establish too.  Were there to be no interruption to this process then, eventually, decades later, we might even see trees growing on this driveway.

It is hard to imagine, but just about every type of terrestrial habitat started out, aeons ago, as a bare rock surface.  Various forms of physical weathering start the process of breaking this up allowing, over time, organisms such as Nostocto get a foothold and convert the virgin surface into a mature ecosystem (you can read about another example in “How to make an ecosystem”).   It may take centuries for this to happen in the natural world, so it is particularly fortuitous to see this human-assisted succession so well developed.  At some stage, I suspect, the owner will decide that enough is enough, and rake the gravel.  Meanwhile, however, we have a rare opportunity to reflect on the role that primitive micro-organisms play in shaping even the grandest of our natural habitats.

Nostoc_commune_Ennerdale_Apr19_#2

A lawn of Nostoc, moss and grass growing on a gravel driveway in Ennerdale Bridge, April 2019.  

Reference

Miles, J. & Walton, D.W.H. (1993).  Primary Succession on Land.  Special Publication of the British Ecological Society 12, Blackwell Scientific Publications, Oxford.

More about measuring biomass …

The previous post showed how the proportions of green algae and diatoms changed as the total quantity of algae in the River Ehen waxed and waned over the course of a year.   The BenthoTorch, however, also measures “blue-green algae” and so let’s look at how this group changes in order to complete the picture.

Before starting, though, we need to consider one of the major flaws of the BenthoTorch: its algorithms purport to evaluate the quantities of three major groups of algae yet, in my posts about the River Ehen I have also talked about a fourth group, the red algae, or Rhodophyta (most recently in “The only way is up …”).  Having pointed a BenthoTorch at numerous stones with thick growths of Audouinella,we can report that Rhodophyta seem to be bundled in with the blue-green alga signal, which is no great surprise given the similarity in their pigments.  It is, however, one of a number of examples of the need to interpret any BenthoTorch results with your brain fully engaged, and not just to treat outputs at face value. Similar questions need to be asked of the Xanthophyta and Chrysophyta, though the latter tend not to be common in UK streams.

cyanos_in_Ehen

Relationship between the proportion of “blue-green algae” (Cyanobacteria and Rhodophyta) and the total quantity of benthic algae (expressed as chlorophyll concentration) in the River Ehen (c.) and Croasdale Beck (d.).  The blue lines show quantile regression fits at p = 0.8, 0.5 and 0.2.  

In contrast to the green algae and diatoms, the Cyanobacteria/Rhodophyta signal shows a strong negative relationship as biomass increases though, again, there is enough scatter in this relationship to make it necessary to approach this graph with caution.  I suspect, for example, that the data points on the upper right side of the data cloud represents samples rich in Audouinella, which tends to occur in winter when biomass, generally, is much greater.   On the other hand, Croasdale Beck, in particular, has a lot of encrusting Chamaesiphon fuscus colonies which are pretty much perennial (see “a bigger splash …”) but whose relative importance in the BenthoTorch output will be greatest when the other two groups of algae are sparse.   I suspect that encrusting members of this genus are favoured by conditions that do not allow a high biomass of other algae to develop, as these will reduce the amount of light that the Chamaesiphonreceives.

Thicker biofilms in the River Ehen often have some narrow Phormidium-type filaments as well as small bundles of nitrogen-fixing Calothrix, but the overall proportion is generally low relative to the mass of diatoms and green algae that predominate.    But that is not really telling us the whole story.  I finished my previous post with a graph showing how the variation in biomass increases as the biomass increases.  The heterogeneity of stream algal communities, however, cannot be captured fully at the spatial scale at which the BenthoTorch works: there is a patchiness that is apparent to the naked eye: one of our sites has distinct mats of Phormidium autumnale towards one margin, and dense Lemaneagrowths in the fastest-flowing sections, largely attached to unmovable boulders, which makes biomass measurement very difficult. I’ve also written about distinct growths of Tolypothrix and its epiphytes (see “River Ehen … again”), another alga which forms discrete colonies at a few locations. I try to collect a random sample of stones from a site but there are constraints, including accessibility, especially when the river rises above base flow.   In the River Ehen we also have to take care not to disturb any mussels whilst removing stones.

Whilst our sampling cannot really be described as “random” I do think that there is sufficient consistency in the patterns we see for the results to be meaningful. We could spend a lot more time finessing the sampling design yet for little extra scientific gain.   I prefer to think of these measurements as one part of a complex jigsaw that is slowly revealing the interactions between the constituents of the dynamic ecosystem of the River Ehen.   The important thing is to not place too much faith in any single strand of evidence, and to have enough awareness of the broader biology of the stream to read beyond the face value indications.

The only way is up …

Ehen_Mill_Feb19

How does an alga move upstream?   I’m curious because, I am now seeing populations of Lemanea fluviatilisabout four kilometres further upstream in the River Ehen than when I first started my regular visits in 2013.   I can explain the presence of the organism partly through changes in the hydrology of the river: a small tributary, Ben Gill, that had been diverted into the lake in Victorian times was reconnected to the river in 2014 and this introduced periodic pulses of intense energy to the river that had immediate effects on the substrate composition.  Lemanea fluviatilisis a species that thrives in the fastest-flowing sections of streams so I am quite prepared to believe that even a small shift in the hydrology of this very regulated river might make the habitat more conducive.

But that does not explain how it got there in the first place.   If the alga was occurring a few kilometres further downstream we would not have any such problems: the upstream populations would provide innocula and, if the habitat conditions changed at the downstream location, then some of those propagules might be able to establish at the downstream locations.   But what about movement in the other direction?

There has been relatively little published on this topic in recent years.  I have a review by Jørgen Kristiansen from 1996 that considers the dispersal of algae but most of the references that he cites are quite a lot older than this and I have not seen much published subsequently.   He lists our options: dispersal by water, by organisms, by air currents and by human activity.   Let’s consider each in the context of Lemaneain the River Ehen.   Lemanea, like most red algae, has a complicated life cycle with the potential for dispersal in both the haploid and diploid phases, but that is probably more detail than we need right now.  We’ll just outline the options in broad terms:

Water:the linear flow of the river means that it is almost impossible for the downstream population to provide inocula for the new upstream locations.  It may be possible for populations from further upstream in the catchment to seed the new locations.  I have not seen Lemaneain any of the streams that flow into Ennerdale Water (from which the Ehen emerges) but my knowledge of the catchment is not exhaustive.   Likelihood: very low to low.

Lemanea_at_Mill_Feb19

Young shoots of Lemanea fluviatillis(bottom right) growing on a submerged boulder in the River Ehen at a location where I have not previously seen it.   These are growing alongside thick growths of diatoms (yellow-brown in colour) and patches of green filamentous algae.

Organisms:much of the older literature is concerned with the possibility of living algae or their propagules being transported in mud attached to bird’s feet or feathers and this cannot be ruled out.   There is also a recent study showing how mink may act as a vector for Didymosphenia geminata in Chile.  The Ehen also has aquatic mammals (such as otters) that could be acting as vectors for Lemanea, as well as migratory fish such as salmon and trout that could move propagules upstream.   There is also some evidence that some algae can survive passage through mammalian and invertebrate guts, and this, too, may provide a means for Lemaneato spread upstream.    Likelihood: low to medium.

Air currents / wind:quite a lot has been written about airborne dispersal of algae, with even Darwin making a contribution (see reference in Kristiansen).  The key hazard in airborne dispersal is desiccation so, in the case of Lemanea, the most likely lifecycle stages that could be dispersed in this way would be the diploid carpospores or haploid monospores. This, however, would assume that there were times during the year when the relevant life-cycle stages were exposed and, as Lemaneais a species that I usually find in the Ehen only fully-submerged, this is not very feasible.  Likelihood: low.

Human activity:there is evidence that Didymosphenia geminatacan be transported between sites attached to waders and new records often correspond with patterns of recreational use (references in Bergey & Spaulding – see below).   When we work in the Ehen we prefer to move downstream in order to minimise the risk of moving organisms on our kit, and we also clean our kit before we start.   However, a lot of people work in this part of the Ehen and it only takes one dirty wader to introduce a propagule.   Likelihood: low to medium.

We’ll almost certainly never know for sure why Lemanea fluviatilisis now thriving four kilometres further upstream than it was five years ago.  It is, however, worth bearing in mind that, given enough time, even a low probability may yield a positive result.   So none of the four hypotheses can be ruled out for sure.   Three of the possibilities are entirely natural, with one – movement by the stream itself – being constrained by the direction of flow.  Biological vectors look like a very plausible means of moving algal propagules around catchments but, for this to work, we need wildlife-friendly corridors around the river to support the animals and birds.  The upper Ehen has these, but many other rivers do not.

Actually, having a number of options all with a relatively low likelihood adds to the sense of mystery that every ecologist should have when they approach the natural world.  When cause and effect are too predictable, we tend to focus on engineering the right “solution”.  The truth, in our muddled and unpredictable world, is often that nudging several factors in the right direction will give us a more resilient outcome, even though we may have to wait longer for it to happen.

Reference

Bergey, E.A. & Spaulding, S.A. (2015). Didymosphenia: it’s more complicated.  BioScience65: 225.

Kristiansen, J. (1996).  Dispersal of freshwater algae – a review.  Hydrobiologia336: 151-157.

Leone, P.B., Cerda, J., Sala, S. & Reid, B. (2014).  Mink (Neovision vision) as a natural vector in the dispersal of the diatom Didymosphenia geminataDiatom Research29: 259-266.

Raven, J.A. (2009).  The roles of the Chantransia phase of Lemanea (Lemaneaceae, Batrachospermales, Rhodophyta) and of the ‘Mushroom’ phase of Himanthalia (Himanthaliaceae, Fucales, Phaeophyta).  Botanical Journal of Scotland46: 477-485.

Mystery, wonder and joy

My Advent reading this year was Michael McCarthy’s The Moth Snowstorm: Nature and Joy (John Murray, 2015), a meditation on the reasons why humans love the natural world, and how engagement with nature can, in turn, be beneficial for our wellbeing.   His personal fascinations with butterflies, moths and birds provide most of the examples but, as I was reading this book at the same time as I was writing the previous post on “little round green things”.  As a result, I found myself reflecting on my own fascinations with the microscopic world.

A characteristic of ecologists, I have realised, is that there is almost always a tension between their scientific training and a primeval emotional response to nature.   This is not unique to ecology: geologists and astronomers certainly share it, but it is not a universal trait of scientists.  In those disciplines where it occurs, however, interactions with the natural world occasionally transcend strictly dispassionate objective observation and spill over into the language of joy and wonder.   “Joy” being, in McCarthy’s words, “concentrated happiness” whilst “wonder” is “a sort of astonished cherishing or veneration … often involving an element of mystery”.   We are straying away from the language of science and towards a religious and spiritual dimension that many ecologists would, I suspect, be reluctant to acknowledge.

“Mystery” is the word that ties together the disparate worlds of science and religion.   It implies “missing knowledge”, but much more than just an absence of necessary facts.   Every time I peer at samples from the River Ehen through my microscope I get the full gamut of joy-wonder-mystery-related emotions even though I have seen similar views many times before.   Part of this can be attributed to “missing knowledge” but not all.  I am acutely aware of my own shortcomings as I struggle to identify the organisms that I see, as well as the limitations of the taxonomic literature on which I depend.  I am, in addition, perpetually astonished that so much diversity can live on such a small scale and, even when I have done my best to name the algae present, I still struggle to explain why the communities differ over the space of a few metres and between our monthly visits.

Ehen_Mill_181212

Regular visits for five years have not diminished my wonder at the microscopic world of the River Ehen: this submerged boulder has obvious patches of brown diatoms and green algae, but also gaps where the algae are much less abundant. We can make coarse predictions about which species are likely to be found in particular locations, but the factors that determine their distribution on much finer scales are still shrouded in mystery.

The word “mystery” in short, carries an emotional heft that simply “not knowing” does not.  It rises above ignorance, partly because mystery, by definition, implies an awareness of this lack of knowledge.   The word “mystery”, in a modern, scientific context, also links to the concept of complexity, recognising that interactions between variables is often such that it is very difficult to predict outcomes.   That “astonished cherishing” that forms part of McCarthy’s definition of wonder needs to include an element of wariness.  We approach – or, at least, we should approach – ecosystems in the same cautious manner that Moses approached the burning bush.   Whether or not you believe in a higher power, recognition of both the complexity of nature and our limited understanding of this is humbling.   Humility, in turn, generates reverence, and we have completed the journey from the hard, dispassionate language of science to the fringes of spirituality and religion.

None of this precludes trying to improve our understanding of the natural world, nor of using this knowledge to inform decision-making.   What I have written above is no more than the Precautionary Principle, albeit expressed in quasi-mystical language.   Whilst the Precautionary Principle is an instrument of policy, my interpretation is more personal.   Each of us, individually, should be finding time to revel in the wonder of nature which, in turn, will fuel the sense of mystery and, in turn, temper any inclination to rush to intemperate conclusions.

Ehen_181212_diatoms

Some of the diatoms that are abundant in the River Ehen.  Top left: colonies of Gomphonema(see “Diatoms and dinosaurs” for more about this species); top right: colonies of Fragilaria tenera, which shares the habitat with at least two other similar representatives of the same genus; bottom left: Tabellaria flocculosa.  Genetic studies suggest that this, too, is probably a complex of morphologically-similar species.   Scale bar: 20 micrometres (= 1/50thof a millimetre).

We should, however, never assume that joy, wonder and a sense of mystery are ever-presents in the make-up of ecologists.   McCarthy makes the point that a love of nature is not a universal human attribute, although a propensity to love nature may be.   Just as that propensity can be nurtured through adolescence into an adult appreciation of the natural world, so a failure to exercise that appreciation as an adult can lead to it withering again.   I am acutely conscious that ecologists of middling seniority and above often spend more time staring at spreadsheets and in teleconferences than they do engaging directly with nature.  Within government agencies the reduction of time available for field ecology since the onset of austerity in the UK means that I often now deal with people who are unable to conjure visual images from the words and numbers that populate their datasets.  And, in my own work, I have to consciously make time to observe the natural world beyond the tight constraints of my professional life.

Above all, never forget that this love of nature exists in the first person, present tense or not at all. Natural history documentaries on the television and (dare I say) blogs such as mine are the herbs and spices that enliven your diet, but the naturalist’s basic sustenance needs a commitment that goes beyond staring at a spreadsheet or sitting on a couch.

 

A day out in Wasdale

Irt_LundBridge_Nov18

A few days after my trip to Weardale I found myself beside the River Irt, a few hundred metres below the point where it flows out of Wastwater, in the western part of the Lake District.   Whereas the River Wear drains a catchment underlain by Carboniferous rocks, including a high proportion of limestone (see “Co. Durham’s secret Karst landscape”), the Irt’s catchment is largely underlain by ancient volcanic rocks, resulting in much softer water.   I was curious to see how different the algae were here compared to those in the Wear.

The river bed at this point is dominated by boulders of granite, which host a patchwork of mosses, filamentous algae and discrete growths of diatoms (visible on the right-hand side of the figure below).  Between these there were areas of pebbles and gravels, suggesting good habitat for freshwater mussels.   The patches of filamentous algae (mostly no more than a couple of centimetres in length) were a mixture of Mougeotiaand Zygnema, similar to the forms that I find in the River Ehen, a 30 minute drive to the north.   These two species differ in the form of their chloroplasts (Mougeotiahas a flat plate whilst Zygnemahas two star-shaped chloroplasts, attached by thin cytoplasmic strands to resemble an animal skin stretched on a frame) but are closely-related, both belonging to the family Zygnemtaceae.

Irt_substratum_Nov18

An underwater photograph of the substratum of the River Irt in November 2018 showing patches of filamentous green algae, mosses and (on the right-hand side) diatoms growing on granite boulders.

Irt_greens_Nov18

Filamentous green algae from the River Irt, November 2018.   The upper photograph shows cells from a filament of Mougeotiawhilst the lower image shows two filaments of Zygnema. Scale bar: 20 micrometres (= 1/50thof a millimetre).

In between the tufts of filamentous algae were apparently bare patches of rock (they almost certainly had a very thin biofilm that would be hard to sample in isolation from the lusher algal growths that shared their habitat) and some conspicuous orange-brown growths of colonial diatoms.  These turned out to be almost pure growths ofGomphonema hebridense, or a close relative (I can’t give a definitive answer until I have examined cleaned material), growing on long mucilaginous, sometimes branched, stalks to create a veritable “bush” of diatoms.  There were a few other species of diatom growing within this bush, most notably some cells of Achnanthidium (cf.) caledonicumthat seemed to be growing on short stalks attached to the Gomphonemastalks, but also a few cells of Gomphonema capitatum(which also grows on long stalks) and some chains of Tabellaria flocculosa.

Gomphonema hebridenseis a diatom that I have written about several times before, as it is also common in the River Ehen, and also presents a number of interesting challenges to taxonomists (see “Diatoms and dinosaurs”). Whatever future studies reveal, however, the presence of colonies of this (or these) species that are visible with the naked eye is something I associate with only the cleanest rivers in the country during the cooler times of year.  It should not have been a great surprise to me to find it flowing out of one of the most pristine lakes in England (see “The Power of Rock …”).

Gomphonema_Irt_x1000_Nov18

A close up of cells within a colony of Gomphonemacf hebridense.  Several mucilaginous stalks are also visible as well as (top left) a cell of Achnanthidiumcf caledonicum.   Scale bar: 10 micrometres (= 100th of a millimetre).

The predominance of boulders over smaller, more easily moved stones, suggests a river that has more energy than the River Ehen, one of my regular Lake District haunts.   Both flow out of lakes whose catchments include some of the wildest and most mountainous terrain in the country.   Lakes tend to act as shock absorbers in catchments, slowing down the water that pours off the fells after heavy rain.   Streams in this part of the world that have no such impediments to flow tend to have rocky, mobile beds and relatively sparse algal communities.   By contrast, the Irt and Ehen just below their respective lakes have relatively lush growths of algae.   The substrates of the two rivers, however, are very different: the Ehen having very few boulders in comparison to the Irt, due to the presence of a weir at the outfall. This allows Ennerdale Water to be used as a water supply for the towns of north west Cumbria but, at the same time, turns the lake into an even more effective hydrological shock absorber.  Yet more of the energy that should be washing smaller stones down the river is no longer available except after the most exceptional storms.

That’s my working hypothesis, then: the Irt is a river that is subject to just enough high energy events to move the rocky substrates around yet no so many that rich algal communities cannot develop between these.  The Ehen, by contrast, has fewer events, leading to fewer opportunities for the algae to be scoured away, whilst unregulated streams such as Croasdale Beck (see “What a difference a storm makes …”) have such regular scouring spates that the algal communities are usually sparse.   I might be wrong, of course and I might be back in a years time with a better hypothesis.  Until then …

 

 

Life in the deep zone …

The view above – looking along Ennerdale Water from the western end – is one that I’ve used before in this blog.  The difference today is that there is about twenty metres of foreshore exposed.  Normally, water covers all the area in the foreground.   Not today: Ennerdale Water is one more victim of our present drought conditions.  During the winter, we often see water splashing over the weir at the outfall; today, the weir head is a metre above the lake level and flow in the River Ehen is maintained only by pumps installed by United Utilities.

When W.H. Pearsall visited Ennerdale Water in the 1920s, he considered it to be one of the most primitive of the Lake District’s lakes (see “The power of rock …”).   However, this supposedly wild lake had been tamed by a weir since the middle of the 19th century in order to provide drinking water for Whitehaven and Workington and surrounding areas.   That, in turn, has consequences for the river downstream, especially at times such as this when, unless augmented by pumps, there would be no water in the River Ehen below the outfall.   At some point in the next decade, a new water infrastructure project will pipe water to west Cumbria from Thirlmere, after which the weir can be removed and fluctuations in water level in both lake and river will be more natural.

The weir at the outfall of Ennerdale Water, with the fish pass at the far end. 

Meanwhile, however, I was able to explore areas of the lake littoral zone that would normally be hidden from me.  My notebook, for example, records my observations that this part of the lake shore has a stony bottom yet, as can be seen from the picture above, these form a belt about 20 metres wide, after which there is firm sand.   Normally, this would be close to the limit of safe wading but, today, I could walk out with just a pair of thin neoprene shoes.   Looking down, I could see a number of tufts of the alga Nitella flexilis growing in this sand.   I’ve written about this species before (see “Finding the missing link in plant evolution …”) and have seen it in the lake before, but not in this particular location. Standing with the lake water lapping against my shins I could bend down and take some photographs of these with my underwater camera that give this usually chilly location a semi-tropical feel.

It is a useful reminder to those of us who dabble in lake littoral zones and think that we understand their ecology that a lot happens beyond the depth in which we can safely wade.   Marco Cantonati and colleagues, for example, have found big changes in the composition of the algal flora of Alpine lakes when they used Scuba diving to explore the depths of their littoral zones.  No doubt, we would see similar changes if we were to try the same in the Lake District.

References

Cantonati, M., Scola, S., Angeli, N., Guella, G. & Frassanito, R. (2009).  Environmental controls of epilithic diatom depth-distribution in an oligotrophic lake characterized by marked water-level fluctuations.   European Journal of Phycology 44: 15-29.

Cantonati, M. & Lowe, R.L. (2014).  Lake benthic algae: toward an understanding of their ecology.  Freshwater Science 33: 475-486.