Baffling Brachysira …

I should have been in the Lake District last week on a sampling trip.  It was not lockdown that stopped me but the weather.   There was rain at the weekend and the river levels went up.  I then watched the hydrographs slowly sink back but not quite to a safe level for wading before Storm Christoph blew through and the river levels went up steeply.   After two days of almost constant rain, the temperature dropped, then the rain turned to snow which is great news for me because snow on the fells means that the river levels start dropping, even if it is fiendishly cold.  As I write (Sunday) the levels are just about back to conditions that permit safe wading, albeit with air temperatures below freezing.  We should get out early next week.  Meanwhile, I am left thinking about the lakes and streams rather than actually experiencing their chilly reality.

My ruminations took me back to Ennerdale Water and along themes that I explored in a post from 2017 (“Lost in detail?”).  Once again, we are looking at the genus Brachysira in this lake but this time, rather than pick up on the identity of a single species, I want to reflect on just how many different species of Brachysira seem to inhabit the littoral habitat of one lake.  This also links with two other recent posts: The stream eats itself … and Curried diatoms?.

As was the case for the 2017 post, the slide was used in the UK/Ireland diatom ring test, which means that it was scrutinised by more analysts than just myself.  All of us, however, agreed that several species of Brachysira were present with, in one case, eight different species being recorded by a single analyst.   I’ve included illustrations of all of them and the first comment that anyone who is not familiar with diatom taxonomy will make is, almost certainly, that they don’t look particularly different from one another.   There are differences but these mostly lie just at the edge of the resolution of the light microscope.  More pertinently, those analysts who do not have light microscopes with the very highest specification will be unlikely to discern these differences.   Whereas 25 years ago, you could probably do a perfectly adequate analysis of a sample such as this with a microscope equipped with a 100x achromatic objective with a numerical objective of 1.4, now you need a plan achromatic objective with a numerical objective of 1.25 and a differential interference contrast condenser and you will still struggle to differentiate some of these species.   

Brachysira intermedia, Ennerdale Water, June 2020. Similar to B. brebissonii (pictured at the top of the post) but slightly drawn-out ends and with undulating longitudinal lines dividing the striae into two, rather than three, areolae.  Scale bar: 10 micrometres (= 1/100^th of a millimetre).  Photos: Bryan Kennedy.

Several questions arise.  One is that if there is so much diversity discernible (albeit only just) with a light microscope (i.e. “semi-cryptic variation”), how much more are we missing because it is not discernible with a light microscope (“cryptic variation”)?   For some freshwater diatom genera we have been able to walk away from taxonomy’s traditional reliance on morphology and approach the problem from a different perspective.  Mostly, in this age, this different perspective comes from molecular genetics and in some cases (e.g. Fragilaria, Nitzschia, Sellaphora) it suggests that there really is variation beyond that readily discernible with light microscopes.  In others (e.g. Gomphonema) the evidence is less clear cut and it is even possible that taxonomist’s enthusiasm may have run ahead of the actuality.  Unfortunately, we don’t have any detailed molecular studies of Brachysira with which to test the ideas of species limits developed with morphological approaches.  

A second question is whether we need all this information in order to draw the ecological insights that catchment managers need in order to decide what measures, if any, are needed to keep the water body in a healthy state.   The autecological information that we have suggests that Brachysira species are mostly associated with healthy, low nutrient, circumneutral water bodies.  Where there are differences, it is primarily due to the hardness of the water rather than to the levels of human-derived pressures.   The very fact that we see several species in one lake is evidence that their preferences for these conditions, in any case, overlap.   So there is little evidence that different species of Brachysira “indicate” starkly different environments.

Brachysira microcephala (formerly B. neoexilis and, further back still, Anomoensis exilis), Ennerdale Water, June 2020. Scale bar: 10 micrometres (= 1/100^th of a millimetre).  Photos: Bryan Kennedy.

As I suggested in Curried diatoms?, however, there may be some value in knowing that there are several species present but for reasons that are not associated with rather naïve associations between species and chemical measurements.  As the present generation of methods for assessing lake or river health are tied to these tired notions of indicator values there is probably not a lot of extra “signal” to be squeezed from splitting Brachysira further.  

Brachysira procera, Ennerdale Water, June 2020. Generally longer and broader than B. microcephala.  Scale bar: 10 micrometres (= 1/100^th of a millimetre).  Photos: Bryan Kennedy.

Having said that water hardness plays a greater role in determining the distribution of Brachysira species than water quality, one of the oddities of this sample was that there were small numbers of two species associated with hard water (B. neglectissima and B. vitrea).  There is absolutely no limestone or any other rocks that might create hard water conditions within the Ennerdale catchment, so why are these species here?   I have found B. vitrea, especially, in small numbers in other soft water areas so these are not isolated occurrences.   Do they represent genuine viable populations hanging on at the extreme edge of their range (and, one assumes, competing for resources with physiologically “fitter” Brachysira species) or are they “noise”, carried into the catchment by wind or on the feet of mammalian vectors such as, er…, myself?   The latter has implications: firstly, it lends support to the controversial “everything is everywhere, environment selects” hypothesis (that I’ve mentioned in the past but never fully explained … I’ll do so in a post in the near future) and it also casts doubt on some of the claims of metabarcoding enthusiasts that they can extract more ecological information because the sequencing depth that is now possible exceeds that of traditional ecological analyses.  If what we hope is “signal” is, in fact, “noise”, then that depth is as likely to confuse or even mislead as it is to inform better decision making. 

Brachysira garrensis, Ennerdale Water, June 2020. Similar to B. microcephala but with denser striae (very difficult to see without very good optics).  Scale bar: 10 micrometres (= 1/100^th of a millimetre).  Photos: Bryan Kennedy.

Studies of the microscopic world are only ever as good as the technology available.  But, equally important, studies of microbial diversity are also complicated by the observer’s expectations.  How different do two cells have to be before you treat them as separate species?   Not only did Hustedt not have such good microscopes as are now available when he wrote his Flora in 1930, but he also assumed that diatom species concepts were much broader than we now know to be the case.  And the final twist is that the ability of anyone to match what s/he sees with the images in the literature.  Hustedt used a single drawing to illustrate “Anomoensis exilis” (what we would now call Brachysira microcephala) in his Flora whereas Bryan Kennedy used 96 photographs, taken with differential interference contrast lighting, along with 31 scanning electron micrographs in his 2017 paper highlighting, in the process, four different morphotypes. Whether these represent different species is yet to be determined.  My point is that “seeing” depends partly on how light stimulates the optical nerve (which depends on the quality of equipment we have), but more on how that raw signal is processed by our brains.  

Brachysira neglectissima, Ennerdale Water, June 2020. Like B. garrensis, this has denser striae than B. microcephala.  Note, too, the undulating pattern of areolae.  Scale bar: 10 micrometres (= 1/100^th of a millimetre).  Photos: Bryan Kennedy.

Postscript: we finally got out into the field on Monday, catching a gap in the weather when the rivers were low but the snow did not impede travel.  We had some spectacular views of snow-covered Pennines, High Street, Blencathra and Skiddaw on the way over, but the western Lakes were virtually snow free.   We even found time for a short walk around Loweswater on the way back, making the most of a rare opportunity to travel during Lockdown 3.0. 

Brachysira vitrea, Ennerdale Water, June 2020. Generally broader (>5.5 micrometres) than B. microcephala.  Scale bar: 10 micrometres (= 1/100^th of a millimetre).  Photos: Bryan Kennedy.

References 

Hustedt, F. (1930).  Susswasserflora von Mitteleuropa 10: Bacillariophyceae.  Gustav Fischer, Jena.

Kennedy, B. & Allott, N. (2017).  A review of the genus Brachysira in Ireland with the description of Brachysira prageriand Brachysira conamarae, new raphid diatoms (Bacillariophyceae) from high status waterbodies.  Phytotaxa 326: 1-27.

Some other highlights from this week:

Wrote this whilst listening to: Brandon Marsalis’ soundtrack album for the film Ma Rainey’s Black Bottom and Arvo Pärt’s Spiegel im Spiegel.

Cultural highlights:  White Tiger, new film set in India.   

Currently reading:  Still reading The Science Delusion by Rupert Sheldrake

Culinary highlight:  Homemade piccalilli pickle.   Too soon to give a definitive opinion on flavour (it needs three weeks to mature) but I’m fantasising about piling it into a thick ham sandwich already.

Lakeland panoramas

I spend most of my time focussed in on the smallest inhabitants of lakes and streams, so I thought I would indulge myself in this post by looking at some grand panoramas in the region of the Lake District where I do most of my fieldwork.  I have often looked up at the hills that surround the lakes and rivers of the western Lake District as I worked but my schedule was usually such that there was not time to climb them, even when the weather smiled upon us.   A combination of fine weather and limitations on travel to more distant places gave us an opportunity to rectify this omission and we’ve climbed a few of the peaks surrounding Crummock Water and Ennerdale Water this summer.   The views that these provide offer a great opportunity to set our fine-scale activities into context.

This involved following the old drove road between Ennerdale and Crummock up as far as Floutern Gap, then following the boundary fence up the side of Great Borne (616 metres) which overlooks Ennerdale Water.  From here, we followed the ridge down and up to Starling Dodd (636 metres) then Red Pike (756 metres) and finally High Stile (806 metres).   The latter two peaks are on the watershed between Ennerdale and the Buttermere/Crummock valleys, affording spectacular views of five lakes (Derwent Water was just visible through a gap in the fells).   Further away, we could see the Solway Firth and the coast of Galloway to the north and, just on the eastern horizon, Cross Fell, the highest point on the Pennines, some 65 kilometres away.  

To the south we could see the River Liza flowing along the flat valley floor of Ennerdale, with Pillar and Scoat Fell rising up on the opposite side and, towards the eastern end, the imposing face of Great Gable, with Scafell Pike just behind.  Looking west, along Ennerdale Water, the high sides of the glacial valley give way abruptly (roughly between Angler’s Crag and Bowness Knott) to a softer landscape of improved pasture criss-crossed with dry stone walls.   This reflects a transition from hard volcanic rocks of the Borrowdale group which form the high fells to softer sandstones and mudstones.   

Buttermere, seen from Red Pike, September 2020.   The photograph at the top of the post shows Ennerdale Water from the same vantage point.

The scenery to the north was equally spectacular and its glacial origins even more obvious.   From the south-east end of Buttermere, the Honister Pass climbing up through a classic U-shaped valley.   There is almost no room for improved pasture on the floor of Buttermere, with the land rising even more steeply on the south side than it does on the north.  At the north-west end, however, there is an area of flat land where Mill Beck has deposited silt over the millenia, dividing the original glacial lake into two.  I remember that from O-level Geography as it was one of the examples we were expected to cite in an essay entitled “Lakes are temporary features of the landscape.  Discuss.”

The entire outline of Crummock Water was also visible, with Grasmoor rising up on the north-eastern flank and Mellbreak on the south-western side.   Grasmoor is the higher of the two peaks (852 metres) but Mellbreak is the more imposing, rising straight up from the lake almost as starkly as the Screes arise from Wastwater.   Wainwright lets his usually measured prose run away when describing Melbreak: “Melbreak is isolated, independent of other high ground, aloof.  Its one allegiance is to Crummock Water”.   I’ve seen it from all four sides over the past month or so, as well as climbing to the summit last week and, though it is not a particularly high peak (511 metres), it punches above its weight.  Forgive me for sharing a few of my photographs to underline Wainwright’s comments… 

Crummock Water from Red Pike, September 2020, guarded by Grasmoor on the right and Melbreak on the right.  Loweswater is just visible to the left of Melbreak.  

Melbreak from Grasmoor, on the other side of Crummock Water, August 2020.

Whiteside, Grasmoor and Melbreak, seen from the shore of Loweswater. 

Beyond Crummock Water we could see the outline of Loweswater, which is unique amongst Lake District lakes because its outflow flows towards, rather than away from, the centre of the Lake District.   Most of the Lake District’s lakes are deep, elongated affairs, formed in the valleys scoured out by glaciatian.  Loweswater, by contrast, is relatively shallow (maximum depth: 16 metres) and set amongst gentler terrain.  “Improved grassland” – enclosed fields where cattle and sheep graze – form almost a quarter of the catchment of Loweswater, compared to just 3 per cent of the catchment of Ennerdale, 2.3% of the catchment of Buttermere and 7.5% of the catchment of Crummock Water. Esthwaite Water, the most productive lake in the Lake District, by contrast, has improved grassland on about a third of its catchment area.   Not surprisingly, Loweswater has had some issues with eutrophication in the past.

Loweswater from the top of Melbreak, photographed a few days before the other pictures in this post, when conditions were more hazy.

And finally, Derwent Water was just visible through a gap in the hills and we could also make out the summits of Skiddaw, Blencathra and Catbells from our vantage points on High Stile and Red Pike.   We got a closer view of Derwent Water a couple of days later when we canoed from Portinscale near Keswick to St Herbert’s Island and back.   A few years back I found my way to the lake impeded by a film set and spent an enjoyable few minutes watching a scene from Swallows and Amazons being filmed.  Apparently, St Herbert’s Island appeared as Wild Cat Island in the film too and was also the fictional Owl Island in Beartrix Potter’s The Tale of Squirrel Nutkin.   Oh yes, and someone called William Wordsworth wrote a poem about it.   I wonder what became of him?

Skiddaw rising above the south end of Derwent Water, September 2020.

On location: filming Swallows and Amazons in Keswick, July 2015.

Some other highlights from this week:

Wrote this whilst listening to:  Shores, the new album by Fleet Foxes

Cultural highlights:  The Farewell, a 2019 film directed by Lulu Wang about a family trying to keep their grandmother from learning about her terminal cancer.  A very warm and humorous film despite the subject matter.   

Currently reading:  Where I Was From, a collection of essays about the history of California by Joan Didion.

Culinary highlight:  Homemade Battenburg cake, inspired by the efforts on the first week of Great British Bake Off.   

Life in the colonies …

Another outcome of my visit to Ennerdale Water a couple of weeks ago in July (see “Life in the Deep Zone”) was some tiny green spheres in the sample I collected from one of the small streams flowing into the lake’s north-west corner.   The stream was very short, little more than a seepage arising from a wet rush-dominated area of a field just twenty metres or so from the lake margin and, at the point which I sampled, there was a tangle of filamentous algae (Stigeoclonium, Mirsrospora and Mougeotia) as well as a distinct diatom-dominated film on exposed stones.    The colonies looked like tiny peas in my sample tray but I suspect that they were attached to rocks or aquatic vegetation before I disrupted them. Under the microscope, these turned out to be colonies of the green alga Chaetophora pisiformis, a relative of Draparnaldia and Stigeoclonium, both of which I have written about before (see “The exception that proves the rule …” and “A day out in Weardale …”.  Like those, Chaetophora has branched filaments but they differ in forming well-defined colonies that are visible to the naked eye.

The pictures below show the form of colonies very clearly.  Chaetophora colonies are firm to the touch and cannot easily be squashed under a coverslip.   I overcame this by using a cavity slide, and taking one of the smallest colonies that I could find in order to photograph it with as little damage as possible.  Note how there is a very clear edge to the colony, whereas Draparnaldia and Stigeoclonium have a mass of filaments and mucilage but no obvious border between the “colony” and the surrounding environment.  Draparnaldia sometimes forms discrete colonies (see “The exception that proves the rule …”) but these are much softer and more easily squashed onto a slide.

Top: colonies of Chaetophora pisiformis from a small stream flowing into Ennerdale Water, with a one cent coin for scale; bottom left: lower power (x40) view of a colony.  The picture frame is about two millimetres across; bottom right: medium power (x100) view of the same colony.

Viewed at higher magnifications, the branches of the filaments are clear. They tend to be clustered towards the tops of the filaments and, in this case at least, end abruptly, rather than tapering to fine hairs.  I explained in the posts mentioned above how these fine hairs are used by the algae as means of capturing the nutrients that they need.  Chaetophora can form these hairs, but it does so less often, in my experience, than Draparnaldia and Stigeoclonium.   There will be dead and decaying vegetation in the rush-dominated swamp from which the stream originates, and the enzymes that these algae produce will be able to harvest any phosphorus from organic particles that result from this decay.  That’s the theory for Stigeoclonium at least, but I suspect that the colonies of Chaetophora are also highly efficient recycling units: the filaments are embedded in a firm mucilage that is far more than an inert polysaccharide gunk.   Any phosphorus that is released from a filament will be far more likely to be hoovered up by another filament than to drift downstream whilst the phosphatase enzymes will also be on hand at the colony surface to savenge any stray nutrients from the seepage.  These tight colonial forms are, in other words, fortresses of plenty in an otherwise inhospitable landscape: well adapted to nutrient-stressed situations and, as a paucity of nutrients is the natural condition of streams, the presence of these colonies is a good sign that this stream is in good condition.

Filaments of Chaetophora pisiformis from a small stream flowing into Ennerdale Water, July 2018.  Scale bar: 20 micrometres (= 1/50^th of a millimetre). 

Reference

Whitton, B.A. (1988).  Hairs in eukaryotic algae.   pp. 446-480.  In: Algae and the Aquatic Environment (edited by F.E. Round).  Biopress, Bristol.

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 19^th 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.

 

More about Platessa oblongella and Odontidium mesodon

As my last post used the conventions of figurative art to describe algal ecology, I thought I would stick to graphs – science’s very own school of abstract art – for this one.   I spent some time in “Small details in the big picture” discussing the ecology of Platessa oblongella (including P. saxonica) but without saying very much about the types of streams where these species were found.  So I am going to take a step away from the Ennerdale catchment in this post and, instead, collate environmental data a large number of sites to get a broader understanding of their habitat preferences.  As these species are often associated with Odontidium mesodon (see “A tale of two diatoms …”), I will summarise the preferences of this species at the same time (but see Annex 1 for a graph of this species’ preferences for still versus standing water).

The first set of graphs show the response of these species to pH and alkalinity and establish both as species typical of circumneutral soft water.  Platessa oblongella can be abundant in more acid conditions (i.e. to the left of the green vertical lines) but most of the records where it is abundant have pH values between 6.5 and 7.5.   Note that P. oblongella can also be found in humic waters, where lower pH thresholds apply (see Annex 2).

Distribution of Odontidium mesodon and Platessa oblongella (including P. saxonica) to pH and alkalinity in UK streams.   Vertical lines for pH indicate threshold values that should support high (blue), good (green), moderate (orange) and poor (red) ecological status classes.  See Annex 2 for more explanation.

The second set of graphs shows how these species respond to inorganic nutrients.   Both are most abundant when inorganic nutrients are present in low concentrations, though the trend is stronger for phosphorus than it is for nitrate-nitrogen.   The graphs for Platessa oblongella, however, both have a few outliers.   I have seen P. oblongella in a few situations where I did not expect it – I remember finding it in the Halebourne, a stream draining heathland around Aldershot and Bagshot in Surrey, where the water was well buffered (mean alkalinity: 61.3 mg L^-1 CaCO₃) and nutrient concentration were high (mean total oxidised nitrogen: 4.01 mg L^-1; dissolved phosphorus: 0.25 mg L^-1) and Carlos Wetzel and colleagues note some other anomalous records from the literature in their paper (cited in my earlier post), including a few from high conductivity and even brackish environments.   So we should treat these plots as indicative of the ecological preferences rather than definitive.

Distribution of Odontidium mesodon and Platessa oblongella (including P. saxonica) to nitrate-N and dissolved phosphorus in UK streams.   Vertical lines indicate threshold values that should support high (blue), good (green), moderate (orange) and poor (red) ecological status classes.  See Annex 2 for more explanation.

The final pair of plots show how the relative abundance of these two species changes over the course of the year.  These plots show the months when each taxon is abundant, by the standards of that taxon.  Because Platessa oblongella tends to be very numerous in samples, the threshold for this taxon (the 90^th percentile of all records) is higher than that for O. mesodon.   This reveals a very clear pattern of O. mesodon thriving in Spring whilst P. oblongella is abundant throughout the year, but with a slight preference for summer and autumn.  We need to reconcile these patterns with the observations in A tale of two diatoms that show that P. oblongella is associated with thinner biofilms than O. mesodon and try to work out whether season is driving the patterns or whether the seasonal patterns are the manifestation of other forces.   My suspicion is that P. oblongella is a classic pioneer species but also has a low-growing prostrate habit which means that it should be resistant to heavy grazing, which may confer an advantage in the summer and autumn when grazers are most active.  However, I may be getting ahead of myself, as we are in the process of analysing data on grazer-algae interactions in the River Ehen and Croasdale Beck that may throw more light on this.  There are clearly more layers to this story yet to be revealed …

Distribution of Odontidium mesodon (i.) and Platessa oblongella (j., including P. saxonica). The solid lines represent relative sampling effort (i.e. the proportion of samples in the dataset collected in a particular month) and the vertical bars represent samples where the relative abundance of taxon in question exceeded the 90^th percentile for that taxon (20% for P. oblongella/P. saxonica and 5% for O. mesodon).

Reference

The dataset used for these analyses is that used in:

Kelly, M.G., Juggins, S., Guthrie, R., Pritchard, S., Jamieson, B.J., Rippey, B, Hirst, H & Yallop, M.L. (2008). Assessment of ecological status in UK rivers using diatoms. Freshwater Biology 53: 403-422.

Annex 1: Odontidium mesodon’s preference for still or standing water

As I included a graph showing the preference of Platessa oblongella / P. saxonica for still or standing water in “A tale of two diatoms …”, I have included a similar graph for Odontidium mesodon here.   I have not included any data from the streams that flow into Ennerdale Water’s north-west corner in this graph as this would give a distorted picture.  To date, I have only seen a single valve of O. mesodon during analyses of 14 samples from these streams but I have not yet sampled these in spring which, as the graph above shows, is the time when O. mesodon is most abundant.   Like Platessa oblongella, O. mesodon is predominately a species of running, rather than standing waters.

Differences in percentage of Odontidium mesodon in epilithic samples from Ennerdale Water and associated streams.  Data collected between 2012 and 2018.

Annex 2: notes on species-environment plots

These are based on interrogation of a database of 6500 river samples collected as part of DARES project.  Vertical lines show UK environmental standards for conditions necessary to support good ecological status: blue = high status; green = good status, orange = moderate status and red = poor status.  Note that there are no environmental standards for alkalinity and the vertical lines show a rough split of the gradient into low alkalinity (“soft water”: < 10 mg L^-1 CaCO₃), low/moderate alkalinity (³ 10, < 75 mg L^-1 CaCO₃), moderate/high alkalinity (³ 75, < 150 mg L^-1 CaCO₃) and high alkalinity (“hard water”: ³ 150 mg L^-1 CaCO₃).

pH thresholds are for clear water (see UK TAG’s Acidification Environmental Standards.  The corresponding thresholds for humic waters are lower (high/good: 5.1; good/moderate: 4.55; moderate/poor: 4.22; poor/bad: 4.03).

Phosphorus thresholds are based on UK TAG’s A Revised Approach to Setting WFD Phosphorus Standards.   Current UK phosphorus standards are site specific, using altitude and alkalinity as predictors.  This means that a range of thresholds applies, depending upon the geological preferences of the species in question.  The plots here show the position of boundaries based on the average alkalinity and altitude measurements in the DARES database.

Note, too, that phosphorus analyses use the Environment Agency’s standard measure, which is unfiltered molybdate reactive phosphorus.  This approximates to “soluble reactive phosphorus” or “phosphorus as orthophosphate” in most circumstances but the reagents will react with phosphorus attached to particles that would have been removed by membrane filtration.

Nitrate-nitrogen: There are, currently, no UK standards for nitrates in rivers.  Values plotted here are derived in the same way as those for phosphorus (see “This is not a nitrate standard”)

 

Small details in the big picture …

I’ve written about Platessa oblongella, a small diatom common in low alkalinity environments, before (see “A tale of two diatoms …” and links therein) but my travels around west Cumbria are gradually revealing more and more about the ecology of this organism, so bear with me as I explain my latest findings.

My first graph shows how the distribution of this diatom varies in different types of water body in the Ennerdale catchment.   I have analysed 223 samples from this small area over the past few years and, within this dataset, there is a very clear distinction between situations where Platessa oblongella is abundant and situations where it is very rare.   I have very few records from Ennerdale Water itself (present in just two out of 27 samples, and never comprising more than 2.7% of all diatoms in the sample) nor from the River Ehen, which flows out of the lake (present in just 16 out of 164 samples, and always £ 1% of all diatoms).  By contrast, in Croasdale Beck and in streams that flow into the north-west corner of the lake, it is present in 28 out of 32 samples, with a maximum relative abundance of 69%.   In ten samples it forms more than 10% of all diatoms present.   Several of my samples from the small streams were collected from just a few metres above the point where they joined the lake, which makes the distinction between these streams and the lake that much more intriguing.

My theory – based on data I showed in A  tale of two diatoms  is that Platessa oblongella is a species of disturbed habitats and that the littoral zone of a lake, whilst subject to some turbulence, is less disturbed than the rough world of an unregulated stream.  The contrast between the River Ehen immediately below the dam at the outfall of the lake and the various small tributary streams also supports this idea.

Differences in percentage of Platessa oblongella (including P. saxonica) in epilithic samples from Ennerdale Water and associated streams.  Data collected between 2012 and 2018 (along with one sample from River Ehen collected in 1997).   The photograph at the top of the post shows Ennerdale Water, photographed in January 2018.

Some of the populations I looked at seemed to consist of two distinct forms, one broader than the other.   This variability is quite common in Platessa oblongella and Carlos Wetzel and colleagues recently published a paper which suggests that these are, in fact, two distinct species.   When I first started looking at diatoms, John Carter, my mentor, used the name Achnanthes saxonica, but Krammer and Lange-Bertalot, in the revised Süsswassserflora, regarded this as a synonym of Achnanthes oblongella, a species first found in Thailand.   Wetzel’s study shows, as well as the difference in valve width, differences in the fine details of the striae between the two species.   They also decided that both species belonged in the genus Platessa, rather than Achnanthes.

Platessa oblongella (top) and P. saxonica (bottom) from Croasdale Beck, October 2017.  Scale bar: 10 micrometres (= 1/100^th of a millimetre).

Valve width is, however, a very useful criterion, as the histograms below show.   The left hand graph shows a distinctly bimodal distribution of widths in specimens from Croasdale Beck, whilst the right hand graph shows a much tighter, and clearly unimodal, range.   This comes from another tributary stream flowing into the Ehen about 500 metres below the lake itself.  Quite why two species can co-exist in one stream but only one is present in another is not clear.

The modes of these populations are very close to the median widths for P. saxonica (narrow, ± 4/5 – 5 mm) and P. oblongella (broader, ± 6.5 mm) respectively but, as the left hand histogram shows, there is some overlap.    You might have trouble, for example, deciding whether a valve that was 5.5 mm wide was a “fat” P. saxonica or a “thin” P. oblongella.   My standard advice in situations such as this is that we should identify populations not individuals although, in the case of Croasdale Beck, this will still leave a grey area between the “fat” and “thin” valves where a judgement call is necessary.   In this case I think I could have done it because the P. saxonica valves in this stream tended to have a greater length:breadth ratio than those of P. oblongella, though I have not actually quantified this.

Width of valves in populations of “Achnanthes oblongella” from a) Croasdale Beck, and b) an unnamed tributary stream of the River Ehen, October 2017. 

There is more to say about the ecology of these species, but I have probably written enough for now.  I will leave you, for now, to bask in the rare sensation that occurs when diatom taxonomists make a situation clearer rather than more opaque, and return to this subject in a future post.

References

Carter, J.R. (1970).   Observations of some British forms of Achnanthes saxonica Krasske.  Microscopy: Journal of the Quekett Microscopical Club 31: 313-316.

Wetzel, C.E., Lange-Bertalot, H. & Ector, L. (2017).  Type analysis of Achnanthes oblongella Østup and resurrection of Achnanthes saxonica Krasske (Bacillariophyta).  Nova Hedwigia, Beiheft 146: 209-227.

The underwater world of Ennerdale Water …

I’ve tried to capture the world of microscopic benthic algae many times but never, until now, attempted the same effect with plankton.   The picture below illustrates the problem that I face: whereas the benthic flora are organised with, for the most part, a clear three-dimensional structure and known dependencies amongst organisms (species A, for example, being epiphytic on species B), plankton are randomly distributed in a very dilute solution.

A representation of the phytoplankton of Ennerdale Water with cells of Rhodomonas and Kephyrion depicted at a realistic density (c. 1000 – 2000 cells per millilitre).

I had to address two issues in producing this image, which is based on four phytoplankton samples collected by the Environment Agency in the summers of 2014 and 2016: depicting the phytoplankton cells at approximately the correct density and making sense of the list of names that appeared on the list.  Ennerdale Water is a very nutrient-poor lake and cell concentrations during the summer are in the order of 1000 to 2000 per millilitre.  That sounds a large number until you consider the scale at which we are working.   For simplicity, I assumed spherical cells of about 20 micrometres diameter (= 1/50^th of a millimetre) at a density of 1000 cells/ml.    That equates to one cell per micrometre which is 1 mm x 1 mm x 1 mm.   Using these assumptions, each cell is 50 diameters distant from its nearest neighbour, which means the foreground of a picture should contain only two small cells and a lot of blue paint.

Next, I need to know what algae to paint and the problem here is that 85 per cent of the cells in the Environment Agency phytoplankton analyses were described as “picoplankton < 2 micrometres diameter” or “nanoplankton 2-20 micrometres diameter” (the latter divided into flagellates and non-flagellates).  There are, apparently, big difficulties in naming many of the cells found as preservation with Lugol’s Iodine coupled with the long time in storage before analysis can lead to loss of useful diagnostic features.   Cells in the nanoplankton category can, in theory, belong to any one of a number of groups of algae but If I focussed just on those organisms that could be named, I see that the Cryptophyta Rhodomonas lacustris var nannoplanctica (formerly R. minuta var. nannoplanctica) predominates, followed by Chrysophytes, of which Kephyrion is the most abundant.   So these are the two cells that I have put in the foreground.

I subsequently turned up a paper from 1912 by the father and son team of William and George West who looked at the phytoplankton of Ennerdale Water and a number of other lakes in the Lake District and Scotland.  The range of taxa that they found was quite different to that recorded in these recent surveys with samples dominated by desmids and almost no Chrysophytes or Cryptophytes recorded at all. That may, in part, be due to differences in methods – they collected samples using a “silken tow net”, which would probably have missed the very small Chrysophyta and Cryptophyta (an earlier paper by them tells us of the size of the nets but not the mesh itself) .  Some desmids that they found were found in the recent surveys but in much smaller quantities and it is possible that this was partly an artefact of the differences in sampling technique.  The idea of comparing count data from old papers with modern records is appealing but, in most cases, separating genuine changes in composition from differences introduced by sampling and analytical methods is always difficult.

Excuse these ramblings … there is, as you can see, not a lot of pictorial interest in the underwater world of an oligotrophic lake.   If you want excitement, tune into Blue Planet II, David Attenborough’s latest series for the BBC You will find sex and violence galore there.  The underwater world of Ennerdale Water is a quieter, more serene and certainly less televisual place.  Maybe that’s not such a bad thing …

References

Lund, J.W.G. (1948) A rarely recorded but very common British alga, Rhodomonas minuta Skuja. British Phycological Bulletin, 2:3, 133-139.

West, W. & West, G.S. (1909). The British freshwater phytoplankton, with special reference to the desmid-plankton and the distribution of British desmids.   Proceedings of the Royal Society of London Series B 81: 165-206.

West, W. & West, G.S. (1912).  On the periodicity of the phytoplankton of some British lakes.  Journal of the Linnaean Society, Botany 40: 395-432.

Buffers for duffers …

In Ecology in the hard rock café I wrote about the challenges of living in an aquatic world where carbon – one of the raw materials for photosynthesis – was in short supply.   What I did not write about in that post is that this carbon also gives freshwater some useful additional properties.   In brief, rainwater is not pure water, but absorbs carbon dioxide from the atmosphere.  This, in turn, makes rainwater slightly acidic and, when it falls onto rocks, this weak acid dissolves the minerals from which the rock is made.  This adds two other forms of carbon to the water – bicarbonate and carbonate (the latter, particularly, from limestone).

Each of these three types of carbon in freshwater can convert to either of the other two types, with the speed of the reaction depending on the balance between the forms (the “law of mass actions”).  In essence, the reactions proceed until equilibrium is obtained, and this equilibrium, in turn, depends upon the pH of the solution.  These processes are summarised in the diagram below.

Relationship between pH and the proportion of inorganic carbon as free carbon dioxide (or carbonic acid, H₂CO₃ – orange line), bicarbonate (HCO₃^– – green line) and carbonate (CO₃^2- – blue line).

The chemistry behind this is not easy to explain but a consequence is that any attempt to shift the pH (e.g. by adding acid) causes an automatic adjustment in the balance between the different forms of carbon.  Some of the hydrogen ions that could make the water acid are, instead , bound up as bicarbonate, and the pH, as a result, does not change.  The greater the quantity of inorganic carbon in the sample, in other words, the greater the capacity of the water to resist changes in pH.   The carbonate, bicarbonate and free carbon dioxide together act as a “buffer”, a chemical shock absorber.   Think of it as equivalent to the responsible use of a credit card or savings account to defer the cost of an unexpected bill (a car repair, for example) so that your current account does not go overdrawn.

Because life largely evolved in well-buffered marine systems, the enzymes that run our cells generally work best within a narrow range of pH (approximately 6-9).   Cells – unicellular life forms in particular – get stressed if pH strays outside this range, so the greater the buffering capacity, the easier it is for cells (life at high pH can bring additional complications, but we don’t have time to go into those here).  “Alkalinity”, as I mentioned in the earlier post, is the measure that ecologists use to assess the strength of the buffer system in a lake or river.  The principle of the measurement is straightforward: we add a dilute acid very slowly and watch what happens to the pH.   At first, nothing happens but, as soon as the water’s natural buffering capacity has been exceeded, pH drops rapidly.

I have a small portable alkalinity titration kit which involves adding drops of bromophenol blue indicator to a sample of stream or lake water.  This gives the water a blue colour when the pH is greater than 4.6.  As the pH falls, the solution becomes colourless and, eventually, turns yellow.   If you look at the graph above you will see that, at pH 4.6 most of the bicarbonate (HCO₃^–) has been converted to carbon dioxide so the buffering capacity is pretty much non-existent.  This means that I can use the quantity of acid that is needed to make the bromophenol blue change colour as a measure of the buffering capacity of the water.

Alkalinity titrations beside Ennerdale Water (see top photograph) using a Hanna HI 3811 alkalinity test kit.  The right hand image shows acid being added to the water sample with a 1 ml pipette.  The blue colour shows that pH has not yet dropped below 4.6.

All this talk of chemical equilibria seems to be a long way from the natural history that is the core business of this blog.  Yet, at the same time, these reactions describe natural phenomena every bit as real as the plants and animals that attract the interest of naturalists.   Geology and chemistry ultimately create the context within which biology flourishes, but it is rare to meet a chemist who can talk with a naturalist’s passion.  I think that this is partly because chemistry tends not to describe tangible features of the landscape but, instead, quickly gets lost in abstract equations.  However, it is also a matter of culture: chemists need clinical separation from the mud and filth to maximise precision, whilst ecologists feel the lure of the field.  There is, nonetheless, a very basic and necessary link between the chemistry and ecology of aquatic systems.   Geology may shape a landscape but chemistry is one of the key mediators that determines the types of plants that cloak the hills and vales.  We ignore it at our peril.

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 (= 1/100^th 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 (= 100^th 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 (= 50^th 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 (= 50^th 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 20^th 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.