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 complexities of measuring mass…

Benthotorch_in_action

Once upon a time, measuring the quantity of algae growing on the beds of streams and rivers was a painstaking, slow process that invariably revealed large amounts of spatial and temporal variation that, very often, obscured the ecological signals you were looking for. That has changed in the last decade thanks to the availability of field fluorimeters such as the BenthoTorch.  This makes it much quicker and easier to measure chlorophyll concentrations, the usual proxy for algal quantity.  Thanks to devices such as this it is now much easier to discover that your ecological signal is masked by spatial and temporal variation.

We’ve generated a lot of data about the fluxes of algae in the River Ehen using a BenthoTorch over the past five years and are in a position where we can start to make some generalisations about how the quantity of algae vary over the course of a year.  In broad terms, the results I showed in “The River Ehen in January” back in 2014 have not varied greatly over subsequent years, with peak biomass in mid-winter and low biomass in the summer (due, we presume, to intense grazing by invertebrates).  Curiously, we see a much less distinctive seasonal pattern in the nearby Croasdale Beck, but that is a story for another day….

The BenthoTorch uses an algorithm to partition the fluorescence signal between three major algal groups and, though this is not without issues (see below), I thought it might be interesting to see how these groups varied with biomass trends, and consider how this links to ecological theory.  The first group I’m considering are the green algae which, in this river, are mainly filamentous forms.   The general pattern, seen in the graph below, is for a gradual increase in the proportion of green algae, which fits with the current understanding of thicker biofilms having greater structural complexity with filamentous algae out-competing attached single celled algae to create a “canopy” of algae that are more effective at capturing light and other resources.  The relationship is, however, strongly wedge-shaped so, whilst many of the thickest biofilms have a lot of green algae, there are also thick biofilms where green algae are scarce or even non-existent.  Croasdale Beck shows a similar, but less pronounced, trend.

green_algae_in_Ehen

Relationship between the proportion of green algae and the total quantity of benthic algae (expressed as chlorophyll concentration) in the River Ehen (a.) and Croasdale Beck (b.).   The blue lines show quantile regression fits at p = 0.8, 0.5 and 0.2.   The image at the top of the post shows Ben Surridge using a BenthoTorch to measure algal biomass beside Croasdale Beck in Cumbria.

The second graph shows that this pattern of a gradual increase in proportion is also the case for diatoms and, once again, there is a broad wedge of points with an upward trend.  But, once again, there are also samples where biomass is high but diatoms are present in very low numbers or are even absent.   What is going on?

The problem is clear I think, if one looks at the final image in “The only way is up …” where the very patchy nature of algal communities in the River Ehen (and, indeed, many other rivers).   There are plenty of algae on this boulder, but not organised in a homogeneous manner: some zones on the boulder are almost pure diatom whilst others are almost pure green algae (and there are also zones that are almost pure Lemanea– I’ll come to that in a future post).   We try to sample the stones as randomly as possible so you can see the potential for getting very different numbers depending on where, on a stone, we point the BenthoTorch’s sensor.

diatoms_in_the_Ehen

Relationship between the proportion of diatoms 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.  

With experience, you can make an educated guess about the types of algae present in a biofilm.  I’ve tried to capture this with my watercolours, using washes of raw sienna for the diatoms and a grass-green for the green algae, which roughly matches the colour of their respective growths in the photo in my earlier post.   The two groups of algae a are relatively distinct on that particular boulder.   The top row roughly matches the upper “edge” of the graph showing variation in diatoms, whilst the bottom row emulates the upper “edge” of the graph showing variation in green algae.  These are the two extreme situations; however, we also often see darker brown growths in the field, which can be recreated by mixing the raw sienna and grass-green together.  When I peer through a microscope I often see green algae smothered in diatoms: genera such as Oedogoniumare particularly prone as they have less mucilage than some of the others we find in the Ehen. Their filaments often host clusters of Fragilariacells as well as Achnanthidium minutissimum, whilst stalked Gomphonemaand chains of Tabellaria flocculosaoften grow through the tangle of green filaments.   The dark brown colour is deepened yet further by the colour of the underlying rock, so my effort on white watercolour paper is a little misleading.

colour_patches

A colour chart showing how different proportions of green algae and diatoms influence the colour of biofilms.

The final graph shows how, as the average biomass increases in the River Ehen, so the variability in biomass also increases.   The River Ehen is one of the cleanest rivers I know but I suspect that this pattern in benthic algal quantity could be reproduced in just about any river in the country. What I would not expect to see in any but the purest and most natural ecosystems is quite so much variation in the types of algae present.   Once there is a little enrichment, so I would expect the algae to become more of a monoculture of a dominant filamentous alga plus associated epiphytes.  Like much that happens in the microscopic world of rivers, it is easier to describe than it is to measure.

That, however, is only part of the story but I’ll come back to explain the patterns in the other main groups of algae in the Ehen and Croasdale Beck in my next post.

mean_biomass_by_stdev

The relationship between mean chlorophyll density and the standard deviation (based on measurements from five separate stones) for samples from the River Ehen and Croasdale Beck. 

 

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.

 

Transitory phenomena …

Fieldwork in the River Ehen has been an unusually pleasurable experience over the past few months, even to the extent of abandoning waders altogether and wearing just a thin pair of neoprene beach shoes and shorts as I worked.   Curiously, there were few obvious signs of the prolonged period of low flow here, but that is partly due to the pumps installed by United Utilities to keep the river running whilst the lake was drawn down (see “Life in the deep zone …”).   I did, however, find some intriguing green patches on fine sediments at the margins.

Most of the bed in this part of the river consists of much coarser sediments than these which are, I suspect, silt and sand deposited on the occasions when Ben Gill (which joins the Ehen immediately below Ennerdale Water) is flowing.   Current velocity is lower at the edges of the river, allowing fine sediments to settle out and create temporary sandbanks.   One decent spate will be all that is needed, I suspect, to wash much of this downstream.  However, there has not been a period of prolonged high flow for several months and there is, as a result, a thin green mat of algae growing on the upper surface of this sediment.

Mats of Oscillatoria on fine sediments beside the River Ehen just downstream from Ennerdale Water, August 2018.   The total length of the mats in the left hand photograph is about one metre. 

I scraped up a small sample to examine under my microscope.  I was expecting to see the broad filaments of the cyanobacterium Phormidium autumnale which I often find at a site about five kilometres downstream (see “’Signal’ or ‘noise’?”) but what I saw was much narrower filaments, some of which were slowly gliding forwards and backwards.   These belong to a species of Oscillatoria, a relative of Phormidium that is common in the plankton.  A few species, however, do live on surfaces and can, as I could see in the Ehen, form mats.  I have, in fact, described a different mat-forming species of Oscillatoria (O. limosa) from the River Wear close to my home (see “More from the River Wear”) and this, too, had been favoured by a long period of warm weather and low flow.   The filaments in the River Ehen were much narrower – just a couple of micrometres wide – and had relatively long cells (two or three times longer than wide) but, in other respects, they clearly belonged to the same genus.

Microscopic views of Oscillatoria filaments from the River Ehen, August 2018.   The upper photograph was taken at medium magnification (400x) and the lower image was taken at 1000x.  The constant motion of the filaments means that it is not possible to use stacking software to obtain a crisp image.  Scale bar: 10 micrometres (= 1/100th of a millimetre). 

The motion that I could see is thought to be due to a layer of tiny fibres (“microfibrils”) which wind around the inner layer of the cell wall in tight spirals.   Movement is caused by waves that are propagated along these fibres, meaning that the filament actually rotates as it moves (though this is almost impossible to see with a light microscope).   The filaments can move either towards or away from light, depending on the intensity, at a speed of up to 11 micrometres per second (that’s about a millimetre a day or, for any petrolheads who are reading, 0.00004 kilometres per hour).  This allows the filaments can adjust their position so that they are neither in the dark nor exposed to so much light that they are likely to do damage to their photosynthetic apparatus (see “Good vibrations under the Suffolk sun” for more about this).   The result is that filaments will tend to converge, Goldilocks-style, at the point where light conditions are “just right”.  You can see some sediment particles settling on the top of the mat in one of the images and we can expect the filaments to gradually adjust their positions, incorporating these particles, over time.

Last year, I wrote about Microcoleus, a relative of Oscillatoria, which formed mats on saltmarshes and explained how this could be the first stage of colonisation of damp habitats by plants (see “How to make an ecosystem”).   We are seeing the same processes happening here, but the life expectancy of these mats is much lower.  They may well be gone next time I visit, depending on how the Cumbrian climate behaves over the next couple of weeks.   They are transitory phenomena, here today and gone tomorrow but, like the subjects of some of my other recent posts, particularly favoured by the long period of settled weather that we have enjoyed over recent weeks.

Reference

Halfen, L.F. & Castenholz, R.W. (1971).  Gliding motility in the blue-green alga Oscillatoria princeps.  Journal of Phycology 7: 133-145.

Note: you can read more about how the heatwave has affected fresh water in the Lake District in Ellie’s MacKay’s recent post on Freshwaterblog

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 90th 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 90th 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”)

 

What a difference a storm makes …

I was back at Croasdale Beck last week and noticed a rather dramatic change to the meander just upstream from our regular sampling spot.   If you look at the photograph that heads the post “A tale of two diatoms …”, you’ll see the stream flowing around this meander.  Now, however, it has cut a new, shorter channel that bypasses the meander altogether.   We visited the stream just a few days after Storm Ophelia had passed through although, judging by the grass growing on the gravel of the abandoned meander, it was not necessarily this particular event that reshaped the stream.

Croasdale Beck is an unruly tributary of the River Ehen, rising on the fells above Ennerdale Water and tumbling down across rough grazing land and some semi-improved pasture (as in the picture above) before joining the Ehen in Ennerdale Bridge.   This is not the first time that we have seen conspicuous changes in the channel after a storm.  The magnitude of the flood is illustrated by the hydrograph below, which went off-scale for a period, as the discharge exceeded 3000 mega litres per day (300 MLD is the approximate limit for safe wading, in my experience).   I noticed that there was much less green algae present than we usually record at this time of year, although the diatom film was still quite thick.   Some of the stones that I picked up to sample had the slimy biofilm on the underside, suggesting that they had been recently rolled by the flooded river.   Croasdale Beck has no lake to buffer the rise and fall of the floodwaters and a huge amount of energy is carried down in a short period of time as the water surges downstream.

By the time we had arrived, the floodwaters had subsided and the sheep were contentedly grazing the surrounding land.  The stream itself was almost back to base flow (in contrast to the River Ehen which was still only just wadable).  Only the meander looked different …

The hydrograph for the River Ehen, as the aftereffects of Storm Ophelia make their way downstream.

The exception that proves the rule …

If you are going to understand river ecology, you need to be able to consider landscapes at several different scales simultaneously.   In the River Ehen, this means looking upstream towards Ennerdale Water and, beyond, to Great Gable and the other Lake District peaks in order to appreciate the geology that gives the catchment its bones.  But, at the same time, you need to look around at the meanders of the river and the bankside vegetation that create the immediate habitat for the organisms, and then to look even more closely at the individual stones that line the river bed.

Peering into the water last week, the pebbles, cobbles and boulders that make up the substratum of the River Ehen looked bare of filamentous algae for the most part.  There were a few clumps but, at this time of year, when grazing invertebrates are active, the algal flora is reduced to a thin film, invisible to the naked eye and apparent only as a slimy sensation when you run your fingers across the stone’s surface.   However, when I picked up a couple of cobbles, I noticed small, pale green gelatinous growths stuck on the upper surface.   Most were just a few millimetres across with the largest up to about a centimetre.

A growth of Draparnaldia glomerata on the upper surface of a cobble in the River Ehen, Cumbria, April 2017.

These growths are composed of the green alga Draparnaldia glomerata.  I have written about this alga before (see “The River Ehen in February”) but, under the microscope, it is such a beautiful organism, that I am not going to apologise for writing about it again.   The alga lives inside the gelatinous mass and consists of a relatively thick central filament from which tufts of narrower side-branches emerge.  The cells that make up these side branches gradually narrow, and the chloroplast becomes smaller until, eventually, the cells form a colourless “hair”.   These hairs are relatively short on the material illustrated below but can be much longer (some longer hairs were present but did not present nicely for photography).  The hairs are, in fact, an adaptation to help the alga acquire phosphorus, something I described in an earlier post about a relative, Stigeoclonium tenue (see “A day out in Weardale”).

Draparnaldia glomerata from the River Ehen, April 2017 showing filaments and side branches. Scale bars: a.: 50 micrometres (= 1/20th of a millimetre); b.: 20 micrometres (= 1/50th of a millimetre).

A low concentration of phosphorus is usually regarded as a Good Thing by aquatic ecologists, as this limits the amount of energy produced  by the plants at the base of the food chain.  This, in turn, means that the microbes and animals that depend on these are not using up all the oxygen in the water, or having other deleterious influences on the ecosystem.   I would usually regard the presence of an organism such as Draparnaldia as a sign of a healthy stream, as it is adapted to thrive when phosphorus is relatively scarce.

I was, however, careful to place “relatively” in front of “scarce”.   Studies by my colleagues (referenced in the earlier post) showed that the production of the phosphatase enzyme that boosts the alga’s ability to acquire phosphorus when it is scarce is determined by the ratio of nitrogen to phosphorus inside the cell itself, rather than in the water.   The physiology of nutrient limitation is all about the balance between the different “ingredients” that a cell needs.   If you have three eggs and 170g of sugar, for example, you can only make one cake, regardless of how much flour you have in your cupboard.   So it is with algae: most of the locations where I find Draparnaldia have very little nitrogen, but even less phosphorus.   There are barely enough ingredients for the algal “cake” so it is advantageous to the organism to pump out some enzyme to order to make up the shortfall.  This means that I can say with confidence that Draparnaldia is usually a good indicator of healthy streams.

Just occasionally, however, I get Draparnaldia in places where I would not usually expect it to be found.   The picture below shows a colleague standing in the Terman River, just before it flows into Lough Erne in Northern Ireland.   She is holding a skein of Cladophora glomerata in her left hand and a skein of Draparnaldia in her right hand.  I associate the former with nutrient-rich rivers where I would not usually expect to find Draparnaldia.  But both were growing prolifically at this site which defied my expectations until I started to think about the physiology of the organism.   Had I had the facilities to analyse the tissues of the algae, I expect that I would have found very high concentrations of nitrogen which, in turn, creates a demand for yet more phosphorus so that it could convert that nitrogen into the proteins it needs to grow.  However, that cannot be the whole story, because normally, under such circumstances, I would expect a competitive alga such as Cladophora to out-compete and overgrow the Draparnaldia.   Here, they were growing side-by-side.   It is, to date, the most luxuriant growth of Draparnaldia that I have seen, and also the only occasion where I have seen these two species co-existing in such abundance.

My colleague, Bernie White, holding skeins of Cladophora glomerata (left hand) and Draparnaldia glomerata (right hand) from the Terman River near Toome.  The border between the Republic of Ireland and the UK runs along the middle of this river.

I can extend my lesson from the first example to say that, to understand the ecology of any particular river you need to have perspectives obtained from many other rivers.   But, in this case, we see a potential limitation: the case of the “rare exception” that clouds an otherwise clear picture of an association between an organism and a particular set of circumstances.   The problem is particularly acute when dealing with the effect of nutrients because we are usually dealing with indirect, rather than direct effects.   Draparnaldia glomerata is usually associated with clean rivers with low concentrations of nutrients but it is not there because nutrient concentrations are low.   As for the diatom Amphora pediculus (see “The challenging ecology of a freshwater diatom?”) a more nuanced understanding of the relationship between an organism and nutrients yields more useful insights than simply assuming a cause-effect relationship.

How green is my river?

Atma_River_July13

I wrote recently about the problems of knowing whether changes we observe in the biology of streams and rivers are signs of long-term shifts caused by human activities or just the result of short-term variation (see “’signal’ or ‘noise’?”). An interesting paper has just been published that allows us to see our observations on the River Ehen into a broader perspective.   The paper was written by my friend Susi Schneider, of the Norwegian Institute for Water Research, and is based on long-term observations by herself and colleagues on the Atna River in Norway (illustrated above; see “A brief excursion to Norway” and subsequent posts).

First of all, here is a graph summarising our observations of biomass in the River Ehen over three years. You can see a fairly regular pattern emerging of low biomass in the summer (when grazing activity is most intense) and high biomass in the winter. But you can also see strong differences between years. There were much greater quantities of algae in winter 2013/14, for example, compared with winter 2014/15 and we are puzzling over why this may have happened.

Ehen_summary_graph

Trends in algal biomasss in the River Ehen, Cumbria between October 2012 and November 2015.   Values are the means of measurements made at four separate sites in a 5 km stretch of the river below the outflow of Ennerdale Water. Arrows indicate the approximate time of year when surveys of the Atma River were performed.

Though the Atna is about 1000 km to the north-east of the Ehen, there are similarities: both rivers have soft water and low levels of human impact and, furthermore, one of the two sites Susi writes about is immediately below a lake, just as our sites on the Ehen are downstream of Ennerdale Water.   The difference is that we visit the Ehen monthly, whereas Susi only visits the Atna once a year, although she takes care to visit at the same time each year.   I’ve indicated the time of year of her visits on the graph of the River Ehen, to aid comparisons between the two datasets.

One of the problems we have in the Ehen is that there is anecdotal evidence of lower quantities of biomass in the past.   The large quantities of algae was the trigger for our study; as is often the case, we generally do not start monitoring until a problem is perceived, which means that we then don’t have the baseline data that we need to understand the causes.   One of the interesting points that arise from Susi’s study is that there have been recent increases in algal cover at both the sites she studied.   Note that the pattern was different at the two sites (one just below a lake, one not). To put the two studies in perspective, the green box on the graphs from the Atma shows the length of time of our study covers, compared to Susi’s.

The reasons for the high algal cover in the Atna include cool, wet summers, driven by the North Atlantic Oscillation, and high discharges in August (i.e. the month before Susi’s regular visits). The former is a natural cyclical phenomenon; high August discharges are, in turn, a consequence of the cool, wet summers, and probably exert their effect by removing the grazers that would naturally control the biomass.   High discharges in the spring (i.e. 10 times the average discharge) also seem to have a major effect on the quantities of algae recorded later in the year.

Atma_long_term_algae_trends

Long term trends in algal cover at two locations in the upper catchment of the Atna River, Norway. The green box indicates the length of time covered by our observations on the River Ehen (2012-2015).  Graph from Schneider (2015).

What should we learn from this comparison?

  • Lesson 1: start monitoring at least 10 years before the problem arises, so that you have a strong baseline.   There is a serious point here, as environmental monitoring is likely to be one major casualty of the cuts in current spending. When problems do arise, the availability of historic data from the site is inevitably very useful but the quid pro quo is that you may need to invest in data collection even when there is no obvious short-term justification for that monitoring.
  • Lesson 2: following on from this, regard environmental monitoring is an insurance policy, insofar as you may not need to make a “claim” on every single site where you monitor.   In effect, this means accepting that some monitoring data that you collect may be redundant. The problem is that you don’t know which data will be redundant until at least a decade or so after you have collected it.   However, the complex nature of many of the problems that we face, particularly where there are interactions with climate (as in the Atna), you will not be able to make evidence-based decisions without long runs of reliable data.
  • Lesson 3: when dealing with algal growth in rivers (which reflects interactions between the physical, chemical and biological environment), do not try to draw any conclusions until you have measurements from years when the North Atlantic Oscillation is in each of its positive and negative phases.   Susi’s paper shows the problems of unravelling the complexities of biological interactions with climate. We need to think in “decades”, not “years” if we are to truly understand environmental change.
  • Lesson 4: simple measurements of criteria that can be easily understood by non-technical stakeholders aid communication. In both the Atna and the Ehen, measurements relate directly to public perceptions of healthy versus unhealthy rivers.   We have all the nerdy details of what algae are found at each site in both the Ehen and Atna, but the take-home message can be put across in terms of “how green is my river?”
  • Lesson 5: if you have to sample at widely-spaced intervals (i.e. yearly, as in Susi’s study), make sure that you sample at the same time every year.
  • Lesson 6: all of these lessons can be ignored if you are a politician with ambitions to create a leaner public sector. The sad truth is that the consequence of failing to invest in monitoring networks is not likely to be apparent for several years (well beyond the next General Election, to be blunt). Almost any aspect of public spending can be hacked away by a skilled political operator, so long as the effects of these decisions are chronic and slow to manifest themselves …

Reference

Schneider S. (2015). Greener rivers in a changing climate? – Effects of climate and hydrological regime on benthic algal assemblages in pristine streams. Limnologica 55: 21-32.

Diatoms and dinosaurs

Ennerdale_looking_north_Apr

Ennerdale Water, looking north from the outflow to the River Ehen, April 2015

My monthly visit to the River Ehen coincided with a week of warm weather, clear skies and low flows.   There were some glorious views of the Lake District fells shrouded in early morning mist when I first arrived and I spent a few minutes trying to capture them with my camera before turning my attention to work.

We are now into our third year of regular sampling and are beginning to get a feel for the seasonal dynamics of the algae in the stream. By this time of year, we expect to see the biomass falling as the invertebrates start to become more active and browse away the luxuriant winter growths (see “A very hungry chironomid”). Many of the stones do appear to have less algae growing on them but, at one site, there were still some very conspicuous growths of diatoms. I did wonder if these were growths of Didymosphenia geminata (see “A journey to the headwaters of the River Coquet”); I have never seen this species in the Ehen but several aspects of the habitat here are such that it is possible that it could thrive.   However, when I got a sample under my microscope, the growths turned out to be long-stalked Gomphonema species, with a variety of other diatoms, including Tabellaria flocculosa, Fragilaria spp., Hannaea arcus, Brachysira neoexilis and at least a couple of members of the Achnnathidium minutissimum complex.

Gomphonema_clumps_Ehen_Apri

Clumps of diatoms – mostly Gomphonema – on a stone from the River Ehen, April 2015.

The Gomphonema species in the River Ehen have perplexed us before. Our best bet is that the larger ones, in particular, are G. gracile.   However, we are straying into areas where names abound although the limits between species are often described in vague terms in the literature. Dawn Rose and Eileen Cox described an experiment recently in which they grew two cultures of a different diatom, Gomphonema parvulum, in culture.   Diatoms, you may remember, get smaller with successive cell divisions (see “Diminishing with age…”) until a point is reached when sexual reproduction is initiated.   When this happened with one of their cultures, they were surprised to see cells that our Floras would have called Gomphonema gracile emerge from the auxospores. They continued to grow these and watched as the G. gracile cells diminished in size until they resembled G. parvulum again. The problem is, as they point out in their paper, most diatomists base their opinions on the structure of the cleaned frustule rather than the behaviour of the living cell.

Gomphonema_Ehen_April15

Gomphonema cells from the River Ehen, Cumbria, April 2015.  The five images on the left show a size reduction series in valve view; the right hand image shows two cells in girdle view with the stalk just visible.   Scale bar: 10 micrometres (1/100th of a millimetre).

Why, you may ask, don’t more people do experiments of this type in order to define the limits between species of diatoms?   It is a good question. The day before my visit to the Ehen, I had read an article in my newspaper about a similar study that had confirmed that the dinosaur Brontosaurus was probably a distinct genus rather than being part of the genus Apatosaurus, as had been assumed for most of the 20th century.   The scientists based their conclusion on careful measurements of different properties of dinosaur skeletons, just as most diatomists measure and describe the diatom frustule. What else can a palaeontologist do?   Their material is, after all, long dead.   Diatomists work with the living world. It is, admittedly, fiddly and time-consuming work to prepare cultures of single species from field communities, and you need special incubators to grow these cultures once they have been isolated. But it is not impossible.

Most of us who study diatoms are over-anxious to get rid of the organic constituents of the cell in order to focus on the silica remains. We are, in effect, contemporary palaeontologists. Most palaeontologists would love the opportunity to get some definitive evidence of species limits (mating experiments on Brontosaurus: that would be quite a sight!).   They must think that us diatomists are a bizarre bunch indeed.

Reference

Rose, D.T. & Cox, E.J. (2014). What constitutes Gomphonema parvulum? Long-term culture studies show that some varieties of G. parvulum belong with other Gomphonema species. Plant Ecology and Evolution 147: 366-373.

Tschopp, E., Mateus, O. & Benson, R.B.J. (2015). A specimen-level phylogenetic analysis and taxonomic revision of Diplodocidae (Dinosauria, Sauropoda).   PeerJ 3: e857 [https://dx.doi.org/10.7717/peerj.857]