The natural history of numbers

I have made a few facetious comments in this blog about the tendency for ecologists to spend more time staring at spreadsheets than engaging directly with the organisms and habitats they are trying to understand.   There is, of course, a balance that needs to be struck.   We can learn a lot from analysing big datasets that would not have occurred to a biologist who spent all his or her time in the field.  And, I have to admit, somewhat grudgingly, there is a beauty to the numerical landscapes that becomes apparent when a trained eye is brought to bear on data.

I’ve been involved in a project for the European Commission which has been trying to find good ways of converting the ecological objectives that we’ve established for the Water Framework Directive into targets for the pressures that lead to ecosystem degradation.   The key principle behind this work is summarised in the graph below: if the relationship between the biology (expressed as an Ecological Quality Ratio, EQR) and a pressure (in this case, the phosphorus concentration in a river or lake) can be expressed as a regression line then we can read off the phosphorus concentration that relates to any point on the biological scale.   (Note that there are many other ways of deriving a threshold phosphorus concentration, but this simple approach will suffice for now.)

PvEQR_1pressure

Relationship between biology (expressed as an Ecological Quality Ratio, EQR) and phosphorus concentration for a hypothetical dataset.  The blue line indicates the least squares regression line, the horizontal green line is the position of the putative good/moderate status boundary and the vertical green line is the phosphorus concentration at this boundary position.  Coefficient of determination, r2= 0.89 (rarely achieved in real datasets!)

This is fine if you have a strong relationship between your explanatory and response variables and you are confident that there is a causal relationship between them.  Unfortunately, neither of these criteria are fulfilled in most of the datasets we’ve looked at; in particular, it is rare for the biota in rivers to be so strongly controlled by a single pressure.  This means that, when trying to establish thresholds, we also need to think about how a second pressure might interact with the factor we’re trying to control.   If this second pressure has an independent effect on the biota then we might expect some sites that would have had high EQRs if we just considered phosphorus might now be influenced by this second pressure, so the EQR at these sites will fall below the regression line we’ve just established.   When we plot the relationship between EQR and phosphorus taking this second pressure into account, our data no longer fits a neat straight line, but now has a “wedge” shape, due to the many sites where the second pressure overrules the effect of phosphorus.   If you were tempted to put a simple regression line through this new cloud of data, you would see the coefficient of determination, r2, drop from 0.89 to 0.35.  Note, too, how the change in slope means that the position of the phosphorus boundary also falls.   More worryingly, we know that, for this hypothetical dataset, the new line does not represent a causal relationship between biology and phosphorus.  That’s no good if you want to use the relationship to set phosphorus targets and, indeed, you now also need to think about how to manage this second pressure.

PvEQR_2pressures

The same relationship as that shown in the previous graph, but this time with an interaction from a second pressure.  The blue line is the regression line established when phosphorus alone was considered, and the red line is the regression between EQR and phosphorus in the presence of this second pressure.

My purpose in this post is not to talk about the dark arts of setting targets for nutrient concentrations that will support healthy ecosystems but, rather, to talk about data landscapes.  Once we saw and started to understand the meaning of “wedge”-shaped data, we started to see similar patterns occurring in all sorts of other situations.   The previous paragraph and graph, for example, assumed that the factor that confounded the biology-phosphorus relationship was detrimental to the biology, but some factors can mitigate the effect of phosphorus, giving an inverted wedge, as in the next diagram.  Once again, the blue line shows the regression line that would have been fitted if this was a pure biology versus phosphorus relationship.

PvEQR_2pressures_#2

The same relationship, but this time with a second factor that mitigates against the effect of phosphorus.  Note how the original relationship now defines the lower, rather than the upper, edge of the wedge. 

Wedge-shaped data crop up in other situations as well.  The next graph shows the number of diatoms I recorded in a study of Irish streams and there is a distinct “edge” to the cloud of data points.   At low pH (acid conditions), I rarely found more than 10-15 species of diatom whereas, at circumneutral conditions, I sometimes found 10-15 species but I could find 30 or more.   Once again, we are probably looking at a situation where, although pH does exert a pressure on the diatom assemblage, lots of other factors do too, so we only see the effect of pH when its influence is strong (< pH 5).

Ntaxa_v_pH_FORWATER

The number of diatom species recorded across a pH gradient in Irish streams.  Unpublished data.

In this case, the practical problem is that the link between species number and pH is weak so it is hard to derive useful information from the number of species alone.   It would be dangerous to conclude, for example, that the ecology at a site was impacted by acidification on the strength of a single sample.  On the other hand, if you visited the site several times and always recorded low species numbers, then you have a pretty good indication that there was a problem (not necessarily low pH; toxic metals would have a similar effect).   Whether such a pattern would be spotted will depend on how often a site is visited and the sad reality is that sampling frequencies in the UK are now much lower than in the past.

However, this post is not supposed to be about the politics of monitoring (evidence-based policy is so much easier when you don’t collect enough uncomfortable evidence) but about the landscapes that we see in our data, and what these can tell us about the processes at work.   Just as a field biologist can look up from the stream they are sampling and gain a sense of perspective by contemplating the topography of the surrounding land, so we should also be aware of the topography of our data before blithely ploughing ahead with statistical analyses.

with_Geoff_&amp;_Heliana

With Geoff Phillips and Heliana Teixaira – fellow-explorers of data landscapes in our project to encourage consistent nutrient boundaries across the European Union.

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Some like it hot …

My reflections on algae that thrive in hot weather continued recently when I visited a river in another part of the country.  As this is the subject of an ongoing investigation, I’ll have to be rather vague about where in the country this river flows; suffice it to say it is in one of those parts of the country where the sun was shining and your correspondent returned from a day in the field with browner (okay, redder) arms than when he started.   Does that narrow it down?

A feature of some of the tributaries, in particular, was brown, filamentous growths which, in close up, could be seen to be speckled with bubbles of oxygen: a sure sign that they were busy photosynthesising.  These were most abundant in well-lit situations at the edges of streams, away from the main flow.   Under the microscope, I could see that these were dominated by the diatom Melosira varians, but there were also several filaments of the cyanobacterium Oscillatoria limosa, chains of the diatom Fragilaria cf capucina and several other green algae and diatoms present.

Melosira varians is relatively unusual as it is a diatom that can be recognised with the naked eye – the fragile filaments are very characteristic as is its habitat – well lit, low-flow conditions seem to suit it well.   It does seem to prefer nutrient-rich conditions (see “Fertile speculations …”) but it can crop up when nutrient concentrations are quite low, so long as the other habitat requirements are right for it.  The long chains of Melosira (and some other diatoms such as Fragilaria capucina and Diatoma vulgare) help the cells to become entangled with the other algae.   I could see this at some sites where the Melosira seemed to grow around a green alga that had been completely smothered by diatoms and was, I presume, withering and dying.  In other cases, the Melosira filaments are much finer and seem to attach directly to the rocks.   Neither arrangement is robust enough for Melosira to resist any more than a gentle current which is why it is often most obvious at the edges of streams and in backwaters.   As is the case for Ulva flexuosa, described in the previous post, I suspect that the first decent rainfall will flush most of this growth downstream.   Another parallel with Ulva is that, despite this apparent lack of adaptation to the harsh running water environment, Melosira varians is more common in rivers and streams than it is in lakes.

Melosira varians-dominated filaments at the margins of a stream.  Top photograph shows the filaments smothering cobbles and pebbles in the stream margins (frame width: approximately one metre); bottom photograph shows a close-up (taken underwater) of filaments with oxygen bubbles (frame width: approximately one centimetre).

Algae from the filaments illustrated above: a. and b.: Melosira varians; c. Fragilaria cf capucina; d. Oscillatoria limosa.  Scale bar: 20 micrometres (= 1/50th of a millimetre).  

The graphs below support my comments about Melosira varians preferring nutrient rich conditions to some extent.  Many of our records are from locations that have relatively high nutrient concentrations; however, there are also a number of samples where M. varians is abundant despite lower nutrient concentrations.   How do we explain this?   About twenty years ago, Barry Biggs, Jan Stevenson and Rex Lowe envisaged the niche of freshwater algae in terms of two primary factors: disturbance and resources.   “Resources” encompasses everything that the organism needs to grow, particularly nutrients and light, whilst “disturbance” covers the factors such as grazing and scour that can remove biomass.   They used this framework to describe successions of algae, from the first cells colonising a bare stone through to a thick biofilm.   As the biofilm gets thicker, so the cells on the stone get denser and, gradually, they start to compete with each other for light, leading to shifts in composition favouring species adapted to growing above their rivals (see “Change is the only constant …”).

The relationship between Melosira varians and nitrate-nitrogen (left: “NO3-N”) and dissolved phosphorus (right: “PO4-P”).   The vertical lines show the average positions of concentrations likely to support high (red), good (green), moderate (orange) and poor (red) ecological status (see note at end of post for a more detailed explanation).

They suggested that filamentous green algae were one group well adapted to the later stages of these successions but these, in turn, create additional opportunities for diatoms such as M. varians which can become entangled amongst these filaments and access more light whilst being less likely to being washed away.   If there is a period without disturbance then the Melosira can overwhelm these green algal filaments.   Nutrients, in this particular case, do play a role but, in this case, are probably secondary to other factors such as low disturbance and high light.  Using the terminology I set out in “What does it all mean?”, I would place M. varians in the very broad group “b”, with the caveat that the actual nutrient threshold below which Melosira cannot survive in streams is probably relatively low.   Remember that phosphorus, the nutrient that usually limits growth in freshwater, comprises well under one per cent of total biomass, so a milligram of phosphorus could easily be converted to 100 milligrams of biomass in a warm, stable, well-lit backwater.

Schematic diagram showing the approximate position of Melosira varians on Biggs et al.’s conceptual habitat matrix.

The final graph shows samples in my dataset where Melosira varians was particularly abundant and this broadly supports all that has gone before: Melosira is strongly associated with late summer and early autumn, when the weather provides warm, well-lit conditions with relatively few spates.

The case of Meloisra varians is probably a good example of the problem I outlined in “Eutrophic or euphytic?”  I have seen similar growths of diatoms in other rivers recently, due to the prolonged period of warm, dry conditions.  It is easy to jump to the conclusion that these rivers have a nutrient problem.  They might have, but we also need to consider other possibilities.   Like Ulva flexuosa in the previous post, Melosira varians is an alga that is enjoying the heatwave.

Distribution of Melosira varians by season.   The line represents sampling effort (percent of all samples in the dataset) and vertical bars represent samples where M. varians forms >7% of all diatoms (90th percentile of samples, ranked by relative abundance). 

Reference

Biggs, B.J.F., Stevenson, R.J. & Lowe, R.L. (1991). A habitat matrix conceptual model for stream periphyton. Archiv für Hydrobiologie 143: 21-56.

Notes on species-environment plots

These are based on interrogation of a database of 6500 river samples collected as part of DARES project.  Phosphorus standards are based on the Environment Agency’s standard measure, which is unfiltered molybdate reactive phosphorus.  This approximates to “soluble reactive phosphorus” or “orthophosphate-phosphorus” in most circumstances but the reagents will react with phosphorus attached to particles that would have been removed by membrane filtration. The current UK phosphorus standards for rivers that are used here 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 boundaries based on the average alkalinity (50 mg L-1 CaCO3) and altitude (75 m) in the whole dataset.

There are no UK standards for nitrate-nitrogen in rivers; thresholds in this report are based on values derived using the same principles as those used to derive the phosphrus standards and give an indication of the tolerance of the species to elevated nitrogen concentrations.  However, they have no regulatory significance.

 

 

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.

The way things were …

Writing the previous post led me to contemplate how much things had changed over the time that I have been working in this field.  Back in the early 1990s when I first set out to look at the response of diatoms to nutrients in streams, few in the National Rivers Authority (NRA, predecessor to the Environment Agency) regarded phosphorus as a serious pollutant in rivers, and most biologists thought about ecological quality solely in terms of organic pollution and invertebrates.   In order to investigate the effect of nutrients, I wanted to visit sites where organic pollution was not a problem.

I was helped in this task by the work done by biologists at the then Institute for Freshwater Ecology (now Centre for Ecology and Hydrology) who had just developed the early versions of RIVPACS (“River Invertebrate Prediction and Classification System”) which established the principle of expressing ecological quality as the observed quality / expected quality.  This, in turn, required an ability to predict the “expected” condition for any stream.   The work that had developed these equations started from a dataset of invertebrate and environmental data collected from a wide range of “unpolluted” running water sites which, in those far off days, was compiled by asking biologists working for the Regional Water Authorities (predecessors to the NRA) for their recommendations of sites that were of “good” or “fairly good” quality.  Nowadays, screening sites to be used for calibrating ecological methods is a much more rigorous procedure but this was the first tentative step on a long journey and “expert judgement” was as good a place to start as any.

The paper that emerged from this exercise (see reference below) analysed data from these “unpolluted” sites and classified them into eight groups.  Each of these groups consisted of sites that shared similar invertebrate assemblages which reflected similarities in the habitat, from upland, fast flowing becks to deep, wide slow-flowing rivers in the lowlands.  The authors included a useful table that listed the physical and chemical characteristics of each of these groups and I noticed that the phosphorus concentrations reported for these spanned a very wide range.   This meant that I could use these as the basis for putting together a sampling program that spanned a long gradient of nutrient pressure without the complications of organic pollution.   The outcome of that work was the first of the two papers referenced in my previous post.

Time has moved on and I thought it would be interesting to revisit these “unpolluted” sites to see how they would be classified using the UK’s current standards for phosphorus.  This highlights a striking difference between the prevailing idea of “unpolluted” in the early 1980s and the present day, as all of these groups had average concentrations that equate to substantial enrichment by modern standards; in half the groups this average concentration would be classified as “poor status” whilst the maximum concentrations in three groups equates to “bad status”.   Whatever way you look at it now, these sites were far from “unpolluted”.

Classification of TWINSPAN end-groups of unpolluted river sites in Great Britain based on Armitage et al. (1984) along with average and maximum phosphorus concentrations recorded in each group and the phosphorus status based on current environmental standards.  M = moderate status; P = poor status; B = bad status.

I am not being critical of the approach taken by Patrick Armitage and colleagues.  In many ways, I regard the work of this group as one of the most significant contributions to the science of ecological assessment in my lifetime.   I am just intrigued to see how the thinking of ecologists and regulators has moved on in the thirty years or so since this paper was published.  I know from my own early conversations with NRA biologists that inorganic nutrients were not perceived as a problem in rivers until the early 1990s.   It was probably the European Community’s Urban Wastewater Treatment Directive (UWWTD) that started to draw the attention of biologists in the UK to these problems, and which led to the development of stricter environmental standards for nutrients, though not without opposition from several quarters.

This, then is a situation where good legislation provided the impetus needed to start the process.  There were places in the UK – rivers in the Norfolk Broads, for example – where nutrients were already being regulated, but these were special circumstances and nutrient problems in most rivers were largely ignored. Indeed, as I said in my previous post, phosphorus was not even measured routinely in many rivers.   I heard via my professional grapevine that it was the Netherlands who had made the case for the clauses in the UWWTD concerning regulating nutrients, as their stretches of the lower Rhine were subject to numerous problems caused by unregulated inputs of nutrients from countries upstream.   I do not know if this is true, but it is certainly plausible.   However, once the need to control eutrophication in rivers was codified in UK law, then the debate about how to evaluate it started, one of the outcomes of which was more funding for me to develop the Trophic Diatom Index (referenced in the previous post).  And, gradually, over time, concentrations in rivers really did start to fall (see “The state of things, part 2”).   I’d like to think the TDI played a small part in this; though this might also mean that I am partially responsible for the steep increase in water charges that everyone endured in order to pay for better water quality …

Reference

Armitage, P.D., Moss, D., Wright, J.F. & Furse, M.T. (1984).  The performance of a new biological water quality score system based on macroinvertebrates over a wide range of unpolluted running-water sites.  Water Research 17: 333-347.

A river is reborn …

I started to tell the story of the Ouseburn in the previous post, but have not yet reached a happy ending.  The Beast that is Newcastle Airport has been transformed, if not by a kiss, then by intelligent regulation, but the river is still far from being beautiful.   The Environment Agency, the Handsome Prince in this particular fairy story (has it ever been described in such terms before?) needs to ride out to find other monsters to slay.

One candidate that my students usually identify in their write-ups is phosphorus, whose concentrations have gradually crept up over the years, as the graph below illustrates.   As in the graphs in my previous posts, I have differentiated between data collected by the Environment Agency and my students.  I have also circled a cluster of points that sit outside the main trend, as a reminder that my students are still learning their craft, and sometimes may make mistakes.   The trend is, nonetheless apparent: the river has had elevated phosphorus concentrations for as long as measurements have been taken, and concentrations are gradually creeping upwards.  The student’s data may exaggerate this slightly, but the trend is definitely there.    Although no sewage works discharge to the stream, there are plenty of storm drains, and there are concerns that domestic “grey water”, and its associated detergent residues, may be entering these rather than the foul sewers.  More recently, a study as part of the Ouseburn River Restoration Project (ORRP) has found that some farmers in the upper part of the catchment are stockpiling farmyard manure on behalf of livery stables and some of the leachate from this may be entering the upper stretches of the river.

phosphorus_in_ouseburn

Trends in concentrations of reactive phosphorus in the Ouseburn over time.  Woolsington is upstream of the airport, Airport tributary (Abbotswood Burn) receives runoff from Newcastle Airport and Jesmond Dene is about 10 km downstream from the airport.  Closed symbols are annual means of data collected by the National Rivers Authority and Environment Agency; open symbols are means of data collected and analysed by Newcastle University Geography students in October (once also in February) of each yearThe lower dashed line is the UK environmental standard for reactive phosphorus to support “good ecological status”; the upper dashed line is the threshold between “moderate” and “poor” status (the threshold between “poor” and “bad” status is at 1.04 mg/L).

In addition to problems such as phosphorus that we can see from our analyses, there are problems that are less obvious because they only happen occasionally, and not necessarily when a sampler is dipping a bottle into the river.  The Pantomime Villain of this story (“He’s behind you …” “oh no he’s not”, “oh yes he is …”) is the overloaded sewerage network and, in particular, the storm sewer overflows which divert foul waste into the river when the sewers are overloaded with surface water from heavy rain.   Even though the graphs in the previous post showed that ammonia and BOD are usually at low levels, there will be short periods when the storm sewers dump raw sewage into the river.  This is a great lesson to my students in why biological monitoring is so necessary: the poor quality of the invertebrate community reflects the state of the river through the whole year, not just the minute or so when the sampler’s bottle is being filled.

A combination of hard impermeable surfaces, the drainage system with its overflows and many artificially-straightened lengths of the river mean that storm water makes its way very quickly to the stream (see “Fieldwork in the rain”).  In extreme cases this can lead to homes and businesses being flooded.   These straightened sections of the river also mean that there is little variation in velocity to create the variation in habitat that would allow a range of organisms to find suitable conditions to thrive.   So another of the objectives of the ORRP is to restore the natural meandering path of the river in the upper stretches as a first step towards creating a more natural river which will, at the same time, slow the flow and reduce the likelihood of flooding downstream.   New property developments such as Newcastle Great Park have been designed with Sustainable Drainage Systems (see “In search of SuDS …”) to create more permeable areas that will soak up rainfall and slow its journey to the river, reducing the size of the flood peaks associated with heavy rainfall.

ouseburn_challenges_oct16

Challenges facing the Ouseburn: left: Newcastle Great Park, one of a number of new or planned housing developments in the upper part of the catchment; right: straightened river channel near Three Mile Bridge beside the Great North Road in Newcastle.

To be honest, there are many grander rivers in the country than the Ouseburn where I would prefer to do my fieldwork.  I feel privileged to be able to visit the River Ehen in the Lake District on a regular basis.   We rightly worry about maintaining fragile ecosystems and rare species in these remote places but the Ouseburn presents equal, if less romantic, challenges.   Most of us are urban, rather than rural dwellers and our most likely interactions with the aquatic world will be with these artificially-straightened extensions to our overloaded sewerage systems.   There is something of Frankenstein’s monster about these rivers: at their worst, in flood, they are our own creations, the result of our own attempts to overrule nature.  So I am very enthusiastic about the work of the ORRP and similar schemes around the country.   These are a small step towards restoring a natural harmony between man and water, and working with, rather than against the powers of nature.  And creating a greener, more pleasant urban milieu in the process.

A day out in Weardale …

Wear_Frosterley_Apr16

Fine weather towards the end of last week lured me away from my computer screen and out to Weardale on the flimsiest of pretexts, and gave me an opportunity to drag this blog away from musing about pandas and Taoism and back to its core business.   Forty minutes drive from home brings me to Frosterley, in the historic mining and quarrying areas of the valley, though enough time has elapsed for the hard edges of the industrial heritage to have been rubbed away by the gradual incursions of nature.   It is a beautiful spot, with the river meandering down a tree-lined channel with wildlife in profusion.

Understanding a river is partly about recognising patterns at different scales and just as one can look at a landscape such as the one above and infer something about its conditions (fast-flowing water? relatively natural channel?  low population density?) so a naturalist should be able to adjust his or her focus and read the story of the river at smaller and smaller scales.   It takes a few moments to adjust – physically and mentally – to searching at these finer scales, but the story of the river started to open up once I looked more closely.

There were a number of small dark green patches on the river bed, most noticeable towards the margins (maybe because my wellingtons limited my explorations to the shallower parts of the channel).   They were slimy to the touch and I could make a guess at their identity, but needed to get them under a microscope before I could confirm this.   That’s one of the problems with my kind of natural history: there is none of the immediacy that a birdwatcher or field botanist gets from putting a name onto organisms in the field.  Underneath the microscope, however, my alga yielded up its secrets: I could see narrow filaments composed of cells each with a single chloroplast lapped around most of the perimeter, with a number of side branches, each gradually tapering to an acute apex.  This is Stigeoclonium tenue, a common alga of streams and rivers although possibly less common now than was the case a couple of decades ago.  It is hard to be sure because it is easily overlooked and there is no systematic recording of these organisms, but I am not the only person to voice this suspicion.

Stig_tenue_Frosterley_Apr16

Stigeoclonium tenue from the River Wear at Frosterley.  Left: macroscopic view showing tufts of green filaments attached to a submerged stone (scale bar: approximately 1 centimetre); right: microscopic view (scale bar: 10 micrometres = 1/100th of a millimetre).

Looking at these filaments, I can read a little more about the state of the river than I could infer from my landscape-scale perspective at the top of this post.   I can suggest to you that this river is, or has been in the recent past, flush with the nutrients that Stigeoclonium and other plants need to thrive.   Why do I know this?   I shared laboratory space as a PhD student with a colleague, Martin Gibson, who was investigating the physiology of this species and its relatives.  This built on earlier work by Brian Whitton and others which showed that when nutrients were scarce, the branches of Stigeoclonium were much longer, tapering gradually into fine, elongated cells that were devoid of chloroplasts.  These had special enzymes which were able to break down organic compounds that contained the phosphorus that the alga needed.   When nutrients were plentiful, these hairs disappeared.

I strongly suspect that, were I to look at recent phosphorus measurements in this part of the Wear, they would indicate low concentrations, which might suggest that my inference is wrong.  However, the Environment Agency’s standard approach to measuring water chemistry is based on a single visit each month and we know, from finer-scale studies, that phosphorus concentrations can vary greatly over short periods of time (particularly related to changes in the weather and flow regime).  We also know that their standard analytical method does not record phosphorus that is tightly bound into molecules.   The Environment Agency’s approach is good enough to give the basic insights into rivers that they need to regulate the environment, but misses many of the nuances.   That’s why an understanding of the ecology of apparently insignificant organisms can be so useful to river managers.

One of the reasons I wanted some samples of algae from the River Wear was to see if we can simplify the process of identifying algae, in order to make these insights more available.  I’ve written already about RAPPER  (see “The Democratisation of Stream Ecology?”) but so far those of us who have tested this have all had access to high power microscopes.   It is possible to buy field microscopes at a much lower cost but these typically only have a maximum of 100x magnification.  One of my objectives for this year is to see just how can be identified with this limitation.    The image below was taken at 100x using my main microscope, and you can see that the basic form of Stigeoclonium can still be resolved and, indeed, differentiated from related genera such as Draparnaldia (see “The River Ehen in February”), so this is an encouraging first step.  If we can repeat this with the other algae used in RAPPER, then all sorts of possibilities for “citizen science” open up …

Stig_x100_Frosterley_Apr16

Stigeoclonium from the River Wear at Frosterley, photographed at 100x magnification.

References

Gibson, M.T. and Whitton, B.A. (1987).   Hairs, phosphatase activity and environmental chemistry in Stigeoclonium, Chaetophora and Draparnaldia (Chaetophorales). British Phycological Journal 22, 11-22.

Gibson, M.T. and Whitton, B.A. (1987).  Influence of phosphorus on morphology and physiology of freshwater Chaetophora, Draparnaldia and Stigeoclonium (Chaetophorales, Chlorophyta). Phycologia 26: 59-69.

Whitton, B.A. & Harding J.P.C. (1978).  Influence of nutrient deficiency on hair formation in Stigeoclonium.  British Phycological Journal  13: 65-68.

The state of things, part 2

If the last post presented a fairly optimistic picture of the quality of Britain’s rivers over the past 30 years, this one is more of a reality check, highlighting some areas where our rivers are still suffering from pollution which means that they are unlikely to meet EU targets.

The first pollutant I will consider in this post is phosphorus.   The data for this and other inorganic nutrients is not summarised by regions but by the predominant type of agriculture in the catchment, reflecting the importance of run-off from land as a source of these pollutants.   However, much phosphorus enters our rivers via sewage works and, as we saw for BOD and other pollutants associated with organic pollution, the trend over the past twenty years or so is generally downwards. Note that the start of the downward trend does not start until the mid 1990s.   This is because reducing phosphorus in effluents only became a legal requirement when the EU’s Urban Wastewater Treatment Directive came into force. The horizontal lines, once again, represent the UK standards for dissolved phosphorus in rivers.   There is a story behind these standards, in which I played a minor role.

HMS_PO4P_trends

Trends in average inorganic phosphorus (PO4-P) concentrations in rivers draining three different land types in Great Britain. Note that the vertical axis is on a logarithmic scale.   The horizontal lines show recent UK standards required to support different classes of ecological status (see text for more details).

You’ll see that the trend lines for the two lowland land types are approaching but have not decisively crossed the line which indicates “good status”.   Presence of a certain phosphorus concentration does not, itself, determine status but these indicate guide values that ecologists believe should be attained if the river is to support a healthy ecosystem. I was involved in setting the original standards and our team originally set values that were much lower than those that were eventually adopted.   We had worked out concentrations that were associated with locations where we had found healthy ecosystems.   However, when these figures made their way through the bureaucracies of our regulatory organisations, they encountered strong resistance from those charged with actually achieving the reductions.   You’ll see that average concentrations have more than halved during the first decade of the 21st century – a considerable achievement. To be told that these concentrations were not low enough was a bitter pill to swallow. To cut a long story short, statistical black arts were performed behind the scenes (not by us, I hasten to add) to justify a higher standard, eventually set at 0.12 milligrams per Litre.

The cavalier fashion in which this original phosphorus standard was set meant that, before long had passed, weaknesses started to become apparent.   Not least of which was that rivers that should, on paper, have had conditions suited to good ecological status continued to have plant and algal communities that were not characteristic of good status.   Eventually, the phosphorus standards were revisited and, in most cases, made more stringent, though not without opposition from the water companies who would bear much of the burden of meeting these.   These new standards would pull each of the horizontal lines on my graph lower, and so make the overall position look yet more pessimistic.

The trend for one other important plant nutrient in GB rivers is equally pessimistic. Concentrations of nitrogen as nitrate have shown virtually no reductions at all over the period that the HMS has been operating.   There are, I think, two reasons for this. The first is that ecologists generally stress the importance of phosphorus over nitrogen as the key nutrient in freshwaters, which means that nitrogen concentrations have received less attention than, perhaps, they deserve. The second reason is more political: the biggest source of nitrates in our rivers is agriculture and DEFRA has to balance the interests of the environment with that of the vocal and politically-powerful farming lobby.   That there is no ecologically-based standard for nitrates in UK rivers tells its own story.   We could easily have produced a standard for nitrates at the same time as we revised the phosphorus standards (the information we needed was in the same spreadsheet) but were warned off. There is, in theory, a standard for nitrates in freshwaters, produced to meet the requirements of the EU’s Nitrates Directive but this was designed to protect human health, not ecology, and is set at a very high concentration. Rather than use this, I have plotted the standards used in the Republic of Ireland on my charts and, assuming that an equivalent UK standard would be of a similar magnitude, these show that the nitrate concentration of lowland rivers is generally much higher than these values.   This is, of course, a very broad-brush picture but it gives us a rough idea of what is going on.

HMS_NO3N_trends

Trends in average inorganic nitrogen (as nitrate) concentrations in rivers draining three different land types in Great Britain. There are no current UK standards for nitrate-N concentrations required to support different classes of ecological status; those plotted here are for the Republic of Ireland.

Looking back at the two posts, the picture that emerges is of reasonably good regulation of those types of pollutants that have been the traditional focus of regulation.   Both the water companies and the regulators understand the processes need to reduce organic pollution levels in sewage effluents, and the necessity for doing this.   The last twenty years or so have seen a change in focus towards inorganic nutrients and here we run into problems. First of all, the benefits for the public are often not immediately clear which, in turn, makes increasing water bills to pay for the expensive process of phosphorus removal more difficult. I have heard staff from water companies raise this argument several times when arguing against tighter regulation of phosphorus.   But the second problem is that regulation of nutrients needs to embrace diffuse inputs from agriculture as well as point sources.   This presents a huge problem as there are many more farms than sewage works to be visited, many of which are struggling to survive in a tough economic climate.   Consequently, there is little political will to drive the process from the top.   These are, indeed, challenging times for freshwater ecologists.