How green is my river?


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.


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.


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 …


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.

A wet afternoon in Berlin …

A happy coincidence brought me to the Gemäldegalerie in Berlin just as I was reading Laura J. Snyder’s book Eye of the Beholder, which is a joint biography of Anton van Leuwenhoek, the pioneer microscopist, and his neighbour (and, most likely, friend) Johannes Vermeer.   The Gemäldegalerie has two fine Vermeers, the culmination of a series of galleries which gives an impressive and coherent overview of the Northern Renaissance which then lead into a series of galleries showing paintings from the Dutch Golden Age.   What we see in the Northern Renaissance can be very roughly summarised as the outcome of experimentation at many levels – with oil paint rather than tempera, with non-religious subject matter and with compositional techniques such as single-point perspective.   Ideas had filtered up to the north from Italy, but the range of outputs is distinctively different from those of their southern European contemporaries.   There is no hard and fast delineation between the Northern Renaissance (roughly 16th century) and the Dutch Golden Age (roughly 17th century) but the Golden Age pictures are distinctively different. Experiments with light and perspective have borne fruit (Vermeer, of course, but also Pieter Saenredam), portraiture becomes more naturalistic and, indeed, intense (Rembrandt and Frans Hals), landscape, the “background” to many Northern Renaissance paintings, becomes a legitimate subject in its own right (Jacob van Ruisdaal, Aelbert Cuyp) and activities hitherto too mundane for consideration become legitimate subjects (Vermeer’s domestic interiors; also Pieter de Hooch).


Johannes Vermeer: Woman with a pearl necklace (1664, left) and The Wine Glass (1660, right). Both in the Gemäldegalerie, Berlin.

Laura Snyder’s book offers some insights. The possibility that Vermeer used optical technology such as the camera obscura to ensure accurate depiction of perspective has been examined before.   The issue, however, may be less to do with the “tricks” that Vermeer used than with the broader intersections between artists and natural scientists at the time, both exploring new ways to “see” the natural world. Look at Jan van Eyck’s Madonna and Child, in the Gemäldegalerie. In this masterpiece of the Northern Renaissance there are aspects of perspective and the proportions of the baby Jesus that suggests that he is following tradition rather than looking afresh at the world.   The priority on direct experience over tradition is key to understanding both the scientific revolution and the art of the Dutch Golden Age and the intersection of the lives of van Leuwenhoek and Vermeer – two men who are remembered for the way in which they saw the world around them – is no mere coincidence.


Jan van Eyck: Madonna in the Church (c. 1440). Gemäldegalerie, Berlin

This, however, is not the whole story.   Snyder and others (Simon Scharma’s Embarrassment of Riches springs to mind) point to the wealth of the Dutch Republic during this period and how this fuelled an art market to provide paintings for the burgeoning and prosperous middle classes to decorate their homes. The market, in other words, fuelled creativity.   This takes us down some interesting paths: is it demand, or is it competition amongst artists to satisfy the demand?   There was an interesting item on the BBC website recently that argued that creativity is, to some extent, dependent upon repetition.   The demand for art, in other words, drives the process.   Vermeer, to be fair, with only 34 paintings unambiguously attributed to him, may be the exception to this rule, but living in an environment where so many artists were simultaneously trying to solve the same problems of perspective, colour and composition must surely have fuelled his own investigations into the depiction of the world around him?

Just to be clear, the free market coupled with craftsmanship may have produced the best art in the seventeenth century.   In our age of mass production and multinational corporations the opposite may well be true. That’s a topic for another day…


Snyder, L.J. (2015). Eye of the Beholder: Johannes Vermeer, Antoni van Leeuwenhoek and the reinvention of seeing. Norton, New York and London.

Hard science in hard water?

Having started to think about the ecology of small Fragilaroid diatoms in a recent post (see “When is a diatom like a London bus?”), I thought that it might pay to look in more detail at the habitats that these taxa do like, in the hope that this will help us to understand why they occur together so often.   I am just looking at two “species” in this post: “Staurosirella pinnata” (which we suspect to be a complex of several species) and “Staurosira construens” (which is also a complex, as the records in my database merge a number of varieties, most of which have subsequently been raised to the status of species in their own right).

One problem has to be confronted at the outset: these taxa also share a propensity to form chains which remain intact even after we’ve made slides.   This means that we often encounter aggregates of five or more cells, which violates the assumptions of random distributions of diatoms that underpin our statistical methods.   No-one, to my knowledge, has found a satisfactory means of dealing with this, but it should be borne in mind when considering the graphs which follow.

The first graph shows the distribution of records of these species in my database along an alkalinity gradient, and generally confirm the preference of both species for hard water.   I have highlighted two outliers on the chart for Staurosira construens. These samples are from the same location, the upper reaches of the River Wey (South) in Surrey, which receive a mixture of soft water, flowing off the Greensand, and harder water from the surrounding areas.   I have encountered anomalies between diatoms and water chemistry in this area before, which are probably the result of the complex hydrology of the area.


The distribution of Staurosirella pinnata (left) and Staurosira construens (right) along an alkalinity gradient. Records from the “DARES” dataset.   Two outliers from the River Wey (South) are highlighted.

The next two graphs show the distribution of records along phosphorus and nitrogen gradients and these show opposite responses: both seem to be most abundant when phosphorus is low and nitrogen is high. Again, we have the problem of the two outliers from soft water sites confusing the view for Staurosira construens but we can generalise and say that neither species is likely to be abundant (meaning > 10 per cent of all valves) except when these conditions are met.

The horizontal red lines on these graphs show the range of phosphorus and nitrogen measured in a single river, the River Wylye, during a study in 2011-2012. I have included these lines to give a rough idea of the precision that we should expect when defining the preferences of a diatom.   The River Wylye is a chalk stream, which tend to have relatively stable hydrology, so the range of nutrient concentrations measured in these streams is probably lower than is the case for many rivers.


The distribution of Staurosirella pinnata (left) and Staurosira construens (right) along an reactive phosphorus gradient. Records from the “DARES” dataset.   Vertical lines represent the approximate position of high (blue), good (green), moderate (orange) and poor (red) status boundaries.   The horizontal line shows the range of concentrations encountered in the River Wylye, Wiltshire in 2011-2012.


The distribution of Staurosirella pinnata (left) and Staurosira construens (right) along a nitrate-N gradient. Records from the “DARES” dataset.   Vertical lines represent the position of the (Irish) high (blue) and good (green) status boundaries.   The horizontal line shows the range of concentrations encountered in the River Wylye, Wiltshire in 2011-2012

Ecological assessment using diatoms is largely based on indices that calculate the relative position of a sample along a quality gradient based on a combination of the known ecology of the species and the representation of that species in the sample.   This means that the result is most strongly influenced by the most common species and anything that occurs below about five per cent has little influence. These charts suggest that Staurosirella pinnata and Staurosira construens will both be good indicators of a combination of low phosphorus and high nitrogen in hard water; however, there are a “tail” of records that extend into other types of water.   One valid question is whether the individuals responsible for these occurrences outside the “optimum” are the same species as those that are abundant at low P / high N / hard water.   Given what I wrote above about both of these taxa probably being complexes, this is a possibility.   However, the generally low numbers means that solving taxonomic riddles will be unlikely to lead to a great increase in precision in ecological assessments.

Personally, I lean towards the options I suggested in Baffled by the benthos (2) – that diversity within samples may be controlled by a wide range of factors unrelated to anthropogenic pressures and that interspecific diversity may give insights into ecological resilience. The problem is that this hypothesis is easier to propose than it is to test. It is not impossible to test; however, the hegemony of taxonomically-inclined diatomists over those with a genuine interest in functional ecology means that will probably remain no more than a theory for some time to come …

The natural history of migration …

A few months ago, I read Nick Lane’s new book, The Vital Question.   In it, he tries to explain the early evolution of life in terms of the biochemical processes that cells need to generate energy. Explaining biochemistry to the masses is a tough call but Lane just about pulls it off. That’s not damning with faint praise as I was turned off biochemistry at university by dull lecturers reciting long lists of equations.   This time around (perhaps because no-one expects me to regurgitate those same equations in an exam), I could focus on the implications of what he was saying.

But I was reading his book at the same time as the migrant crisis began to dominate headlines. Boatloads of Africans and Syrians were being pulled from the Mediterranean or washed up on beaches in Greek Islands.   Every news bulletin seemed to have its quotient of pundits offering their opinion on how to deal with the situation.   There seemed to be an assumption, particularly amongst those of the right, that migrants could (or should) be controlled by tighter border controls. This is ironic, as the same politicians can also often be heard pushing for free trade across borders, but it also resonated with some of the messages that I was picking up from Nick Lane’s book.

Nick Lane’s thesis is that to understand life, we first have to understand how cells obtain the energy that they need in order to function.   He then takes a step back and explains how explains how, in nature, there is almost constant movement of molecules in solution. It is not random movement, but follows thermodynamic and electrochemical principles to even out imbalances. Any high concentration of molecules with a particular property (it could be concentration, it could be reactabilty) will naturally disperse until the concentration is the same throughout the solution. Adding sugar to tea is a good example, although the tea will be cold if you leave the sugar to disperse naturally, which is why sweet tea aficionados stir their cups to hasten dispersal.

The same principles apply to molecules with electric charges and, as Lane explains, this is the key to understanding life.   Insert a barrier to this free flow of energy and you create a gradient; make the barrier slightly permeable and the gradient between the two sides will ensure a constant flow that will, in turn, produce the energy that a cell needs.   OK, so you need to read the book to get all the nuances; the point I want to make is that, if we replace “molecules” with “people” and “membranes” with “national borders”, we have a fair model of the migrant crisis.   There are strong gradients across Europe and the near East in prosperity (fuelling economic migrants) and in fear (fuelling the Syrian refugee crisis).   Expressed in these terms, the flow of people is entirely predictable, as is their massing at semi-permeable national borders that can be crossed if you have enough money (to pay smugglers) or initiative.

The response of nationalist politicians is to try to make the borders less permeable (razor-wire fences, more security) but this is not getting to the root of the problem.   If we use the biochemical analogies from Nick Lane’s book, this will just create a barrier to the flow and mean that the migrants accumulate in ever increasing numbers on one side of the border. Country A may have temporarily averted the problem, but Country B now faces an ever bigger problem.   The gradient that drives the flow still exists.

So we need to address the gradient, not the flow?   All credit to the UK government for putting extra money into refugee camps in countries around Syria, as creating safe enclaves in the region has got to be part of the solution to stopping refugees starting their perilous journeys in the first place. The sheer number of refugees and the instability of the region means that this is no easy task.   Making Western Europe a less attractive destination for refugees would also reduce the gradient but the situation in Syria is now so awful that we would seriously compromise our own humanity before we solved the problem from this direction.   Or we could accept that the gradient exists and that barriers are, at best, temporary obstacles to desperate refugees, and adapt.   Ironically, the political right is happy to encourage this option when it suits. There are some on the right (Matt Ridley, for example) who accept the argument for climate change but who argue that we should be prepared to adapt rather than combat it via legislation. Why not apply the same principle to migration, adapt a humane and welcoming approach to those who make it to our borders and react to what transpires, rather than second-guess the outcome of a complex problem?   My surname is a clue to my own family’s origins: our ancestors responded to a famine by migrating across the Irish Sea, along with hundreds of thousands of others who, in turn, made their own contributions to the UK’s economy.   Fighting against nature is never a good idea (see previous post and links therein on invasive alien plants); learning from experience is, in the long-term, a much better bet.


Nick Lane (2015).   The Vital Question: Why Is Life the Way It Is?   Profile Books, London.

In praise of Japanese knotweed …


Autumn colours on Pelaw Wood, photographed from Durham Racecourse, October 2015.

There, I’ve said it.   Now to redeem my professional reputation …

It has been a particularly fine autumn, with a long period of warm, dry, calm weather allowing leaves to stay on trees and giving us plenty of opportunities to stroll through Durham and enjoy their colours.   Some of the most vivid yellow-orange colours were right down beside the river, large heart-shaped leaves growing from a tangle of erect stems readily recognisable as Japanese knotweed, Fallopia japonica. There is not normally much good to say about Japanese knotweed, introduced to Britain in about 1850 via Kew Gardens, and now crowding out native species but the colours this autumn were so spectacular that I thought it deserved at least a brief mention here.   We can, perhaps, see the plant from the perspective of the Victorian horticulturists who thought that it would be an attractive addition to the garden flora.


Japanese knotweed on Durham riverbanks, November 2015.

It is all a matter of perspective: autumnal landscapes around Durham are enhanced by introduced maples and sycamores and, indeed, beech trees (not native to the north of England). Even larch, an introduced deciduous conifer, adds to the overall aesthetic, so long as it is planted in small, unregimented patches. But these are not vigorous hard-to-eradicate colonisers like Japanese knotweed or Himalayan balsam (see “The future is pink …”), so we do not damn them with the epiphet “weed”. “Weed” is not a botanical term; it is an anthropocentric word, a declaration of war, if you like, against a plant that intrudes into spaces where we do not think it belongs.   Just for a few weeks each Autumn, however, perhaps we should declare a truce?

Costing the earth’s pantomime villain …

Some of the themes I wrote about in recent blogs came together in the latest edition of BBC Radio 4’s Costing the Earth , which unpacked the issue of river water quality and, in particular, the pernicious effects of nitrogen and phosphorus on aquatic ecosystems.   Overall, I thought that the program did a good job of explaining a complicated issue but it also served as a case study of the issues I wrote about in “Wide Sargassum Sea …”: algae are forever portrayed in the media as if they were A Bad Thing. Paul Knight, Chief Executive of Salmon and Trout Conservation UK, interviewed during the program, described algae in the River Itchen as “… a brown sort of claggy stuff …” and then went on to explain that excessive nutrients “… speed up the growth of algae and the wrong sort of weed”. From which I infer that there is a right sort of weed, but that all algae are universally bad?   Pedantic? Maybe …

However, a few minutes later we hear John Slader, also associated with Salmon and Trout Conservation UK, bemoaning the lack of invertebrate life in the River Itchen: “You’ve got to recognise that this is part of a food chain and if these insects aren’t there, what would happen to your swallows, your martins, your wagtails …”   The same subtle (or careless) omission: the food chain, of course, extends down to the algae as well as upwards to fish and birds. Successful restoration of chalk streams needs to be based on an understanding of the right sort of algae, as these ultimately create the habitat within which the insects and fish will thrive.

Later in the same programme, Mike Bowes of the Centre for Ecology and Hydrology was interviewed and he pointed out some of the practical problems associated with reducing nutrient concentrations to levels that would reduce the quantities of algae, based on the fine research he has performed over many years in southern England.   He then went on to echo some of the points I made in my previous post, “An embarrassment of riches …”: it is possible to reduce the quantities of algae not just by reducing nutrients but also by planting bankside trees in order to create more shade. This would, incidentally, have bonus effects for wildlife and aquatic diversity, and would undoubtedly be much cheaper than removing nutrients.   We should, however, remember that this may reduce the risk of eutrophication although the hazard that the nutrients presented would remain.

The program did a good job of presenting the complexity of river pollution and therein lies the challenge: if a problem is complex, there will not be straightforward cause-effect relationships.   It should not, perhaps, surprise us that the interviewees representing the pressure group were the ones that simplified the story to a cause-effect relationship (“high nutrients = bad fishing and fewer birds”) whilst the independent academic scientist offered a more nuanced view.   And it is perhaps inevitable that algae, the most diverse component of the river ecology story (see “The sum of things …”) are overlooked except when the narrative demands a convenient villain.


Bowes, M.J., Ings, N.L., McCall, S.J., Warwick, A.,Barrett, C., Wickham, H.D., Harman, S.A., Armstrong, L.K., Scarlett, P.M., Roberts, C., Lehmann, K., Singer, A.C. (2012). Nutrient and light limitation of periphyton in the River Thames: implications for catchment management. Science of the Total Environment 434: 201-212.

An embarrassment of riches …

A couple of years ago, on a snowy day in January, I gave a talk and ran a practical on algal-based ecological assessment for an MSc class at the University of Bristol. As part of the practical class, I asked my colleague, Marian Yallop, who organised the session, if she could set some algal cultures growing a few days ahead of my arrival, in order to stimulate some discussion amongst the students on what we understood by “eutrophication”.

I asked for four flasks, each with a standard algal growth media, into which an inoculum of algae was pipetted. I can’t remember what species of algae we used, except that it was a common green algae.   The growth medium was naturally low in nutrients, so Marian augmented the media in two of the flasks with an extra squirt of nutrients, so we had two with “background” nutrients and two with high nutrients.   Then she placed one of each pair on a windowsill, and the other in a refrigerator and left them for a few days.


Our eutrophication “thought experiment” in Bristol in January 2013

I had given the students the following definition of eutrophication, used in a key EU Directive (the Urban Waste Water Treatment Directive): “the enrichment of water by nutrients, especially compounds of nitrogen and/or phosphorus, causing an accelerated growth of algae and higher forms of plant life to produce an undesirable disturbance to the balance of organisms present in the water and to the quality of the water concerned.”

My question to the students was simple.   Which of the four flasks is “eutrophic”?   The first part of the definition says “the enrichment of water by nutrients …”, so we could have argued that both of the “plus nutrient” treatments were eutrophic. However, the definition then goes on to say “… causing accelerated growth of algae ….”.   The plus nutrient treatment that was kept in the refrigerator did not fulfil this criterion; however, the one kept on the window ledge is the greenest of all the flasks. So could we claim that only the “plus nutrient, window ledge” flask was eutrophic?

What about the “minus nutrient window ledge” flask?   That looked quite green, even though the nutrient levels were low.   This illustrates a further important point: the quantity of phosphorus, in particular, that a plant needs to grow is very small and if other conditions are right (i.e. a windowsill in a centrally-heated laboratory), then you can get high biomasses of algae, even without excess nutrients (see “A brief excursion to Norway”).   In nature, such natural abundance would not last for long: it would be scoured away by a flood or eaten by hungry invertebrates on the river or lake bed. Based on my experience of northern English rivers, I would only get concerned if the high biomass persisted beyond a few weeks.

The flask with added nutrients that was kept in the refrigerator offers another perspective. Even if we could argue that it is not strictly “eutrophic”, we should acknowledge that there is a risk of a high biomass developing.   In our thought experiment, the cool dark environment of the refrigerator minimised the risk of a high biomass developing.   However, for as long as there are elevated nutrient concentrations, we have to acknowledge that both “plus phosphorus” treatments represent a hazard to healthy ecosystem functioning.

A final, and slightly pedantic, interpretation was that none of these treatments fulfilled the final criterion in the definition of causing “… an undesirable disturbance to the balance of organisms …”. In this case, of course, there was only a single organism present, so it was impossible to demonstrate this particular criterion. However, a quick scan of the literature on eutrophication does show a strong bias towards establishing a link between nutrients and aquatic plants and algae, and the secondary effects are often neglected.   These, however, provide the essential “so what?” to our arguments.   I live in a world of algae-obsessed nerds and we sometimes need a sharp poke in the ribs to be reminded that most people need a better return on the expensive investment in better water treatment than just to be told that the algae have changed (see “So what?”).