Comparing algae on a summer’s day …

I wrote about the effect of the long period of low flow in the River Wear a few weeks ago (see “Summertime Blues …”) and have, now, completed two dioramas depicting the state of the river in the main channel and in a filamentous algae-dominated backwater.  The first of these is dominated by free-living green algae, either single cells or coenobia (see note at end), which is a big contrast to the situation I recorded two months earlier when the assemblage was dominated by diatoms, with patches of filamentous green algae (see “Spring comes slowly up this way” and “A question of scale”).

I sent a small sample of the Wolsingham biofilm to Dave John for his opinion on the green algae, and he sent back a list with twenty one different green algae that he had found.  Fortunately, this confirmed my own original list, with Keratococcus bicaudatus, Scenedesmus, Desmodesmus and Monoraphidium all featuring.   He also commented that Keratococcus is hard to differentiate from Chlorolobium (which is also in his list) and that most of the green alga on his list are usually considered to be planktonic (Keratococcus and Chlorolobium are exceptions) although, as my earlier post suggested, these definitely formed a distinct biofilm on the surface of stones this year in the River Wear.

A diorama showing the biofilm in the River Wear at Woslingham, July 2018.   You can see coenobia of Demodesmuss communis (centre), Scenedesmus sp. (left) and Coelastrum microporum (right – half tucked behind a mineral particle, along with single cells of Keratococcus bicaudatus (upright cells) and Monoraphidium.  There are also some cells of Achnanthidium minutissimum on short stalks in the foreground and a cluster of Fragilaria gracilis cells in the background.

There seems to be little hard evidence on the habit of Keratococcus and Chlorobium apart from references to a preference for benthic habitats.   I have drawn them as upright cells, drawing on their similarity in form to Characium, for which there is better evidence of an upright habit (although Characium tends to grow on other algae, rather than on hard surfaces).  Whereas I often have a strong sense of the three dimensional arrangement of organisms within benthic biofilms, so little has been written about the preferences of these green algae that, apart from the Keratococcus, I have had to show them as a jumble of cells and coenobia across the picture frame.

The second diorama depicts the tangle of filamentous green algae that I found in the pools beside the main channel.  As I mentioned in my earlier post, these are species that I do not normally find at this site and are here, I presume, due to the long period of unusually warm weather and low flows.   One difference between these communities and that captured in my first diorama is that there is a more obvious organisation of the constituents here: the Cladophora filaments, though appearing as a tangle to us, form the foundation upon which epiphytes grow directly, but also around which Melosira filaments are entangled, rather like the lianas in a tropical rain forest.   The quantity of diatoms around the Cladophora is so great that their brown pigments completely mask the Cladophora’s green cells but note how the density of Cocconeis cells reduces towards the tips – the youngest parts of the filaments.

Depiction of filamentous algae growing in the margins of the River Wear at Wolsingham in July 2018, showing epiphytic Cocconeis pediculus and entangled Melosira varians.

There have been some recurring themes in my posts this summer: one is that UK rivers have been behaving quite differently to previous years, due to a combination of low flows (more accurately, a lack of the scour associated with high flows) and warm, well-lit conditions.   The low flows have also resulted, to some extent, in rivers becoming more physically heterogeneous, with side-pools and silty areas developing distinct assemblages of algae quite different to those encountered in the main channel.   Sometimes, the sum of these effects is for rivers to look less healthy than is usually the case.

The Wear at Wolsingham is one of those sites that I like to think I know well, having visited the location so many times over the past 30 years.  It is reassuring, in a rather humbling way, to know that it still has the capacity to surprise me.

Dave’s list of green algae from the Wolsingham biofilm, July 2018

Desmids
Closterium moniliferum
Closterium acerosum
Cosmarium botrytis
Cosmarium venustum
Staurastrum striatum

‘Chlorococcalean’ algae
Acutodesmus dimorphus
Coelastrum astroideum
(very small and atypical)
Coelastrum microporum (very small and atypical)
Chlorolobion braunii
Desmococcus olivaceum (subaerial species)
Desmodesmus communis
Desmodesmus subspicatus
Keratococcus bicaudatus

Monoraphidium arcuatum
Monoraphidium contortum
Monoraphidium griffithsia
Monoraphidium irregulare
Scenedesmus arcuatus
Pseudopediastrum boryanum
Tetradesmus obliquus
Tetraedron minimum

Note

A coenobium is a colony in which the cell number is fixed at the time of formation and not augmented subsequently.   Coenobia are particularly common in the Chlorococcalees.

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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.

 

 

Spring comes slowly up this way …*

I took a few minutes out on my trip to Upper Teesdale to stop at Wolsingham and collect one of my regular samples from the River Wear.  Back in March, I commented on the absence of Ulothrix zonata, which is a common feature of the upper reaches of rivers such as the Wear in early Spring (see “The mystery of the alga that wasn’t there …”).   I put this down to the unusually wet and cold weather that we had been experiencing and this was, to some extent, confirmed by finding prolific growths of Ulothrix zonata in late April in Croasdale Beck (see “That’s funny …”).   Everything seems to be happening a little later than usual this year.   So I should not have been that surprised to find lush growths of green algae growing on the bed of the river when I waded out to find some stones from which to sample.

These growths, however, turned out to be Stigeoclonium tenue, not Ulothrix zonata (see “A day out in Weardale”): it is often hard to be absolutely sure about the identity of an alga in the field and, in this case, both can form conspicuous bright green growths that are slimy to the touch.   Did I miss the Ulothrix zonata bloom in the River Wear this year?   Maybe.   Looking back at my records from May 2009 I see that I recorded quite a lot of narrow Phormidium filaments then but none were apparent in this sample.   That taxon thrived throughout the summer, so perhaps, again, its absence is also a consequence of the unusual weather.

Growths of Stigeoclonium tenue on a cobble in the River Wear at Wolsingham, May 2018.  

The photograph illustrates some of the problems that ecologists face: the distribution of algae such as Ulothrix zonata and Stigeoclonium zonata is often very patchy: there is rarely a homogeneous cover and, often, these growths are most prolific on the larger, more stable stones.   I talked about this in Our Patchwork Heritage; the difference now is that the patchiness is exhibited by different groups of algae, rather than variation within a single group.   Ironically, the patchiness is easier to record with the naked eye than by our usual method of sampling attached algae using toothbrushes.   That’s partly because we tend to sample from smaller substrata (the ones that we can pick up and move!) but also because of the complications involved in getting a representative sample.   We have experimented with stratified sampling approaches – including some stones with green algae, for example, in proportion to their representation on the stream bed – but that still means that we have to make an initial survey to estimate the proportions of different types of growth.

Under the microscope, therefore, the algal community looks very different.   There are fewer green cells and more yellow-brown diatom cells, these dominated by Achnanthidium minutissimum, elegant curved cells of Hannaea arcus and some Navicula lanceolata, still hanging on from its winter peak.   The patterns I described in The mystery of the alga that wasn’t there … are still apparent although the timings are all slightly adrift.

A view of the biofilm from the River Wear, Wolsingham in May 2018.

The schematic view below tries to capture this spatial heterogeneity.  On the left hand side I have depicted the edge of one of the patches of Stigeoclonium.   Healthy populations of Stigeoclonium do no support large populations of epiphytes, probably as a result of the mucilage that this alga produces.  My diagram also speculates that the populations of Gomphonema olivaceum-type cells and Ulnaria ulna may be living in the shadow of these larger algal growths, as neither is well adapted to the fast current speeds on more exposed rock surfaces.  Finally, on the right of the image, there are cells of Achnanthidium minutissimum, small fast-growing cells that can cope with both fast currents and grazing.   I have not included all of the taxa I could see under the microscope, partly because of the space available.  There is no Hannaea arcus or Navicula lanceolata and I have also left out the chain of Diatoma cells that you can see on the right hand side of the view down the microscope.

The speckled background in the image of the view down the microscope is, by the way, a mass of tiny bacteria, all jigging around due to Brownian motion.  The sample had sat around in the warm boot of the car for a few hours after collection so I cannot be sure that these were quite as abundant at the time of collection as they were when I came to examine it.  However, some people have commented on the absence of bacteria – known to be very abundant in stream biofilms – from my pictures, so these serve as a salutary reminder of an extra dimension that really needs to be incorporated into my next images.

Schematic view of the biofilm from the River Wear at Wolsingham, May 2018.  a. Stigeoclonium tenue; b. Gomphonema olivaceum complex; c. Ulnaria ulna; d. Meridion circulare; e. Achnanthidium minutissimum.   Scale bar: 10 micrometres (= 1/100th of a millimetre).

* Samuel Taylor Coleridge, Christabel (1816)

 

Eutrophic or euphytic?

A paper has just been published that should be required reading for anyone interested in the management of nutrients in in ecology.   It is a follow-up of a 2006 paper with the catchy title “How green is my river” that set out to provide a conceptual framework for how rivers responded to enrichment by nutrients.   That original paper contained several good ideas but, crucially, not all of them were underpinned by evidence.  A decade on, several of the predictions and statements made in that original paper have been tested, and the time has come to re-examine and modify that original conceptual model.

My reaction to the 2006 paper was that it was very interesting but not fully reflective of the rivers in my part of Britain, whose rougher topography produced quite different responses to nutrient enrichment than that proposed in their original model.   That criticism has been addressed in the revised version, which places greater emphasis on the physical habitat template, which means that it is more broadly applicable than the original version.   But that, in turn, got me wondering about the continued relevance of a term such as “eutrophication” to rivers.

People have been using the term “eutrophic” to describe lakes with high concentrations of nutrients since early in the 20th century.   As the century progressed, evidence of a causal relationship between inorganic nutrients and algal biomass, and the consequences for other components of lake ecosystems grew.   With this foundation, it has then become possible to predict the benefits of reducing nutrients and there are plenty of case studies, particularly from deep lakes, that demonstrate real improvements as nutrient concentrations have declined.

Attempts to apply the same rationale to rivers have, however, met with far less success.   Legislation to reduce nutrients in rivers has been in force in Europe since 1991 (the Urban Wastewater Treatment Directive, followed by the Water Framework Directive) and whilst this has led to reductions in concentrations of phosphorus in rivers (see  “The state of things, part 2”), there has, in most cases, not been a corresponding improvement in ecology.   There are a number of reasons for this but, at the heart, there was a failure to understand that the tight coupling between nutrients and biology that was the case in deep lakes did not also occur in running waters.   What was needed was recognition of fundamental differences between lakes and rivers, and “How green is my river?” and, now, this new paper have both contributed to this.

However, one consequence of recognising the importance of the physical habitat template alongside nutrients is to challenge the relevance of the term “eutrophic” when describing rivers.   “Eutrophic” literally means “well-nourished” so is appropriate in situations where high nutrients cause high plant or algal biomass.   This high biomass (strictly speaking, the primary production arising from this biomass) then creates problems for the rest of the ecosystem (night-time anoxia caused by plants consuming oxygen being a good example).   If high biomass can arise due to, let’s say, removal of bankside shade or alteration to the flow regime, perhaps (but not always) in combination with nutrients, then perhaps we need a term that does not imply a naïve cause-effect relationship with a single pressure?

My suggestion is to shift the focus from nutrients to plant growth by using the term “euphytic” (“too many plants”) as this would shift the emphasis from simply driving down nutrient concentrations (expensive and not always successful) towards reducing secondary effects.  It is possible that strategies such as planting more bankside trees, for example, or altering the flow regime or channel morphology (see “An embarrassment of riches …”) may be just as beneficial, in some cases, as reducing nutrient concentrations.   That said, we also have to bear in mind that nutrients may have an effect well downstream, so focus on amelioration of effects within a particular stream segment will never be a complete solution.

I should emphasise that a lot of work has been done in recent years to understand the concentrations of nutrients that should be expected in undisturbed conditions, and also to understand the nutrient concentrations that lead to changes in community structure in both macrophytes and algae.   These show that many rivers around Europe do have elevated concentrations of nutrients and I am not trying to side-step these issues.  I do, however, think it is important that regulators can prioritise those rivers in greatest need of remediation and, in most cases, they do this without considering the risk of secondary effects.

It is, largely, a matter of semantics.   I have been involved in many conversations over the past couple of decades about how to improve the state of our rivers.  Many of those have centred on the importance of reducing nutrient concentrations (which would be, indisputably, a major step towards healthier rivers).  But there is more to it than that.  And Mattie O’Hare and colleagues are helping to open up some new vistas in this paper.

Note: the photograph at the top of this post shows the River Wear at Wolsingham.  This stretch of the river captures many of the challenges facing river ecologists: nutrient concentrations are relatively low and there is good bankside shade.  However, the flow of the river is highly altered due to impoundments upstream and a major water transfer scheme.  How do all these factors interact to create the often prolific algal growths that can be seen here, particularly in winter and spring?

References

Hilton, J., O’Hare, M., Bowes, M.J. & Jones, J.I. (2006).  How green is my river?  A new paradigm of eutrophication in rivers.   Science of the Total Environment 365: 66-83.

O’Hare, M.T., Baattrup-Pedersen, A., Baumgarte, I., Freeman, A., Gunn, I.D.M., Lázár, A.N., Wade, A.J. & Bowes, M.J. (2018).  Responses of aquatic plants to eutrophication in rivers: a revised conceptual model.   Frontiers in Plant Science.   9: 451

The mystery of the alga that wasn’t there…

I was back at the River Wear at Wolsingham a few days ago for my second visit of the year (see “The River Wear in January” and “The curious life of biofilms” for accounts of the first visit).   I had wanted to go out earlier in the month but we’ve had a month of terrible weather that has translated into high river flows.  Even this trip was touch and go: the river was about 30 cm higher than usual and the gravel berm that usually stretches out under the bridge on the left bank was largely submerged.

Compare the image of the substratum with the one I took in January: that one had a thick film with a chocolate-brown surface whilst the March substratum had a much thinner film lacking any differentiation into two layers.  When I put a small sample of the biofilm under my microscope, I could see that it was dominated by diatoms with only a few strands of green algae.   Many of the diatoms that I saw in January were still here in March but Navicula lanceolata, which comprised over half the algal cells I saw in January was now just 15 per cent of the total whilst Achnanthidium minutissimum was up from about 15 per cent to about 40%.    However, as A. minutissimum is a much smaller cell, N. lanceolata still formed more of the total biovolume.   One other difference that I noticed as I peered down my microscope was that there was much less amorphous organic matter in the March sample compared with the one from January.

The substratum at the River Wear, Wolsingham on 24 March 2018.   The photograph at the top shows the view from the road bridge looking downstream.

When I looked back at notes I had taken after my visit in March 2009, I saw that the riverbed then had been covered with lush growths of the green alga Ulothrix zonata (you can see a photograph of this in “BollihopeBurn in close-up”).   I did not see this on my visit last week.  That might be because the high water level means that I could not explore as much of the river as I wanted, but it was more likely a consequence of the preceding conditions.   The graph below shows at least three separate high flow events during March, the first of which associated with the melting of the snow that fell during the “Beast from the East”.   I suspect that these high flow events would have both moved the smaller substrata (the ones I usually pick up to sample!) scouring away the biofilms in the process.

A view of the biofilm from the River Wear, Wolsingham in March 2018.

River levels at Stanhope, 20 km upstream from Wolsingham across March 2018 showing three separate high flow events.  A screenshot from www.gaugemap.co.uk.

The final graph shows the trend in the three algae that I’ve been talking about over the course of 2009, which is similar to what I am seeing in 2018 except that that the timing of the decline in Navicula lanceolata and Ulothrix zonata along with the increase in Achnanthidium minutissimum is slightly different.   In very broad terms N. lanceolata is typical of winter / early spring conditions, favoured by thick biofilms partly created by the matrix of stalks that Gomphonema olivaceum and relatives creates.   Achnanthidium minutissimum, on the other hand, is the most abundant alga through the summer and early autumn.  It is a species that thrives in disturbed conditions, such as we would expect after the weather we’ve experienced this March.   However, we must not forget that the grazing invertebrates that thrive

during the summer months also represent a type of disturbance.  Ulothrix zonata thrives in the late winter / early spring window (see “The intricate ecology of green slime”).   I would have expected it to have persisted beyond March but, as I said earlier in the post, I may have missed some as it was difficult to get a good impression of the whole reach due to high flows.

This moveable switch between a “winter” and “summer” state creates a problem when we are sampling for ecological status assessments.   The Environment Agency has, for as long as I have worked with them, had a “spring” sampling window that starts on 1 March and runs to the end of May.  As you can see, this straddles the period when there is a considerable shift in the composition of the flora.   I’ve always suggested that they wait as long as possible within this window to collect diatom samples to increase the chance of being past the switch.  However, with a huge network to cover in a short period, along with other logistical considerations, this was always easier said than done.   I’ve worked closely with the Environment Agency to manage as much of the variation in their diatom analyses as is possible (see “Reaching a half century …”); one of the mild ironies is that simply being a huge Behemoth of an organisation can, itself, be the source of some of the variation that we are trying to manage.

Trends in approximate biovolume of three common taxa discussed in this post in the River Wear at Wolsingham during 2009.  

Certainly uncertain …

Back in May I set out some thoughts on what the diatom-based metrics that we use for ecological assessment are actually telling us (see “What does it all mean?”).  I suggested that diatoms (and, for that matter, other freshwater benthic algae) showed four basic responses to nutrients and that the apparent continua of optima obtained from statistical models was the result of interactions with other variables such as alkalinity.   However, this is still only a partial explanation for what we see in samples, which often contain species with a range of different responses to the nutrient gradient.  At a purely computational level, this is not a major problem, as assessments are based on the average response of the assemblage. This assumes that the variation is stochastic, with no biological significance.  In practice, standard methods for sampling phytobenthos destroy the structure and patchiness of the community at the location, and our understanding is further confounded by the microscopic scale of the habitats we are trying to interpret (see “Baffled by the benthos (1)”).  But what if the variability that we observe in our samples is actually telling us something about the structure and function of the ecosystem?

One limitation of the transfer functions that I talked about in that earlier post is that they amalgamate information about individual species but do not use any higher level information about community structure.  Understanding more about community structure may help us to understand some of the variation that we see.   In the graph below I have tried to visualise the response of the four categories of response along the nutrient/organic gradient in a way that tries to explain the overlap in occurrence of different types of response.   I have put a vertical line on this graph in order that we can focus on the community at one point along the pollution gradient, noting, in particular, that three different strategies can co-exist at the same level of pollution.  Received wisdom amongst the diatom faithful is that the apparent variation we see in ecological preferences amongst the species in a single sample reflects inadequacies in our taxonomic understanding.  My suggestion is that this is partly because we have not appreciated how species are arranged within a biofilm.  I’ve tried to illustrate this with a diagram of a biofilm that might lead to this type of assemblage.

Schematic diagram showing the response of benthic algae along a nutrient/organic gradient.  a.: taxa thriving in low nutrient / high oxygen habitats; b.: taxa thriving in high nutrient / high oxygen habitats; c.: taxa thriving in high nutrient / low oxygen habitats; d.: taxa thriving in high nutrients / very low oxygen habitats.   H, G., M, P and B refer to high, good, moderate, poor and bad ecological status.

The dominant alga in many of the enriched rivers in my part of the world is the tough, branched filamentous green alga Cladophora glomerata.   This, in turn, creates micro-habitats for a range of algae.  Some algae, such as Rhoicosphenia abbreviata, Cocconeis pediculus and Chamaesiphon incrustans, thrive as epiphytes on Cladophora whilst others, such as C. euglypta are often, but not exclusively, found in this microhabitat.  Living on Cladophora filaments gives them better access to light but also means that their supply of oxygen is constantly replenished by the water (few rivers in the UK are, these days, so bereft of oxygen to make this an issue).   All of these species fit neatly into category b. in my earlier post.

Underneath the Cladophora filaments, however, there is a very different environment.  The filaments trap organic and inorganic particulate matter which are energy sources for a variety of protozoans, bacteria and fungi.   These use up the limited oxygen in the water, possibly faster than it can be replenished, so any algae that live in this part of the biofilm need to be able to cope with the shading from the Cladophora plus the low levels of oxygen.   Many of the species that we find in highly polluted conditions are motile (e.g. Nitzschia palea), and so are able to constantly adjust their positions, in order to access more light and other resources.   They will also need to be able to cope with lower oxygen concentrations and, possibly, with consequences such as highly reducing conditions.  These species will fit into categories c. and d. in the first diagram.

A stylised (and simplified) cross-section through a biofilm in a polluted river, showing how different algae may co-exist.   The biofilm is dominated by Cladophora glomerata (i.) with epiphytic Rhoicosphenia abbreviata (ii.), Cocconeis euglypta (iii.) and Chamaesiphon incrustans (iv.) whilst, lower down in the biofilm, we see motile Nitzschia palea (v.) and Fistulifera and Mayamaea species (vi.) growing in mucilaginous masses.

However, as the cross-section above represents substantially less than a millimetre of a real biofilm, it is almost impossible to keep apart when sampling, and we end up trying to make sense of a mess of different species.   The ecologists default position is, inevitably, name and count, then feed the outputs into a statistical program and hope for the best.

A final complication is that river beds are rarely uniform.  The stones that make up the substrate vary in size and stability, so some are rolled by the current more frequently than others.  There may be patches of faster and slower flow associated with the inside and outsides of meanders, plus areas with more or less shade.   As a result, the patches of Cladophora will vary in thickness (some less stable stones will lack them altogether) and, along with this, the proportions of species exhibiting each of the strategies.  The final twist, therefore, is that the vertical line that I drew on the first illustration to illustrate a point on a gradient is, itself, simplistic.  As the proportions vary, so the position of that line will also shift.  Any one sample (itself the amalgamation of at least five microhabitats) could appear at a number of different points on the gradient.  Broadly speaking, uncertainty is embedded into the assessment of ecological status using phytobenthos as deeply as it is in quantum mechanics.  We can manage uncertainty to some extent by taking care with those aspects that are within our control.   However, in the final analysis, a sampling procedure that involves an organism 25,000 times larger than most diatoms blundering around a stream wielding a toothbrush is invariably going to have limitations.

The same schematic diagram as that at the start of this article, but with the vertical line indicating the position of a hypothetical sample replaced by a rectangle representing the range of possibilities for samples at any one site. 

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.