Summertime blues …

My reflections on the effects of the heatwave on freshwater algae continued with the latest of my regular visits to the River Wear at Wolsingham.  A comparison of the picture above with that at the head of “Spring comes slowly up this way …” says it all: the sun was shining and the gravel berms that I usually use to enter the river were occupied by families with barbeques whilst their children splashed around in the water.   At times such as this, a grown man picking up stones and then vigorously brushing their tops with a toothbrush would have invited too many questions, so I slunk off 100 metres or so downstream and found a quieter spot to explore.

The biofilm in the main channel of the River Wear at Wolsingham, July 2018. 

The first thing I noticed was that the biofilm coating the submerged stones at the bottom of the river had a greenish tinge, rather than its usual chocolate brown appearance.  It also was more crusty and less slimy to the touch than I usually see in this river.  When I got a specimen under the microscope, I could see that the composition was completely different to that which I had observed in previous months.   Most samples from this location that I’ve looked at in the past have been dominated by diatoms, with occasional spring flourishes of filamentous green algae.  Today, however, the sample was dominated by small green algae – a group that I am not very confident at identifying.   My rough estimate is that these formed about three quarters of all the algae that I could see, with diatoms and cyanobacteria each accounting for about half of the remainder.   The most abundant greens were a tiny single-celled alga that I tentatively identified as Keratococcus bicaudatus, along with a species of Scenedesmus and Desmococcus communis.   There were also a number of cells of Monoraphidium arcuatum and some of Ankistrodesmus sp.

Two views of biofilms from the River Wear, Wolsingham in January 2018.   Left: from the main channel; right: from pools at the edge of the channel.

Green algae from the River Wear at Wolsingham, July 2018: a. Desmococcus communis; b. Monoraphidium arcuatum; c. Scenedesmus sp.; d. unidentified, possibly Keratococcus bicaudatus.  Scale bar: 10 micrometres (= 1/100th of a millimetre).

However, there were also pools at the side of the channel, away from the main current but not so cut off that they were isolated from the river itself.   These were dominated by dense, brown filamentous growths, very similar in appearance to the Melosira varians flocs I described in “Some like it hot …”.  The filaments, however, felt coarser to the touch and, in close-up, could be seen to be branched, even without recourse to a microscope.   Once I got these under the microscope, I could see that they were filaments of Cladophora glomerata, another green alga, but so smothered with epiphytic diatoms (mostly Cocconeis pediculus) that they appeared brown in colour.

This combination of Cladophora glomerata and Cocconeis pediculus in the backwaters were as much of a surprise as the green-algae-dominated biofilms in the main channel.   These are species usually associated with enriched rivers (see “Cladophora and friends”) and, whilst I have seen Cladophora in the upper Wear before, it is an unusual occurrence.   Just as for the prolific growths of Melosira varians described in “Some like it hot …” it is tempting to leap to the conclusion that this must be a sign that the river is nutrient-rich.  However, the same conditions will apply here as there: “nutrients” are not the only resource that can limit plant growth and a steady trickle of phosphorus combined with warm, sunny conditions is just as likely to lead to prolific growths as a more conventionally “polluted” river.

Cladophora filaments smothered by the diatom Cocconeis pediculus in a pool beside the River Wear at Wolsingham, July 2017.   The frame width of the upper image is about 1 cm; the scale bar on the lower images is 20 micrometres (= 1/50th of a millimetre).

Another way to think of these situations is that, just as even healthy people are occasionally ill, so healthy streams can go through short periods when, based on a quick examination of plants and animals present, they exhibit symptoms associated with polluted conditions or simply (as for the first sample I described) different to what we usually expect to find.   A pulse of pollution might have passed downstream or, as seems to be happening at the moment, an unusual set of conditions lad to different organisms thriving.   Just as the ability to fight off infection forms part of a doctor’s understanding of “health”, so I expect that the River Wear will, in a few weeks time, be back to its usual state.   Healthy ecosystems, just like healthy humans, show “resilience”.   The irony is that, in this case, the “symptoms” are most obvious at a time when we are enjoying a summer better than any we’ve had in recent years.


Loving the low flows …

It is a long time since we have had a heatwave in the UK that has lasted as long as the present one.  The last that compares was in 1995, when the A1 near my house was busy with tankers ferrying water from Kielder Water in Northumberland to the drought-afflicted areas of Yorkshire.   Three weeks in and gardeners are staring anxiously at the parched soil and quietly praying for rain whilst, at the same time, trying to make the most of the rare luxury of warm weather.   Rivers, too, are showing the effects of the weather.  In some parts of the country, rivers are drying out and fish stocks are threatened.   That is not the case here in the north-east, but the River Wear is showing some signs.

The medieval Prebends Bridge is one of the most picturesque sights in Durham but, at the moment, the water underneath the bridge – and, indeed, throughout Durham – is bright green with flocs of algae.   Closer inspection showed this to be fronds of Ulva flexuosa: the cells are arranged to form a hollow tube, like a sausage skin, which traps the oxygen released by photosynthesis to give the alga an integral buoyancy aid.  You can see how this clearly in the image below. The common name for this alga is “gutweed”, which offers us another metaphor for the appearance of these semi-inflated sacs of cells.   This broad thallus is loosely attached to the river bed (see lower picture below) but is easily dislodged, after which the thalli drift downstream until they become entangled in other water plants or submerged branches.  In the current state, Durham’s rowers are grumbling that it is becoming entangled with their oars

We often see a little Ulva flexuosa in the Wear during the summer, but rarely as much as this.  It is a species that thrives under still, warm conditions and which also benefits from the weirs which regulate the flow of the river in Durham.   It is an alga that we tend to associate with nutrient-rich conditions, but this might be because the type of slow-flowing lowland rivers where it can become common are more likely to be nutrient rich than faster-flowing upland rivers where it is rarely found.   The current weather, in other words, creates the “perfect storm” for Ulva flexuosa.   Ironically, a storm – perfect or otherwise – will probably alter the flow regime in the Wear enough to flush it all downstream.  Another curiosity is that, despite being favoured by low flows and the near-standing water behind the weirs in Durham, Ulva flexuosa seems to be more likely to form mass growths in rivers rather than in lakes or ponds.

Ulva flexuosa in the River Wear, July 2018: the upper picture shows free-floating thalli, inflated by oxygen released by photosynthesis; the lower photograph shows thalli still attached to the river bed.

In my experience, the genus Ulva tends to be absent from nutrient-poor conditions which is subtly different to saying that it thrives when nutrients are abundant.  There are other factors – warm, stable conditions in particular that determine the success of the genus in any particular place.  The Wear has seen a significant decrease in nutrients in recent years yet here we are, in 2018, with a river full of Ulva.   I could say that, despite this reduction in nutrients, the Wear is still, relatively speaking, nutrient-rich, but the coincidence with an altered flow regime, a prolonged spell of warm weather and low flow conditions seems too great to ignore.   Ulva flexuosa is, in other words, a fine example of an alga where we need to think of a multifactorial “habitat template” rather than just in terms of single stressors.   We also need to think in terms of probabilities of mass proliferations increasing or decreasing as habitat factors vary, rather than a simple likelihood of finding Ulva at any particular location.

That means we need to look at climate change forecasts and, if there is a likelihood of more long, warm, dry summers, then we should expect more frequent blooms of Ulva in our rivers.  We may tinker with nutrient concentrations and even try to restore more natural flows (though Durham’s rowers will have a view if that was tried here!) but, ultimately, Ulva flexuosa is a species that enjoys a heatwave as much as any other resident of these islands.

A high magnification (x 400) view of the thallus of Ulva flexuosa from the River Wear.   Scale bar: 20 micrometres (= 1/50th of a millimetre).


Rybak, A.S. & Gąbka, M. (2017).  The influence of abiotic factors on the bloom-forming alga Ulva flexuosa (Ulvaceae, Chlorophyta): possibilities for the control of the green tides in freshwater ecosystems.  Journal of Applied Phycology

A question of scale …

It has taken some time to convert the observations from my last visit to the River Wear (see “Spring comes slowly up this way …”) into a picture.  Then, if you remember, the river was balanced between its “spring” and “summer” guises, the cool, wet weather that we experienced in March seems to have held the plants and animals that I usually see at this time of year back.   The result was a patchiness that was easy to see with the naked eye, but harder to visualise at the microscopic level.

First there were quite a few diatoms, Achnanthidium minutissimum in particular, – that suggested a thin biofilm subject to grazing by invertebrates (and I could see some chironomid larvae moving amongst the biofilm as I was sampling).   However, there were also diatoms such as Ulnaria ulna and Gomphonema olivaceum that suggested a thicker biofilm.    And finally there were filaments of the green alga Stigeoclonium tenue, mostly in discrete patches.   I never see healthy filaments of Stigeoclonium tenue smothered in epiphytes, which I have always assumed to be due to the copious mucilage that surrounds the plants.  However, I wondered if, nonetheless, Stigeoclonium contributes to overall habitat patchiness for the diatoms, as they subtly alter the way that water flows across the stone, reducing drag and shear stress in a way that favours Gomphonema and Ulnaria.   This is just speculation, of course…

That brings me back to a familiar theme: the problems of understanding the structure of the microscopic world (see “The River Wear in January” and “Baffled by the benthos (1)”) and, tangentially, to a paper on organisms’ responses to climate that was quoted at a scientific meeting I attended recently.   In this, Kristen Potter and colleagues demonstrated that there was typically a 1000 to 10,000 fold difference between the scale at which the distribution of organisms is studied and the size of those organisms.   That might be enough to draw out some coarse-scale patterns in distribution of species, but organisms actually live in microclimates, which may be patchy and which can often be quite different to the prevailing macroclimate (the difference between being exposed to full sun in open grassland and in the shade of a forest being a good example).   They suggested that the ideal spatial resolution is between one and ten times the organism’s length/height.

I see no reason why the same challenge should not also apply to the pressures faced by organisms in rivers where, again, we can get a certain amount of useful information from a coarse analysis of distribution in relation to (let’s say) average nutrient concentrations in a reach, but cannot really understand the reasons behind the spatial and temporal variation that we see in our data.  This mismatch between the scale at which organisms respond and the scale at which we study them is, I suspect, an even bigger problem for those of us who study the microscopic world.

A second illustration came at the same meeting in a talk by Honor Prentice from the University of Lund in Sweden.  She was dabbling in molecular biology years before this became a fashionable pastime for ecologists and has, over her career, developed some fascinating insights into how the structure of both plant communities and populations of individuals vary over short distances.  Her work has focussed on the island of Öland in Sweden which has the largest extent of alvar (limestone pavement) in Europe.   The system of grikes (the slabs) and clints (the fissures which separate the grikes) create quite different microclimates – the cool, moist conditions in the latter can create bog-like conditions with much lower pH than the limestone clints.   These differences influence not just the composition of the community but the genetic structure of species within these communities too.  I left thinking that if she could detect such differences at a scale barely more than one order of magnitude greater than the organisms, then how much more variation am I missing, with perhaps a five order of magnitude difference between organism size and sampling scale?

Based on these two studies, we would need to sample biofilms at a scale of about 1 mm2 in order to get a meaningful understanding of habitat patchiness in stream benthic algae.  That might just be possible with Next Generation Sequencing technologies, though I am not sure how one would go about collecting environmental data at that scale needed to explain what is going on.  Meanwhile, I am left with the coarse approach to sampling that is inevitable when you are five orders of magnitude bigger than the organism that you want to collect, and my imagination.


Potter, K.A., Woods, H.A. & Pincebourde, S. (2013).  Microclimatic changes in global change biology.   Global Change Biology 19: 2932-2939.

Prentice, H.C., Lonn, M., Lefkovitch, L.P. & Runyeon, H. (1995).   Associations between allele frequencies in Festuca ovina and habitat variation in the alvar grasslands on the Baltic island of OlandJournal of Ecology 83: 391-402.


The Catchment Data Explorer

One of the things I like to do on this blog is to draw out the links between the microscopic and human worlds, and also to explain how we measure the extent of human impacts on the aquatic environment, and what we can do to reverse significant negative impacts.   My professional life is largely concerned with how the evidence for these evaluations is gathered and used to arrive at decisions.  Lip service has always been paid to the importance of transparency in this process but it has not always been easy to find information about the condition of your local environment.  So a few months ago I was pleased to find a new website from the Environment Agency that makes this process a lot easier.

The Catchment Data Explorer starts with some intuitive navigation panes that let you search for your part of England, and then to locate particular streams, rivers and lakes and see how these match up to current environmental targets.   Navigating to my local river, the River Wear, and, more specifically, to the section closest to my house (“Croxdale Beck to Lumley Park Burn”), I find a table with drop-down tabs that give a brief overview of its state.    I see from this that the overall condition of the river is “moderate” and, then, by opening-up further levels, see that the various components of the ecology are all good (I’m not sure that I agree with that for the microscopic algae but that’s a story for another day) but that “physico-chemical supporting elements” are “moderate”.   Classification of rivers and lakes follows the “one out, all out” rule, so it is the lowest class that is measured that determines overall status.   In this case, opening up the physico-chemical elements levels in the table, I see that all is well, except for phosphorus, which is moderate and, therefore, determines the classification.

The home page of the Catchment Data Explorer

From here we can also download a file of “reasons for not achieving good status” in order to understand why phosphorus levels are elevated which tells us that it is waste water treatment and urban drainage that is the most likely source of phosphorus in the catchment.    Control that and, in theory, all should be well.   However, these are just two rows of 147 in a spreadsheet which deals with the lower Wear catchment and its tributaries, so the scale of overall challenge facing the Environment Agency becomes clear.    Moreover, the Wear has already had over £7M investment to install phosphorus stripping from the larger sewage works, to comply with the Urban Wastewater Treatment Directive, so the potential for further improvement is already limited.   Go back to the original table and look at the right hand column, which is labelled “objectives”.   The ecological target is “good by 2027”; however, if you hover the cursor over this, a box pops up telling you that this is “disproportionately expensive” and “technically infeasible”, invoking the WFD’s notorious “Get Out of Jail Free” card which lets countries bypass the need to achieve good status in certain specified situations (clause 4 paragraph 5 – “Less Stringent Objectives”).

Water body classification information from the Catchment Data Explorer for the River Wear, between Croxdale Beck and Lumley Park Burn.

All good so far.   The problems come when you start burrowing deeper into the Catchment Data Explorer and, in particular, when you download data.   There is a lot of information in Excel spreadsheets (which is great); however, it is riddled with jargon and not very well interpreted.   Then there are some apparent contradictions that are not explained. I searched for one stream that interested me, and found the overall ecological status to be moderate, despite the status of the fish being poor.  There is probably a good reason for this (perhaps there was low confidence in the data for fish, for example) but, again, it is not very well explained.

Then there are those water bodies that are, apparently, “good status” but, when you delve deeper into the Catchment Data Explorer, you find that there is no evidence to support this.   This is a surprisingly common situation, not just in the UK but across Europe.  The phrase “expert judgement” is invoked : probably meaning that someone from the local Environment Agency office went along for a look around and could not see any obvious problems.   It seems to be used, in the UK at least, mostly for smaller water bodies and is probably a pragmatic decision that limited resources can be better used elsewhere.

These are relatively minor niggles when set against the positives that the Catchment Data Explorer offers.   There is already quite a lot of information in the Help pages, and there is also a Glossary, so you should be able to work out the situation for your local water bodies with a little patience.   A struggle with terminology is, perhaps, inevitable, given the complexities of managing the environment.  We would all do well to remember that.


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)


Our patchwork heritage* …

The problem with the case I set out for a “switch” from a winter / early spring biofilm community to a summer / autumn assemblage is the sample that I was writing about contained elements of both.   This, I think, is another aspect of an issue that I touched upon in “The River Wear in January”: that the scale that we work at is much greater than the scales at which the forces which shape biofilms operate.   There is no intrinsic driver for this switch beyond the physical forces in the river but each stone will have a slightly different history.  A smaller cobble will be more likely to be rolled than a boulder, as will one that is not sheltered from the main current, or not well bedded into the substratum.  The sample I collect is a composite from the upper surface of five separate cobbles so will blend these different histories.   The more stable stone might have more Navicula lanceolata and Gomphonema olivaceum whilst the recently rolled might be dominated by early colonisers such as Achnanthidium minutissimum.

The same processes can even work on a single stone.   Arlette Cazaubon, a French diatomist, now retired, wrote several papers on this topic (see references at the end of this post).  She highlighted how the diatom assemblages differed across the surface of a boulder, depending on the exposure to the current.  However, that is only part of the story.  The picture at the top of the post was taken in January, when I was collecting my first samples of the year.  You can see the streak where I ran my finger through the biofilm and some other marks, perhaps where the heel of my wader had scuffed the stone (I’m trying to keep my balance in the middle of a northern English river in January whilst holding a waterproof camera underwater, remember).   But such damage could have arisen just as easily from twigs or stones that were being washed downstream.   Taken together with Arlette’s work, it shows how a mature Navicula lanceolata / Gomphonema olivaceum assemblage can live alongside a pioneer Achnanthidium minutissimum assemblage.

A schematic view of the biofilm in the River Wear at Wolsingham, March 2018.   a. Navicula lanceolata; b. Gomphonema olivaceum complex; c. Fragilaria gracilis; d. Achnanthidium minutissimum.   Scale bar: 10 micrometres (= 1/100th of a millimetre).

I’ve tried to depict that in the schematic diagram above.   On the left-hand side there is a mature biofilm, with long-stalked Gomphonema species creating a matrix within which motile diatoms such as Navicula lanceolata live whilst, on the right, there is a pioneer community dominated by Achnanthidium minutissimum.   However, whilst this patchiness is a natural phenomenon, it can contribute to the variability we see in ecological data and, indirectly, to an impression that ecological data are not precise.   If I were to divide the diagram above into two halves, the left-hand side would return a higher TDI than the right.  This is because the diatoms on that side have broader ecological tolerances than those on the other (the sample size, by the way, is far too small to do this seriously but I just want to make a point).   In practice, however, the entire diagram represents little more than the width of a single bristle of the toothbrush that I use to collect samples so a sample is, inevitably, an amalgam of many different microhabitats on a stone.  Our assessment of the condition of the river represents the average of all the patches across the five stones that form a typical sample on that day.

The importance of patchiness in determining the structure and composition of stream communities has been known for some time (see review by Alan Hildrew and Paul Giller in the reference list).   What we have to remember when trying to understand phytobenthos is that patchiness is, to some extent, embedded in the samples we collect, rather than being something that our present sampling strategies might reveal.

* “… for we know our patchwork heritage is a strength not a weakness ..” Barack Obama: inaugural address, 2009


A useful review on patchiness in stream ecosytems (several other papers in this volume also discuss patchiness in freshwater and marine environments):

Hildrew, A.G. & Giller, P.S. (1994).  Patchiness, species interactions and disturbance in the stream benthos.  pp. 21-62.  In: Aquatic Ecology: Scale, Pattern and Process (edited by P.S. Giller, A.G. Hildrew & D.G. Rafaelli).   Blackwell Scientific Publications, Oxford.

Some of Arlette Cazaubon’s papers on variability in diatom assemblages across the surfaces of single stones:

Rolland, T., Fayolle, S., Cazaubon, A. & Pagnetti, S. (1997). Methodological approach to distribution of epilithic and drifting algae communities in a French subalpine river: inferences on water quality assessment. Aquatic Science 59: 57-73.

Cazaubon, A. & Loudiki, M. (1986). Microrépartition des algues épilithiques sur les cailloux d’un torrent Corse, le Rizzanese. Annals de Limnologie 22: 3-16.

Cazaubon, A. (1986). Role du courant sur la microdistribution des diatomées epilithiques dans une Riviere Méditerranéenne, L’Argens (Var, Provence). pp. 93-107.   Proceedings of the 9th Diatom Symposium.   Bristol.

Cazaubon, A. (1988). The significance of a sample in a natural lotic ecosystem: microdistribution of diatoms in the karstic Argens Spring, south-east France.  pp. 513-519.   In: Proceedings of the 10th Diatom Symposium, Joensuu, Finland.

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

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.