That’s funny …

The most exciting phrase to hear in science, the one that heralds new discoveries, is not “Eureka!” but “That’s funny”
Attributed to Issac Asimov

I have visited Croasdale Beck, in western Cumbria, twenty-eight times since 2015 and I thought I was beginning to understand it’s character (see “A tale of two diatoms” and “What a difference a storm makes”).   It is the unruly sibling of the River Ehen which, usually, offers a far less amenable environment for freshwater algae.  Last week, however, as we walked down the track towards the stream, we were confronted with the unexpected sight of a river bed that was bright green.  Our measurements, too, showed that not only was there a lot of algae in absolute terms, but there was far more here than we had measured in the River Ehen.  Usually, the situation is reversed, with the Ehen having more than Croasdale Beck.

Croasdale Beck at NY 087 170 looking upstream in April 2018.   The position of the gravel bar has shifted over the time that we have visited, with the wetted channel originally being at the right hand side, rather than being split into two.

It was hard to capture the extent of the algae growing on the river bed in a photograph, but the macroscopic image below captures the colour of the growths well, and you’ll have to use your imagination to scale this up to cover half of the stream bed.  Under the microscope, these growths turned out to be virtual monocultures of the green alga Draparnaldia glomerata.  This is common in clean rivers in spring time, and I often find it in the nearby River Ehen (see “The River Ehen in February”).  What my images do not show is the mucilage that surrounds the filaments.   In some cases, the growths can be almost jelly-like, so prolific is this mucilage.   One of the roles of this mucilage plays is to serve a matrix within which enzymes released by the fine hairs at the end of the filaments can act to release nutrients bound into tiny organic particles (see “A day out in Weardale …”).

Growths of Draparnaldia glomerata in Croasdale Beck (NY 087 170) in April 2018.  The upper image shows the filaments growing on submerged stones and the lower image shows the bushy side-branches growing from a central filament.  Scale bar: 100 micrometres (= 1/10th of a millimetre).

We also sample a site a couple of kilometres downstream on Croasdale Beck and, here again, the river bed was smothered in green growths.  I assumed that this, too, was Draparnaldia glomerata but, when I examined the filaments under the microscope, it turned out to be a different alga altogether: Ulothrix zonata (see “Bollihope Bhavacakra” and links therein).   There is little difference between the two sites that might explain this: the latter is slightly lower and is surrounded by rough pasture whilst the other is closer to the fells.   However, I have seen both Ulothrix zonata and Draparnaldia glomerata at several other sites in the vicinity, and a simplistic interpretation based on agricultural enrichment does not really work.

There were also a few obvious differences in the diatoms that I saw in the two samples.   In both cases, we sampled stones lacking green algae but, instead, having a thick brown biofilm.  Several taxa were common to both sites – Odontidium mesodon, for example (broadly confirming the hypothesis in “A tale of two diatoms …”) and Meridion circulare was conspicuous in both.   However, the lower site had many more cells of “Ulnaria ulna” than the upper site.   Again, there is no ready explanation but, at the same time, neither green algae or diatoms at either site suggests anything malign.

Filaments of Ulothrix zonata at Croasdale Beck (NY 072 161).   The upper filament is in a healthy vegetative state (although the cell walls are not as thickened as in many populations).  The lower filament is producing zoospores.   Scale bar: 25 micrometres (= 1/40th of a millimetre).

Diatoms in Croasdale Beck, April 2018.   a. upper site: note the abundance of Odontidium mesodon, plus cells of Gomphonema cf exilissimum, Achnanthidium minutissimum and Meridion circulare; b. lower site: note the presence of “Ulnaria ulna” as well as several of the taxa found at the upper site.   Scale bar: 25 micrometres (= 1/40th of a millimetre).  

So where does this take us?  I talked about the benefits of repeat visits to the same site in “A brief history of time wasting …” and I think that these data from Croasdale are making a similar point.  By necessity, most formal assessments of the state of ecology are based on very limited data, from which, at best, we get an estimate of the “average” condition of a water body over a period of time.  Repeat visits might lead to a more precise assessment of the “average” state but also give us a better idea of the whole range of conditions that might be encountered.  Here, I suspect, we chanced upon one of the extremes of the distribution of conditions.   Cold, wet weather in early spring delayed the growth of many plants – aquatic and terrestrial – as well as the invertebrates that graze them.   Then the period of warm, dry conditions that preceded our visit gave the algae an opportunity to thrive whilst their grazers are still playing “catch-up”.  I suspect that next time we visit Croasdale Beck will have its familiar appearance.   It is, nonetheless, sobering to think that this single visit could have formed fifty-percent of the evidence on which a formal assessment might have been made.

 

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Hilda Canter-Lund competition 2018

The competition for the 2018 Hilda Canter-Lund Award is now underway and you have until 25 May to submit images of algae – microscopic or macroscopic, marine or freshwater – for consideration.  I’ve written several posts over the last few years with advice on what makes a great image of an algae and thought that it might be useful to list them all here.   Although I suspect that the winning photograph is already nestling on someone’s hard drive, careful use of editing software could well make the difference between an image that gets onto the shortlist and one that does not.  So, read on:

How to win the Hilda Canter-Lund competition

How to win the Hilda Canter-Lund competition (2)

How to win the Hilda Canter-Lund competition (3)

How to win the Hilda-Canter-Lund competition (4)

Finally, you may find inspiration in the archive of previous winners and short-listed entires on the British Phycological Society’s website.

Good luck!

(The image at the top of this post is Luis Henriquez’s short-listed entry from last year showing Carpoglossum confluens emerging from a mass of Caulpera triferia in the coastal waters off Tasmania.)

Cypriot delights …

I could not return from my visit to Cyprus without an algal sample and a fine opportunity presented itself early last week when we visited the Avgás Gorge, on the Akámas peninsula at the west coast of Cyprus, just north of Pathos.  This is a spectacular limestone ravine whose steep sides offered welcome relief from the Mediterranean sun.  At points, as in the photograph above, the ravine narrowed to just a few metres wide, reminiscent of the siq which guards the entrance to Petra except that instead of exquisite carvings we stumbled across a Russian team conducting a glamour shoot.

A small stream made its way down the gorge.  The presence of woody debris at intervals suggested a considerable head of water during the winter months but, at this time of year the water has reduced in power, tumbling across a series of boulders into stagnant pools, interspersed with short runs shaded either by the high cliffs or the vegetation that flourished away from the harsh glare of the sun.

In the sections where water was still running there were clumps of Chara tangled up with filamentous algae, with plenty of bubbles of oxygen as evidence that both were busily photosynthesising away.  The filamentous algae was a coarse unbranched filament that was clearly a relative of Cladophora but which did not match any of the genera or species that I had encountered before (see “Fieldwork at Flatford” for similar situation).   There were pebbles and cobbles between these clumps, their surfaces criss-crossed by the galleries of caseless caddis larvae – probably Psychomyiidae, according to Richard Chadd.

Chara growing in the stream at Avgás Gorge in western Cyprus, April 2018.

Cladophora or a relative growing in the stream at Avgás Gorge in western Cyprus, April 2018.  The scale bar is 50 micrometres (= 1/20th of a millimetre). 

There were a number of diatoms present too, the most abundant of which was a chain-forming Ulnaria.  Unfortunately, despite having just co-authored a paper on Ulnaria from Cyprus, I cannot name the species, as I saw no cells in valve view.  I will have to return to this subject once I have prepared a permanent slide from the sample that I brought back.   The chloroplasts in the illustration below are not in a very healthy state because the sample lived in a fridge for almost a week before I was able to get it under a microscope.  Had I looked at this sample 20 years ago, I would have assumed that I was looking at a species of Fragilaria, as most keys then stated categorically that Synedra (the former generic name) were solitary rather than chain-forming.  However, we now know that there are several Ulnaria species that form chains although most that I see in my regular haunts in the UK do not.   Our paper states that the species we describe form short chains although, as we worked from cleaned samples collected by other people, I now wonder if that was an artefact of the preparation process and whether these, too, formed longer chains in their living state.

A chain of Ulnaria from the stream at Avgás Gorge in western Cyprus, April 2018.  Scale bar: 20 micrometres (= 1/50th of a millimetre).

Though it is rare for me to stray into describing the invertebrate life of streams, the Psychomyiidae are actually an important part of the story here, as the larvae graze algae from the surface of the stones.  They create a silk tube and this, in turn, becomes covered with fine sediment to create the galleries that are visible in the picture.   We know that invertebrates can change the composition of the attached algae by grazing but I sometimes wonder if the caseless caddis larvae also change the composition by creating myriad patches of very fine sediment across the rock surface.   If you look closely you can also see a couple of Simuliidae (blackfly) larvae.  These attach themselves to the rock by a circle of hooks at their last abdominal segment (I think that is “bum” in entomological language) and then use fan-like structures around their mouths to filter tiny particles from the water.  However, I have also seen Simuliidae larvae bent double on rock surfaces in order to hoover up particulate matter and algae that live there.

That, I thought, was just about enough natural history for one day.   I put my toothbrush and bottle back into my rucksack and made my way back down until, turning the corner, I stumbled across the Russian glamour models.   I’ve often written about the similarities between freshwater ecosystems in different parts of Europe but you don’t often see a bikini – or less – in April in my cold, damp corner.

Galleries of Psychomyiidae larvae on the top of a limestone cobble from the stream at Avgás Gorge in western Cyprus, April 2018.

Reference

Cantonati, M., Lange-Bertalot, H., Kelly, M.G. & Angeli, N. (2018).  Taxonomic and ecological characterization of two Ulnaria species (Bacillariophyta) from streams in Cyprus.   Phytotaxa 346: 78-92.

Wallace, I. (2003).   The Beginner’s Guide to Caddis (order Trichoptera).  Bulletin of the Amateur Entomologists’ Society 62: 15-26.

Letter from Cyprus

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Travel can be one of the most rewarding forms of introspection ….
Laurence Durrell, Bitter Lemons, 1957

When I stepped off flight EZY1973 from Manchester to Paphos on Saturday night I passed a personal milestone. Arriving in Cyprus means that I have now visited all 28 Member States of the European Union. Starting with (West) Germany in 1972 on an exchange visit before the UK was even a member of the European Economic Community, followed shortly after by a family holiday to southern Austria (where my father had been stationed just after the war) with a day trip to Slovenia (then part of Yugoslavia), the number started to increase in the late 1990s when I became involved in the work of CEN, the European Standards Agency and, from the mid-2000s onwards, with the intercalibration exercise associated with the Water Framework Directive. A few years ago I made a list and realised just how many I had visited, after which, I have to admit, my choice of conference and holiday destinations was driven by this rather childish whim. Latvia, Malta and Bulgaria, all subjects of posts on this blog, were ticked off, leaving just Cyprus. This year, a family holiday to celebrate my mother’s 80th birthday provided the opportunity and, after some shameless lobbying, we had booked a villa near Paphos via AirBnB and were on our way.

How Europe has changed in the 47 years since my first overseas trip. Twelve countries were behind the Iron Curtain, three of the remainder were right-wing dictatorships. Two have merged (East and West Germany) whilst seven have become disentangled from previous relationships (the Baltic States from the USSR, Slovenia and Croatia from the former Yugoslavia and the two former constituents of Czechoslovakia from each other). Cyprus, from where I am writing, was in political chaos in the early 1970s. A former British colony whose territory was argued over by Greece and Turkey, it was soon to be split into two, separated by a buffer zone. I used to browse my Collins World Atlas assuming national borders to be fixed and immutable; the older and wiser me wonders where (and when) the next changes will come from.

The intercalibration exercise, in particular, was an opportunity for an exchange of ideas and I counted co-authors from 23 of the 28 EU states on my publication list. Looking back, these papers show remarkable consistency in some aspects of ecology across Europe whilst, in other respects, I am much more cautious about assuming that knowledge gained in my damp corner of north-west Europe can be applied to warmer and more continental regions. This publication list includes, incidentally, two papers about Cyprus, despite never having either visited before or having a native Cypriot on my list of co-authors. In the first paper, we worked with an Austrian employed by the Ministry of the Environment but, for the second, the samples were collected and analysed by Italians and Germans whilst I helped out with data analysis. Scientific colonialism is not, perhaps, dead?

My favourite? I don’t think I should single one of the 28 out. The food and culture of the warm lands of the Mediterranean basin draw me but I think that the parched summer landscapes would lose their appeal if I was there for too long. I find the grey, damp climate of my own corner of Europe wearisome but the greenness of the Spring and Summer, and the Autumn colours almost compensate. My ideal, in other words, seems like it should be a semi-nomadic existence but that, too, would pale with time. The truth is that, for me, elsewhere, being wanted, is always more wondered at ….

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A brief history of time-wasting …*

Having talked about diversity on a microscale in the previous post, I thought it would be interesting to place this in context by looking at the variations that I have observed in the River Wear at Wolsingham over the past decade or so.   The River Wear has seen some significant improvements in water quality over this period, but those have mainly affected sections of the river downstream from Wolsingham.  Most of the changes at Wolsingham are, therefore, giving us some insights into the range of natural variation that we should expect to see in a river.

I’ve got 31 samples from the River Wear at Wolsingham on my database, collected since 2005.  Over this period, nine different diatom species have dominated my counts: Achnanthidium minutissimum on 21 occasions, Nitzschia dissipata twice and Cocconeis euglypta, Encyonema silesiacum, Gomphonema calcifugum, Navicula lanceolata, Nitzshia archibaldii, N. paleacea and Reimeria sinuata once each.   I also have records for non-diatoms during 2009, during which time the green alga Ulothrix zonata, and two Cyanobacteria, Phormidium retzii and Homeothrix varians were the dominant alga on one occasion each.   In total, I have recorded 131 species of diatom from this one reach, although only I’ve only found 91 of them more than once, and only 59 have ever formed more than one percent of the total.   I’ve also got records of 22 species other than diatoms.

This – along with my comments in “The mystery of the alga that wasn’t there …” raises questions about just how effective a single sample is at capturing the diversity of algae present at a site.  .    In 2009 I collected a sample every month from Wolsingham and the graph below shows how the total number of species recorded increased over that period.   Typically, I find between 20 and 30 species in a single sample, and each subsequent month revealed a few that I had not seen in earlier samples.   Importantly, no single sample contained more than 40 per cent of the total diversity I observed over the course of the year.  Part of this high diversity is because of the greater effort invested but there is also a seasonal element, as I’ve already discussed.   The latter, in particular, means that we need to be very careful about making comments about alpha diversity of microalgae if we only have a single sample from a site.

Increase in the number of diatom taxa recorded in successive samples from the River Wear at Wolsingham.  In 2009 samples were collected monthly between January and December whilst in 2014 samples were collected quarterly. 

This seasonal pattern in the algal community also translates into variation in the Trophic Diatom Index, the measure we use to evaluate the condition of streams and rivers.  The trend is weak, for reasons that I have discussed in earlier posts, but it is there, nonetheless.   Not every river has such a seasonal trend and, in some cases, the community dynamics results in the opposite pattern: higher values in the summer and lower values in the winter.  It is, however, something that we have to keep in mind when evaluating ecological status.

Variation in the Trophic Diatom Index in the River Wear at Wolsingham between 2005 and 2015, with samples organised by month, from January (1) to December (12).   The blue line shows a LOESS regression and the grey band is the 95% confidence limits around this line.

All of these factors translate into uncertainty when evaluating ecological status.   In the case of the River Wear at Wolsingham, this is not particularly serious as most of the samples indicate “high status” and all are to the right of the key regulatory boundary of “good status”.  However, imagine if the histogram of EQRs was slid a little to the left, so that it straddled the good and moderate boundaries, and then put yourself in the position of the people who have to decide whether or not to make a water company invest a million pounds to improve the wastewater coming from one of their sewage treatment plants.

At this point, having a long-term perspective and knowing about the ecology of individual species may allow you to explain why an apparent dip into moderate status may not be a cause for concern.  Having a general sense of the ecology of the river – particularly those aspects not measured during formal status assessments – should help too.  It is quite common for the range of diatom results from a site to encompass an entire status class or more so the interpretative skills of the biologists play an important role in decision-making.   Unfortunately, if anything the trend is in the opposite direction: fewer samples being collected per site due to financial pressures, more automation in sample and data analysis leading to ecologists spending more time peering at spreadsheets than peering at stream beds.

I’ve never been in the invidious position of having to make hard decisions about how scarce public sector resources are used.  However, it does strike me that the time that ecologists used to spend in the field and laboratory, though deemed “inefficient” by middle managers trying to find cost savings, was the time that they learned to understand the rivers for which they were responsible.  The great irony is that, in a time when politicians trumpet the virtues of evidence-led policy, there is often barely enough ecological data being collected, and not enough time spent developing interpretative skills, for sensible decisions to be made.   Gathering ecological information takes time.   But if that leads to better decisions, then that is not time wasted …

Ecological Quality Ratio (EQR: observed TDI / expected TDI) of phytobenthos (diatoms) at the River Wear, Wolsingham) between 2005 and 2015.   Blue, green, orange and red lines show the positions of high, good, moderate and poor status class boundaries respectively.

* the title is borrowed from the late Janet Smith’s BBC Radio 4 comedy series

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

Reference

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 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.