Gastropods and gastropubs …

This post sees me entering the unfamiliar territory of intertidal rock pools for the first time in many years (see “Epiphytes with epiphytes” for my last post on this topic), following a walk along the Northumberland coast starting near Dunstanburgh Castle and making our way, via the Jolly Fisherman pub at Crastor, to Howick Bay.  We’ve been visiting the Northumberland coast for many years now but have never seen it so busy: unsurprising on a beautiful sunny day in a year when many people are holidaying in the UK.  However, “busy” on the Northumberland coast is hardly the same as “busy” in Bournemouth and Brighton so everyone had plenty of space to enjoy the views and at least a modicum of solitude.

Howick Bay, a couple of kilometres south of Crastor,  is a place that is of particular interest to geologists as there is a very clear discontinuity where the Whin Sill – responsible for so much of north-east England’s most dramatic scenery – drops away to expose steeply sloped beds of Carboniferous limestone.  The boulders of basalt that the sea erodes away, and the cracks and gaps in the limestone beds, however, then create a rich mosaic of rock pools teeming with intertidal life forms.  

Howick Bay with the Whin Sill in the background and the shelf of Carboniferous limestone emerging on the right hand side of the image.  The picture at the top of the post shows Dunstanburgh Castle on a crag of Whin Sill a few kilometres north of Howick Bay.

My eye was caught by the trails left by some periwinkles on top of cobbles of Whin Sill at the bottom of one of the rock pools.   Rather than the slimy trails we associate with terrestrial snails, this one seemed to have rasped a path across the top of the stone as it fed on the thin layer of algae that coated the surface.   That, of course, piqued my interest so I decided to take a closer look at whatever it was feeding upon.  As I had not anticipated this turn of events, I had not come prepared, meaning that I had to cram a small stone into a plastic bag lurking in our rucksack and carrying this around for the rest of the afternoon.  When I was back, I scrubbed the stone with a toothbrush, just as I would if I had found it at the bottom of a stream.

A Common Periwinkle (Littorina littorea) grazing on a Whin Sill cobble in a rock pool at Howick Bay, Northumberland, August 2020.

The film that I managed to brush off the top surface of a cobble had the same yellow-brown colour that I find when I look at stones from lakes and streams so I expected to see plenty of diatoms when I peered at a sample through my microscope.   What I saw, instead, was a community dominated by narrow branched filaments of the brown alga, Ectocarpus.   These have similar pigments to the diatoms and are, in fact, members of the same Phylum (“Ochrophyta”) so my snap judgement may be excused.  I did also find several  types of diatom were also present in the sample, though the Ectocarpus was by far the most abundant organism.   Those diatoms that I saw included some Navicula along with Tabularia and an occasional cell of Licmophora.  There were also a few green algae cells and some cyanobacteria, but I’ll write more about those in the next post.   

Left: the trail made by another periwinkle as it browses its way across a Whin Sill cobble at Howick Bay (if you look very closely, you’ll also see a trail of eggs that it has deposited as it moves) and, right: the scraped biofilm suspended in sea water.
Filaments of Ectocarpus from the biofilm on a Whin Sill cobble from Howick Bay, August 2020.   Scale bar: 20 micrometres (= 1/50th of a millimetre).    

The periwinkle is really “mowing” the biofilm: its mouth is on its underside, surrounded by the strong muscles that enables it to move across the rock surface.   The mouth contains a long, ribbon like-structure, the “radula”, which can be thought of as a tongue with minute teeth.  These enable the organism to scrape the algae off the rock as it moves albeit relatively unselectively.  Brown algae, diatoms, green algae and anything else small enough to be pulled into the gullet will be removed in the process.

Diatoms from the rock pool biofilm at Howick Bay, August 2020.  a. & b.: two cells of Tabularia in girdle view; c. a girdle view of a cell of Licmophora.   Scale bar: 20 micrometres (= 1/50th of a millimetre).  

Our unanticipated rook pooling session over, we scrambled back up the cliff to retrace our steps to Dunstanburgh.   We had delayed our purchase of Crastor’s famous kippers earlier in order to avoid having to carry them in our rucksack for too long on a warm August day but, by the time we got back to Crastor there was a long queue snaking out of the shop and, as our time was limited and our legs tiring, we pushed on.   

As we made our way back past the Whin Sill outcrop on which Dunstanburgh Castle stands, and skirted the links golf course just beyond, I reflected that my crab salad at the Jolly Fisherman put me two stages in the marine food chain away from the molluscs of Howick Bay.   I eat crabs, they eat molluscs, molluscs eat algae and algae feed on sunshine.  Or, to turn this around, algae sustain us all.   But if you read this blog regularly, you probably knew that already… 

Some other highlights from this week:

Wrote this whilst listening to:  Michael Kiwanuka, who we should have seen live at Green Man a week ago.

Cultural highlights:  Our first visit to a cinema in six months: to see Babyteeth.  Not sure that many films are going to be so unmissable that it is worth wearing a mask for two hours.   The  low budget ‘subhero’ indie films that we prefer will probably work as well streamed on a laptop anyway.  

Currently reading:  Afua Hirsch’s Brit(ish).  Her series “Africa Renaissance”, currently on the BBC iPlayer, is also worth watching..

Culinary highlight:  a five-course tasting menu at Jöro in Sheffield: a very belated Christmas present from our daughter and her partner Sam.

But …

An article in The Guardian last week reported a talk by James Bevan, the Chief Executive of the Environment Agency, in which he highlights one aspect of the Water Framework Directive, the so-called “one-out-all-out-rule”, as being a “candidate for thoughtful reform”.   As this is an integral, albeit rather technical, part of how our rivers are managed, I was intrigued to see what he had to say.  

The one-out-all-out-rule means that, of all the biological, chemical and physical measurements that the Environment Agency make on a river, the worst determines the final quality that is reported.   If I go to the Environment Agency’s Catchment Data Explorer for my local river (the Wear from Croxdale Beck to Lumley Park Burn) for example, I see that its overall condition is “moderate”.  Digging a little deeper into the table, I see that most of the components measured are “high” or “good” with “moderate” recorded just for “phosphate”.   This one dip below good status (the WFD’s target condition) is enough for the whole water body to be designated “moderate”.   Repeat this over the whole country, and we find that only 14% of rivers in England are assessed as “good” because there are so many criteria which might pull down the status.   Bevan said that the “one-out-all-out-rule “can underplay where rivers are in a good state, or where improvements have been made to those that aren’t”.     

First of all, I would question whether the ecology of my local stretches of the River Wear is “good”: there are some flaws with the approaches that the Environment Agency uses and the monitoring program has been cut back so much that many of their evaluations need to be treated with caution.   I would put the ecology of these stretches of the Wear, as a result, at no better than “moderate”. Leaving that aside, however, let’s imagine the Catchment Data Explorer’s results as being a doctor’s summary of the state of your health: for the Wear this would be: “you are in generally good shape but. ….”.  All of us will be paying close attention as that “but” is unpacked.   If your cholesterol is too high, for example, then you will think about cutting back on fatty foods; if blood pressure is too high, then you’ll head to the pharmacy with a prescription and so on.   Bevan, by comparison, seems to want us to focus on the “in generally good health” part of the diagnosis.    

It is important to say that the one-out-all-out-rule is one of the most controversial parts of the Water Framework Directive and it is recognised to give over-precautionary outcomes in many cases.  The European Commission itself recognises that there are difficulties in communicating results of efforts to implement the Directive as a result; however, the answer is not to ditch the principle, but rather to find better ways of reporting progress.   It would not be difficult, for example, to rework the Catchment Data Explorer to generate summary statistics that better represent overall progress without compromising the all-important “but …” that I mentioned above.    

Melbreak rising above Crummock Water in the English Lake District; photographed from Grasmoor.  The photograph at the top of the post shows Crummock Water and Buttermere and distant fells including Great Gable and Scafell Pike, viewed from the cairn at the summit of Grassmoor. 

So “thoughtful reform” should not, in principle, alarm us.   The concern is that any such review could become a means by which the high standards set by the EU are diluted.    The fact that only 14 per cent of rivers in England achieve good status does not seem unrealistic to those of us who spend a lot of time working around rivers but the problems that remain are generally those that have proved most intractable over the 30 years that the Environment Agency has been operating.   Playing with the statistics will never, ultimately, disguise the fact that the Environment Agency is now spread too thinly to make serious inroads into challenges such as diffuse pollution.

Bevan makes the point that “some of England’s heavily engineered rivers in urban centres, for example, will never be restored to their natural state”  The “thoughtful reform” needs to acknowledge frankly that such urban rivers are far from their natural state (rather than redefine standards) but also include derogations that recognises where such targets are not feasible.   The Water Framework Directive, in fact, already acknowledges this via options for “ecological potential” and “less stringent objectives” where good status is not possible so, again, there are frameworks already in place on which these reforms could be based.  The US regulatory framework may offer some lessons, with water bodies assigned a “designated use” and water quality criteria then applied to protect this designated use.    

So long as “good enough” is recognised as being fundamentally different from “good”, and such designations are applied in a transparent manner, then it may then it may represent a compromise that allows the efforts of the Environment Agency to be more widely recognised.  The question I ask, as I peer into the River Wear, is how “good enough” will be interpreted.   I would argue that a diffuse nutrient management plan such as that Bevan describes for Pool Harbour could pull the Wear up from “moderate” to “good” status.  My fear is that desk-based apparatchiks will decide that the likelihood of this happening at current funding levels is low and try to slap a “good enough” designation on this and many other rivers in a similar state.   

The second point of contention is Bevan’s claim that the one-out-all-out rule can “force regulators and others to focus time and resources on indicators that may not make much difference to the actual water quality”.  The implication is that the Environment Agency spends too much money collecting unnecessary data to confirm the bleeding obvious.   Maybe that is sometimes the case: I have long argued that there are short-cuts to effective decision-making, particularly when rivers are very polluted (see “Winning hearts and minds …”).    However, the inverse is not true: you cannot say that a river is in a healthy state if you only use a limited number of indicators.   None of us would be happy if a doctor gives us a clear bill of health based only on our blood pressure.  Limit the number of indicators and the risk of “false negatives” in complex systems such as the human body and river catchments will increase.  So, by all means, rethink what evidence is needed but don’t use this as a quick fix to boost the number of rivers in good status.   If anything, “thoughtful reform” might identify needs for more evidence in some cases, not less.  

A simple test for James Bevan is to test out any ideas for “thoughtful reform” with Public Health England.   If it sounds preposterous as a strategy for reducing obesity (to take one example), then it probably won’t work for rivers either.   Public health challenges require addressing root causes, not redefining notions based in sound science.   You would not want your doctor applying anything less than the “one-out-all-out rule” when evaluating your own health, so why should we let our environmental regulator work to a lower standard?


Carvalho, L., Mackay, E.B., Cardoso, A.C., Baattrup-Pedersen, A., Birk, S., Blackstock, K.L., Borics, G., Borja, A., Feld, C.K., Ferreira, M.T., Globevnik, L., Grizzetti, B., Hendry, S., Hering, D., Kelly, M., Langaas, S., Meissner, K., Panagopoulos, Y., Penning, E., Rouillard, J., Sabater, S., Schmedtje, U., Spears, B.M., Venohr, M., van de Bund, W., Solheim, A.L. (2019). Protecting and restoring Europe’s waters: An analysis of the future development needs of the Water Framework Directive. Science of the Total Environment .

Some other highlights from this week:

Wrote this whilst listening to:  Mozart’s Clarinet Concerto, prompted by watching Out of Africa (see below).  That film is directly responsible for delaying our purchase of a CD player by about 10 years because it came out when people would proudly demonstrate the clarity of their expensive new CD players by putting on a Dire Straits CD and telling you that there was no background noise at all.  I remember thinking: “yes, but it’s still Dire Straits whereas Dennis Finch-Hatton played Mozart’s clarinet concerto on a wind-up gramophone in the bush and it sounded sublime, despite the crackle and hum.

Cultural highlights:  We revisited the film Out of Africa, curious to see how our response to this glossy portrayal of colonial-era Africa had changed in the 34 years since we first saw it.  

Currently reading:  Andrea Levy’s The Long Song.

Culinary highlight:  Crab salad with chips triple cooked in beef fat at the Jolly Fisherman in Crastor.  But I’m getting ahead of myself as the expedition that took us to the Northumberland Coast will be the subject of my next post … 

Around the village in 80 days …


Mildy riled by an “explorer” talking on the radio about derring-dos in far-flung tropical forests, I decided to write this post about an inauspicious back lane about 300 metres from my house.   The whole point of this blog, after all, is been to point out that there are more strange plants and creatures in your own neighbourhood than you are ever likely to encounter in the jungles of Borneo.   Much of what I write about is microscopic and, therefore, there is no disgrace in not noticing it.  But sometimes, we nonchalantly ignore nature because it is out of our sightline or simply too small and prosaic to attract our interest.

A patch of liverwort on a narrow strip of wasteland between garden fences and footpath in our back lane is a case in point.   It is possible to walk this past hundreds of times without noticing but, once you care to take a closer look, the sights in front of you are as weird as anything you see on a David Attenborough documentary.   Liverworts are relatives of mosses, and I talked briefly about a couple in a post in 2019 (see “A thousand little mosses …”).  Like these, today’s liverwort, Marchantia polymorpha, does not have a distinct stem or leaves but, rather, the whole plant looks like a lobed leaf, albeit lying flat on the ground.   The technical term for this is “thallus”.    If you pull one up, you’ll see thin white hairs (“rhizoids”) on the underside which anchor it to the soil, but there are no roots.    Peering even closer, you’ll see that the upper surface is dotted with white spots which, if you have a hand lens, resolve into air pores.   These look superficially like stomata (see “Whatever doesn’t kill you …”) but differ in that they are permanently open, allowing air to penetrate to the inner layers of the thallus.


Left: a thallus of Marchantia polymorpha with a young female reproductive structure present (approximately 3 cm across); right: close up of thallus surface showing air pores. 

Several of the thalli had structures that looked like minuscule palm trees arising from them.  These are the female reproductive structures.  The plant is dioecious, meaning that there are separate male and female plants but, curiously, all those in our back lane appear to be female.  On the underside of the umbrella-like structure there are a number of flask-like archegonia into which a sperm cell will, if very lucky, swim.   This all happens before the stalk elongates (as in the left-hand image above) and when the liverwort is damp.   Another whole phase of the life cycle takes place in these sacs, resulting in the production of spores which are eventually ejected with the assistance of spring-like structures called “elators” but only after the stalk has raised itself a couple of centimetres above the thallus.   The reason for the apparent absence of male reproductive structures from our back lane is something of a mystery, as it seems to make the possibility of congress even more remote than usual.   Fortunately, Marchantia polymorpha has a Plan B.


Left: female reproductive structures on the surface of a Marchantia polymorphus thallus and, right, close-up of the underside of the female reproductive structure showing the antheridia (photos: H. Kelly).

If you look at the left-hand image below you’ll see a circular shape on the thallus near the base of the palm-tree like female reproductive structure.  This is a gemma cup which contains small bundles of cells called gemmae which can also disperse to form new thalli.  These new thalli will be genetically-identical to their parents (clones, in other words).   That has some disadvantages but, as we have seen, it is a common phenomenon in the algae (see “The River Ehen in March”) and is a good strategy for organisms that want to occupy a lot of vacant ground in a short period of time.   What this also means is that the patch of Marchantia polymorpha is not necessarily a lot of individual plants but, rather a few (possibly even a single) “plant”, albeit not one formed of contiguous units.


A close-up of a gemma cup on the surface of a thallus of Marchantia polymorpha, with some gemmae visible inside. 

Finally, I’ve included n ant’s eye view of the Marchantia patch to finish off the post.  On the right foreground you can see the surface of the leaf with an air pore; on the left-hand side there is a gemma cup and, dominating the centre of the image, there are female reproductive bodies towering over the micro-landscape.   Where, I still wonder, are the male reproductive bodies?  Could it be that a female Marchantia polymorpha needs a male Marchantia polymorpha like a … a fish needs a bicycle?


An ant’s eye view of a Marchantia polymorpha thalli. 

Some other highlights from this week:

Wrote this whilst listening to:  live music!   Well not “live” as in happening in the same room, but in the sense that it was happening in real time as we listened via a YouTube link.  It was from local musician Emma Fisk and the Hot Club du Nord- playing Django Reinhardt / Stephane Grappelli -inspired jazz.

Cultural highlights:  The film Green Book.  Winner of three Oscars in 2019.   And the BBC adaptation of Vikram Seth’s A Suitable Boy.  Beautifully filmed and very evocative of our visits to India.

Currently reading:  A Single Thread by Tracey Chevalier.

Culinary highlight:  Butter beans in salsa verde.  Meera Sodha’s recipe from last week’s Guardian.  And Cumberland sausage, mash and a rich onion gravy from The Shepherd’s Arms in Ennerdale Bridge, our fieldwork home-from-home.



The wrong kind of green?


Back in 2016 I wrote a post entitled “The camera never lies?” which mused on the difficulties in interpreting photographs of stream beds taken with waterproof cameras.  My point was that quantities of algae can be highly variable and interpretation of conditions from a single photograph is fraught with difficulties.   Because waterproof cameras are now relatively cheap it is easy to take a photograph whilst out doing fieldwork and then include this in a report, making a point, perhaps, about the poor condition of a site.  However, the frequency of visits to a site is often such that the person who took the photograph has little idea of whether that growth is persistent or not.   The photograph at the top of the post (from the River Irt, Cumbria, just below Wastwater) shows just how variable the cover can be at a single site over the course of a year.

The question is still pertinent because, last year, an article appeared in FBA News suggesting that percent benthic algal cover, measured from bed photographs, was correlated with the level of phosphorus enrichment.   The example from the River Irt challenges this: the cover recorded here (average: 17%, maximum: 36%, based on direct estimates of the percent of the stream bed covered with algae, rather than via photographs) suggests quite a high level of enrichment, whereas the Irt is, in fact, just downstream of one of the most oligotrophic lakes in the country.   There are two plausible reasons for this: first, my experience of West Cumbrian rivers suggests that regulation can have a big effect on the quantity of filamentous algae present regardless of the amount of nutrients and, second, the article makes no differentiation between “good” algae and “bad” algae.   Not all algae are indicators of nutrient enrichment, so you really need to know the identity of the algae before leaping to a conclusion (see “The democratisation of stream ecology?” for one way of addressing this.

A further issue is illustrated by the second image – this time, showing the bed of the River Ehen, about 15 kilometres north of the River Irt and, once again, downstream from an oligotrophic lake.  Here, the total cover is even higher (average: 46%, maximum: 87%) but green algae represent a relatively small proportion of this (typically about a quarter).  The FBA News article makes little reference to the type of algae, just referring to “benthic algal growth”. Again, identity is important: the Ehen site has a lot of red algae and Cyanobacteria as well as green algae and, again, these are not necessarily indicators of enrichment.   Copious Lemanea fluviatilis in the Spring is, in fact, usually a sign of healthy conditions.


Seasonal variation in the appearance of the bed of the River Ehen, Cumbria.   Photographs taken at approximately the same spot at two monthly intervals.  The photograph at the top of the post shows variation in the River Irt, also in Cumbria.

One extra twist to the story comes when we compare visual estimates of cover with the biomass, as measured using a BenthoTorch (see “The complexities of measuring mass”).   Both show a trend of increased measured biomass linked to increased observed cover, but the relationships are very noisy.  Of particular interest is the row of points bouncing along the x axis, indicating visits where the eye saw lots of green algae but the BenthoTorch measured very little.  What I think is happening is that the green algae can form bright but relatively thin layers on the rocks on these occasions.  On other occasions, the measured biomass is much higher but if the green algae are covered with epiphytic diatoms, then they may not be so obvious to the naked eye, possibly leading to percent cover being under-estimated during surveys.   A final possibility, certainly plausible in the Irt, is that the green algal growths are associated with boulders whereas our biomass measurements are made on moveable cobbles.


Measuring biomass with a BenthoTorch in West Cumbrian streams.

There are, in other words, good reasons for the mismatch between observed and measured quantities of algae.  Neither is a perfect indicator of what is going on in a river.  If we are interested in the effect of algae on ecosystems, then measured biomass would seem to be the better indicator of the extent to which other ecological processes in a stream are likely to be affected by the algae that are present.  However, the general patchiness of algal growths, plus the relative ease with which visual estimates can be made probably outweighs their problems, at least from the point of view of basic assessments of ecosystem health.   Following on from this, I doubt that a photograph offers any  advantage over an inspection of the river bed for basic ecological health assessments.   You need to explore a reasonable length of river, including variations in velocity, substratum type and shade, to get some idea both of the quantity of algae, and its variability.


The relationship between visual estimates of cover (x axis) and biomass measured using a BenthoTorch; a. all algae (Spearman’s rank correlation, r = 0.55); b. just green algae (r = 0.65).  Data from West Cumbrian lakes and streams, 2019-20.

There are, in other words, good reasons for the mismatch between observed and measured quantities of algae.  Neither is a perfect indicator of what is going on in a river.  If we are interested in the effect of algae on ecosystems, then measured biomass would seem to be the better indicator of the extent to which other ecological processes in a stream are likely to be affected by the algae that are present.  However, the general patchiness of algal growths, plus the relative ease with which visual estimates can be made probably outweighs their problems, at least from the point of view of basic assessments of ecosystem health.   Following on from this, I doubt that a photograph offers any  advantage over an inspection of the river bed for basic ecological health assessments.   You need to explore a reasonable length of river, including variations in velocity, substratum type and shade, to get some idea both of the quantity of algae, and its variability.

After a quarter of a century spent contemplating the relationship between algae and nutrients in rivers, however, I’m reluctant to endorse any system that implies that a “green” riverbed equates to poor ecosystem health based on a single visit.   These proliferations are, to use a human health analogy, the sniffles and colds of river ecosystems: most rivers suffer from them from time to time and most recover quite quickly.  The concern is when rivers are persistently green, as this suggests a long-term breakdown in the links between trophic levels.  The first response on seeing a green riverbed, in other words, should be to schedule a follow-up visit a month later to see if the situation persists rather than leaping to a rash conclusion.   The second response should be to work out which alga is responsible for the bloom as this, too, can help with subsequent diagnoses.

On a more positive note, anything that makes people notice and record algae in rivers has got to be a good thing.  Just don’t be seduced by any simplistic correlations that assume that algal growths are, by definition, a bad thing.  Algae are key components of the ecological engine of rivers and if that is not part of the thinking behind any river health assessment, then treat it with extreme caution.


Everall, N.C., Johnson, M.F., Clarke, A. & Gray, J. (2019).  The visual state of river beds and their associated invertebrate community biosignatures.  FBA News 77: 11-15.

Some other highlights from this week:

Wrote this whilst listening to:  early Fleetwood Mac, as I commemorated the death of Peter Green last weekend.

Cultural highlights:  Once Upon A Time In Iraq, on the BBC iPlayer, is a gruelling exposé of Western policy blunders and ideological naivity that brought the Middle East to its present dire predicament.

Currently reading:  Watching the English by Kate Fox.   Cultural anthropology on our own doorstep.

Culinary highlight:  Red Dragon (Aduki Bean) Pie – one of the best veggie alternatives to a shepherd’s pie that I have come across.  Recommended by Julie Gething.