Quantifying our ignorance …

Petta_Water_May19

I am fairly sure that I am not a popular person after my latest choice of slide for the “ring test”, the regular calibration exercise that UK and Irish diatomists perform.   I had noticed a few taxa that we had not seen in previous ring tests in a sample I collected during my visit to the Shetland Islands back in May 2019 (see “Hyperepiphytes in the Shetland Islands”) but, on closer examination, the sample proved to be both highly diverse and very challenging.  The seven experienced analysts who provide the benchmark analyses for the ring test found, between them, over 150 different species: some we could name with confidence, but others we could match to no published description.  Amongst those was the species of Achnanthidium photographed below.   It might be Achnanthidium digitatum or possibly A. ertzii but, then again, it does not quite match the characteristics of either of these so, once again, we have left it unnamed (you can find the original descriptions of both these species in the reference list).

According to Algaebase there are 116 species of Achnanthidium that are currently accepted but descriptions of these are scattered through the literature so it is really hard to be confident that you have found a new species during a routine survey.  This is particularly the case when we only have light microscopical analyses with which to work, as the small size of Achnanthidium species means that you really need a scanning electron microscope to see the fine details clearly.  This, however, assumes that the pool of unnamed Achnanthidium species is finite and that the 116 species on Algaebase is a significant proportion of the total number of Achnanthidium species.  A recent study by Eveline Pinseel and colleagues based on samples from Arctic regions offers hints that there is still plenty of diversity within the genus that cannot be linked to named species

This may, however, be a naïve assumption.   My colleague Maria Kahlert, who works in Sweden, comments that she is quite happy looking at samples that I send her from polluted sites in the UK as she can name most of the species (Achnanthidium and otherwise) from her own experience.   It is the samples from pristine habitats that fox her because so many of the forms are different to anything she has encountered in Sweden.  We have, in other words, a neat reversal of the opening line of Anna Karenina (“All happy families are alike, each unhappy family is unhappy in its own way”), with very high beta and gamma diversity of diatoms (probably other microalgae too) as a characteristic of regions with low population density (see “Baffled by the benthos (2)”).  We often miss this in our enthusiasm to fit all that we see down the microscope to published descriptions, but when we take time to look hard, that diversity – and those differences between sites – start to mount up.

Achnanthidium_Petta_Water_May19

The unknown Achnanthidium species from Petta Water, Mainland, Shetland Islands (pictured at the top of the post).  Scale bar: 10 micrometres (= 1/100th of a millimetre).   Photographs: Lydia King

Let’s think of this as an ecological experiment to understand the diversity of Achnanthidium, following the capture-mark-capture approach.   Capture-mark-recapture is a technique used by ecologists to assess the size of a population.   As it is rarely possible to count all individuals, a portion of the population is collected, marked (a dab of paint on a snail’s back, for example) and released.   Some time later, the population is sampled again, and the proportion of those that bear the mark in this second sample is used as an indicator of the proportion of the population captured by the original sample.   Though devised for population biology, some have used the same principles to understand diversity in other contexts too so might it work as a means of understanding the yet-to-be discovered diversity of diatoms?

What we have in the scattered taxonomic literature is a record of all the Achnanthidium species that have been “captured” (i.e. observed) and “marked” (i.e. described) by taxonomists.   Suppose we now go some locations not previously visited by taxonomists, take some new samples and see 1) how many different forms of Achanthidium we can see and b) how many of these are “recaptured” (i.e. forms that align with previously described species).   Or, thinking about the problem in a different way, the number of named species could be compared with the number of distinct “operational taxonomic units” (“OTUs”) detected by metabarcoding.   More relevantly, how many extra OTUs are added when more lakes and streams are added to the dataset?   There are well-established methods for deriving “rarefaction curves” that might be useful in understanding regional diversity of diatoms, and modifications of “capture-mark-recapture” have been used to understand taxonomic diversity in palaeobiolgoical contexts, so why not in contemporary ecology too?

The Shetland Islands would make an ideal test ground for such a study as they are geologically-diverse habitats providing the types of conditions where Achnanthidium species thrive (low population density and agricultural intensity.   The diatoms of the region were studied about 40 years ago by my late mentor John Carter and although one of his samples yielded the type material for Achnanthidium caledonicum there have been so many developments in Achnanthidum taxonomy subsequently that this archipelago represents a tabula rasa for a modern taxonomist.   Its many remote lochs and streams offer the setting for a natural experiment which sets out, to put it bluntly, to quantify our ignorance.

Achnanthidium_caledonicum_Osgaig

Achnanthidium caledonicum from Loch Osgaig, Highland Region, Scotland.   Originally described as Achnanthes microcephala f. scotica Carter & Bailey-Watts 1981 (Scale bar: 10 micrometres (= 100th of a millimetre).  Photographs: Lydia King.

References

Carter J. R., Bailey-Watts A. E. (1981). A taxonomic study of diatoms from standing freshwaters in Shetland. Nova Hedwigia. 33: 513-630.

Pinseel, E., Vanormelingen, P., Hamilton, P. B., Vyverman, W., Van de Vijver, B., & Kopalova, K. (2017). Molecular and morphological characterization of the Achnanthidium minutissimum complex (Bacillariophyta) in Petuniabukta (Spitsbergen, High Arctic) including the description of A. digitatum sp. nov. European Journal of Phycology 52: 264-280. https://doi.org/10.1080/09670262.2017.1283540

Van der Vijver, B., Jarlman, A., Lange-Bertalot, H., Mertens, A., de Haan, M. & Ector, L. (2011).  Four new European Achnanthidium species (Bacillariophyceae).  Algological Studies 136/137: 193-210.

Liow, L.H. & Nichols, J.D. (2010). Estimating Rates and Probabilities of Origination and Extinction Using Taxonomic Occurrence Data: Capture-Mark-Recapture (CMR) Approaches.  The Paleontological Society Papers 16: 81-94).

This week’s other highlights:

Wrote this whilst listening to: Sheku Kanneh-Mason’s recording of Elgar’s Cello Concerto.   Taking me back to his performance at the proms on a warm evening last summer.

Cultural highlight: Sam Mendes’ film 1917 which, coincidentally, uses the River Tees (as featured sporadically in this blog) as one of its locations

Currently reading: I have just finished Good Economics for Hard Times by Abhijit V. Banerjee and Esther Duflo, which I mentioned a couple of weeks ago.  It left me with the feeling that, had both Boris Johnson and Jerermy Corbyn read it and taken on its messages, the election campaign and the UK political landscape might have been very different.

Culinary highlight: OK Diner on the southbound side of the A1 near Grantham.  Felt like we were walking into the opening scene from Pulp Fiction (the one where Tim Roth jumps up onto a table and attempts to rob all the customers).   Escaped with wallet intact.

 

Fit for purpose?

 

Durham_School_boathouse_Jan2020

It is sobering to think that the Water Framework Directive (WFD) will be twenty years old this year (23 October, to be precise).  The 70 pages of legalese that comprise this directive have, to a large extent, determined the course of my career over the past two decades (it is a few sentences in Annex V, to be precise, but unravelling and interpreting these has been enough).  Just before this anniversary arrives, however, the European Commission has published a “fitness check”, giving the Directive a thorough once-over before reaching a mixed verdict on its performance.

The report’s conclusion is that the WFD has provided a governance framework for water management but, overall, the condition of Europe’s water bodies has shown little significant improvement since the WFD passed into law.   The original objective – grossly optimistic in hindsight – was for all Europe’s water bodies to be at least at good status by 2015.  Instead, we are still in the situation where less than half are at good status.  There is no doubt that there have been local improvements, and the rate of deterioration may have decreased but this is not the same as a general trend towards better ecological quality in our water bodies.   I’ll offer three possible reasons for the shortcomings, based on my own experience of WFD implementation, in the hope that lessons learned from turning a well-intentioned policy instrument from theory into practice will have some broader lessons as we tackle the climate emergency.

The first lesson is that complex problems, by necessity, spawn complicated legislation.  The Water Framework Directive arose from an attempt, in the early 1990s, to produce a directive addressing the Ecological Quality of Waters.  As debates about this progressed, people realised that you cannot consider the state of the aquatic environment in isolation, without also considering broader economic issues such as water pricing and, indeed, all aspects of catchment management that respects the rights of other legitimate users.  Each of these issues requires a small army of bureaucrats to unpack and apply within the 28 Member States.   In some countries and for some aspects of the legislation, there were procedures in place that simply needed tweaking to be fit-for-purpose.  Some other aspects were, however, completely new for almost everyone.

The whole idea of using the health of an aquatic ecosystem (“ecological status”) as a measure of the long-term sustainability, for example, was something never attempted on such a scale before.  It had been advocated in the academic literature, and there were a few localised attempts to apply the system (RIVPACS in the UK, for example) but, as the sun rose on 23 October 2000, the task of working out how the fine words of Article 4 had to be translated to a practical reality that was both faithful to the intentions of the WFD and that worked within public sector budgets had to start.

rower_on_Wear

A second big issue that was relatively under-acknowledged in the fitness check is that solving environmental problems cannot be achieved without engaging other sectors as well.   A recent review, to which I contributed, highlighted this, emphasising the need, first, to integrate water policy with other sectors (such as agriculture) whilst, at the same time, emphasising the need to demonstrate tangible benefits that extend beyond the subtleties of shifts in ecological parameters.  Bring agriculture on board to achieve more sympathetic management of catchments, in other words, recognise the contributions that farmers make (“public money for public goods”) but also back this up with substantial demonstrations of reduced flood risk for urban areas downstream.   That calls for a level of joined-up thinking across sectors that has not yet been achieved in Europe and which is, perhaps, an opportunity that the UK, shortly to be freed from the leviathan that is the Common Agricultural Policy, may be in a better position to address.  We live in hope.

The third reason may be that the ambition of the WFD may be higher than many politicians and civil servants are happy with.   Article 1 sets out the objective of promoting “sustainable water use based on a long-term protection of available water resources”.  A phrase such as that could have appeared in any of the party manifestos for our recent election but when the scientists unpack this and explain that this will mean that every river in the country needs to have average phosphorus concentrations of well under 0.1 milligrams per litre, and the water planners put a price on this, alone, that runs into hundreds of millions (if not billions) of euros, then that ambition falters.   More particularly, the noisy nature of much ecological and environmental data gives ample opportunity for bureaucrats to prevaricate rather than take steps that are unlikely to play well with the media (the WFD enshrines the “polluter pays” principle and, as we all contribute to urban wastewater loading, this translates to “voter pays”).

As its 20th anniversary approaches, the WFD will have spanned four electoral cycles (assuming national parliaments have five-year terms), at each of which policy wonks will have been thinking less about long term sustainability of water resources and more about short-term swings in voting preferences.   Moreover, since 2008, much of Europe has felt the consequences of the banking crises, with public sector finances often badly affected.  Again, the scientific challenges that the WFD creates provides easy excuses for cash-strapped regulators to kick the can down the road rather than make potentially unpopular decisions.

Governance may be in place, in other words, but a willingness to push this governance to deliver may be lacking.  That, in turn, reflects a perceived unwillingness on the part of the electorate to accept the costs.  Imperfect democracies will always deliver imperfect solutions, particularly when the underlying problems are complex and the opportunity costs are high.

ducks_on_Wear

Pictures in this post are from a New Year’s Day walk around the riverbanks in Durham.  New feature for 2020 is a few notes on what else I’ve been up to during the week in which this post gestated:

Wrote this whilst listening to:  Bob Dylan’s John Wesley Harding; Bruce Springsteen’s Nebraska

Cultural highlight: Greta Gerwig’s Little Women.

Currently reading: Good Economics for Hard Times by Abhijit V. Banerjee and Esther Duflo – two Nobel Prize winners setting global problems into a broader economic framework.  Not an easy read but very stimulating.   A good follow-up to Kate Raworth’s Doughnut Economics, which I mentioned in a couple of posts last year.

Culinary highlight: followed a recipe in The Guardian which involved cramming all the leftovers from our Christmas dinner (turkey, stuffing, roast potatoes, parsnips, brussel sprouts) into a loaf tin along with some breadcrumbs and two eggs to bind.  This created a meatloaf which I froze and then produced on New Year’s Day to provide a final reminder of the festive season before the realities of 2020 intruded.  Doubly enjoyable as West Ham had their first win of the Festive Season as it was being demolished.

Reference

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 658 1228-1238. https://doi.org/10.1016/j.scitotenv.2018.12.255

Messy bedrooms …

Sand_Loch_May19

When I was tramping around the Shetland Islands earlier this year (see “Hyperepiphytes in the Shetland Islands“), looking at the algae that live in the freshwater lochs, I noticed some meandering hieroglyphs made from fine sediment on the tops of some of the stones in the littoral zone.   I see these occasionally at other places too, and know that they are the “galleries” of caseless caddis flies.  Caddis flies are close relatives of the butterflies and are best known because many of their larvae use “found materials” (in contemporary art jargon) to construct cases to protect themselves.  Some species use fine gravel, silt and sand, some use fragments of plants, some have cases that are very neat, some have a more haphazard approach to construction.  However, a few families of caddis flies eschew cases and, instead, build these galleries.

Many caddis fly larvae, whether cased or not, are grazers, scraping the algae off the rocks on the bed of the stream or lake.   There is evidence that the cases offer some protection against predators such as trout which, by increasing survival rate, means that it is worthwhile for the caddis larvae to divert some of their hard-earned energy into building these.   Presumably, their caseless cousins gain the same advantage to building their galleries but recent research has suggested that these galleries offer a further benefit.

Think of caddis larvae as adolescent caddis flies.  Now imagine that the caddis gallery is the equivalent of an adolescent’s bedroom.   Horribly messy, in other words.   Let’s leave that image of a teenager behind (as most human teenagers know their way to the bathroom) and consider what happens to all that waste material that emerges from the far end of a caddis larva’s digestive system.   This nutrient-rich “ manure” encourages algae, meaning that our caseless caddis flies are, in fact, gardeners and are able to tap into this extra energy resource within their galleries in order to grow.   That brings us back to the analogy with teenagers, as these also frequently graze in their bedrooms (the diatom Campylodiscus is even the same shape as a Pringle, whose empty containers litter the bedroom floor of my own progeny).   I guess it is a good thing that caddis larvae don’t wear socks as, with six legs and two prolegs, the mess inside the gallery would be indescribable.

Psychomiiddae_Sand_Loch_May19

Galleries of caseless caddis flies (possibly Psychomiidae) on the top surface of a cobble from Sand Loch, Shetland Islands with (right) a close-up of a single gallery. The photograph at the top of the post shows Sand Loch in May 2019.

A recent study in the Lake District has shown that this “gardening” means that the algae which grow in the fine sediment from which the galleries are constructed are different to those found elsewhere on the rock surface, with a greater proportion of diatoms, which are considered to be more palatable to invertebrates than other types of algae.  Some caddis flies are thought to go even further, and can selectively remove and discard the algae that are least palatable (some Cyanobacteira, for example).

It is possible that up to 40% of the larva’s energy needs are met from the gallery itself.   The tube is, in fact, not a static construction: the larva pokes its head out in order to graze the algae immediately in front of the gallery, and extends the gallery as the food supply within easy (and safe) reach is exhausted.   At the same time, it is consuming the alga-rich rear part of the gallery (reminiscent of Hansel and Gretel eating the gingerbread house?).   A gallery only has a life-span of 10 days in the laboratory; whether this is the same under field conditions is not clear but that gives us some idea of the transience of these structures.   This rapid turnover means that the caddis larva is always feeding on succulent early-succession species, rather than the tougher and less digestible algae that might appear in more mature biofilms.

I also see similar galleries on the bed of the River Ehen from time to time but have been told that these are formed by non-biting midge (chironomid) larvae, rather than by caddis.  I presume that the same processes are happening in these although I have not been able to find much written in the literature.

Organisms that can significantly alter the habitat in which they live, and affect the conditions experienced by other species in the habitat are termed “ecosystem engineers”.  Beavers are good examples, as their dams can have significant effects on organisms extending for hectares.  Yet, in their own small way, caseless caddis larvae are also ecosystem engineers.  As are adolescent boys.   Which makes me wonder, having only talked until now about the algae in their galleries, whether caseless caddis larvae also have patches of mould extending up their walls.

Chironomid_galleries_Ehen_March19

Galleries made by chironomid larvae on a boulder in the River Ehen, March 2019.

References

Hart, D. D. (1985). Grazing insects mediate algal interactions in a stream benthic community. Oikos 44: 40-46. https://doi.org/10.2307/3544041

Johansson, A. (1991). Caddis larvae cases (Trichoptera, Limnephilidae) as anti-predatory devices against brown trout and sculpin. Hydrobiologia 211: 185-194. https://doi.org/10.1007/BF00008534

Ings, N. L., Hildrew, A. G., & Grey, J. (2010). Gardening by the psychomyiid caddisfly Tinodes waeneri: Evidence from stable isotopes. Oecologia 163: 127-139. https://doi.org/10.1007/s00442-009-1558-8

Ings, N. L., Grey, J., King, L., McGowan, S., & Hildrew, A. G. (2017). Modification of littoral algal assemblages by gardening caddisfly larvae. Freshwater Biology 62: 507-518. https://doi.org/10.1111/fwb.12881

Otto, C., & Johansson, A. (1995). Why do some caddis larvae in running waters construct heavy, bulky cases? Animal Behaviour 49: 473-478. https://doi.org/10.1006/anbe.1995.0061

The presence of absence in Castle Eden Dene

CED_Aug19

Some of my strongest impressions of Castle Eden Burn after last week’s visit concerned not what I found in the stream, but what was not there.  I mentioned in my previous post that I had not seen the mosses that I associated with streams in northern England in Castle Eden Burn, but there were other species, too, that I had expected to see but had not noticed.   Once I have noticed that something is absent, this absence becomes present.  I have noticed the presence of absence.  Woohoo: I’ve shoehorned Jean-Paul Sartre’s Being and Nothingness into a blog about ecology.

When I got back home I had read a chapter about the FBA’s study of the Winterbourne in Dorset, an intermittent stream flowing off the chalk downland, and noticed that they had recorded plants there that I knew from north-east English rivers, but which I could not remember seeing in Castle Eden Burn.  Was this because I had not searched the stream environs thoroughly, or is this a real difference between intermittent streams on chalk and on Magnesian limestone?

I went back this weekend to try to answer these questions, taking Heather with me, as her skills with the higher plants far exceed mine, and walked as much of the stream bed as we could, starting near the remains of a footbridge at NZ 424 389, and making our way downstream to Denemouth, where Castle Eden Burn joins the North Sea.  If my original intention was to better understand the burn by traversing space within the Dene, my first lesson concerned time: a week with some heavy rainfall separated my two visits and it was clear straight away that the Burn had been flowing during the week, with a fine layer of silt and mud spread across much of the surface, making parts of it slippery to walk upon.  There were standing pools of water at several points in the upper part of the Burn too.   Within a week the stream had come and gone, offering scant opportunities for any water-loving organism to establish.

We made our way along the Burn through the delicious silence of the forest.  The banksides were richly vegetated: masses of opposite-leaved golden saxifrage plus the mosses I described last time and many others, along with plenty of harts-tongue fern (Asplenium solopendrium).   Then, with a very clear demarcation, there was the stony stream bed with very little vegetation at all.    We looked hard for three plants, in particular, that I associated with the damp margins of streams, and which I had expected to see here: Verronica beccabunga (brooklime or water speedwell), Rorippa nasturtium-aquaticum (water cress) and Mentha aquatica (water mint).  None seemed to be present in any of the stretches we visited apart from a single sorry looking brooklime in the freshwater marsh at Denemouth .

What we did find, a little further downstream, was a pebble and gravel-dominated stretch with a straggly array of plants, all bent over in the direction of flow.   These included broad-leaved dock (Rumex obtusifolius), nettles (Urtica diocia), a few shoots of Himalayan balsam (Impatiens glandulifera) and some grasses.    Were I not standing on a dry stream bed I would have assumed that this was a bare piece of ground being colonised by typical ruderal species.  And that, I think, offers some insights into the ecology of Castle Eden Burn.   This is not a stream that occasionally dries out: it is a long-thin terrestrial habitat that is occasionally flushed through by water.   Welcome to north-east England’s premier wadi.

Rumex_obtusifolius_CED

Rumex obtusifolius and other ruderal vegetation on the stream bed of Castle Eden Burn, August 2019.

This hypothesis really needs corroboration by a hydrologist, but the graph I showed in “Out of my depth …” shows that, despite flow being generally low,  episodes of high flow are scattered throughout the year, and I suspect that these keep the substratum mobile and, more important, stop organic matter accumulating to give amphibious plants an opportunity to establish.   The water table, too, I guess, is too far below the stream bed in between the spates to make it easy for plants to stay hydrated.   This is one of the main differences between Castle Eden Burn and the southern chalk streams, which are characterised by very stable flow regimes

From the point at which Blunt’s Burn enters Castle Eden Burn (NZ 436 396) there does seem to be permanent flow down to the sea.  Still, however, there was very little in-stream vegetation.  That was in contrast to the forest around us, which was floristically-rich (Heather has written more about this on her blog) and, on this warm summer morning, positively humming with bees and aflutter with butterflies.

A large embankment takes the busy A1086 over the Dene, the Burn passing through a long culvert at this point, after which there is a viaduct taking the coastal railway line across before the dene widens out into a large area of meadow just before it reaches the sea.   The stream’s path to the sea is, however, blocked by mine waste that was dumped from the coal mines that used to line the Durham coast.  This forces the stream to turn ninety degrees south for a few hundred metres before finding a way through and, gradually, trickling and percolating through the beach. The mines have all gone now and the sea is gradually eroding this compacted mass of waste.  Before the waste arrived, apparently, there was an area of saltmarsh at the mouth of the burn.   Now, there is a freshwater marsh, dominated by reeds (Phragmites australis).  When the mine waste finally goes, maybe the saltmarsh will return.  Meanwhile, Castle Eden Burn has no grand finale: it ends on a whimper not a bang.

We climbed a narrow, steep pathway up through gorse and brambles onto the clifftops overlooking these final stages of Castle Eden Burn to get a view that was, in light of all that had passed through my mind earlier, oddly symbolic.  The stream flowed almost due east until it encountered the bar, and the gentle arc which it then describes looks just like a question mark.   How ironic, I thought, for a stream that raises more questions than answers to sign off in that way ….

Denemouth_CED_Aug19

Denemouth, at the end of Castle Eden Dene, just above the point where the stream joins the North Sea.

Out of my depth …

Castle_Eden_Dene_March19

I was about to start writing up an account of my latest visit to Castle Eden Dene, when I realised that I had forgotten to describe my previous visit, back in March.   I’ve already described a visit in January, when the stream was dry (see “Castle Eden Dene in January” and “Tales from a dry river bed”) and promised regular updates through the year.   It seems that, amidst all the travel that filled my life over the last three months, I overlooked the post that I should have written about the visit that I made in early March.

Whereas the river was dry in January, rain during February meant that, when I returned to the Dene on 11 March, some rather turbid water was flowing down the channel on its short journey to the North Sea.   There is, finally, something more like a stream habitat from which I can collect some diatoms.

Many of the diatoms that I found in March belonged to taxa that I had also seen in January; however, the proportions were quite different.   In some cases, species that were common in January were less common now (e.g. Humidophila contenta*) but there was a small Nitzschia species with a slightly sigmoid outline that was very sparse in the January sample but which was the most abundant species in the March sample.  I’ve called this “Nitzschia clausii” but the Castle Eden Dene population does not fit the description of this perfectly.   A lot can change in a couple of months, especially when dealing with fast-growing organism such as these, as my posts on the River Wear showed (see “A year in the life of the River Wear”).  Castle Eden Burn’s highly variable discharge just adds another layer of complication to this.

CED_diatoms_Mar19

Diatoms from Castle Eden Dene, March 2019:   a. – e.: Nitzschia cf clausii; f. Tabularia fasiculata; g. Tryblionella debilis; h. Luticola ventricosa; i. Luticola mutica; j. Ctenophora pulchella.  Scale bar: 10 micrometres (= 1/100thof a millimetre).   The picture at the top of the post shows Castle Eden Burn at the time that the sample was collected.   

Nitzschia clausii is described as being “frequent in brackish freshwater habitats of the coastal area and in river estuaries, as well as in inland waters with strongly increased electrolyte content”.   A couple of the other species from this sample – Ctenophora pulchella and Tabularia fasiculata (both illustrated in the diagram above) – have similar preferences.    My experience is that we do often find a smattering of individuals belonging to “brackish” species in very hard water, as we have in Castle Eden Burn.  Average conductivity (based on Environment Agency records) is 884 µS cm-1; however, values as high as 1561 µS cm-1.   The fluctuating discharge plays a role here, as any evaporation will serve to concentrate those salts that are naturally present in hard freshwater.   This should probably not be a big surprise: life in brackish waters involves adapting to fluctuating osmotic regimes so species that can cope with those conditions are also likely to be able to handle some of the consequences of desiccation.

Average values of other chemical parameters from 2011 to present, based on Environment Agency monitoring are: pH: 8.3; alkalinity: 189 mg L-1 CaCO3; reactive phosphorus: 0.082 mg L-1; nitrate-nitrogen: 1.79 mg L-1; ammonium-nitrogen: 0.044 mg L-1.   There is some farmland in the upper catchment, and the burn also drains an industrial estate on the edge of Peterlee but, overall, nutrient concentrations in this stream are not a major concern.   The Environment Agency classifies Castle Eden Burn as “moderate status” due to the condition of the invertebrates but does not offer any specific reason for this. I suspect that the naturally-challenging habitat of Castle Eden Burn may confound assessment results.

I’ve also been given some data on discharge by the Environment Agency which shows how patterns vary throughout the year.  The two sampling locations are a couple of kilometres above and below the location from which I collect my samples and both have more regular flow.  However, we can see a long period between April and September when discharge is usually very low.   The slightly higher values recorded in July are a little surprising, but are spread across a number of years.   It is also, paradoxically, most common for the burn to be dry in July too: clearly, a month of extremes.  As my own visits have shown, it is possible for the burn to be dry at almost any time of the year, depending on rainfall in the preceding period   The dots on the graph (representing ‘outliers’ – records that exceed 1.5 x interquartile range) show that it is also possible to record high discharges at almost any time during the year too.  I should also add that, as I am not a hydrologist, I am rather outside my comfort zone when trying to explain these patterns.  I would have said ‘out of my depth’ though that’s not the most appropriate phrase to use in this particular situation.

CED_discharge

Discharge in Castle Eden Burn, as measured by the Environment Agency between 2007 and present.   Measurements are from NZ 4136 2885 (‘upstream’) and NZ 45174039 (‘downstream’).  

* Note on Humidophila contenta:it is almost impossible to identify this species conclusively with the light microscope as some key diagnostic characters can only be seen with the scanning electron microscope.   However, all members of this complex of species share a preference for intermittently wet habitats so these identification issues are unlikely to lead to an erroneous ecological interpretation.  It is probably best to refer to this complex as “Humidophila contenta sensu lato” rather than “Humidophilasp.” order to distinguish them from those species within the genus that can be recognised with light microscopy.

Reference

Lange-Bertalot, H., Hofmann, G., Werum, M. & Cantonati, M. (2017).  Freshwater Benthic Diatoms of Central Europe: over 800 Common Species Used in Ecological Assessment. English edition with updated taxonomy and added species.  Edited by M. Cantonati, M.G. Kelly & H. Lange-Bertalot.  Koeltz Botanical books, Schmitten-Oberreifenberg.

The complexities of measuring mass…

Benthotorch_in_action

Once upon a time, measuring the quantity of algae growing on the beds of streams and rivers was a painstaking, slow process that invariably revealed large amounts of spatial and temporal variation that, very often, obscured the ecological signals you were looking for. That has changed in the last decade thanks to the availability of field fluorimeters such as the BenthoTorch.  This makes it much quicker and easier to measure chlorophyll concentrations, the usual proxy for algal quantity.  Thanks to devices such as this it is now much easier to discover that your ecological signal is masked by spatial and temporal variation.

We’ve generated a lot of data about the fluxes of algae in the River Ehen using a BenthoTorch over the past five years and are in a position where we can start to make some generalisations about how the quantity of algae vary over the course of a year.  In broad terms, the results I showed in “The River Ehen in January” back in 2014 have not varied greatly over subsequent years, with peak biomass in mid-winter and low biomass in the summer (due, we presume, to intense grazing by invertebrates).  Curiously, we see a much less distinctive seasonal pattern in the nearby Croasdale Beck, but that is a story for another day….

The BenthoTorch uses an algorithm to partition the fluorescence signal between three major algal groups and, though this is not without issues (see below), I thought it might be interesting to see how these groups varied with biomass trends, and consider how this links to ecological theory.  The first group I’m considering are the green algae which, in this river, are mainly filamentous forms.   The general pattern, seen in the graph below, is for a gradual increase in the proportion of green algae, which fits with the current understanding of thicker biofilms having greater structural complexity with filamentous algae out-competing attached single celled algae to create a “canopy” of algae that are more effective at capturing light and other resources.  The relationship is, however, strongly wedge-shaped so, whilst many of the thickest biofilms have a lot of green algae, there are also thick biofilms where green algae are scarce or even non-existent.  Croasdale Beck shows a similar, but less pronounced, trend.

green_algae_in_Ehen

Relationship between the proportion of green algae and the total quantity of benthic algae (expressed as chlorophyll concentration) in the River Ehen (a.) and Croasdale Beck (b.).   The blue lines show quantile regression fits at p = 0.8, 0.5 and 0.2.   The image at the top of the post shows Ben Surridge using a BenthoTorch to measure algal biomass beside Croasdale Beck in Cumbria.

The second graph shows that this pattern of a gradual increase in proportion is also the case for diatoms and, once again, there is a broad wedge of points with an upward trend.  But, once again, there are also samples where biomass is high but diatoms are present in very low numbers or are even absent.   What is going on?

The problem is clear I think, if one looks at the final image in “The only way is up …” where the very patchy nature of algal communities in the River Ehen (and, indeed, many other rivers).   There are plenty of algae on this boulder, but not organised in a homogeneous manner: some zones on the boulder are almost pure diatom whilst others are almost pure green algae (and there are also zones that are almost pure Lemanea– I’ll come to that in a future post).   We try to sample the stones as randomly as possible so you can see the potential for getting very different numbers depending on where, on a stone, we point the BenthoTorch’s sensor.

diatoms_in_the_Ehen

Relationship between the proportion of diatoms and the total quantity of benthic algae (expressed as chlorophyll concentration) in the River Ehen (c.) and Croasdale Beck (d.).   The blue lines show quantile regression fits at p = 0.8, 0.5 and 0.2.  

With experience, you can make an educated guess about the types of algae present in a biofilm.  I’ve tried to capture this with my watercolours, using washes of raw sienna for the diatoms and a grass-green for the green algae, which roughly matches the colour of their respective growths in the photo in my earlier post.   The two groups of algae a are relatively distinct on that particular boulder.   The top row roughly matches the upper “edge” of the graph showing variation in diatoms, whilst the bottom row emulates the upper “edge” of the graph showing variation in green algae.  These are the two extreme situations; however, we also often see darker brown growths in the field, which can be recreated by mixing the raw sienna and grass-green together.  When I peer through a microscope I often see green algae smothered in diatoms: genera such as Oedogoniumare particularly prone as they have less mucilage than some of the others we find in the Ehen. Their filaments often host clusters of Fragilariacells as well as Achnanthidium minutissimum, whilst stalked Gomphonemaand chains of Tabellaria flocculosaoften grow through the tangle of green filaments.   The dark brown colour is deepened yet further by the colour of the underlying rock, so my effort on white watercolour paper is a little misleading.

colour_patches

A colour chart showing how different proportions of green algae and diatoms influence the colour of biofilms.

The final graph shows how, as the average biomass increases in the River Ehen, so the variability in biomass also increases.   The River Ehen is one of the cleanest rivers I know but I suspect that this pattern in benthic algal quantity could be reproduced in just about any river in the country. What I would not expect to see in any but the purest and most natural ecosystems is quite so much variation in the types of algae present.   Once there is a little enrichment, so I would expect the algae to become more of a monoculture of a dominant filamentous alga plus associated epiphytes.  Like much that happens in the microscopic world of rivers, it is easier to describe than it is to measure.

That, however, is only part of the story but I’ll come back to explain the patterns in the other main groups of algae in the Ehen and Croasdale Beck in my next post.

mean_biomass_by_stdev

The relationship between mean chlorophyll density and the standard deviation (based on measurements from five separate stones) for samples from the River Ehen and Croasdale Beck. 

 

Croasdale Beck in February

Ennerdale_Feb19

My latest trip to the west Cumbria coincided with the period of freakily warm weather that marked the end of February (in marked contrast to a year previously when we were in the midst of the “Beast from the East”).   It felt like spring had come early although the skeletal outlines of leafless trees were incongruous against the backdrop of blue skies and, despite feeling the warmth of the sun on our faces as we worked, the water still had a wintery chill when the time came to plunge in my arm.

There were thick growths of algae on the bed of Croasdale Beck: a quick check with my microscope later showed this to be mostly Odontidium mesodonand Gomphonema parvulumand this piqued my curiosity to see how different species responded to the fluctuations in biomass that we observe in the streams in this region. I’ve talked about this before (see “A tale of two diatoms …”), suggesting that Platessa oblongellatended to dominate when biofilms were thin whilst Odontidium mesodon preferred thicker biofilms.  That was almost two years ago and I now have more data with which to test that hypothesis, and also to see if any other common taxa had an equally strong preference for particular states.

Croasdale_cobble_Feb19

A cobble from the bed of Croasdale Beck in February 2019 showing a brown biofilm (approx. 1.7 micrograms per square centimetre) dominated by Gomphonema parvulumand Odontidium mesodon.   The photograph at the top of the post shows Ennerdale Water photographed on the same day.

I should also be clear that, in Croasdale Beck especially, diatoms are the main algal component of the biofilm, so they are not so much responding to a particular state of the biofilm as actively contributing biomass to create that state.  The other photosynthetic organism that is obvious to the naked eye in this part of Croasdale Beck is the cyanobacterium Chamaesiphon fuscus (see “A bigger splash …”) but this forms crusts on stone surfaces rather than contributing to the superstructure of the biofilm itself. We do find other filamentous algae, but intermittently and in smaller quantities.

We’ll look at Platessa oblongellafirst, bearing in mind that this was shown to be a mixture of two species about halfway through our study (see “Small details in the big picture …”).   The graph below, therefore, does not differentiate between these two species although, from my own observations, I have no reason to believe that they behave differently.   What I have done in these graphs is to divide the biomass measurements and the percent representation of these taxa in each sample into three categories: low, middle and high.   In each case, “low” represents the bottom 25 per cent of measurements, “high” represents the top 25 per cent of measurements and “middle” represents all the rest. The left-hand graph shows biomass (as chlorophyll a concentration) as a function of the relative abundance of the diatom whilst the right-hand graph shows the opposite: the relative abundance of the diatom as a function of the biomass.  These graphs bear out what I suggested in my earlier post: that Platessa oblongella(and P. saxonica) are species whose highest relative abundances occur when the biofilm is thin.  So far, so good.

P.oblongella

Relationship between relative abundance of Platessa oblongella (including P. saxonica) and biomass in Croasdale Beck, Cumbria.  a. shows biomass (as chlorophyll a) as a function of the relative abundance of the two species (Kruskal-Wallis test, p = 0.047) whilst b. shows the relative abundance as a function of biomass (p = 0.057).

My second prediction in my earlier post was that Odontidium mesodonpreferred moderate or thick biofilms; however, whilst there is a clear trend in the data, differences between low, middle and high values of neither biomass nor relative abundance are significant.   The explanation may lay in the strong seasonality that O. mesodondisplays, thriving in spring but less common at other times of year (see “More about Platessa oblongella and Odontidium mesodon”).  However, there are no strong seasonal patterns in biomass in Croasdale Beck, and this disjunction introduces enough noise into the relationship to render it not significant.

O.mesodon

Relationship between relative abundance of Odontidium mesodon and biomass in Croasdale Beck, Cumbria.  a. shows biomass (as chlorophyll a) as a function of the relative abundance of O. mesodon (Kruskal-Wallis test, p = 0.568) whilst b. shows the relative abundance as a function of biomass (p = 0.060).

I then tried looking at the relationship between relative abundance and biomass for a few other common taxa but with mixed results.   None of Achnanthidium minutissimum, Gomphonema parvulum complex or Fragilaria pectinalis showed any clear relationship; however, when I looked at Fragilaria gracilis, a different pattern emerged, with a significant relationship between the quantity of biomass and the proportion of this species in the sample.  That, too, is not a great surprise as I often see clusters of Fragilaria gracilis cells growing epiphytically on filamentous algae within the biofilm.  Whilst Platessa oblongella, which sits flat on the stone surface, seems to be a species that thrives when the biofilm is thin, so Fragilaira gracilisis favoured by a more complex three-dimensional structure, where it can piggy-back on other algae to exploit the light.   I suspect, however, that in a stream such as Croasdale Beck, where the substratum is very mobile, Fragilaira gracilis will also be one of the first casualties of a scouring spate which will, in turn, open up the canopy allowing Platessa oblongella back.   Even though my results for Odontidium mesodonare not significant, I still think it plays a part in this sequence, occupying the intermediate condition when some biomass has accumulated.  It looks to me as if it also likes cooler conditions which then complicates interpretation of my results.

Indeed, I am being rather selective in the results that I have included here.  Three of the six species I investigated showed no response and one of the three that I did include showed a trend rather than a statistically-convincing effect.  I suspect that the situation will rarely be as simple as I have shown for Platessa oblongella and Fragilaira gracilis.  Nonetheless, there is enough here to make me want to scratch a little more and try to understand this topic better.

F.gracilis

Relationship between relative abundance of Fragilaria gracilis and biomass in Croasdale Beck, Cumbria.  a. shows biomass (as chlorophyll a) as a function of the relative abundance of F.gracilis (Kruskal-Wallis test, p = 0.010) whilst b. shows the relative abundance as a function of biomass (p = 0.036).

Croasdale_Beck_Feb19

Croasdale Beck, photographed in February 2019.