Pesticide-induced myopia …

The previous post ended with a reflection on how algal communities in rivers changed.   This one picks up that theme and takes it forward, looking at a very different reason.   First, we need to move from the shadow of the Wastwater screes to a chalk stream flowing through Dorset, an altogether more gentle landscape.    It was beside this stream that Helen Rosenkranz, a PhD student I co-supervised, ran some experiments on what happens to stream algae when they are exposed to herbicides.  

The graph below  summarises the situation.   The bars represent  (from left to right): the control treatment (water from the River Frome), samples treated with the herbicide chlorotoluron, samples treated with glyphosate and samples treated with both herbicides.   The two herbicides were applied at quite high concentrations for 12 hours, simulating the situation if the spray had accidentally drifted to the water course or if the herbicide had washed off the plants and soil following rainfall.   Helen measured a number of characteristics of the algae, but I have just shown one of her measurements.   

Ratio between total number of cells of diatoms and green algae in control (CON) and herbicide-treated biofilms (CLT = chlorotoluron, GBH = glyphosate, GHBC = chlorotoluron and glyphosate combined.   Bars show mean values; vertical lines show standard errors.   See Rosenkranz et al. (2023) for more details.   The photograph at the top of the post shows the upper reaches of the River Wylye in Wiltshire, an example of a southern English chalk stream. 

Because a lot of stream monitoring around Europe uses diatoms, we know less about how pollutants affects other groups of algae.   In this case, we did see an effect of the herbicides (particularly glyphosate) on the diatom species but more interesting was an overall shift in the proportions of green algae and diatoms.  The glyphosate seems to have encouraged the green algae but suppressed the diatoms whilst not affecting the overall quantity of algae at all.  Helen also showed that the changes to the diatom assemblage affected the Trophic Diatom Index (TDI), the basis for UK’s assessments of the health of benthic algae. This is important because the TDI is used primarily to assess the impact of nutrients from rivers, so another pollutant that affects the TDI potentially influences the decisions that the Environment Agency make about how a river should be managed.  

The problem is that pesticides have always been a blind spot in the UK’s river management strategy.   Everyone knows that they are there, but measuring their effect is difficult.  Our knowledge of the chemical composition of river water is based on a network of sites sampled four times a year.   They used to be sampled monthly, but that was trimmed back a few years ago.  Let’s say it takes ten seconds to dip a bottle into a river to collect a sample.   Four samples spread through the year means that we have an accurate idea of conditions for 0.001 per cent of the year.   That’s not a problem if we can assume that those four brief moments are representative of the year as a whole.   The effluent from a sewage works, for example, runs continuously, so there is no particular reason to presume that one sample is wildly different from another.   

However, we know that pesticides are only applied at certain times of year, and also that rainfall events around those peak spraying periods are when the risk to river ecology is greatest.   That means we really need to focus our sampling on those brief periods.  That’s the ecologist in me speaking, but try scheduling that into the work programmes of a team of public sector workers.   The result is a very imperfect understanding of the extent to which pesticides affect stream biology.  

Two recent German studies tried to overcome this by using automatic samplers that switched on when it started to rain.   These measured substantially more pesticides in the water than a conventional sampling programme, and they found that pesticide concentrations in the water were probably the most important source of stress on aquatic invertebrate communities.   If that is the situation in Germany, then there is no reason why this is not also the case in the UK.   

It’s a question that’s easy to ask but which will be difficult to answer.   As in many situations with environmental health, the issue is not the technology itself but finding the resources and the willingness to deploy it.   The importance of agriculture to the UK economy and food security means that pesticides in rivers s a subject that the government would prefer to kick into the long grass.  It is a manifestation of what philosophers call the “trolley problem”: there are losers whatever they do and that does not make for easy political messaging.   

References

Liess, M., Liebmann, L., Vormeier, P., Weisner, O., Altenburger, R., Borchardt, D., … & Reemtsma, T. (2021). Pesticides are the dominant stressors for vulnerable insects in lowland streams. Water Research, 201, 117262.

Rosenkranz, H., Kelly, M. G., Anesio, A. M., & Yallop, M. L. (2023). A Multi-Faceted Approach to Quantifying Recovery of Stream Phytobenthos Following Acute Herbicide Incidents. Phycology, 3(1), 25-46.

Weisner, O., Arle, J., Liebmann, L., Link, M., Schäfer, R. B., Schneeweiss, A., … & Liess, M. (2022). Three reasons why the Water Framework Directive (WFD) fails to identify pesticide risks. Water Research, 208, 117848.

Some other highlights from this week: 

Wrote this whilst listening to: Elderly vinyl copy of Elgar’s Cello Concerto with Jacqueline du Pre and Daniel Barenboin which has a scratch on the final bars.   It does mean that a piece of music that you wish could last forever literally does, so long as you don’t mind listening to endless repetitions of half a crescendo.

Currently reading:   The Wonderful Adventures of Mrs Seacole in Many Lands: the autobiography of a pioneer nurse working during the Crimean War.  Realised, amidst all the grim news from Ukraine that the Crimean War (1853-1856) was also a confrontation between Russia and the West fought out in Ukraine, and that I knew very little about it.

Cultural highlight: Tár, film starring Cate Blanchett exploring the question of whether a monster can make great art.  The plot centres on rehearsals for performances of Mahler’s 5^th Symphony and Elgar’s Cello Concerto (see above).  

Culinary highlight:   Vegan ragu, made to recipe by Felicity Cloake in The Guardian, with homemade tagliatelle 

Temporary foundations …

I spent the quiet period between Christmas and New Year converting my comments on the structure of diatom communities in the River Irt (see “Cold comforts …”) into a picture, as a way of illustrating how Gomphonema exilissimum acts as a “foundation species”.  Gomphonema exilissimum grows on long branched stalks which create a matrix upon which other species can attach.   The most abundant of these epiphytes were Achnanthidium and Fragilaria tenera, both of which I’ve included in the picture.   There were also a few ribbons of Tabellaria flocculosa and occasional green cells (top left corner of the final illustration in “Cold Comforts …”) but I have not included them in this image.   Neither the photos nor the painting really conveys the patchiness of the actual colonies.   Some fields of view were dominated by Gomphonema cells, others had as many Fragilaria or Achnanthidium cells as Gomphonema.  Any illustration is, necessarily, a simplification.   As is a conventional “name and count” analysis.  You may be able to produce a longer list of species and, because the samples are better homogenized during the preparation steps, better estimates of the relative abundance of each, but you lose the sense of their interrelatedness.   

Interrelatedness is the key here.   Remove Fragilaria tenera from the mix, and another Fragilaria species will come in and replace it.  I’ve found several other Fragilaria species in samples from these streams over the years, often growing as epiphytes, and there is no obvious reason why they would not also thrive on the stalk of a Gomphonema.   However, remove Gomphonema exilissimum, however, and there would be nowhere for Fragilaria tenera to gain a foothold.  This distinctive diatom-dominated assemblage, visible with the naked eye, would disappear.  Fragilaria tenera might not disappear from the reach as a whole, as it also grows as an epiphyte on some of the green algae that grow in these streams (e.g. Oedogonium).   However, that would be a different community.   The old habitat is demolished and a new one built in its place.   From the coarse perspective of a biologist interested in a list of species from a location, Fragilaria tenera is still there.  From a more focussed perspective, however, it has been “rehoused” in a different suburb, a community built around a different foundation species.   

The Gomphonema community viewed down the microscope.  The image at the top is the result of scanning multiple fields of view such as this.

It is not always easy to get an impression of interrelatedness from simply peering through a microscope.   Gomphonema exilissimum was neither the most abundant species in the sample nor the largest, at least based on cell size.  Diatom stalks are extracellular structures which disappear when samples are prepared by the standard cleaning procedures.  It might well be that G. exilissimum plus stalks constituted more biomass than the other species present, but I have not measured that.   However, it does not need to be the case because the stalks are, themselves, supported by the water and by each other.  We cannot simply extrapolate from terrestrial habitats where a plant needs to invest in structural tissues in order to stand up straight.   The tangled mass of stalks creates a matrix that is greater than the sum of its parts.  

My experience is that it is the stalk-forming species that are the most obvious diatoms to fit the term “foundation species”.   In freshwaters these tend to be in the order Cymbellales.   Examples covered in this blog include Didymosphenia geminata (see “A journey to the headwaters of the River Coquet …” and Gomphosinica (see “Subaquatic landscapes in Pangong Tso”).   It is important to distinguish between a “foundation species”, which shapes a community, and a “dominant species” which is the most abundant species (number, biomass, whatever) but which is replaceable, in terms of their control of population and community dynamics.  But that opens up another problem: this community is not always present.  It waxes and wanes (see graph in “Cold comforts …”).   A terminology that was developed by biologists concerned with the visible world sometimes struggles when fitted to the microscopic domain.   Describing Gomphonema exilissimum as a foundation species works right here right now, but not when applied across the whole year.   Algal communities in lakes and rivers are often transient, with an ever-shifting patchwork of growth forms.  In  essence, however, the same processes govern these and the forests which extend up the hillside beside the river.   We just need to recalibrate our minds in order to understand what we are seeing.    

Reference

Record, S., McCabe, T., Baiser, B., & Ellison, A. M. (2018). Identifying foundation species in North American forests using long‐term data on ant assemblage structure. Ecosphere, 9(3), e02139.

Some other highlights from this week: 

Wrote this whilst listening to: Simon and Garfunkel Live in Central Park.  

Currently reading:   A Ballet of Lepers by Leonard Cohen.   A short novel and stories published posthumously.  Just finished Takeaway by Angela Hui, a memoir of growing up in a Chinese Takeaway in the valleys of South Wales. 

Cultural highlight: Catherine Called Birdy, 2022 film by Lena Dunham.  

Culinary highlight:   Old-school Chinese takeaway, inspired by reading Angela Hui’s memoir.   We’ve had plenty of Chinese food since our visits to China but have gravitated towards Sichuan restaurants and this meal reignited my love of Cantonese cooking.