More about Platessa oblongella and Odontidium mesodon

As my last post used the conventions of figurative art to describe algal ecology, I thought I would stick to graphs – science’s very own school of abstract art – for this one.   I spent some time in “Small details in the big picture” discussing the ecology of Platessa oblongella (including P. saxonica) but without saying very much about the types of streams where these species were found.  So I am going to take a step away from the Ennerdale catchment in this post and, instead, collate environmental data a large number of sites to get a broader understanding of their habitat preferences.  As these species are often associated with Odontidium mesodon (see “A tale of two diatoms …”), I will summarise the preferences of this species at the same time (but see Annex 1 for a graph of this species’ preferences for still versus standing water).

The first set of graphs show the response of these species to pH and alkalinity and establish both as species typical of circumneutral soft water.  Platessa oblongella can be abundant in more acid conditions (i.e. to the left of the green vertical lines) but most of the records where it is abundant have pH values between 6.5 and 7.5.   Note that P. oblongella can also be found in humic waters, where lower pH thresholds apply (see Annex 2).

Distribution of Odontidium mesodon and Platessa oblongella (including P. saxonica) to pH and alkalinity in UK streams.   Vertical lines for pH indicate threshold values that should support high (blue), good (green), moderate (orange) and poor (red) ecological status classes.  See Annex 2 for more explanation.

The second set of graphs shows how these species respond to inorganic nutrients.   Both are most abundant when inorganic nutrients are present in low concentrations, though the trend is stronger for phosphorus than it is for nitrate-nitrogen.   The graphs for Platessa oblongella, however, both have a few outliers.   I have seen P. oblongella in a few situations where I did not expect it – I remember finding it in the Halebourne, a stream draining heathland around Aldershot and Bagshot in Surrey, where the water was well buffered (mean alkalinity: 61.3 mg L-1 CaCO3) and nutrient concentration were high (mean total oxidised nitrogen: 4.01 mg L-1; dissolved phosphorus: 0.25 mg L-1) and Carlos Wetzel and colleagues note some other anomalous records from the literature in their paper (cited in my earlier post), including a few from high conductivity and even brackish environments.   So we should treat these plots as indicative of the ecological preferences rather than definitive.

Distribution of Odontidium mesodon and Platessa oblongella (including P. saxonica) to nitrate-N and dissolved phosphorus in UK streams.   Vertical lines indicate threshold values that should support high (blue), good (green), moderate (orange) and poor (red) ecological status classes.  See Annex 2 for more explanation.

The final pair of plots show how the relative abundance of these two species changes over the course of the year.  These plots show the months when each taxon is abundant, by the standards of that taxon.  Because Platessa oblongella tends to be very numerous in samples, the threshold for this taxon (the 90th percentile of all records) is higher than that for O. mesodon.   This reveals a very clear pattern of O. mesodon thriving in Spring whilst P. oblongella is abundant throughout the year, but with a slight preference for summer and autumn.  We need to reconcile these patterns with the observations in A tale of two diatoms that show that P. oblongella is associated with thinner biofilms than O. mesodon and try to work out whether season is driving the patterns or whether the seasonal patterns are the manifestation of other forces.   My suspicion is that P. oblongella is a classic pioneer species but also has a low-growing prostrate habit which means that it should be resistant to heavy grazing, which may confer an advantage in the summer and autumn when grazers are most active.  However, I may be getting ahead of myself, as we are in the process of analysing data on grazer-algae interactions in the River Ehen and Croasdale Beck that may throw more light on this.  There are clearly more layers to this story yet to be revealed …

Distribution of Odontidium mesodon (i.) and Platessa oblongella (j., including P. saxonica). The solid lines represent relative sampling effort (i.e. the proportion of samples in the dataset collected in a particular month) and the vertical bars represent samples where the relative abundance of taxon in question exceeded the 90th percentile for that taxon (20% for P. oblongella/P. saxonica and 5% for O. mesodon).

Reference

The dataset used for these analyses is that used in:

Kelly, M.G., Juggins, S., Guthrie, R., Pritchard, S., Jamieson, B.J., Rippey, B, Hirst, H & Yallop, M.L. (2008). Assessment of ecological status in UK rivers using diatoms. Freshwater Biology 53: 403-422.

Annex 1: Odontidium mesodon’s preference for still or standing water

As I included a graph showing the preference of Platessa oblongella / P. saxonica for still or standing water in “A tale of two diatoms …”, I have included a similar graph for Odontidium mesodon here.   I have not included any data from the streams that flow into Ennerdale Water’s north-west corner in this graph as this would give a distorted picture.  To date, I have only seen a single valve of O. mesodon during analyses of 14 samples from these streams but I have not yet sampled these in spring which, as the graph above shows, is the time when O. mesodon is most abundant.   Like Platessa oblongella, O. mesodon is predominately a species of running, rather than standing waters.

Differences in percentage of Odontidium mesodon in epilithic samples from Ennerdale Water and associated streams.  Data collected between 2012 and 2018.

Annex 2: notes on species-environment plots

These are based on interrogation of a database of 6500 river samples collected as part of DARES project.  Vertical lines show UK environmental standards for conditions necessary to support good ecological status: blue = high status; green = good status, orange = moderate status and red = poor status.  Note that there are no environmental standards for alkalinity and the vertical lines show a rough split of the gradient into low alkalinity (“soft water”: < 10 mg L-1 CaCO3), low/moderate alkalinity (³ 10, < 75 mg L-1 CaCO3), moderate/high alkalinity (³ 75, < 150 mg L-1 CaCO3) and high alkalinity (“hard water”: ³ 150 mg L-1 CaCO3).

pH thresholds are for clear water (see UK TAG’s Acidification Environmental Standards.  The corresponding thresholds for humic waters are lower (high/good: 5.1; good/moderate: 4.55; moderate/poor: 4.22; poor/bad: 4.03).

Phosphorus thresholds are based on UK TAG’s A Revised Approach to Setting WFD Phosphorus Standards.   Current UK phosphorus standards are site specific, using altitude and alkalinity as predictors.  This means that a range of thresholds applies, depending upon the geological preferences of the species in question.  The plots here show the position of boundaries based on the average alkalinity and altitude measurements in the DARES database.

Note, too, that phosphorus analyses use the Environment Agency’s standard measure, which is unfiltered molybdate reactive phosphorus.  This approximates to “soluble reactive phosphorus” or “phosphorus as orthophosphate” in most circumstances but the reagents will react with phosphorus attached to particles that would have been removed by membrane filtration.

Nitrate-nitrogen: There are, currently, no UK standards for nitrates in rivers.  Values plotted here are derived in the same way as those for phosphorus (see “This is not a nitrate standard”)

 

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What a difference a storm makes …

I was back at Croasdale Beck last week and noticed a rather dramatic change to the meander just upstream from our regular sampling spot.   If you look at the photograph that heads the post “A tale of two diatoms …”, you’ll see the stream flowing around this meander.  Now, however, it has cut a new, shorter channel that bypasses the meander altogether.   We visited the stream just a few days after Storm Ophelia had passed through although, judging by the grass growing on the gravel of the abandoned meander, it was not necessarily this particular event that reshaped the stream.

Croasdale Beck is an unruly tributary of the River Ehen, rising on the fells above Ennerdale Water and tumbling down across rough grazing land and some semi-improved pasture (as in the picture above) before joining the Ehen in Ennerdale Bridge.   This is not the first time that we have seen conspicuous changes in the channel after a storm.  The magnitude of the flood is illustrated by the hydrograph below, which went off-scale for a period, as the discharge exceeded 3000 mega litres per day (300 MLD is the approximate limit for safe wading, in my experience).   I noticed that there was much less green algae present than we usually record at this time of year, although the diatom film was still quite thick.   Some of the stones that I picked up to sample had the slimy biofilm on the underside, suggesting that they had been recently rolled by the flooded river.   Croasdale Beck has no lake to buffer the rise and fall of the floodwaters and a huge amount of energy is carried down in a short period of time as the water surges downstream.

By the time we had arrived, the floodwaters had subsided and the sheep were contentedly grazing the surrounding land.  The stream itself was almost back to base flow (in contrast to the River Ehen which was still only just wadable).  Only the meander looked different …

The hydrograph for the River Ehen, as the aftereffects of Storm Ophelia make their way downstream.

The exception that proves the rule …

If you are going to understand river ecology, you need to be able to consider landscapes at several different scales simultaneously.   In the River Ehen, this means looking upstream towards Ennerdale Water and, beyond, to Great Gable and the other Lake District peaks in order to appreciate the geology that gives the catchment its bones.  But, at the same time, you need to look around at the meanders of the river and the bankside vegetation that create the immediate habitat for the organisms, and then to look even more closely at the individual stones that line the river bed.

Peering into the water last week, the pebbles, cobbles and boulders that make up the substratum of the River Ehen looked bare of filamentous algae for the most part.  There were a few clumps but, at this time of year, when grazing invertebrates are active, the algal flora is reduced to a thin film, invisible to the naked eye and apparent only as a slimy sensation when you run your fingers across the stone’s surface.   However, when I picked up a couple of cobbles, I noticed small, pale green gelatinous growths stuck on the upper surface.   Most were just a few millimetres across with the largest up to about a centimetre.

A growth of Draparnaldia glomerata on the upper surface of a cobble in the River Ehen, Cumbria, April 2017.

These growths are composed of the green alga Draparnaldia glomerata.  I have written about this alga before (see “The River Ehen in February”) but, under the microscope, it is such a beautiful organism, that I am not going to apologise for writing about it again.   The alga lives inside the gelatinous mass and consists of a relatively thick central filament from which tufts of narrower side-branches emerge.  The cells that make up these side branches gradually narrow, and the chloroplast becomes smaller until, eventually, the cells form a colourless “hair”.   These hairs are relatively short on the material illustrated below but can be much longer (some longer hairs were present but did not present nicely for photography).  The hairs are, in fact, an adaptation to help the alga acquire phosphorus, something I described in an earlier post about a relative, Stigeoclonium tenue (see “A day out in Weardale”).

Draparnaldia glomerata from the River Ehen, April 2017 showing filaments and side branches. Scale bars: a.: 50 micrometres (= 1/20th of a millimetre); b.: 20 micrometres (= 1/50th of a millimetre).

A low concentration of phosphorus is usually regarded as a Good Thing by aquatic ecologists, as this limits the amount of energy produced  by the plants at the base of the food chain.  This, in turn, means that the microbes and animals that depend on these are not using up all the oxygen in the water, or having other deleterious influences on the ecosystem.   I would usually regard the presence of an organism such as Draparnaldia as a sign of a healthy stream, as it is adapted to thrive when phosphorus is relatively scarce.

I was, however, careful to place “relatively” in front of “scarce”.   Studies by my colleagues (referenced in the earlier post) showed that the production of the phosphatase enzyme that boosts the alga’s ability to acquire phosphorus when it is scarce is determined by the ratio of nitrogen to phosphorus inside the cell itself, rather than in the water.   The physiology of nutrient limitation is all about the balance between the different “ingredients” that a cell needs.   If you have three eggs and 170g of sugar, for example, you can only make one cake, regardless of how much flour you have in your cupboard.   So it is with algae: most of the locations where I find Draparnaldia have very little nitrogen, but even less phosphorus.   There are barely enough ingredients for the algal “cake” so it is advantageous to the organism to pump out some enzyme to order to make up the shortfall.  This means that I can say with confidence that Draparnaldia is usually a good indicator of healthy streams.

Just occasionally, however, I get Draparnaldia in places where I would not usually expect it to be found.   The picture below shows a colleague standing in the Terman River, just before it flows into Lough Erne in Northern Ireland.   She is holding a skein of Cladophora glomerata in her left hand and a skein of Draparnaldia in her right hand.  I associate the former with nutrient-rich rivers where I would not usually expect to find Draparnaldia.  But both were growing prolifically at this site which defied my expectations until I started to think about the physiology of the organism.   Had I had the facilities to analyse the tissues of the algae, I expect that I would have found very high concentrations of nitrogen which, in turn, creates a demand for yet more phosphorus so that it could convert that nitrogen into the proteins it needs to grow.  However, that cannot be the whole story, because normally, under such circumstances, I would expect a competitive alga such as Cladophora to out-compete and overgrow the Draparnaldia.   Here, they were growing side-by-side.   It is, to date, the most luxuriant growth of Draparnaldia that I have seen, and also the only occasion where I have seen these two species co-existing in such abundance.

My colleague, Bernie White, holding skeins of Cladophora glomerata (left hand) and Draparnaldia glomerata (right hand) from the Terman River near Toome.  The border between the Republic of Ireland and the UK runs along the middle of this river.

I can extend my lesson from the first example to say that, to understand the ecology of any particular river you need to have perspectives obtained from many other rivers.   But, in this case, we see a potential limitation: the case of the “rare exception” that clouds an otherwise clear picture of an association between an organism and a particular set of circumstances.   The problem is particularly acute when dealing with the effect of nutrients because we are usually dealing with indirect, rather than direct effects.   Draparnaldia glomerata is usually associated with clean rivers with low concentrations of nutrients but it is not there because nutrient concentrations are low.   As for the diatom Amphora pediculus (see “The challenging ecology of a freshwater diatom?”) a more nuanced understanding of the relationship between an organism and nutrients yields more useful insights than simply assuming a cause-effect relationship.

How green is my river?

Atma_River_July13

I wrote recently about the problems of knowing whether changes we observe in the biology of streams and rivers are signs of long-term shifts caused by human activities or just the result of short-term variation (see “’signal’ or ‘noise’?”). An interesting paper has just been published that allows us to see our observations on the River Ehen into a broader perspective.   The paper was written by my friend Susi Schneider, of the Norwegian Institute for Water Research, and is based on long-term observations by herself and colleagues on the Atna River in Norway (illustrated above; see “A brief excursion to Norway” and subsequent posts).

First of all, here is a graph summarising our observations of biomass in the River Ehen over three years. You can see a fairly regular pattern emerging of low biomass in the summer (when grazing activity is most intense) and high biomass in the winter. But you can also see strong differences between years. There were much greater quantities of algae in winter 2013/14, for example, compared with winter 2014/15 and we are puzzling over why this may have happened.

Ehen_summary_graph

Trends in algal biomasss in the River Ehen, Cumbria between October 2012 and November 2015.   Values are the means of measurements made at four separate sites in a 5 km stretch of the river below the outflow of Ennerdale Water. Arrows indicate the approximate time of year when surveys of the Atma River were performed.

Though the Atna is about 1000 km to the north-east of the Ehen, there are similarities: both rivers have soft water and low levels of human impact and, furthermore, one of the two sites Susi writes about is immediately below a lake, just as our sites on the Ehen are downstream of Ennerdale Water.   The difference is that we visit the Ehen monthly, whereas Susi only visits the Atna once a year, although she takes care to visit at the same time each year.   I’ve indicated the time of year of her visits on the graph of the River Ehen, to aid comparisons between the two datasets.

One of the problems we have in the Ehen is that there is anecdotal evidence of lower quantities of biomass in the past.   The large quantities of algae was the trigger for our study; as is often the case, we generally do not start monitoring until a problem is perceived, which means that we then don’t have the baseline data that we need to understand the causes.   One of the interesting points that arise from Susi’s study is that there have been recent increases in algal cover at both the sites she studied.   Note that the pattern was different at the two sites (one just below a lake, one not). To put the two studies in perspective, the green box on the graphs from the Atma shows the length of time of our study covers, compared to Susi’s.

The reasons for the high algal cover in the Atna include cool, wet summers, driven by the North Atlantic Oscillation, and high discharges in August (i.e. the month before Susi’s regular visits). The former is a natural cyclical phenomenon; high August discharges are, in turn, a consequence of the cool, wet summers, and probably exert their effect by removing the grazers that would naturally control the biomass.   High discharges in the spring (i.e. 10 times the average discharge) also seem to have a major effect on the quantities of algae recorded later in the year.

Atma_long_term_algae_trends

Long term trends in algal cover at two locations in the upper catchment of the Atna River, Norway. The green box indicates the length of time covered by our observations on the River Ehen (2012-2015).  Graph from Schneider (2015).

What should we learn from this comparison?

  • Lesson 1: start monitoring at least 10 years before the problem arises, so that you have a strong baseline.   There is a serious point here, as environmental monitoring is likely to be one major casualty of the cuts in current spending. When problems do arise, the availability of historic data from the site is inevitably very useful but the quid pro quo is that you may need to invest in data collection even when there is no obvious short-term justification for that monitoring.
  • Lesson 2: following on from this, regard environmental monitoring is an insurance policy, insofar as you may not need to make a “claim” on every single site where you monitor.   In effect, this means accepting that some monitoring data that you collect may be redundant. The problem is that you don’t know which data will be redundant until at least a decade or so after you have collected it.   However, the complex nature of many of the problems that we face, particularly where there are interactions with climate (as in the Atna), you will not be able to make evidence-based decisions without long runs of reliable data.
  • Lesson 3: when dealing with algal growth in rivers (which reflects interactions between the physical, chemical and biological environment), do not try to draw any conclusions until you have measurements from years when the North Atlantic Oscillation is in each of its positive and negative phases.   Susi’s paper shows the problems of unravelling the complexities of biological interactions with climate. We need to think in “decades”, not “years” if we are to truly understand environmental change.
  • Lesson 4: simple measurements of criteria that can be easily understood by non-technical stakeholders aid communication. In both the Atna and the Ehen, measurements relate directly to public perceptions of healthy versus unhealthy rivers.   We have all the nerdy details of what algae are found at each site in both the Ehen and Atna, but the take-home message can be put across in terms of “how green is my river?”
  • Lesson 5: if you have to sample at widely-spaced intervals (i.e. yearly, as in Susi’s study), make sure that you sample at the same time every year.
  • Lesson 6: all of these lessons can be ignored if you are a politician with ambitions to create a leaner public sector. The sad truth is that the consequence of failing to invest in monitoring networks is not likely to be apparent for several years (well beyond the next General Election, to be blunt). Almost any aspect of public spending can be hacked away by a skilled political operator, so long as the effects of these decisions are chronic and slow to manifest themselves …

Reference

Schneider S. (2015). Greener rivers in a changing climate? – Effects of climate and hydrological regime on benthic algal assemblages in pristine streams. Limnologica 55: 21-32.

Diatoms and dinosaurs

Ennerdale_looking_north_Apr

Ennerdale Water, looking north from the outflow to the River Ehen, April 2015

My monthly visit to the River Ehen coincided with a week of warm weather, clear skies and low flows.   There were some glorious views of the Lake District fells shrouded in early morning mist when I first arrived and I spent a few minutes trying to capture them with my camera before turning my attention to work.

We are now into our third year of regular sampling and are beginning to get a feel for the seasonal dynamics of the algae in the stream. By this time of year, we expect to see the biomass falling as the invertebrates start to become more active and browse away the luxuriant winter growths (see “A very hungry chironomid”). Many of the stones do appear to have less algae growing on them but, at one site, there were still some very conspicuous growths of diatoms. I did wonder if these were growths of Didymosphenia geminata (see “A journey to the headwaters of the River Coquet”); I have never seen this species in the Ehen but several aspects of the habitat here are such that it is possible that it could thrive.   However, when I got a sample under my microscope, the growths turned out to be long-stalked Gomphonema species, with a variety of other diatoms, including Tabellaria flocculosa, Fragilaria spp., Hannaea arcus, Brachysira neoexilis and at least a couple of members of the Achnnathidium minutissimum complex.

Gomphonema_clumps_Ehen_Apri

Clumps of diatoms – mostly Gomphonema – on a stone from the River Ehen, April 2015.

The Gomphonema species in the River Ehen have perplexed us before. Our best bet is that the larger ones, in particular, are G. gracile.   However, we are straying into areas where names abound although the limits between species are often described in vague terms in the literature. Dawn Rose and Eileen Cox described an experiment recently in which they grew two cultures of a different diatom, Gomphonema parvulum, in culture.   Diatoms, you may remember, get smaller with successive cell divisions (see “Diminishing with age…”) until a point is reached when sexual reproduction is initiated.   When this happened with one of their cultures, they were surprised to see cells that our Floras would have called Gomphonema gracile emerge from the auxospores. They continued to grow these and watched as the G. gracile cells diminished in size until they resembled G. parvulum again. The problem is, as they point out in their paper, most diatomists base their opinions on the structure of the cleaned frustule rather than the behaviour of the living cell.

Gomphonema_Ehen_April15

Gomphonema cells from the River Ehen, Cumbria, April 2015.  The five images on the left show a size reduction series in valve view; the right hand image shows two cells in girdle view with the stalk just visible.   Scale bar: 10 micrometres (1/100th of a millimetre).

Why, you may ask, don’t more people do experiments of this type in order to define the limits between species of diatoms?   It is a good question. The day before my visit to the Ehen, I had read an article in my newspaper about a similar study that had confirmed that the dinosaur Brontosaurus was probably a distinct genus rather than being part of the genus Apatosaurus, as had been assumed for most of the 20th century.   The scientists based their conclusion on careful measurements of different properties of dinosaur skeletons, just as most diatomists measure and describe the diatom frustule. What else can a palaeontologist do?   Their material is, after all, long dead.   Diatomists work with the living world. It is, admittedly, fiddly and time-consuming work to prepare cultures of single species from field communities, and you need special incubators to grow these cultures once they have been isolated. But it is not impossible.

Most of us who study diatoms are over-anxious to get rid of the organic constituents of the cell in order to focus on the silica remains. We are, in effect, contemporary palaeontologists. Most palaeontologists would love the opportunity to get some definitive evidence of species limits (mating experiments on Brontosaurus: that would be quite a sight!).   They must think that us diatomists are a bizarre bunch indeed.

Reference

Rose, D.T. & Cox, E.J. (2014). What constitutes Gomphonema parvulum? Long-term culture studies show that some varieties of G. parvulum belong with other Gomphonema species. Plant Ecology and Evolution 147: 366-373.

Tschopp, E., Mateus, O. & Benson, R.B.J. (2015). A specimen-level phylogenetic analysis and taxonomic revision of Diplodocidae (Dinosauria, Sauropoda).   PeerJ 3: e857 [https://dx.doi.org/10.7717/peerj.857]

 

A winter wonderland in the River Ehen

I have just finished another visualisation of the underwater life of the River Ehen, following up my observations in Food for Thought from the River Ehen.   In that post I commented on the prominent brown biofilms that I saw on rocks which, under the microscope, turned out to be mixtures of the green alga Bulbochaete and diatoms.   I have tried to convey this in my illustration, with the long-stalked Gomphonemas risking through the mat of Bulbochaete filaments (many of which end in long hairs) and, on the left hand side, a colony of Fragilaria tenera cells, loosely attached to the green alga.   Our measurements showed a conspicuous increase in the quantity of algae present on the stream bed compared to our previous visit, though we are still puzzling about why this is the case.

Ehen_141106

The submerged world of the River Ehen, November 2014 showing diatoms growing on and around Bulbochaete: left: Fragilaria tenera, central foreground, Gomphonema truncatum, left foreground: Gomphonema acuminatum.   The Gomphonema truncatum cells are about 40 micrometres (one fortieth of a millimetre) long.

We do know that many of the stream algae found in the UK are able to grow at low temperatures and so thrive throughout the winter.   One reason why we expect the increased quantity at this time is that the grazers which feed on the algae are less active in cold weather so the ratio between algal biomass that is produced and that which is consumed decreases.   Just how effective grazers can be is illustrated by two photographs taken the following month, one of which shows even more algae present at the same site where the samples on which my picture is based were collected. The other was taken about 100 metres upstream on the same day and shows another stone covered with tiny Simuliidae larvae.   These will, in time, develop into the swarms of blackflies that can plague river users, as the adult females of many species feed on human blood.

Simuliidae_Ehen_141203

Left: a stone from the bed of the River Ehen in December 2014 covered with Simuliidae larvae (scale bar: one centimetre) and, right, the head of a Simuliidae larva (scale bar: half a millimetre).

Of course, the abundance of Simuliidae larvae does throw some doubt on the idea that lack of grazing is primary reason for the abundance of algae.   Yet this is the first time we have seen such an abundance of Simuliidae at this particular site (which has sometimes had the greatest quantity of algae of all the sites on the Ehen that we visit) which means that we are puzzling over variability in time as well as in space.   I suspect that the long hairs on the Bulbochaete, as we know that other species with similar structures use these to secrete enzymes into the water that enable them to release phosphorus from organic molecules. My suspicion is that the wetter conditions of autumn and winter flush more of these molecules from the peaty soils of the Ennerdale catchment into the river and these, coupled with the lower grazing intensity, fuel the burst of growth that we see.

Bulbochaete_Ehen_141203

A mass of green algae (predominately Bulbochaete sp) smothering stones on the bed of the River Ehen, December 2014.   The effect of the numerous colourless hairs of the algae is to create a translucent “haze” over the top of the mass of green filaments.

Simuliidae larvae are often described as filter-feeders which attach to the substratum and have mouthparts adapted into “fans” that capture particles as they drift downstream. However, I have seen Simuliidae bent double so that they can feed directly on the benthic algae. Look at the difference in the quantity of algae between stones with and without Simuliidae, which were only about 100 metres apart.

We have been visiting the Ehen regularly now for about two years and it continues to surprise us.   Simuliidae were certainly not here in such obvious quantities at this time last year or the year before.   I suspect that we are dealing here with nothing more sinister than nature’s complex dynamics, and it makes the case, again, for a slower and more patient study of the natural world than is often possible with the present grant-awarding system (see “Slow science and streamcraft”).   Two and a half years into my explorations of the River Ehen and I’m still sometimes surprised by what I am seeing.