Notes from the Serra de Estrela

At the end of my last post I suggested that the next time I wrote it may be from Portugal.   In reality, tiredness and, to be frank, a steady consumption of Vino Verde intervened and this post may be about Portugal but is not, alas, written from that country.   Our travels took us from Lisbon northwards to Covilhã, a town on the edge of the Serra da Estrela mountain range, then onwards to the Duoro valley and Porto, and finally back to Lisbon.   The lower part of the Duoro is the home to many of the Vino Verde vineyards, although our focus was mostly on the vineyards further upstream from which the grapes for port are grown.  I’ll write more about the Duoro in a later post but, first, I want to take you on a journey to the Serra da Estrela.

These are the highest mountains in mainland Portugal (there is a higher point in the Azores) but summit with a summit at 1993 metres at Torre.  Unusually, for the highest peak in a mountain range, there is a road all the way to the top, along with a couple of shops and a small bar/restaurant.   On the day we visited, a couple of hardy cyclists had toiled their way up from the plains but most of the visitors had driven up.   We had stopped on our route up from Covilhã to explore the granite landscape and botanise so felt that we had earned our bica and Pastéis de Nata by the time we got to the very top.

Much as I appreciate a summit that satisfies a caffeine addiction, the real interest lies elsewhere, with the road up from Covilhã passing through some dramatically-eroded granite outcrops, composed of huge boulders apparently perched precariously on top of each other.  These resemble the granite “tors” we find in Dartmoor in south-west England, and have a similar origin.   The area around the tors had distinctive vegetation that will, no doubt, be described in greater length in a post on Heather’s blog before too long.   The free-draining sandy soils that the granite landscape creates mean that there was not a lot of surface water for me to indulge my own passions, so I will have to take you to another part of the Serra da Estrela for the remainder of this post.

Granite landscapes near Torre in the Serra da Estrela Natural Park in northern Portugal, September 2018.  

We found an inviting stream as we were walking near Unhais de Serra, at the southern end of the Natural Park.  The first plants to catch our eye were a submerged Ranunculus species with finely-divided leaves and five-petelled white flowers sitting at the water surface.   As well as these, we could see shoots of patches of water dropwort (Oenanthe sp.) and, looking more closely, several of these appeared to be growing out of dark coloured patches which turned out to be a submerged moss overgrown with algae (more about which a little later).   I am guessing that, once the rains come, much of these mini-ecosystems will be washed downstream leaving just a few moss stems to be colonised again next year.

Submerged vegetation in the stream at Unhais de Serra in September 2018 (40°15’44” N 7°37’21” W).  The top photograph shows a Ranunculus species and the lower photograph shows mosses overgrown with algae (a mixture of Cyanobacterial filaments, diatoms and coccoid green algae), within which young plants of Oenanthe sp. have taken root (top photograph: Heather Kelly).

Somewhat to my surprise there were also some patches of Lemanea.   This is a red algal genus that I usually associate with late winter and spring in my own part of the world, so I had not expected to find such prolific growths at this time of year at lower latitudes.   Maybe Iberian species of Lemanea behave differently to those with which I am familiar?

The Lemanea species found in the stream at Unhais de Serra in September 2018.  The top photograph shows it growing in situ and the lower photograph is a close-up.  The filaments are about a millimetre wide.

The dark film itself contained a variety of algae, some of which I have put in a plate below.   There were some cyanobacterial filaments which looked like Oscillatoria to me but which were not moving (their life between collection and examination was less than ideal).  There were also a large number of diatoms, mostly Navicula and Surirella.  Again, both would have been moving around in a healthy sample but were static when I got around to examining them; the chloroplasts in the Surirella, in particular, were not in very good condition).  I also saw some chains of Fragilaria species and several small green algae (especially Monoraphidium, discussed in the previous post).  I’ll return to the diatoms in a future post, once I have been able to get permanent slides prepared and examined but first impression is that I am looking at a community from a low nutrient, circumneutral environment.

Some of the algae living in the dark films overgrowing mosses in the stream at Unhais de Serra in September 2018.   a. – c.: Navicula angusta; d. –g. Surirella cf. roba; h. – i. two different chain-forming Fragilaria sp.; j. – k.: Navicula cf cryptocephala; l. – m.: Oscillatoria sp.    Scale bar: 20 micrometres (= 1/50th of a millimetre). 

The diatoms, in particular, reiterate the important point that notwithstanding the huge number of new species that have been described in recent years, it is possible to peer through a microscope at a sample from anywhere in Europe and see a familiar set of outlines that, for the most part, give a consistent interpretation of environmental conditions wherever you are (see, for example, “Lago di Maggiore under the microscope”).   That same rationale applies, to some extent to other organism groups too: we have recently shown this for macrophytes in shallow lakes for example.   Likewise, the geology here was shaped by the same broad forces that created the landscape of south-west England even if local climate means that the flora surrounding the tors in the Serra da Estrela is adapted to more arid conditions than that on Dartmoor.    It is important that, when we travel, we see the differences but, perhaps even more important in this fractured age, that we see the similarities too.

References

Chapuis, I.S., Sánchez-Castillo, P.M. & Aboal Sanchero, M. (2014).  Checklist of freshwater red algae in the Iberian Penisula and the Balearic Islands.   Nova Hedwigia 98: 213-232.

Poikane, S., Portielje, R., Deny, L., Elferts, D., Kelly, M., Kolada, A., Mäemets, H., Phillips, G., Søndergaard, M., Willby, N. & van den Berg, M. (2018).   Macrophyte assessment in European lakes: Diverse approaches but convergent views of ‘good’ ecological status.  Ecological Indicators 94: 185-197.

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Talking about the weather …

September is here.  When I visited this site two months ago we were in the midst of the heatwave and the samples I collected from the Wear at Wolsingham were different to any that I have seen at this location before, dominated by small green algae (see “Summertime blues …”).   As I drove to Wolsingham this time, I could see the first signs of autumn in the trees and the temperatures are more typical of this time of year.   We have had rain, but there has not been a significant spate since April and this means that there has been nothing to scour away these unusual growths and return the river to its more typical state.

That does not mean, however, that there have been no changes in the algae on the submerged stones.  Some of these differences are apparent as soon as I pick up a stone.  Last month, there was a thin crust on the surface of the stones; that is still here but now there are short algal filaments pushing through, and the whole crust seems to be, if anything, more consolidated than in July, and I can see sand grains amidst the filaments.   Biofilms in healthy rivers at this time of year are usually thin, due to intense grazing by invertebrates, so I’m curious to know what is going on here this year.

A cobble from the River Wear at Wolsingham, showing the thick biofilm interspersed with short green filaments.   Note, too, the many sand grains embedded in the biofilm.  The bare patch at the centre was created when I pulled my finger through it to show how consolidated it had become.  The cobble is about 20 centimetres across.

Many of the organisms that I can see when I peer at a drop of my sample through my microscope are the same as those I saw back in July but there are some conspicuous differences too.   There are, for example, more desmids, some of which are, by the standards of the other algae in the sample, enormous.   We normally associate desmids with soft water, acid habitats but there are enough in this sample to suggest they are more than ephemeral visitors.   And, once I had named them, I saw that the scant ecological notes that accompanied the descriptions referred to preferences for neutral and alkaline, as well as nutrient-rich conditions.  Even if I have not seen these species here before, others have seen them in similar habitats, and that offers me some reassurance.    In addition to the desmids, there were also more coenobia of Pediastrum boryanum and Coelastrum microporum compared to the July sample.

A view of the biofilm from the River Wear at Wolsingham on 1 September 2019. 

There were also more diatoms present than in my samples from July – up from about 13 percent of the total in July to just over 40 per cent in September.   The most abundant species was Achnanthidium minutissimum, but the zig-zag chains of Diatoma vulgare were conspicuous too.  The green filaments turned out to be a species of Oedogonium, not only a different species to the one I described in my previous post but also with a different epiphyte: Cocconeis pediculus this time, rather than Achnanthidium minutissimum.   I explained the problems associated with identifying Oedogonium in the previous post but, even though I cannot name the species, I have seen this form before (robust filaments, cells 1.5 to 2 times as long as broad) and associate it with relatively nutrient-rich conditions.  That would not normally be my interpretation of the Wear at Wolsingham but this year, as I have already said, confounds our expectations.   I did not record any Cladophora in this sample but am sure that, had I mooched around for longer in the pools at the side of the main channel, I would have found some filaments of this species too.

Desmids and other green algae from the River Wear at Wolsingham, 1 September 2019.  a. Closterium cf. acerosum; b. Closteriumcf. moniliferum; c. Cosmarium cf. botrysis; d. Closterium cf. ehrenbergii; e. Coelastrum microporum; f. Pediastrum boryanum.   Scale bar: 50 micrometres (= 1/20th of a millimetre).  

It is not just the differences between months this year that I’m curious about.  I did a similar survey back in 2009 and, looking back at those data, I see that my samples from August and September in that year had a very different composition.   There was, I remember, a large spate in late July or early August, and my August sample, collected a couple of weeks later had surprised me by having a thick biofilm dominated by the small motile diatom Nitzschia archibaldii.   My hypothesis then was that the spate had washed away many of the small invertebrates that grazed on the algae, meaning that there were few left to feed on those algae that survived the storm (or which had recolonised in the aftermath)..   As the algae divided and re-divided, so they started to compete for light, handing an advantage to those that could adjust their position within the biofilm.   This dominance by motile diatoms was, in my experience of the upper Wear, as uncommon as the assemblages I’m encountering this summer, though probably for different reasons.

Other algae from the River Wear at Wolsingham, September 2018.    The upper image shows Diatoma vulgare and the lower image is Oedogonium with epiphytic Cocconeis pediculus.   Scale bar: 20 micrometres (= 1/50th of a millimetre).

I suspect that it is the combination of high temperatures and low flows (more specifically, the absence of spates that might scour away the attached algae) that is responsible for the present state of the river.  This, along with my theory behind the explosion of Nitzschia archibaldii in August 2009, both highlight the importance of weather and climate in generating some of the variability that we see in algal communities in rivers (see “How green is my river?”).   The British have a reputation for talking about the weather.   I always scan the weather forecasts in the days leading up to a field trip, mostly to plan my attire and make sure that I will, actually, be able to wade into the river.  Perhaps I also need to spend more time thinking about what this weather will be doing to the algae I’m about to sample.

A hitchhiker’s guide to algae …

One of the recurring themes of this blog is the hidden delights of natural history for anyone prepared to take a closer look at unprepossessing locations, so it is appropriate that we have found some quite rich habitats within walking distance of our home in County Durham.   I’ve written before about visits to Crowtrees, a local nature reserve (see “More pleasures in my own backyard” and “Natural lenses”) and Heather is also writing a series of posts about the ever-changing flora of this small vale at the foot of the Permian limestone escarpment (see “Crowtrees LNR July 2018 part 2: gentians to grasses” for the most recent and links back to previous ones).   I visited again last week, taking Brian Whitton along for company.

His interest was the red alga Chroothece ricteriana, which I described in one of my earlier posts about Crowtrees but we did not find it on this particular visit.   Instead, my eye was drawn to soft clouds of green filaments that floated just above the bed of the pond.   When I looked closely under my microscope, I saw that these were thin filaments of Oedogonium.  Typically, these had no reproductive organs, so cannot be named (see “Love and sex in a tufa-forming stream” for a rare exception), but all showed characteristic “cap cells” (see lower illustration).

Growths of Oedogonium in Crowtrees pond, August 2018.   The frame width is about 30 centimetres.   The photograph at the top of the post shows Brian Whitton searching for algae during our visit.

The diatom Achnanthidium minutissimum was growing on small stalks attached to the Oedogonium filaments, often alone but also in pairs and stacks of four, as the diatom cells divided and re-divided.  Oedogonium is a rougher alga to the touch than filamentous genera such as Draparnaldia, Stigeoclonium and Spirogyra, and often carries epiphytes, and I presume the lack of mucilage is a factor in this.   Achnanthidium minutissimum is a diatom that is very common on the upper surface of submerged stones in both lakes and rivers, but it is not fussy and I often see it as an epiphyte if conditions are right.  In this case, I suspect that the very hard water of Crowtrees Pond is a factor: calcium carbonate is constantly being precipitated from the water to create a thin layer of “marl” (see photo in “Pleasures in my own backyard”).   This makes life difficult for a tiny diatom that cannot move, so hitch-hiking a ride on the back of a filamentous alga that floats about the lake bottom makes a lot more sense.

Oedogonium filaments with epiphytic Achnanthidium minutissimum, from Crowtrees pond, August 2018.  Scale bar: 20 micrometres (= 1/50th of a millimetre).  

Oedogonium is an adaptable genus.  It is also common in the River Ehen (soft water, low nutrients) and I also find it in lowland polluted rivers too.  Being able to name the species would, I am sure, help us to better understand the ecology but this is, as I have already mentioned, problematic (see “The perplexing case of the celibate alga”).   However, in each of the cases I’ve mentioned, the epiphytes are different (Achnanthidium minutissimum here, Tabellaria flocculosa and Fragilaria species in the Ehen, Rhoicosphenia and Cocconeis placentula in enriched lowland rivers) and I suspect that these might offer an easier way to interpret the habitat than the filaments themselves, at least until someone finds a stress-free way of naming them.

Comparing algae on a summer’s day …

I wrote about the effect of the long period of low flow in the River Wear a few weeks ago (see “Summertime Blues …”) and have, now, completed two dioramas depicting the state of the river in the main channel and in a filamentous algae-dominated backwater.  The first of these is dominated by free-living green algae, either single cells or coenobia (see note at end), which is a big contrast to the situation I recorded two months earlier when the assemblage was dominated by diatoms, with patches of filamentous green algae (see “Spring comes slowly up this way” and “A question of scale”).

I sent a small sample of the Wolsingham biofilm to Dave John for his opinion on the green algae, and he sent back a list with twenty one different green algae that he had found.  Fortunately, this confirmed my own original list, with Keratococcus bicaudatus, Scenedesmus, Desmodesmus and Monoraphidium all featuring.   He also commented that Keratococcus is hard to differentiate from Chlorolobium (which is also in his list) and that most of the green alga on his list are usually considered to be planktonic (Keratococcus and Chlorolobium are exceptions) although, as my earlier post suggested, these definitely formed a distinct biofilm on the surface of stones this year in the River Wear.

A diorama showing the biofilm in the River Wear at Woslingham, July 2018.   You can see coenobia of Demodesmuss communis (centre), Scenedesmus sp. (left) and Coelastrum microporum (right – half tucked behind a mineral particle, along with single cells of Keratococcus bicaudatus (upright cells) and Monoraphidium.  There are also some cells of Achnanthidium minutissimum on short stalks in the foreground and a cluster of Fragilaria gracilis cells in the background.

There seems to be little hard evidence on the habit of Keratococcus and Chlorobium apart from references to a preference for benthic habitats.   I have drawn them as upright cells, drawing on their similarity in form to Characium, for which there is better evidence of an upright habit (although Characium tends to grow on other algae, rather than on hard surfaces).  Whereas I often have a strong sense of the three dimensional arrangement of organisms within benthic biofilms, so little has been written about the preferences of these green algae that, apart from the Keratococcus, I have had to show them as a jumble of cells and coenobia across the picture frame.

The second diorama depicts the tangle of filamentous green algae that I found in the pools beside the main channel.  As I mentioned in my earlier post, these are species that I do not normally find at this site and are here, I presume, due to the long period of unusually warm weather and low flows.   One difference between these communities and that captured in my first diorama is that there is a more obvious organisation of the constituents here: the Cladophora filaments, though appearing as a tangle to us, form the foundation upon which epiphytes grow directly, but also around which Melosira filaments are entangled, rather like the lianas in a tropical rain forest.   The quantity of diatoms around the Cladophora is so great that their brown pigments completely mask the Cladophora’s green cells but note how the density of Cocconeis cells reduces towards the tips – the youngest parts of the filaments.

Depiction of filamentous algae growing in the margins of the River Wear at Wolsingham in July 2018, showing epiphytic Cocconeis pediculus and entangled Melosira varians.

There have been some recurring themes in my posts this summer: one is that UK rivers have been behaving quite differently to previous years, due to a combination of low flows (more accurately, a lack of the scour associated with high flows) and warm, well-lit conditions.   The low flows have also resulted, to some extent, in rivers becoming more physically heterogeneous, with side-pools and silty areas developing distinct assemblages of algae quite different to those encountered in the main channel.   Sometimes, the sum of these effects is for rivers to look less healthy than is usually the case.

The Wear at Wolsingham is one of those sites that I like to think I know well, having visited the location so many times over the past 30 years.  It is reassuring, in a rather humbling way, to know that it still has the capacity to surprise me.

Dave’s list of green algae from the Wolsingham biofilm, July 2018

Desmids
Closterium moniliferum
Closterium acerosum
Cosmarium botrytis
Cosmarium venustum
Staurastrum striatum

‘Chlorococcalean’ algae
Acutodesmus dimorphus
Coelastrum astroideum
(very small and atypical)
Coelastrum microporum (very small and atypical)
Chlorolobion braunii
Desmococcus olivaceum (subaerial species)
Desmodesmus communis
Desmodesmus subspicatus
Keratococcus bicaudatus

Monoraphidium arcuatum
Monoraphidium contortum
Monoraphidium griffithsia
Monoraphidium irregulare
Scenedesmus arcuatus
Pseudopediastrum boryanum
Tetradesmus obliquus
Tetraedron minimum

Note

A coenobium is a colony in which the cell number is fixed at the time of formation and not augmented subsequently.   Coenobia are particularly common in the Chlorococcalees.

Two-faced diatoms …

Back in March I reflected on the challenges involved in discriminating species of Gomphonema (see “Baffling biodiversity …”).   That there were several species in the sample which prompted the article was indisputable; that some of those species were, individually, quite variable was also clear.  The former issue I resolved, to some extent, by reference back to Hutchinson’s “Paradox of the Plankton” but the latter was harder to explain.

Part of the problem stems, I suspect, from the reliance on morphology to characterise species.  We assume that, because a group of organisms share a set of visible characteristics, then they must also share genes which determine those characteristics and that, in turn, implies a common ancestry.   Turning that assumption on its head, we assume that groups of microscopic algae that appear different to each other belong to different species.   However, a dog lover might point out that Chihuahuas and Great Danes look very different but are, in fact, the same species.   One of the challenges of those of us who study algae is deciding just how much variation in form is typical within a species, and at what point differences are such that they represent more than one species.

Gomphonema sarcophagus from Pitsford Water, Northamptonshire, showing Janus cells.  Photographs by Ingrid Jüttner.  Scale bar: 10 micrometres (= 1/100th of a millimetre).

So what should we make of the diatom valves in the image above?   The valve outlines and breadths are similar but the striae densities are so different that we might think that they belong to two separate species.   However, I recently stumbled, by chance, on a 1998 paper by Stacy McBride and Robert Edgar which discussed the topic of “Janus cells”.  Janus, you may remember, is the Roman god of time and is depicted with two faces, one looking back to the past and the looking to the future. His name has been appropriated, in this context, to describe diatoms that have frustules comprising two valves with different characteristics.   A few genera show consistent differences between the two valves – in Cocconeis and Planothidium, for example, one valve has a raphe whilst the other does not – and there are also differences in striae densities between the raphe and rapheless valves.   The term “Janus cell” is applied to diatoms where there are marked differences between the two valves but this is not a fundamental characteristic of the species or genus.   So, in the example above, we see some forms with much denser striae (11-13 in 10 mm) than others (7-8 in 10 mm).

We don’t know, from just looking at variability in populations, that this is not polymorphism within the species, in much the same way that some humans have attached ear lobes and others do not.   But, as diatom populations grow in number by repeated divisions of single cells, we can assume that most are clones of a small number of genotypes and, therefore, that the differences are due to ontogenetic variation.   What is interesting here is that this variation seems to create two distinct outcomes – coarsely or finely striated valves.  Some have suggested that such variation may be determined by differences in environmental conditions; however, the co-existence in a single population argues against this.

Gomphonema, as I have mentioned in earlier posts, is a genus that challenges taxonomists.  And, because ecologists depend upon taxonomists to give them a means of sorting diatom valves and frustules into meaningful categories, the environmental signals we get from Gomphonema species are often quite confused too.   The possibility of encountering Janus cells just throws one more curve ball into the mix.

Reference

McBride, S.A. & Edgar, B.K. (1998).   Janus cells unveiled: frustular morphometric variability in Gomphonema angustatum.   Diatom Research 13:293-310.

Round, F.E., Crawford, R.M. & Mann, D.G. (1990).  The Diatoms: Biology and Morphology of the Genera.   Cambridge University Press, Cambridge.

Summertime blues …

My reflections on the effects of the heatwave on freshwater algae continued with the latest of my regular visits to the River Wear at Wolsingham.  A comparison of the picture above with that at the head of “Spring comes slowly up this way …” says it all: the sun was shining and the gravel berms that I usually use to enter the river were occupied by families with barbeques whilst their children splashed around in the water.   At times such as this, a grown man picking up stones and then vigorously brushing their tops with a toothbrush would have invited too many questions, so I slunk off 100 metres or so downstream and found a quieter spot to explore.

The biofilm in the main channel of the River Wear at Wolsingham, July 2018. 

The first thing I noticed was that the biofilm coating the submerged stones at the bottom of the river had a greenish tinge, rather than its usual chocolate brown appearance.  It also was more crusty and less slimy to the touch than I usually see in this river.  When I got a specimen under the microscope, I could see that the composition was completely different to that which I had observed in previous months.   Most samples from this location that I’ve looked at in the past have been dominated by diatoms, with occasional spring flourishes of filamentous green algae.  Today, however, the sample was dominated by small green algae – a group that I am not very confident at identifying.   My rough estimate is that these formed about three quarters of all the algae that I could see, with diatoms and cyanobacteria each accounting for about half of the remainder.   The most abundant greens were a tiny single-celled alga that I tentatively identified as Keratococcus bicaudatus, along with a species of Scenedesmus and Desmodesmus communis.   There were also a number of cells of Monoraphidium arcuatum and some of Ankistrodesmus sp.

Two views of biofilms from the River Wear, Wolsingham in January 2018.   Left: from the main channel; right: from pools at the edge of the channel.

Green algae from the River Wear at Wolsingham, July 2018: a. Desmodesmus communis; b. Monoraphidium arcuatum; c. Scenedesmus sp.; d. unidentified, possibly Keratococcus bicaudatus.  Scale bar: 10 micrometres (= 1/100th of a millimetre).

However, there were also pools at the side of the channel, away from the main current but not so cut off that they were isolated from the river itself.   These were dominated by dense, brown filamentous growths, very similar in appearance to the Melosira varians flocs I described in “Some like it hot …”.  The filaments, however, felt coarser to the touch and, in close-up, could be seen to be branched, even without recourse to a microscope.   Once I got these under the microscope, I could see that they were filaments of Cladophora glomerata, another green alga, but so smothered with epiphytic diatoms (mostly Cocconeis pediculus) that they appeared brown in colour.

This combination of Cladophora glomerata and Cocconeis pediculus in the backwaters were as much of a surprise as the green-algae-dominated biofilms in the main channel.   These are species usually associated with enriched rivers (see “Cladophora and friends”) and, whilst I have seen Cladophora in the upper Wear before, it is an unusual occurrence.   Just as for the prolific growths of Melosira varians described in “Some like it hot …” it is tempting to leap to the conclusion that this must be a sign that the river is nutrient-rich.  However, the same conditions will apply here as there: “nutrients” are not the only resource that can limit plant growth and a steady trickle of phosphorus combined with warm, sunny conditions is just as likely to lead to prolific growths as a more conventionally “polluted” river.

Cladophora filaments smothered by the diatom Cocconeis pediculus in a pool beside the River Wear at Wolsingham, July 2017.   The frame width of the upper image is about 1 cm; the scale bar on the lower images is 20 micrometres (= 1/50th of a millimetre).

Another way to think of these situations is that, just as even healthy people are occasionally ill, so healthy streams can go through short periods when, based on a quick examination of plants and animals present, they exhibit symptoms associated with polluted conditions or simply (as for the first sample I described) different to what we usually expect to find.   A pulse of pollution might have passed downstream or, as seems to be happening at the moment, an unusual set of conditions lad to different organisms thriving.   Just as the ability to fight off infection forms part of a doctor’s understanding of “health”, so I expect that the River Wear will, in a few weeks time, be back to its usual state.   Healthy ecosystems, just like healthy humans, show “resilience”.   The irony is that, in this case, the “symptoms” are most obvious at a time when we are enjoying a summer better than any we’ve had in recent years.

Some like it hot …

My reflections on algae that thrive in hot weather continued recently when I visited a river in another part of the country.  As this is the subject of an ongoing investigation, I’ll have to be rather vague about where in the country this river flows; suffice it to say it is in one of those parts of the country where the sun was shining and your correspondent returned from a day in the field with browner (okay, redder) arms than when he started.   Does that narrow it down?

A feature of some of the tributaries, in particular, was brown, filamentous growths which, in close up, could be seen to be speckled with bubbles of oxygen: a sure sign that they were busy photosynthesising.  These were most abundant in well-lit situations at the edges of streams, away from the main flow.   Under the microscope, I could see that these were dominated by the diatom Melosira varians, but there were also several filaments of the cyanobacterium Oscillatoria limosa, chains of the diatom Fragilaria cf capucina and several other green algae and diatoms present.

Melosira varians is relatively unusual as it is a diatom that can be recognised with the naked eye – the fragile filaments are very characteristic as is its habitat – well lit, low-flow conditions seem to suit it well.   It does seem to prefer nutrient-rich conditions (see “Fertile speculations …”) but it can crop up when nutrient concentrations are quite low, so long as the other habitat requirements are right for it.  The long chains of Melosira (and some other diatoms such as Fragilaria capucina and Diatoma vulgare) help the cells to become entangled with the other algae.   I could see this at some sites where the Melosira seemed to grow around a green alga that had been completely smothered by diatoms and was, I presume, withering and dying.  In other cases, the Melosira filaments are much finer and seem to attach directly to the rocks.   Neither arrangement is robust enough for Melosira to resist any more than a gentle current which is why it is often most obvious at the edges of streams and in backwaters.   As is the case for Ulva flexuosa, described in the previous post, I suspect that the first decent rainfall will flush most of this growth downstream.   Another parallel with Ulva is that, despite this apparent lack of adaptation to the harsh running water environment, Melosira varians is more common in rivers and streams than it is in lakes.

Melosira varians-dominated filaments at the margins of a stream.  Top photograph shows the filaments smothering cobbles and pebbles in the stream margins (frame width: approximately one metre); bottom photograph shows a close-up (taken underwater) of filaments with oxygen bubbles (frame width: approximately one centimetre).

Algae from the filaments illustrated above: a. and b.: Melosira varians; c. Fragilaria cf capucina; d. Oscillatoria limosa.  Scale bar: 20 micrometres (= 1/50th of a millimetre).  

The graphs below support my comments about Melosira varians preferring nutrient rich conditions to some extent.  Many of our records are from locations that have relatively high nutrient concentrations; however, there are also a number of samples where M. varians is abundant despite lower nutrient concentrations.   How do we explain this?   About twenty years ago, Barry Biggs, Jan Stevenson and Rex Lowe envisaged the niche of freshwater algae in terms of two primary factors: disturbance and resources.   “Resources” encompasses everything that the organism needs to grow, particularly nutrients and light, whilst “disturbance” covers the factors such as grazing and scour that can remove biomass.   They used this framework to describe successions of algae, from the first cells colonising a bare stone through to a thick biofilm.   As the biofilm gets thicker, so the cells on the stone get denser and, gradually, they start to compete with each other for light, leading to shifts in composition favouring species adapted to growing above their rivals (see “Change is the only constant …”).

The relationship between Melosira varians and nitrate-nitrogen (left: “NO3-N”) and dissolved phosphorus (right: “PO4-P”).   The vertical lines show the average positions of concentrations likely to support high (red), good (green), moderate (orange) and poor (red) ecological status (see note at end of post for a more detailed explanation).

They suggested that filamentous green algae were one group well adapted to the later stages of these successions but these, in turn, create additional opportunities for diatoms such as M. varians which can become entangled amongst these filaments and access more light whilst being less likely to being washed away.   If there is a period without disturbance then the Melosira can overwhelm these green algal filaments.   Nutrients, in this particular case, do play a role but, in this case, are probably secondary to other factors such as low disturbance and high light.  Using the terminology I set out in “What does it all mean?”, I would place M. varians in the very broad group “b”, with the caveat that the actual nutrient threshold below which Melosira cannot survive in streams is probably relatively low.   Remember that phosphorus, the nutrient that usually limits growth in freshwater, comprises well under one per cent of total biomass, so a milligram of phosphorus could easily be converted to 100 milligrams of biomass in a warm, stable, well-lit backwater.

Schematic diagram showing the approximate position of Melosira varians on Biggs et al.’s conceptual habitat matrix.

The final graph shows samples in my dataset where Melosira varians was particularly abundant and this broadly supports all that has gone before: Melosira is strongly associated with late summer and early autumn, when the weather provides warm, well-lit conditions with relatively few spates.

The case of Meloisra varians is probably a good example of the problem I outlined in “Eutrophic or euphytic?”  I have seen similar growths of diatoms in other rivers recently, due to the prolonged period of warm, dry conditions.  It is easy to jump to the conclusion that these rivers have a nutrient problem.  They might have, but we also need to consider other possibilities.   Like Ulva flexuosa in the previous post, Melosira varians is an alga that is enjoying the heatwave.

Distribution of Melosira varians by season.   The line represents sampling effort (percent of all samples in the dataset) and vertical bars represent samples where M. varians forms >7% of all diatoms (90th percentile of samples, ranked by relative abundance). 

Reference

Biggs, B.J.F., Stevenson, R.J. & Lowe, R.L. (1991). A habitat matrix conceptual model for stream periphyton. Archiv für Hydrobiologie 143: 21-56.

Notes on species-environment plots

These are based on interrogation of a database of 6500 river samples collected as part of DARES project.  Phosphorus standards are based on the Environment Agency’s standard measure, which is unfiltered molybdate reactive phosphorus.  This approximates to “soluble reactive phosphorus” or “orthophosphate-phosphorus” in most circumstances but the reagents will react with phosphorus attached to particles that would have been removed by membrane filtration. The current UK phosphorus standards for rivers that are used here 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 boundaries based on the average alkalinity (50 mg L-1 CaCO3) and altitude (75 m) in the whole dataset.

There are no UK standards for nitrate-nitrogen in rivers; thresholds in this report are based on values derived using the same principles as those used to derive the phosphrus standards and give an indication of the tolerance of the species to elevated nitrogen concentrations.  However, they have no regulatory significance.