Return to the Serra da Estrela


Back in October I wrote about the algae and other plants that I had found in a small stream draining the Serra da Estrela mountains in Portugal (see “Notes from the Serra da Estrela”).  I’ve now had a chance to look more closely at the diatoms that I found there, and can offer a few thoughts on the ecology of the stream.

I collected two samples from the stream: one by brushing the top surface of the granite stones with a toothbrush and the other from the darker patches that I described in the earlier post.   These were a mix of algae and mosses, with the former dominated by cyanobacterial filaments and diatoms.   I merged the two samples prior to digesting them, but the biofilm on the submerged rocks was very thin so it is the diatoms from the dark patches that dominate the slide that I prepared from this stream.   As my preliminary observations suggested, motile diatoms were very abundant in this sample, with Surirella roba, Navicula angustaand N. exilis all common, along with some Pinnularia and Nitzschia.   I do not often find motile diatoms to be quite so abundant in fast-flowing upland streams, but I suspect that this is because I look in the wrong places.   Our standard sampling method involves scrubbing the tops of submerged stones which, in this type of stream at least, are not situations where motile diatoms thrive.  By contrast, the tangle of cyanobacterial filaments and dead organic matter creates a very different environment, where an ability to adjust position in order to move away from densely-shaded areas and, perhaps, from situations where bacteria and fungi had used up all the available oxygen, was an advantage.


Surirella robafrom the stream at Unhais de Serra, September 2018; a. – f.: valve views; g. – i.: girdle views. Scale bar: 10 micrometres (= 1/100thof a millimetre). The photo at the top of the post shows the view along the valley of the Rio Zêzere towards Mantiegas in the Serra da Estrela.


Miscellaneous diatoms from the stream at Unhais de Serra, September 2018: a. – d.: Cocconeis placentula, complete frustule, rapheless valve and two raphe valves; e. – g.: Navicula exilis; h. N. angusta; i. – k.: Pinnularia subcapitata, two valve views and a girdle view.  Scale bar: 10 micrometres (= 1/100thof a millimetre). 

A chain-forming species of Fragilariawas abundant in the original sample although, by the time I had prepared a slide, the chain had disintegrated into individuals or pairs of cells.  These all belonged to a member of the Fragilaria capucinacomplex, though I am not sure which one. There were also a few cells of the free-living (i.e. non-chain-forming) Fragilaria gracilis.    Eunotia minoror a close relative was also present, sometimes also forming short chains and, finally, I found a number of cells of Cocconeis placentula(possibly var. klinoraphis).

These are all diatoms that I would expect to find in a stream draining a hard rock such as granite in an area that is remote from any industrial or mining influences that might lead to artificial acidification.   There are mines in the area, but these are further south.  These do have a measurable effect on the biology of local streams, as the references at the end of this post attest.   However, this particular stream appears to be in rude health.

A curious side-effect of the years that I have spent looking at diatoms is that a sample such as this can evoke the environments from which it came: an assemblage of soft-water circumneutral diatoms conjures, in my mind, a particular landscape.   The label on the slide, of course, takes me straight back to our time in the Serra da Estrela but, in a more general sense, the diatoms capture an essence that transcends any one particular time or place.   Analysing diatom slides can become an escape from the humdrum and a chance to remember warmer days …


Fragilaria species from the stream at Unhais de Serra, September 2018: a. – g.: chain-forming member of Fragilaria capucina complex (a.-c.: valve views; d.-g.: girdle views); h.-j.Fragilaria gracilis.  Scale bar: 10 micrometres (= 1/100th of a millimetre).


Eunotiacf. minorfrom the stream at Unhais de Serra, September 2018: j. – n.: valve views; o. valve view of a related species; p. girdle views. Scale bar: 10 micrometres (= 1/100thof a millimetre). 


Luis, A.T., Teixeira, P., Almeida, S.F.P., Matos, J.X. & Silva, E.F. (2004).  Environmental impact of mining activities in the Lousal area (Portugal): Chemical and diatom characterization of metal-contaminated stream sediments and surface water of Corona stream.  Science of the Total Environment409: 4312-4325.

Silva E.F., Almeida, S.F.P., Nunes, M.L. & Fredrik, A.T.L. (2009). Heavy metal pollution downstream the abandoned Coval da Mó mine (Portugal) and associated effects on epilithic diatom communities.  Science of the Total Environment407: 5620-5636.


A year in the life of the River Wear …

After six bimonthly visits to the River Wear at Wolsingham during 2018, I can now step back and have a look at the complete dataset to see what patterns emerge.   Over the course of the year, I have visited the site six times and recorded a total of 107 species: 5 Cyanobacteria, 32 green algae, 69 diatoms and one red alga.  The true figure is probably higher than this, as the green algae include a number of “LRGT” (see “Little round green things …”) and certainly did not receive the same level of attention as the diatoms.

This crude enumeration of species, however, disguises some interesting seasonal patterns with, as I described in “Summertime Blues” and “Talking about the weather …”, abundant growths of green algae during the heatwave and associated low flow periods.  This can be seen clearly in the bar chart showing the seasonal changes in the river: diatoms predominate in the early part of the year whilst green algae are very scarce.  The bloom of the green filamentous alga Ulothrix zonata that I expected to see in March was missing due, I suspected, to the hard weather we experienced in late Feburary (see “The mystery of the alga that wasn’t there …”) but, by the summer, the river had taken on a very different complexion and was dominated by small green algae.   The last sample of the year, collected in November, showed a return to diatom dominance with a late autumn showing of Ulothrix zonata(see “The River Wear in November …”).


Relative proportions (by approximate biovolume) of the main groups of algae found in the River Wear at Wolsingham during 2018.  

Looking back at records of a similar exercise in 2009, I see that the beginning and end of the year were quite similar, with thick biofilms dominated by diatoms; however, the algae in the summer of 2009 were very different to those I found in 2018.  My 2009 exercise involved monthly rather than bimonthly visits and I see that I recorded more Cyanobacteria in June and July than I found in Summer 2018.  These were mostly filaments of Phormidium retziiand tufts of Homoeothrix varians, which I assumed to be a consequence of intense grazing (there is evidence that invertebrates find Cyanobacteria to be less palatable than other algae).  By July, Cyanobacteria comprised over half the total biovolume of algae; however, there was a major spate soon after my visit.  I was surprised to find, when I visited in August, a noticeably thicker biofilm smothering the rocks and, when I looked closely, this was dominated by the small motile diatom Nitzschia archibaldii.   The Cyanobacteria had disappeared almost completely.   I attributed this change to the invertebrate grazers being washed away by the spate, allowing the algae to grow unhindered.  As the biofilm grew in thickness, so the algal cells start to shade each other, and a diatom that can glide through the biofilm has an advantage over any that are stuck to one place.  Diatoms remained dominant for the remainder of the year, although my November sample came just after another storm and the stones I sampled were completely bare.


Relative proportions (by approximate biovolume) of the main groups of algae found in the River Wear at Wolsingham during 2009.   A sample was collected in November but no living algae were recorded from it.

Overall, however, the similarities between the years outweighed the differences in the summer assemblages, whilst the composition of communities between late autumn and late spring was remarkably similar across the two years.   The changes in summer 2018 extended beyond just a shift in the balance of algae in favour of greens: there were also changes in the composition of diatoms too.  In fact, the changes in diatoms proved to be quite powerful mirrors of the changes in the community as a whole.  I have demonstrated this in datasets spanning a number of sites in the past but it is reassuring to see that they are also reflecting patterns within one site.   On the other hand, if I only had examined the diatoms, I would have missed some of the most interesting changes in the river over the course of the year.

Another observation is that no single sample from 2018 contained more than a quarter of the total algal diversity that I recorded over the course of the year.  Every month saw some new arrivals and some departures (or, more likely in some cases, a few taxa that were present had dropped below my analytical detection limit).  Some of these were expected (the seasonal dynamics of Ulothirx zonata, for example); others not (e.g. dominance by Keratococcus bicaudatusin the summer).  I discussed this in “A brief history of time-wasting …” and, in honour of that post, am not going to repeat myself here. In an age when our environmental regulators are cutting back on the amount of data that they gather, I shall go into 2019 reflecting on Yuval Noah Harari’s comment that “the greatest scientific discovery was the discovery of ignorance”.