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.