The Nordic Diatom Meeting, Ratnieki, Latvia, May 2015.
The motivation for my recent trip to Latvia (see “Following in Arthur Ransome’s footsteps …”) was to attend the Nordic Diatom Meeting, which took place in the University of Riga’s conference centre, Ratnieki, set in the Latvian countryside. It was a small, informal and very friendly meeting, fuelled by enormous quantities of food and enlivened by an excursion that managed to compress an overview of Latvian history from the Iron Age to the present into one afternoon of sightseeing.
It is a little unfair to pick out one of the presentations, but there was a moment on the first afternoon when I sat up with a start as the story of a fossil lake in the Saharan desert unfolded. There is, in the middle of the Western Great Erg (a huge sand-covered area of the Sahara in southern Algeria), within which there are several depressions. Some of these contain deposits that suggest that they were once lakes. An Algerian PhD student, Nassima Yahiaoul, told us about her study of an outcrop in one of these depressions, Guern Touil, which was composed largely of diatomite, a rock consisting largely of the remains of dead diatoms. This is good evidence that, in a moister period perhaps 7000 years ago (the precise date is not yet known), this area was not a bleak, unforgiving desert, but a freshwater or brackish lake.
What made me take particular notice, however, was the diatoms that she found when she analysed these deposits. These included Cymbella cymbiformis, Epithemia argus, Denticula tenuis, three species of Mastogloia and Navicula oblonga, a very large and distinctive species. None of these are particularly common in the streams and lakes that I study in temperate Europe but, curiously, several of these occur together in a small pond about 30 kilometres away from where I lived. This pond is, itself, botanically quite distinctive, and it was a strange sensation to sit in the Latvian countryside and hear about another with such an uncanny resemblance but which is so far distant in both space and time.
The outcrop of diatomite in the Guern Touil depression, Western Great Erg, Algeria, studied by Nassima Yahiaoul.
The place that Nassima’s description evoked for me is Croft Kettle, a small pond is fed by subterranean springs emanating from the Permian limestone. It is fringed by the saw sedge, Cladium mariscus but the edges of pond then shelve very steeply and the submerged vegetation is dominated by Chara hispida and C. vulgaris. Whether Nassima’s pond ever looked like the illustration below is debatable (there are fossil forests near Guern Touil so the idea of a tree-fringed oasis at some point in the Holocene is not wholly fanciful). The bare evidence that palaeoecologists produces often needs to be catalysed by the imagination, and the imagination, in turn, feeds off analogies. So long as we treat these speculations with a healthy dose of caution, all is good.
Croft Kettle, a Site of Special Scientific Interest in County Durham, just south of Darlington, photographed in May 2015.
As the pond is fed by subterranean springs, the water in Croft Kettle is very clear, allowing the dense Chara beds to extend into the depths. I could only reach the very edge of these beds when I visited a few days ago, but I was struck by the large quantity of yellow-brown diatom growths that smothered the Chara. Under the microscope, these proved to be composed of a dense tangle of a stalked diatom, probably Cymbella cymbiformis, within which other diatoms such as Rhopalodia gibba and Navicula radiosa were moving. The Cymbella is the same one that Nassima found in Guern Touil and I could also see representatives of three of the other genera that she described. I have recorded some of the other species that Nassima recorded from here, but they were not showing themselves today.
An underwater view of the margins of Croft Kettle, showing the dense beds of Chara, smothered by growths of diatoms, May 2015.
The quantity of diatoms that I saw in Croft Kettle was surprising, especially as I normally expect grazers to be very active at this time of year. The yellow-brown growths resembled those that I reported from the River Ehen in April (see “Diatoms and dinosaurs”). Those were of a Gomphonema species which, like Cymbella cymbiformis, grows on the end of long stalks. These, in turn, create a tangled matrix within which other species of diatom can live.
The Cymbella cells become detached from their stalks very easily, which means that it is easier to photograph isolated cells than the complete stem plus stalk complex. The tangle of stalks is also difficult to capture in a photograph due to the very shallow depth of field available when you are using medium-and high-magnifications. That brings me back to the topic of imagination: the microscopist needs this just as much as the palaeoecologist, if s/he is to gain an insight into the nature of communities that have been wrenched out of their natural habitat and squashed under a cover slip. More so, indeed, for the diatomist, who habitually marinades samples in a sauce of oxidising agents to leave just the silica frustule behind. But here I go again … droning on about the need to understand diatoms in their living state. Forgive me …
Enough for today: Croft Kettle is a pond with many fascinating – and one or two very unexpected – stories to tell. Plenty to keep me going for a few more posts …
Cymbella spp. growing on Chara in Croft Kettle, May 2015; a. – c.: Cymbella cf. cymbiformis; d. – e.: two as yet unknown Cymbella sp. Scale bar: 10 micrometres (= 1/100th of a millimetre).
Wheeler, B.D. & Whitton, B.A.(1971). Terrestrial and Sub-aquatic vegetation. The Vasculum 56: 25-37.
Hudson, J.W., Crompton, K.F. & Whitton, B.A. (1971). Ecology of Hell Kettles; 2. The Ponds. The Vasculum 56: 38-45.