Diatom of the Month: April 2017 - Staurosirella pinnata

by Jennifer Fitchett*

At very high altitudes and relatively high latitude (~30°S), the eastern Lesotho highlands comprise am ecologically restricted, yet biologically diverse, environment for plant growth. The terrestrial vegetation is described as the Drakensberg Alpine Centre, hosting a considerable number of endemic montane to alpine species. This region of South Africa is dotted by numerous small tarns (lakes) and wetlands, each host to thriving communities of largely cosmopolitan diatom species. Of these, a very common taxa, Staurosirella pinnata (Ehrenberg), has an interesting story to tell.

                                                 

Fig. 1. Photographs of Mafadi summit, with the white diatomite outcrops 

visible in the foreground.

Fig. 2. A photograph of a typical microscope slide of diatoms from Mafadi Summit, indicating a predominance of Staurosirella pinnata and Fragilaria construens.

At one of the highest summits of eastern Lesotho, and the highest point of bordering South Africa, Mafadi Summit represents the contemporary terminal altitude for vegetation. The summit is covered by a very sparse distribution of ice-tolerant Helichrysum species, a small Asteraceae shrub which is dwarfed at high altitudes. The landscape is most notably marked by a series of diatomite stripes¹, potentially indicating the presence of a palaeolake (Fig. 1). Today, a shallow wetland marks the foot of the summit.

Of the 37 diatom taxa identified from an 8,000 year profile from Mafadi Summit, Staurosirella pinnata represented 22.4% of the total count. When grouped with Fragilaria construens (Ehrenberg), a species with markedly similar morphology and ecology, the total count reaches 41.4%; a significant portion of the diatom assemblage, and no doubt a key indicator of past conditions in the wetland. Staurosirella pinnata are benthic yet facultative planktonic species, which together with a large proportion of Aulacoseira species tentatively supports hypotheses from the diatomite exposures of a palaeolake. More important, however, is the climatic story that this species might tell. Both Staurosirella pinnata and Fragilaria construens are r-strategist species (small fast reproducing), able to tolerate harsh and frequently changing conditions. For the eastern Lesotho highlands, the harshest contemporary conditions are the seasonal fluctuations in ice cover (highest during past cold periods including the Little Ice Age), and prolonged periods of permafrost.  

 Fig. 3. Staurosirella pinnata.

Fig. 4. Map of the study region.

S. pinnata are similarly common in the alpine lakes of East Africa, and in a number of locations globally have been found to survive prolonged periods of ice cover and very cold water temperatures. If this is the predominant climatic factor responsible for the predominance of this diatom species in the alpine wetlands of eastern Lesotho, this would make the species an excellent temperature marker for the region.  However, determining the nature of the stress that r-strategists are able to withstand is complicated. It is also possible that the fluctuations in the dominance of S. pinnata reflect a change in insolation levels, or rapid seasonal moisture fluctuations. For this reason, detailed studies of contemporary fluctuations in the seasonal and inter-annual variations in the species predominance across spatial and altitudinal transects in the region are of tremendous importance in improving the capacity to infer past climatic conditions. 

*School of Geography, Archaeology and Environmental Studies, University of the Witwatersrand, South Africa

1. Fitchett, J.M., Mackay, A.W., Grab, S.W. and Bamford, M.K., 2016. Holocene climatic variability indicated by a multi-proxy record from southern Africa’s highest wetland. The Holocene, p.0959683616670467.