Ennerdale Water, looking north from the outflow to the River Ehen, April 2015
My monthly visit to the River Ehen coincided with a week of warm weather, clear skies and low flows. There were some glorious views of the Lake District fells shrouded in early morning mist when I first arrived and I spent a few minutes trying to capture them with my camera before turning my attention to work.
We are now into our third year of regular sampling and are beginning to get a feel for the seasonal dynamics of the algae in the stream. By this time of year, we expect to see the biomass falling as the invertebrates start to become more active and browse away the luxuriant winter growths (see “A very hungry chironomid”). Many of the stones do appear to have less algae growing on them but, at one site, there were still some very conspicuous growths of diatoms. I did wonder if these were growths of Didymosphenia geminata (see “A journey to the headwaters of the River Coquet”); I have never seen this species in the Ehen but several aspects of the habitat here are such that it is possible that it could thrive. However, when I got a sample under my microscope, the growths turned out to be long-stalked Gomphonema species, with a variety of other diatoms, including Tabellaria flocculosa, Fragilaria spp., Hannaea arcus, Brachysira neoexilis and at least a couple of members of the Achnnathidium minutissimum complex.
Clumps of diatoms – mostly Gomphonema – on a stone from the River Ehen, April 2015.
The Gomphonema species in the River Ehen have perplexed us before. Our best bet is that the larger ones, in particular, are G. gracile. However, we are straying into areas where names abound although the limits between species are often described in vague terms in the literature. Dawn Rose and Eileen Cox described an experiment recently in which they grew two cultures of a different diatom, Gomphonema parvulum, in culture. Diatoms, you may remember, get smaller with successive cell divisions (see “Diminishing with age…”) until a point is reached when sexual reproduction is initiated. When this happened with one of their cultures, they were surprised to see cells that our Floras would have called Gomphonema gracile emerge from the auxospores. They continued to grow these and watched as the G. gracile cells diminished in size until they resembled G. parvulum again. The problem is, as they point out in their paper, most diatomists base their opinions on the structure of the cleaned frustule rather than the behaviour of the living cell.
Gomphonema cells from the River Ehen, Cumbria, April 2015. The five images on the left show a size reduction series in valve view; the right hand image shows two cells in girdle view with the stalk just visible. Scale bar: 10 micrometres (1/100th of a millimetre).
Why, you may ask, don’t more people do experiments of this type in order to define the limits between species of diatoms? It is a good question. The day before my visit to the Ehen, I had read an article in my newspaper about a similar study that had confirmed that the dinosaur Brontosaurus was probably a distinct genus rather than being part of the genus Apatosaurus, as had been assumed for most of the 20th century. The scientists based their conclusion on careful measurements of different properties of dinosaur skeletons, just as most diatomists measure and describe the diatom frustule. What else can a palaeontologist do? Their material is, after all, long dead. Diatomists work with the living world. It is, admittedly, fiddly and time-consuming work to prepare cultures of single species from field communities, and you need special incubators to grow these cultures once they have been isolated. But it is not impossible.
Most of us who study diatoms are over-anxious to get rid of the organic constituents of the cell in order to focus on the silica remains. We are, in effect, contemporary palaeontologists. Most palaeontologists would love the opportunity to get some definitive evidence of species limits (mating experiments on Brontosaurus: that would be quite a sight!). They must think that us diatomists are a bizarre bunch indeed.
Rose, D.T. & Cox, E.J. (2014). What constitutes Gomphonema parvulum? Long-term culture studies show that some varieties of G. parvulum belong with other Gomphonema species. Plant Ecology and Evolution 147: 366-373.
Tschopp, E., Mateus, O. & Benson, R.B.J. (2015). A specimen-level phylogenetic analysis and taxonomic revision of Diplodocidae (Dinosauria, Sauropoda). PeerJ 3: e857 [https://dx.doi.org/10.7717/peerj.857]