I was back at the River Team yesterday, pushing my way past Himalayan balsam, in order to collect some of the brown biofilm from the tops of some stones. There was one species that I was particularly interested in finding, because of a strong historical link with north-east England. The species I was looking for was a small diatom called Navicula gregaria. “Navicula” is a Latin word which means “small ship”, an apt description as these small boat-shaped organisms were serenely gliding around the field of my microscope (see an earlier post “Coxhoe” for a short video of some close relatives in action). Look out, too, for the two parallel chloroplasts which, in the case of N. gregaria, are slightly offset in respect to one another. When the sample is digested to remove organic matter and a permanent slide is prepared, we can see more detail of the ornamentation on the silica cell wall (“valve”).
Navicula gregaria, from the River Team at Causey Arch (approximately 50 m downstream from the location photographed in my post of 31 July. Scale bar: 10 micrometres (= 1/100th of a millimetre).
Navicula gregaria, cleaned valves from Bradgate Brook, Newton Linford, Nottinghamshire, October 2011. Scale bar: 10 micrometres (= 1/100th of a millimetre).
Navicula gregaria is one of the most common freshwater diatoms in Britain, with a range that extends from very clean rivers to highly polluted streams such as the River Team. It does not like very soft water or water that is acidic but, otherwise, is almost cosmopolitan, particularly in spring, though it can be found all year round.
I wanted to write about Navicula gregaria today partly because it gives me an opportunity to tell you about what we know about Arthur Scott Donkin, the Victorian microscopist who we met briefly in earlier posts (see “In the footsteps of a Victorian microscopist”, “Prime time diatoms”, “Sampling the surf at Alnmouth”). I mentioned in the last of these that we knew little about his life; however, along with colleagues at the Royal Botanic Gardens in Edinburgh, I have managed to unearth a few more details. As try to limit my posts to about 500 words, I’ll spread these across two posts.
The main source of information about his early life comes from a publication called “Men of Mark ‘Twixt Tweed and Tyne”, published in 1895, which has a chapter about his father, Samuel Donkin (1801-1888), a farmer and auctioneer based ,for most of his adult life at Felton, a village between Morpeth and Alnwick in Northumberland. Arthur, the eldest of two sons and a daughter, chose a career in medicine whilst his younger brother continued the family business. The family farm at Felton is only about 10 kilometres from Druridge Bay, where he collected many of his specimens, as I mentioned in earlier posts. Towards the north end of Druridge Bay a small stream, Chevington Burn, flows through the sand dunes and into the Bay at Chibburn Mouth. It was from here that Arthur Scott Donkin first recorded Navicula gregaria in 1861, describing it as “very abundant in localities where small streams pass over the sandy beach into the sea below the high-water level. Donkin pointed out that these localities would have wide daily fluctuations in salinity as the tide ebbed and flowed, and also commented that it was “the species which occurred in most abundance on our coasts.” We now also know that it is very common in freshwaters as well, although there is a strong suspicion that that marine and freshwater forms may be different species (see Cox, 1987, listed below). He described Navicula gregaria as “exceedingly minute”, though now, with the advances in optics, we now know that there are many species that are much smaller than N. gregaria but which Donkin, with his primitive equipment, could not see.
Cox, E.J. (1987). Studies of the diatom genus Navicula Bory. VI. the identity, structure and ecology of some freshwater species. Diatom Research 2: 159-174.
Donkin, A.S. (1861) On the marine diatomaceae of Northumberland, with a description of several new species. Quarterly Journal of the Microscopical Society 1: 1-15.
Welford, R. (1895). Men of Mark ‘Twixt Tweed and Tyne. Volume 2. Walter Scott, London