Some part-time sleuthing on a sample that I was sent a couple of weeks ago have resulted in a new addition to the UK freshwater diatom flora. The slide came to me from an Environment Agency laboratory with a question mark over a small diatom that was quite abundant but which did not match any of the species with which they were familiar. It was a small diatom, only about 10 micrometres (1/100th of a millimetre) long, with very fine features, but there were enough features visible for me to realise that it was not something that I had seen before either. I sent images off to a couple of colleagues and we decided that it was a species of Nupela, probably N. neglecta.
Nupela was only established in 1991. Before that, species that we now place in this genus were spread between Achnanthes and Navicula, as people struggled to understand its characteristics. If you look at diatom keys written before Nupela was established (and several written subsequently – including my own), the presence of a raphe on either or both valves is seen as an important distinguishing characteristic, and the small number of genera that have a raphe on just one valve were generally assumed to be related. Nupela, however, has some representatives that have a raphe on one valve (formerly placed in Achnanthes) and representatives that have a raphe on both valves (formerly placed in Navicula). Nupela neglecta has a raphe on both valves, but one of the valves has raphe slits that only extend for about half the total length. Stir in the small size and morphological details that are barely visible with the light microscope and there is ample scope for confusion.
Valves of Nupela cf neglecta with a full raphe. Scale bar: 10 micrometres (= 1/100th of a millimetre). The double lines indicate a single valve at different focal points. Photographs: Chris Carter.
Valves of Nupela cf neglecta with a short raphe. Scale bar: 10 micrometres (= 1/100th of a millimetre). The double lines indicate a single valve at different focal points. Photographs: Chris Carter.
The sample was collected from the River Stor, a tributary of the River Arun, in West Sussex, downstream of Storrington sewage works (NGR: TQ 0681 1641). This is a small hard water stream (average pH: 7.9; average alkalinity: 170 mg L-1 CaCO3) with very high concentrations of nutrients (average dissolved reactive phosphorus: 0.99 mg L-1; average total oxidised nitrogen: 5.0 mg L-1). These observations are similar to those made by Marina Potapova and her colleagues for habitatas where they found N. neglecta in the USA. And this raises an interesting paradox: normally, the presence of a rare and exotic organism is considered to be a reason for conserving a habitat. In this case, however, the rare species seems to be associated with a polluted habitat and, as a result, the Environment Agency will be doing their best to drive any organism that thrives here (however rare) to extinction. Discuss.
Krammer, K. & Lange-Bertalot, H. (1991). Süswasserflora von Mitteleuropa2: Bacillariophyceae; 4. Teil: Achnanthaceae, Kritische Ergänzungen zu. Achnanthes s.l., Navicula s. str., Gomphonema, Gesamtliteraturverzeichnis Teil 1-4. Spektrum Akademischer Verlag, Heidelberg. (see p. 440 for account of Nupela)
Potapova, M.G., Ponader, K.C., Lowe, R.L., Clason, T.A. & Bahls, L.L. (2003). Small-celled Nupela species from North America. Diatom Research 18: 293-306.
Vyverman, W. & Compère, P. (1991). Nupela giluwensis gen. & spec. nov. A new genus of naviculoid diatoms. Diatom Research 6: 175-179.