It has been over three weeks since I last wrote about Croft Kettle. However, with the diversions to Milan and Trento behind me, I can now settle down and continue to sort out the images I had taken from the samples I collected back in May. I’ve also dug out some other slides from previous visits in order to show the full range of diversity that we’ve encountered over the years.
One of the most abundant diatoms gliding through the mass of Cymbella stalks surrounding the Chara stems was Navicula radiosa, illustrated below. The name derives from this diatom’s strongly radiate striae, which are not visible in the live specimens that I photographed. What you can see clearly are the two long, narrow plastids (chloroplasts), one on either side of the valve. Another Navicula species which I have seen in this habitat at Croft Kettle, but which was not obvious in the samples I collected in May is illustrated in the next illustration: Navicula oblonga. This is enormous by diatom standards: the largest individual I found in a sample I collected in May 1999 was just over a fifth of a millimetre long. It is hard to fit an entire valve of N. oblonga into a field of view to photograph at 1000x magnification, so the images below were all taken at 400x. N. oblonga is a relatively rare diatom in the UK in my experience, with an apparent preference for hard waters, extending into slightly brackish conditions.
More diatoms associated with Chara hispida stems in Croft Kettle, May 2015: a, b., c.: Navicula radiosa; d. Amphipleura pellucida. Scale bar: 10 micrometres (= 1/100th of a millimetre).
My return to Croft Kettle was prompted, you may remember, by a talk about a fossil lake in the Sahara which had a similar diatom flora (see “The desert shall rejoice and bloom …”). Both Cymbella cymbiformis and Navicula oblonga featured in the assemblages that Nassouma Yahiaoui found there, along with representatives of Epithemia and Mastogloia, both of which I found in my May 2015 samples, though neither presented themselves in particularly photogenic poses. I did include some photographs of another species of Epithemia in my description of Cassop Pond, which is also associated with the Permian limestone.
Navicula oblonga associated with Chara stems in Croft Kettle, May 1999. Scale bar: 25 micrometres (= 1/40th of a millimetre).
One other interesting species that I found at Croft Kettle, though it was not present in Nassouma’s Guern Toil profile, was Amphipleura pellucida. This is a long, delicately-featured diatom with a small, H-shaped plastid in the centre. A silica rib runs along the centre of the valve; this splits as it approaches the poles to contain a short raphe slit. The combination of silica rib and raphe resembles the end of a sewing needle. Microscopists have long been interested in Amphipleura pellucida because the striae are extremely-closely spaced (37-40 in 10 micrometres). This means that they can only be resolved by the very best optics and, consequently, slides containing A. pellucida are used as test objects. The slide I used to photograph the specimens below was given to me by John Carter (see “Remembering John Carter”) and was made from material collected in 1872 by “Firth”. Some hunting on the internet suggests that this was William Allott Firth, who was a Quaker from Yorkshire. Croft Kettle is less than a kilometre from the Yorkshire – Durham county boundary and Darlington, the nearest town, has strong Quaker connections too.
It is a good idea to have a test slide with Amphipleura pellucida to hand when you are buying a microscope. The sales reps whose job it was to demonstrate new microscopes used to breeze into our lab in shiny suits reeking of cheap aftershave and talk the talk about how wonderful their product was. We would then hand them our test slide and say “resolve that”. After twenty minutes fiddling with the microscope set-up they would usually make some excuse and retreat with their tails between their legs. I got a certain sadistic pleasure from watching these fast-talking laboratory sales representatives being defeated by a handful of gunk collected by a Victorian amateur natural historian.
Ampipleura pellucida from a sample collected from Hell Kettles in 1872 by Robert Issac Frith. Scale bar: 10 micrometres (= 1/100th of a millimetre). Bottom right: the slide from which the specimens were photographed.