Meetings with remarkable Gomphonema …

Having written about Gomphonema rhombicum in my previous post, I thought it would be worth staying with Gomphonema and showing some images of G. vibrio.   This is a diatom that I had rarely encountered previously but which cropped up in separate email conversations with Chris Carter and Geoff Phillips in the space of a couple of months.  Chris’ samples come from a small man-made pond at Yardley Chase, an SSSI in Northamptonshire (photographed above), whilst Geoff’s was from Phragmites stems in a Norfolk marsh dyke.  Both have hard water (Geoff’s location: pH: 7.6; alkalinity: 275 mg L-1 CaCO3; conductivity: 700 mS cm-1) and good water quality (TP: 60 mg L-1; TN: 1.5 mg L-1).   This set of conditions prompted me to dig out some samples from Croft Kettle, a location I wrote about a couple of years ago (see “The desert shall rejoice and blossom …”) where I had a vague memory of having seen something similar.

Valves of Gomphonema vibrio are relatively large (30 – 95 x 7 – 10 mm, according to Hofmann et al., 2017) and club-shaped with a slight swelling at the centre.  Overall, the valves are more slender than was the case for G. rhombicum (see illustrations in the previous post).   The striae are coarse (7 – 10 in 10 mm) and mostly radiate, but there is a distinct central area where there is a single stria on each side more distantly spaced from the adjacent striae than in the rest of the valve.  On one side, this stria is very short (sometimes it can be hard to see); on the other side, it is longer and ends with a distinct stigmoid (an isolated pore).    The central endings of the raphe are often turned to the same side.

Cleaned valves of Gomphonema vibrio from a pond at Yardley Chase, Northamptonshire.  Yardley Chase is shown in the image at the top of the post.   Images are in pairs, each at a slightly different focus plane.   All photos by Chris Carter.

Chris also sent me some photographs of the living cells, showing a clear stalk protruding from the narrower “foot” pole, as well as a beautifully-clear H-shaped chloroplast.  The presence of a stalk in this species just doubles my annoyance at not having checked for the same in G. rhombicum before cleaning the valves.

There are, it seems, remarkably few records of Gomphonema vibrio from the UK.  I can find no other records from rivers and Helen Bennion found just two other records of recent samples in the UCL database, both from Scotland: Loch Levan and Loch Davan.  Three of the five records are from ponds, which may be significant, and two of these were epiphytes, though there are not enough records here to make any firm pronouncements about habitat preferences.  However, the picture that is emerging is of a species that definitely has a preference for moderately hard to hard water with relatively low nutrients. If that is the case, then it could well be a species that used to be more common that it is now, as many habitats such as these will have deteriorated in recent decades due to agricultural enrichment.   It is certainly a very different habitat from the soft water, fast-flowing stream from which I recorded G. rhombicum in Bulgaria.

Live cells of Gomphonema vibrio from a pond at Yardley Chase, Northamptonshire.  Photos by Chris Carter. 

That makes a total of five records from the UK which, even allowing for the muddled taxonomy (which I’ll talk about in the next post) and the fact that the diatoms of small ponds are rarely studied, suggests that this may be a genuinely rare. It is listed as an “endangered species with persistent risk factors” on the German red list, with a forecast of further decline over the next ten years.   I’ve voiced my concerns about “rarity” and red lists before (see “A red list of endangered British diatoms?”) but will stick my neck out on this one and suggest that Gomphonema vibrio might be a candidate.

Reference

Lange-Bertalot, H., Hofmann, G., Werum, M. & Cantonati, M. (2017).   Freshwater Benthic Diatoms of Central Europe: Over 800 Common Species Used In Ecological Assessment (edited by M. Cantonati, M.G. Kelly & H. Lange-Bertalot).   Koeltz Botanical Books, Schmitten-Oberreifenberg.

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