After my sojourn in East Durham, described in the previous post, I have travelled back to the Pennines for this one, crossing the River Wear at Wolsingham before driving up onto the fells and finally dropping down to the woodlands that are Hamsterley Forest. This is a large man-made plantation, dating from the 1930s and popular for recreation. In January, however, the forest is quiet, and I only have a few mountain bikers and a lone dog walker for company as I peer into the peaty waters of Euden Beck. This stream rises on the open fells of Hamsterley Common, between Weardale and Teesdale, before flowing through the forest and joining Spurl’s Wood Beck just downstream from where I am standing, to become Hamsterley Beck. This then joins the Wear a few kilometres downstream from Wolsingham.
Euden Beck, just above the forest drive in Hamsterley Forest, January 2019. The photograph at the top of the post shows a view towards Hamsterley Forest.
There is a mixture of diatoms growing on the stones here but I am most interested in the genus Fragilaria today. One of the curiosities of this genus is that we often find several representatives growing at the same site at the same time, reminiscent of the old adage about London buses (“you wait ages, and then three come along at once”). I’ve written about this before (see “Baffled by the benthos (2)” and “When is a diatom like a London bus?”) and Euden Beck is another good example of this conundrum in practice.
Today, I could see quite a few cells of Fragilaria teneraand smaller numbers ofF. gracilisplus a newly-described species that I will talk more about later in the post. Fragilaria teneraforms long, needle-like cells, often clustering together to form sea urchin-like masses growing out from either a filamentous alga or particle to which they are attached (see “Food for thought in the River Ehen” for an illustration). Most of the ones that I saw in my samples from Euden Beck were either single cells or pairs of cells, presumably following a recent division. Note how the second cell from the left in the figure below is not as straight as the others. This is something that I often see with Fragilaria populations in streams in the northern Pennines, and indicates that there may be heavy metal pollution in the water. There are a lot of abandoned lead mines in the northern Pennines and, sure enough, when I looked at a large scale map, I found one that I had not previously noticed in the upper part of Euden Beck’s catchment.
Live cells of Fragilaria tenera(a. – d.) and F. heatherae from Euden Beck, January 2019. a., b. and e. are valve views; c. and d. are girdle views. Scale bar: 10 micrometres (= 1/100thof a millimetre).
The next image shows these valve abnormalities even more clearly, with almost all of the cells showing aberrations in their outline. These images are from an older sample; the curiosity here is that whilst most of the Fragilaria tenera valves were twisted, fewer of the valves of Fragilaria gracilisare twisted, whilst few of the valves of the third Fragilaria species show any abnomality in their outline at all. This species is very common in northern Pennine streams, and I have often seen distorted valves of this species in streams polluted by mine discharges. This makes the discrepancy between the outlines of this and Fragilaria tenera in Euden Beck particularly intriguing.
Fragilaria tenera from a sample collected from Euden Beck in June 2012. Scale bar: 10 micrometres (= 1/100thof a millimetre). Photographs: Lydia King.
I say “Fragilaria gracilis” with a modicum of trepidation as a recent study in which I have been involved, suggests that there may well be at least two species. These are, as far as we can tell, indistinguishable using characteristics that can be seen with the light microscope though we know that they are genetically quite distinct, and both are widespread, turning up not just in the UK but in other parts of Europe too.
The third species, to the best of our knowledge, does not match the description of any other Fragilaria species, and we are in the process of publishing it as a new species, Fragilaria heatherae. We have found it a number of samples, not just from the UK but also from sites elsewhere in Europe. These, by comparison with the other two species, show very little distortion at all. Whilst several authors have noted this phenomenon in the past, the physiological cause is still not understood. My guess is that the metal ions are displacing a metal co-factor in an enzyme that is involved in the process of laying down the silica cell wall. Fragilaria seems to be particularly susceptible, but this may be because their long needle-like cells show the distortions more clearly than in some genera but, based on the evidence from Euden Beck, there are clearly differences in susceptibility between species.
Once again, I seem to be ending a post having asked more questions than I have answered. That is always frustrating but another way of looking at this is to realise that the frontiers of ecology are only ever a short drive away from where you are now. It is very nice to cross oceans to visit rain forests and coral reefs, but there are adventures to be had closer to your doorstep.
Fragilaria gracilis from a sample collected from Euden Beck in June 2012. Scale bar: 10 micrometres (= 1/100thof a millimetre). Photographs: Lydia King.
“Fragilaria heatherae” from a sample collected in Euden Beck in June 2012. Scale bar: 10 micrometres (= 1/100thof a millimetre). Photographs: Lydia King
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Falasco, E., Bona, F., Ginepro, M., Hlúbiková, D., Hoffmann, L. & Ector, L. (2009). Morphological abnormalities of diatom silica walls in relation to heavy metal contamination and artificial growth conditions. Water SA35: 595-606.
McFarland, B.H., Hill, B.H. & Willingham, W.T. (1996). Abnormal Fragilaria spp. (Bacillariophyceae) in Streams Impacted by Mine Drainage. Journal of Freshwater Ecology 12: 141-149.