Seatoller, in Borrowdale, is the wettest place in England, so we should not have been surprised by the persistent drizzle that accompanied us as we set off hunting for desmids last week. The combination of Borrowdale’s hard volcanic rocks and a damp climate combine to create ideal habitats for bog-loving desmids and I had intelligence that Dock Tarn, on the fells above Borrowdale, was a hot spot of desmid diversity. Getting there, however, was no easy task. Though just a couple of kilometres from Stonethwaite on the map, there were an awful lot of contour lines awfully close together between the beginning and end of our walk. The footpath zig-zagged through ancient woodland clinging to a steep hillside until we emerged onto the moorland above. We then made our way across a plateau covered with heather moorland until we saw the tarn stretching away into the mist in front of us.
You know you are in good desmid habitat when there is water percolating into your body from both ends: rain dripping down from the hood of your cagoule and dampness seeping in through your shoes. They are organisms that love marshy, boggy conditions, especially in areas where the water is as soft as it is here. The alternative to damp feet would be to either climb up from Borrowdale in Wellingtons or waders or carry them up that steep hillside in a rucksack. However, I suspect that the mud at the bottom of the tarn was too soft and deep for Wellington boots and lugging waders up that hillside would have been hard work so damp feet was the price I had to pay. I leaned out as far as I could from the shore to grab some of the sedge stems which had a visible coating of attached algae, and also squeezed the peaty water from a few handfuls of Sphagnum that I pulled from a boggy pool. That would have to do on this particular morning as the rain was now soaking through my trousers and, in any case, there were places I needed to be later that morning. I shoved the bottles containing my samples into my rucksack and followed the path back down the hillside.
Epiphytic algae growing around a sedge stem in the outflow of Dock Tarn, Cumbria, July 2017. The width of the stem plus epiphytes is about half a centimetre.
Dock Tarn is one of a number of sites identified as an “Important Plant Area” (IPA) on the basis of the rich desmid flora, largely due to work over the years by David Williamson. It qualifies as an IPA on four criteria: the presence of threatened species, high diversity, a long history of study and because it represents a “threatened habitat”. David Williamson has recorded over 50 species from this location, 13 of which are candidates for a “potential Red Data List”. A few of these are illustrated in the figures below. One of the species in the first image, Haplotaenium minutum, belongs to a genus only recently separated from Pleurotaenium, which looks very similar to the untrained eye (the difference lies in the structure of the ridges on the chloroplast). Looking at these long cylindrical cells serves to emphasise just how much dexterity Chris Carter needed to produce his Hilda Canter-Lund prize winning image. Images in the second plate include two more species of the genus Xanthidium, which we met in “Desmids on the defensive …”.
Dock tarn desmids: a. Netrium digitus var. latum; b. Tetmemorus brebissonii; c. Haplotaenium minutum. Scale bar: 25 micrometres ( = 1/40th of a millimetre).
The desmids in the lower plate, in particular, show one of their key characteristics very clearly: their cells are divided into two distinct lobes (“semicells”) joined by an isthmus (the word desmid comes from the Greek desmos, meaning “bond”). The image of Staurastrum manfeldtii var. productum also shows a number of bacteria growing on the cell: these are probably growing within the mucilage that desmids secrete around themselves whilst there are distinct pyrenoids in the two Xanthidium species. Their predilection for soft water means that they need the carbon-concentrating mechanisms that these contain if they are to thrive. Not all desmids live in water as soft as this, and some are able to use inorganic bicarbonate to fuel their photosynthetic engine, but there will be little or no bicarbonatae in a habitat such as Dock Tarn. I wrote about these carbon concentrating mechanisms in algae from Ennerdale Water (see “Concentrating on carbon …”) and the two filamentous algae that featured in that post, Mougeotia and Spirogyra, both belong to the same class within the green algae as the desmids (Conjugatophyceae or Zygnemtetophyceae).
There will be more about desmids on this blog over the next few months in preparation for a the weekend of 15-17 September when I am organising a joint meeting of the British Phycological Society and Quekett Microscopical Club in Windermere. We’ll be visiting some other Lake District tarns known to be rich in desmids during this weekend and have Dave Johns and Allan Pentecost on hand, amongst others, to offer expert advice on what we find. There are still a few places left, so hurry up to book your place. I haven’t done a great job of selling the Cumbrian climate in this post but we have the use of the Freshwater Biological Association facilities, including a laboratory and the library, so no one need get damper than they want. See you there…
More desmids from Dock Tarn: d. Euastrum cuneatum; e. Xanthidium cristatum var. uncinatum; f. Xanthidium antilopaeum; g. Staurastrum manfeldtii var. productum. Scale bar: 25 micrometres
( = 1/40th of a millimetre).
Coesel, P.F.M. (1994). On the ecological significance of a cellular mucilaginous envelope in planktic desmids. Algological Studies 73: 65-74.
Kiemle, S.N., Domozych, D.S. & Gretz, M.R. (2007). The extracellular polymeric substances of desmids (Conjugatophyceae, Streptophyta): chemistry, structural analyses and implications in wetland biofilms. Phycologia 46: 617-627.
Spijkerman, E., Maberly, S.C. & Coesel, P.F.M. (2005). Carbon acquisition mechanisms by planktonicdesmids and their link to ecological distribution. Canadian Journal of Botany 83: 850–858.