I spent part of last weekend wandering in the vicinity of a small reservoir on Bollihope Common in Weardale. It is one of many small manmade water bodies in this part of the northern Pennines constructed to power the mills that served the lead mines in the region.
Rocks on the northern shore of the reservoir had tufts of a dark green, almost black, moss inhabiting the splash zone. Under the microscope, I saw the characteristic wavy-edged cells which indicated that this was a Racomitrium. This is Racomitrium aciculare, a semi-aquatic cousin of the species we encountered on rocks in Teesdale last year (see “Upper Teesdale in March”). The southern shore of the lake, by contrast, was not fringed with rocks, but with rushes and Sphagnum moss, along with some Polytrichum. This side of the reservoir receives the drainage from the fells above and, I suspect, the constant supply of sediment has led to the gradual infilling of the original shoreline. There were at least a couple of species of Sphagnum present here, but I was most interested in the submerged moss, S. cuspidatum.
Looking north towards the unnamed reservoir on Bollihope Common (NY 989 348). The road on the left hand side of the image leads to Stanhope.
Aquatic mosses from the unnamed reservoir on Bollihope Common. The left hand image shows Racomitrium aciculare on the tops of boulders and the right hand image shows Sphagnum cuspidatum from the boggy areas on the southern shore.
I shook portions of both mosses vigorously in a small amount of water from the reservoir to dislodge the attached algae. The clear water quickly turned brown and I sucked up a few drops of each with a pipette and dropped them onto a microscope slides. First up was the sample from the Racomitrum. This was dominated by the small diatom Achnanthidium minutissimum (a – e in the figure below). When I had looked at the Racomitrium leaves under the microscope, I had seen many of these attached to the leaves by short stalks. These comprised just over half of all the diatom cells that I counted. Long needle-like cells of Fragilaria rumpens (or something similar) which attached to the leaf by their base formed another 27% and another genus, Gomphonema (one or more forms in the G. parvulum complex), formed about 16%. Most interesting to me were a few gracefully-curved cells of Hannaea arcus, as these are good indicators of a relatively pristine habitat.
Next up was the sample I had obtained from the Sphagnum. Sphagnum usually favours acid habitats so I was intrigued to see what diatoms would be associated with it, having seen that the diatoms associated with Racomitrium, a hundred metres or so away, mostly suggested neutral or slightly alkaline conditions.
Once again, it was Achnanthidium, Fragilaria and Gomphonema that comprised the majority of the diatom cells (54, 19 and 16% respectively) but this time, about 8% of the total belonged to at least three species of a different genus, Eunotia, which is often associated with acid habitats, and the curved cells of Hannaea were conspicuous by their absence. Interestingly, Sphagnum does not only favour acid conditions, peculiar features of its cell wall chemistry also helps to create those acid conditions and the diatoms living in the microhabitats around the submerged Sphagnum were clearly indicating a slight change in conditions, compared to those I found on the Racomitrium.
Diatoms growing on and around mosses in the unnamed reservoir at Bollihope Common; a – e: Achnanthidium minutissimum complex; f,g: Gomphonema parvulum complex; h. Eunotia spp (probably E. implicata); i. Navicula (probably N. cryptocephala); j. Fragilaria (probably F. gracilis); k. Hannaea arcus. Scale bar: 10 micrometres (1/100th of a millimetre). Note, particularly for h and k, healthier specimens were present in the samples but none presented in a manner amenable to photography.
There was much more Sphagnum underfoot as I walked over Bollihope Common. Given time – a couple more centuries, maybe – and the gradual invasion of Sphagnum from the moorland around the reservoir might continue and, we can hypothesise, the acid-loving diatom species might become more abundant. Indeed, we could even argue that this would simply be nature re-establishing its influence, the reservoir being an unnatural and – in the grand scheme of things – temporary intrusion into the landscape.