The other excursion during our weekend of microscopy at Malham Tarn described in the previous post saw us walking in the opposite direction from Tarn House, not towards Tarn Moss but following the Pennine Way south, skirting the edge of the Tarn, then heading striking along the ancient Mastiles Lane to the point where it crossed Gordale Beck. We were on the Carboniferous Limestone here, but at a point where the water percolating through met the impervious Silurian slates and bubbled to the surface. That water had, however, been enriched with carbon dioxide released by the microorganisms in the soil and the carbonic acid that resulted from this encounter dissolved the limestone so that the water that emerged from the numerous springs and seepages in the area was saturated with calcium carbonate.
Once exposed to the air, however, the carbon dioxide diffuses out of the water, shifting the equilibrium so that the calcium carbonate deposits as a fine layer on any available surface. Organisms that live in these streams have to cope with at least two problems as a result: the steady accretion of calcite around them, and the tendency for any phosphorus in the water to precipitate at the same time, thus depriving them of vital nutrients.
Conspicuous on the bed of Gordale Beck at this point were dark hemispherical colonies of Rivularia haematites, a member of a cyanobacterium genus that has featured in this blog on several occasions (see “Blue skies and blue flowers in Upper Teesdale”). The colonies here were amongst the largest I had seen – all easily visible to the naked eye against the pale coloured limestone boulders and cobbles in the stream. The hemispherical colonies are really tiny stromatolites, with clear laminations visible in sections showing their growth over several years alternating between favourable and unfavourable conditions.
These are the most obvious of the cyanobacteria of the area, but there are others hiding in plain sight – in particular fine filaments of Schizothrix calcicola and Phormidium incrustatum, which coat the bed of the stream and seepages but which also contribute to more impressive structures. Had we made our way downstream from where we were standing we would have entered Gordale Scar, a narrow limestone valley created when the roof of an underground canyon collapsed. At a few points Gordale Beck or its tributaries tumbles down waterfalls where the aeration of the water hastens the loss of carbon dioxide, encouraging the deposition of calcite. These waterfalls are colonised by mosses – principally Cratoneuron commutatum – amongst which cyanobacteria also grow and on which the calcite is deposited. Over time these mixtures of moss, cyanobacteria and lime grow into substantial “screens” of tufa. The most impressive of these in the Malham area are found in Gordale Scar and, a little further downstream, at Janet’s Foss.
There was a debate, for a long time, about the role that mosses and algae played in the deposition of calcite in tufa. It was thought that photosynthesis by these organisms would, by removing carbon dioxide, shift the chemical equilibrium and encourage precipitation of calcite. Now, however, the consensus is that the mosses and algae play a physical role, providing surfaces onto which the calcite deposits, rather than actively promoting precipitation. Whatever the nuances, the cyanobacteria are playing a role as “ecosystem engineers” (see “How to make an ecosystem” and “How to make an ecosystem (2)”). Thousands of visitors look at Janet’s Foss or scrabble over the waterfall in Gordale Scar every year, but how many are aware that these are living structures, built, in part, by some of the most ancient organisms on our planet?
Pentecost, A. (1981). The tufa deposits of the Malham district. Field Studies 5: 365-387.
Some other highlights from this week:
Wrote this whilst listening to: P.J. Harvey via YouTube
Cultural highlights: Norwegian black comedy, Ninja Baby. Will not be to everyone’s tastes, but we enjoyed it.
Currently reading: Three Men in a Boat by Jerome K. Jerome.
Culinary highlight: Two contenders this week: haggis, neeps and tatties served in a whisky cream sauce in Tynholm Inn just off the A82 in Scotland and a south Indian meal at Dosa Kitchen in Jesmond, Newcastle.