Looking down my microscope at a sample from the upper Ehen a couple of months ago, I saw something that I was not expecting. The geology of the catchment of the River Ehen means that the water in this area is very soft yet here was an alga I usually associated with strongly calcareous geology. I double checked to make sure that I had not made a mistake and, once I was confident of my identification, I wondered if it was a freak occurrence (see “When is a record not a record?”).
The organism that I was looking at was a species of the cyanobacterium (blue-green alga) Rivularia, which we last saw in Upper Teesdale (“Blue skies and blue flowers in Upper Teesdale”). In Upper Teesdale it formed distinct hemispherical colonies yet here I could just see bundles of filaments. Maybe they were fragments of colonies washed in from elsewhere or which I had disrupted as I had collected my samples? One other difference is that the Upper Teesdale samples all had calcite crystals within the colonies whereas my samples from the soft water of the River Ehen lacked these.
The cyanobacterium Rivularia from the upper River Ehen, March 2013. The main image shows a bundle of filaments in sheaths with a single filament in the inset. The arrows indicate the heterocysts. Scale bar: 10 micrometres ( = 100th of a millimetre).
Brian Whitton and Alan Pentecost summarised their records of Rivularia in a short article in The Phcyologist and commented that all but one came from catchments where there was some limestone (the exception was Haweswater in the Lake District). This sample from the River Ehen is, therefore, unusual for the UK although we do know that Rivularia is found in soft waters in Norway, including the River Atma, subject of several posts back in July 2013.
Since I first noticed it in November, I have seen it at the most upstream of my four sites on the River Ehen every time I have visited. It is possible that “chance favours the prepared mind”: I didn’t “see” what I was not expecting but, having found it once, I was alert to its presence on every subsequent visit. On the other hand, I have looked at many other samples from soft water habitats and am fairly confident that there was no Rivularia present in these. And I am sure that Brian and Alan have also looked at enough soft water habitats for their generalisation about Rivularia’s preference for calcareous habitats to be sound.
And why just at this one site and not at the others, all within about four kilometres of the lake outfall? What is so special about this particular location? Part of my fascination with the lower plants is that we can still make discoveries, still turn preconceptions on their heads, still approach a visit even to a familiar site with anticipation …