I was back at the River Ehen last week, fully expecting to find rocks scoured clean of algae following the heavy rain that had preceded our visit. Two surprises were awaiting me: first, almost all the stones had a cover of algae that was thicker than on several of my previous visits and, second, these particular green films were composed of different species to those that I had found on those visits. The most abundant species were a form of Oedogonium (though not the same one as I had seen in the Wear a few weeks ago) and a close relative of Oedogonium, called Bulbochaete.
Bulbochaete shares Oedogonium’s characteristic rows of barrel-shaped cells but differs in two important respects: the filaments of Bulbochaete have several short side-branches and several of the cells have long, colourless hairs protruding from them. The function of the hairs has never been investigated, to my knowledge but, like the hairs we saw on Draparnaldia, they could be associated with nutrient acquisition. Indeed, looking at the prolific growths in the Ehen, I did wonder if the recent heavy rain had flushed nutrients off the surrounding fells, giving the normally hungry algae the brief feast which allowed this dramatic flourish on the stream bed.
A branched filament of Bulbochaete from the River Ehen, August 2013. Scale bar: 10 micrometres (1/100th of a millimetre).
A kilometre or so downstream, I picked up a stone from the river bed that had a distinct blue-green tinge. However, this was not a film lying on top of the rock: it seemed that the rock itself was coloured. I had seen this before but never in a stream with such soft water as the River Ehen. I had to scrape the rock with a razor blade in order to remove some of this crust and, once under the microscope, it was difficult to get a clear view due to the quantity of rock particles that had come off along with the film. However, there was enough for me to make out some narrow filaments, similar to the Phormidium and Oscillatoria filaments we have seen in earlier posts. This is a species called Lyngbya vandenberghenii. The inset photograph shows some of the empty “sheaths” which surround the filaments of cells – the possession of a distinct sheath is the main character which distinguishes this genus from Phormidium and Lyngbya although, as the right hand picture shows, the filaments often glide free of this sheath.
Left: Lyngbya vandenberghenii growing on a limestone cobble in the River Ehen, August 2013. Right: individual trichomes (the technical term for the chain of cells) as seen under the microscope (the rock particles make crisp focus impossible) and, inset, empty sheaths of L. vandenberghenii. Scale bar: 10 micrometres (1/100th of a millimetre).
Although it looks as if the alga was growing in the rock, it is actually growing on the surface of the rock but depositing calcite crystals as it grows. I was surprised to see this species here as I generally associate it with harder water. However, the stone itself was a surprise: a single piece of limestone amidst a river bed composed almost entirely of the igneous rocks that underlie the Ennerdale catchment. I presume that some limestone had been imported, perhaps for a building, and that this piece had somehow found its way into the river. It had, in the process, created a tiny microenvironment which enabled this exile from harder water to survive.