I diverted from my usual haunts in the upper River Ehen in Cumbria recently in order to explore Ennerdale Water in greater detail. I am used to see it from the western end as we do our fieldwork, but the length of the journey to and from the River Ehen means that we rarely have time to linger. Finally, however, we found a July day when we could circumnavigate the lake. “July day”,”Lake District” and “fieldwork” sounds like an intoxicating combination. However, the photograph above shows it was not quite as idyllic as it might have been (or, even, as it was on the day before). Hence the title of this post, borrowed from a beautiful Joni Mitchell song which includes the line “But clouds got in my way”.
In the far past, the lakes of the Lake District were thought to have “evolved” at different speeds following their formation at the end of the last Ice Age. Ennerdale Water and Wastwater, surrounded by hard volcanic rocks which erode very slowly, were regarded as the two most “primitive” lakes, whilst Windermere and Esthwaite Water were thought to be the two most “evolved”. That is now known not to be the case: the geology is very important in determining the type of lake, not just because erosion is the source of the inorganic salts that give the water a particular chemical character, but also because this influences how man uses the lake. In the case of Ennerdale Water, only about five per cent of the catchment is cultivatable, and this, in turn, influences the amount of inorganic fertiliser that is added to the meagre supply of salts provided by the underlying rocks. Ennerdale is, as a result, one of the least chemically-disturbed of all English lakes.
At the far south east end of the lake, close to where the River Liza enters the lake, I was intrigued to see some very dark spots on the rocks. They looked like they might be cyanobacterial colonies, so I picked a few off with my forceps and put them into a bottle for later investigation. When I was able to look at them, the following day, I saw clumps with brown filaments radiating out, and each gradually narrowing towards the tip. Closer examination showed that the cells that made up each filament had a blue-green colour, but were each enclosed in a brown pigmented sheath. The filaments showed a characteristic form of “false” branching, in which the daughter filament breaks off from the mother, but is contained within the same sheath. At the base of many of the filaments, I could see a modified cell (slightly lighter and less granular than the others) called a “heterocyst”, which was responsible for nitrogen-fixation. These are all characteristics of the genus Rivularia, which is a good indicator of very high status water.
A cobble (about 15 cm long) from the littoral zone of Ennerdale Water, SW end. The dark cyanobacterial colonies are about 3-4 mm across.
Rivularia biasolettiana from the littoral zone at the south east end of Ennerdale Water, Cumbria, UK. a. low power (x100) image showing radiating filaments gradually narrowing in width; b. filaments showing false branching (a heterocyst is present, but hard to see); c. false branching in a filament of Rivularia with heterocyst arrowed. Scale bar (b. and c.): 10 micrometres (= 1/100th of a millimetre).
The presence of Rivularia here was interesting to me for several reasons. First, it continues a series of observations that suggest that this genus is not confined to hard water habitats in Britain and Ireland, as once thought (see “more about Rivularia” and links) although earlier posts have also referred to its presence in soft water habitats in Norway. I’m also fairly sure that the organism that I collected from this trip to Ennerdale is different to the one that I find in the River Ehen and, indeed, at other locations around the Ennerdale perimeter, but that is a subject for another day.
The second comment to make is that the presence of cyanobacteria (blue-green algae) is usually a sign of an unhealthy, enriched habitat, not the very high quality habitat that Ennerdale, in fact, represents. Blue-green algae in the phytoplankton is, very often, a bad sign, suggesting enrichment by inorganic nutrients. Several species combine the ability to fix nitrogen with positive buoyancy, which means that they have two distinct advantages over other algae as they compete to exploit limited light and nutrients. Some of these blue-green algae are also toxic, which has implications for how the lake and its water is used.
Those blue-green algae that live attached to surfaces in streams and in the littoral zone of lakes play by a different set of rules, however. As they cannot use positive buoyancy to compete for light, they are more likely to be overgrown by faster-growing algae in the scrabble to capture available light. This means that investment in expensive nitrogen fixation machinery is only an advantage when other algae, too, are very short of nutrients. And a shortage of nutrients is the natural state for most freshwater ecosystems.
I chose the title of this piece, originally, because I felt that I had looked at Ennerdale Water from both sides now. However, the same lyric could just as well apply to blue-green algae. The water manager sees them as a problem; in some situations, however, they can be a positive sign. So we can, in fact, look at blue-green algae from both sides too … from give and take and still somehow … I really don’t know life (microscopic life especially) at all ….