loading . . . Green days … The casual bankside observer of the west Cumbrian streams that I wrote about in my previous post may well have noticed the prolific algae, which seemed to smother much of the stream bed wherever we looked. What they might not have realised is that these growths were not homogeneous and, when examined under a microscope, their composition varied even across quite small distances.
Join me for a short walk beside the River Ehen, starting just above the point where it flows out of Ennerdale Water. In the lake itself there are almost no visible algae at all; however, walk past the weir and into the River Ehen itself, and the river bed rapidly changes from a dull brown colour to a vivid green. When I checked these growths under the microscope, they were composed largely of a single alga, _Bulbochaete_ , characterised by short side branches each bearing a long, colourless hair.
**_Bulbochaete_ from the River Ehen, just below Ennerdale Water, August 2025. Scale bar: 20 micrometres (= 1/50th of a millimetre). The photograph at the top of the post shows the appearance of the river bed at this site.**
Walk along the bank for a further kilometre downstream to a small footbridge and have another look. We can see, even with the naked eye, that the flocs are a duller green here and, with a microscope, we see that whilst _Bulbochaete_ is still present, two other green algae are also abundant, _Oedogonium_ (a relative of _Bulbochaete_) and a thin form of _Mougeotia_. There are also many more diatoms than we saw at the previous site, predominately _Tabellaria flocculosa_ but also clusters of _Fragilaria_ cells growing on the _Oedogonium_ filaments.
**_Oedogonium_ , a thin _Mougeotia_ and _Tabellaria flocculosa_ from the River Ehen about a kilometre downstream from the location where the first sample was collected.** **Scale bar: 20 micrometres (= 1/50 th of a millimetre). **
Scramble out and walk a couple of kilometres further (the river meanders a lot, so better to use the roads and footpaths) then have another look at the river just before it reaches the village of Ennerdale Bridge. The river bed here is bright green again but, this time, the composition is a mix of _Oedogonium_ and _Zygnema,_ most likely distinct, almost pure, clumps of each but difficult to tell apart in the field and, as a result, mingled together in my sample vial. Diatoms are present but in smaller quantities than at the previous site.
That’s three sites offering a three different variations on a basic theme of prolific green algae. The water chemistry is practically the same at each location, as are the proportions of light and shade and, being just a few kilometres apart, they all have similar physical properties. Quite what makes one species of filamentous green alga proliferate at one location, but another thrive a kilometre further downstream continues to baffle me, and I’ve never yet found an explanation in the literature.
**_Zygnema_ and _Oedogonium_ from the third site on the River Ehen, about two kilometres further downstream.** **Scale bar: 20 micrometres (= 1/50 th of a millimetre). **
One way of considering this problem is to say that conditions at these three sites do not strongly favour any of these species, and their patchiness along the river reflects the “founder effect”. A few filaments take hold on a particular stone and proliferate, to the extent that other species cannot easily get a foothold. Filaments of a different species lands on another stone, a few metres away, and thrives there instead. Because I could write similar stories after most visits, albeit with a rotating cast of players, patchiness may be an inherent property of these systems. I have, in an earlier post, compared this to the “multiverse” (see: “Something, somewhere, just for a moment …”): the dynamic nature of stream beds allows myriad different combinations of the same basic filamentous-green alga-dominated formula to thrive.
What was different on my latest visit was the quantity of algae that I recorded. Again, I’m just speculating but the warmth lifts a growth constraint, and all the algae will benefit until the point where they are competing with one another for space. As many of the algae that I am looking at are growing as monospecific clumps, competition between species – “survival of the fittest” – is not a factor that is readily apparent at the scale that most of us work at. We see a river turn green, but mostly do not have the wherewithal to investigate the many forms of algae that contribute to that greening. Seen from an alga’s perspective, climate change is not a “pressure” – it is an opportunity. Warmth and light let algae do what evolution has programmed algae to do so well and whatever is growing there will benefit. Summer 2025 turned many of our rivers into “all inclusive resorts” for algae, who have been having the times of their lives.
Some other highlights from the past week:
**Wrote this while listening to:** Van Morrison’s _Moondance_ ; an appropriate soundtrack for a visit to Northern Ireland.
**Currently reading:** Toby Harnden’s _Bandit Country_ , because we were staying in South Armagh and this book sets out, in often gruesome detail, the bloodshed that stained this beautiful countryside within my own living memory.
**Cultural highlight:**_The Titanic Experience_ in Belfast, which is as much about the cultural history of Belfast as it is about the fateful maiden voyage.
**Culinary highlight:** Tasting menu at the Muddler’s Club, Michelin-starred restaurant in Belfast.
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### _Related_ https://microscopesandmonsters.wordpress.com/2025/09/06/green-days/
