The diatoms I saw in my sample from the littoral of Lake Popovo (described in the previous post) reminded me of an assemblage that I had seen in another lake which, apart from its location, has much in common with Popovo. This lake is Wastwater, in the western part of the English Lake District (see “The Power of Rock …”). Like Popovo, it is situated in a remote a region of hard volcanic rocks and, as such, has very soft water and is subject to few of the pressures to which most of our freshwaters are subject. The photograph above shows me sampling Wastwater in about 2006 (more about this photograph, by the way, in “A cautionary tale …”).
I wrote about Wastwater when I was writing my book Of Microscopes and Monsters, the precursor of this blog. I focussed, in particular, on an experiment that my friend Lydia King had performed as part of the research towards her PhD. Her previous work had established that there were relationships between the types of algae that she found in lakes in the Lake District and the amount of nutrients that they contained. She also saw that the types of algae she found depended upon how acid or alkaline the water was. But the water chemistry only explained a part of the variation in the algae and now she wanted to find out about the variation that was not explained by this. In particular, she wanted to know how much of the variation was due to the way that the algae interacted with each other.
Lydia’s experiment involved putting clay pots into the shallows at the edge of Wastwater and then watched how the algal communities changed over the course of six weeks. She also examined small parts of the pots at extremely high magnifications using a scanning electron microscope. These micrographs, and subsequent conversations with her, had inspired some of my early paintings and I returned to this subject several times, finally producing a series of three pictures that showed changes in the algae over time.
The microbial world of the littoral zone of Wastwater after two weeks of colonisation showing unidentified small unicellular blue-green alga, unidentified small unicellular green alga; thin filaments of Phormidium, Achnanthidium minutissimum and Gomphonema parvulum.
The first of these shows the surface of the plant pot after being submerged in Wastwater for two weeks. You could think of this as a patch of waste ground that was, at the start of the experiment, bare of vegetation. If we watched this patch over a number of weeks, we would notice some plants appearing: scattered stalks of grass, perhaps some rosebay willow herb, dock or plantains. A gardener might dismiss these as “weeds”, although this term has no ecological meaning but ecologists prefer to think of these as “pioneers”: plants adapted to colonising new habitats, growing quickly (which might mean producing lots of seeds in a short space of time or producing rhizomes or runners) and covering the ground. This same process has taken place on Lydia’s plant pot in Wastwater: the “weeds” in this case are scattered thin filaments of the blue-green alga Phormidium, the diatoms Achnanthidium minutissimum and Gomphonema parvulum plus a number of spherical green and blue-green cells that she couldn’t identify. Such is the scale that we are working at that this open landscape still contains about 92000 cells per square centimetre.
The microbial world of the littoral zone of Wastwater after three weeks of colonisation. The composition is similar to that in the previous figure but the density of cells is greater.
When she came back a week later, much of the empty space had been infilled; there were now about 300,000 cells per square centimetre. These mostly belonged to the same species that she had found the week before. The difference is that they are now rubbing up against each other and this has some important consequences. All plants need light and nutrients to grow and algae are no exceptions. Sunlight provides the energy for photosynthesis but now, at week three, the density of algae is such that there is a chance that some of the light will be intercepted by a neighbouring cell. The total amount of sunlight that filters through the water to the pot surface is already much lower than that available at the lake surface; now it has to be shared out between many more cells. At this point, properties such as fast growth rates that helped our pioneers to colonise the plant pot become less relevant, and it is algae that are better adapted to capturing the limited light that will survive.
So when Lydia came back to Wastwater after six weeks, she saw a very different community of algae on her pots. There was still a lot of Achnanthidium minutissimum, but rising above these was the elegant art deco shape of Gomphonema acuminatum (also found in Lake Popovo) which, importantly for our story, grows on a long stalk. There are also cells of “Cymbella affinis” (the correct name at the time that Lydia was working but see comments in the previous post about the nomenclatural history of this species). This, too, grows on a long-stalk, the better to grow above the Achnanthidium and other pioneers. If we continue to use the analogy of a patch of wasteland, then it has now reached the point where it has been invaded by shrubs such as hawthorn and blackthorn. However, in a terrestrial habitat this would happen two or three years after the first pioneers had arrived, not six weeks as Lydia had observed for the algae. She also found the diatom called Tabellaria flocculosa which forms filaments. These often start out loosely-attached to the substratum but more often break free and become entangled around the other algae. In our “wasteland” analogy, these would be the brambles.
The microbial world of the littoral zone of Wastwater after five weeks of colonisation. Gomphonema acuminatum, “Cymbella affinis” and Tabellaria flocculosa have now joined the assemblage seen in the two earlier dioramas.
The experiment finished shortly after this, terminated when the apparatus was overturned. Whether by a wave or by vandalism, Lydia will never know but this event is, itself, a metaphor for the harsh world in which benthic algae have to survive. In real life, the many cobbles in the littoral zone will be rolled by wave action or, as we have seen in other posts, invertebrate grazers could have removed much of the “shrubbery”, leaving a “pasture” composed of the tough, fast-growing species such as Achnanthidium minutissimum to dominate samples. The “successions” we see in the microscopic world not only take place much more quickly than those in the macro world, but they also rarely have a stable “climax”: just a brief pause before the next onslaught from the physical, chemical and biological processes that shape their existence.
King, L., Barker, P. & Jones, R.I. (2000). Epilithic algal communities and their relationship to environmental variables in lakes of the English Lake District. Freshwater Biology 45: 425-442.
King, L., Jones, R.I. & Barker, P. (2002). Seasonal variation in the epilithic algal communities from four lakes of different trophic state. Archiv für Hydrobiologie 154: 177-198.