If you look just behind the horse in Constable’s landscape (on display at Tate Britain) showing the River Stour with Flatford Mill in the background (1816-1817), you’ll see a man crouching down. He’s just disconnected the rope between the barge and the horse to enable the barge to pass under Flatford Bridge but, by coincidence, it is roughly the point where, on 30 December, I crouched down to pull out a handful of weed from the River Stour for some post-Christmas natural history.
I pulled up a handful of Canadian pondweed, Elodea canadensis, which happened to have some filaments of a green alga tangled around it. It was tough, wiry stuff and, as far as I could see with the naked eye, unbranched, which ruled out the most likely candidate, Cladophora glomerata, so I stuffed a handful of the weed into a sample bottle that I had brought along and brought it back home for a closer look. However, what I thought would be a straightforward identification task proved problematic. Rhizoclonium hieroglyphicum is an unbranched alga in the same order as Cladophora (Cladophorales) but the Flatford material was too broad to fit the description. Another candidate, a marine/brackish species occasionally found in freshwaters, is Chaetomorpha linum. The Flatford material was broad enough to qualify as this species, but the cells of C. linum are typically less than twice as long as broad, whereas the Flatford alga had cells with a longer length:breadth ratio. So, even after consulting Dave John, who wrote the relevant section of the Freshwater Algal Flora of the British Isles, I am still not confident that I know the genus, let alone the species.
Flatford Mill algae: a. the tangle of Elodea canadensis and filamentous algae that I pulled from the River Stour; b. the unidentified alga with epiphytic diatoms (Cocconeis spp. and Rhoicosphenia abbreviata); c. a filament of Ellerbeckia arenaria, also tangled amidst the E. canadensis. Scale bars: 10 micrometres (= 100th of a millimetre).
The filaments were, in turn, smothered with diatom epiphytes – mostly Cocconeis placentula but also some C. pediculus and Rhoicosphenia abbreviata and there were also a few chains of the large centric diatom Ellerbeckia arenaria (see Ellerbeck and Ellerbeckia) alongside a variety of other diatoms including chains of Melosira varians. The tangle of Elodea and Cladophorales filaments presumably creates a “micro-backwater” within which these other organisms can thrive away.
Unidentified Cladophorales from River Stour near Flatford Mill, 30 December 2015; scale bar: 50 mm (= 1/20th of a millimetre).
My final picture for this post is a view down my microscope during my examination of this material – a homage to John Constable’s landscapes, of sorts, except that distances in my image are measured in micrometres rather than metres. You can see a filament of the Cladophorales complete with epiphytes at one side and, on the other, a few cells of the diatom Melosira varians. Understanding of the composition of a sample of algae comes from examination of many fields of view such as this, partly because, as you can see, there is usually a lot of non-algal material present that would obscure the algae if preparations were much denser. I’ve also taken some liberties with depth of field in putting this sketch together. The experienced microscopist is constantly adjusting the fine focus to get an insight into the three-dimensional structure of his samples. With microscopy, what you see is rarely what you get … but I’ve written about that quite recently so perhaps it is time for me to stop…
A view down the microscope (400x magnification) whilst examining the material from Flatford Mill. Watercolour, gouache and pencil.