One of the features of the upper River Ehen is thick brown growths, visible to the naked eye on the submerged stones. Diatoms often just form part of a homogeneous, slimy layer on the top of stones and it is unusual to see growths that are so distinctive. Under the microscope, these resolve into masses of diatoms – mostly the mass of Gomphonema on their long stalks and zig-zag filaments of Tabellaria that we have already met but, as you can see in the photograph below, they are not always covered by green algae such as Spirogyra (see post). Perhaps – and this is just a hunch – this site has heavier shade from the surrounding trees which favours the diatoms over the green algae?
The left hand image shows a diatom-covered stone in the upper River Ehen, February 2013; the right-hand image is a low-power view of one of these growths under the microscope showing the mass of diatoms (mostly Gomphonema and Tabellaria). The scale bar is 50 micrometres (1/20th of a millimetre).
Amongst this dense forest there were a few chironomid larvae, greedily feeding amongst the diatom stalks. These are the young stages of the non-biting midges often seen swarming close to rivers on summer evenings. I don’t know if these cold-blooded larvae would have been so vigorous in the freezing cold water of the Ehen but as soon as they were exposed to the warmth from the microscope lamp they became surprisingly active, grabbing the mass of stalks with the finely-hooked “toes” on their front prolegs and using their mandibles to shovel the diatoms into their mouths.
To give you some idea of scale, the head of the larva is about 100 micrometres (1/10th of a millimetre) across, and the whole organism is between two or three millimetres long. The video was taken at 100x magnification; none of my videos taken at higher magnifications were successful due to the shallow depth of field. However, you can still just about make out the motion of the mandibles, the larva’s jaws, which, unlike vertebrate jaws, work from side to side rather than up and down.
I have made sporadic attempts to incorporate grazing animals into my pictures but have not yet produced an image that I am happy with. It is partly a problem of my own unfamiliarity with insect mouthparts, which makes portraying them in a “lifelike” manner very difficult, and partly simply a matter of scale: the invertebrates, though still only a few millimetres in length, are enormous in comparison to the algae that have generally formed the foreground of my art. It is the equivalent of trying to incorporate both a blade of grass and a cow into the same photograph. But I need to persist. Aquatic ecology too often seems to default to lists of species, whether algae, invertebrates, higher plants or fish. We can get a good sense from such lists of the health of an ecosystem, but we never get a complete picture. Today I was watching the ecosystem functioning: energy, originally from the sun, passing from one trophic level (the algae) to another (the invertebrates). In due course, this chironomid might itself be eaten by a trout, or die and decompose, letting bacteria and fungi feast on the energy it contained. The “nouns” that make up an ecosystem are easier to describe, but it is the “verbs” that drive it along and it is good to be able to peer down my microscope and be reminded of this.