Fieldwork in the River Ehen has been an unusually pleasurable experience over the past few months, even to the extent of abandoning waders altogether and wearing just a thin pair of neoprene beach shoes and shorts as I worked. Curiously, there were few obvious signs of the prolonged period of low flow here, but that is partly due to the pumps installed by United Utilities to keep the river running whilst the lake was drawn down (see “Life in the deep zone …”). I did, however, find some intriguing green patches on fine sediments at the margins.
Most of the bed in this part of the river consists of much coarser sediments than these which are, I suspect, silt and sand deposited on the occasions when Ben Gill (which joins the Ehen immediately below Ennerdale Water) is flowing. Current velocity is lower at the edges of the river, allowing fine sediments to settle out and create temporary sandbanks. One decent spate will be all that is needed, I suspect, to wash much of this downstream. However, there has not been a period of prolonged high flow for several months and there is, as a result, a thin green mat of algae growing on the upper surface of this sediment.
Mats of Oscillatoria on fine sediments beside the River Ehen just downstream from Ennerdale Water, August 2018. The total length of the mats in the left hand photograph is about one metre.
I scraped up a small sample to examine under my microscope. I was expecting to see the broad filaments of the cyanobacterium Phormidium autumnale which I often find at a site about five kilometres downstream (see “’Signal’ or ‘noise’?”) but what I saw was much narrower filaments, some of which were slowly gliding forwards and backwards. These belong to a species of Oscillatoria, a relative of Phormidium that is common in the plankton. A few species, however, do live on surfaces and can, as I could see in the Ehen, form mats. I have, in fact, described a different mat-forming species of Oscillatoria (O. limosa) from the River Wear close to my home (see “More from the River Wear”) and this, too, had been favoured by a long period of warm weather and low flow. The filaments in the River Ehen were much narrower – just a couple of micrometres wide – and had relatively long cells (two or three times longer than wide) but, in other respects, they clearly belonged to the same genus.
Microscopic views of Oscillatoria filaments from the River Ehen, August 2018. The upper photograph was taken at medium magnification (400x) and the lower image was taken at 1000x. The constant motion of the filaments means that it is not possible to use stacking software to obtain a crisp image. Scale bar: 10 micrometres (= 1/100th of a millimetre).
The motion that I could see is thought to be due to a layer of tiny fibres (“microfibrils”) which wind around the inner layer of the cell wall in tight spirals. Movement is caused by waves that are propagated along these fibres, meaning that the filament actually rotates as it moves (though this is almost impossible to see with a light microscope). The filaments can move either towards or away from light, depending on the intensity, at a speed of up to 11 micrometres per second (that’s about a millimetre a day or, for any petrolheads who are reading, 0.00004 kilometres per hour). This allows the filaments can adjust their position so that they are neither in the dark nor exposed to so much light that they are likely to do damage to their photosynthetic apparatus (see “Good vibrations under the Suffolk sun” for more about this). The result is that filaments will tend to converge, Goldilocks-style, at the point where light conditions are “just right”. You can see some sediment particles settling on the top of the mat in one of the images and we can expect the filaments to gradually adjust their positions, incorporating these particles, over time.
Last year, I wrote about Microcoleus, a relative of Oscillatoria, which formed mats on saltmarshes and explained how this could be the first stage of colonisation of damp habitats by plants (see “How to make an ecosystem”). We are seeing the same processes happening here, but the life expectancy of these mats is much lower. They may well be gone next time I visit, depending on how the Cumbrian climate behaves over the next couple of weeks. They are transitory phenomena, here today and gone tomorrow but, like the subjects of some of my other recent posts, particularly favoured by the long period of settled weather that we have enjoyed over recent weeks.
Halfen, L.F. & Castenholz, R.W. (1971). Gliding motility in the blue-green alga Oscillatoria princeps. Journal of Phycology 7: 133-145.
Note: you can read more about how the heatwave has affected fresh water in the Lake District in Ellie’s MacKay’s recent post on Freshwaterblog