A few days after my trip to Weardale I found myself beside the River Irt, a few hundred metres below the point where it flows out of Wastwater, in the western part of the Lake District. Whereas the River Wear drains a catchment underlain by Carboniferous rocks, including a high proportion of limestone (see “Co. Durham’s secret Karst landscape”), the Irt’s catchment is largely underlain by ancient volcanic rocks, resulting in much softer water. I was curious to see how different the algae were here compared to those in the Wear.
The river bed at this point is dominated by boulders of granite, which host a patchwork of mosses, filamentous algae and discrete growths of diatoms (visible on the right-hand side of the figure below). Between these there were areas of pebbles and gravels, suggesting good habitat for freshwater mussels. The patches of filamentous algae (mostly no more than a couple of centimetres in length) were a mixture of Mougeotiaand Zygnema, similar to the forms that I find in the River Ehen, a 30 minute drive to the north. These two species differ in the form of their chloroplasts (Mougeotiahas a flat plate whilst Zygnemahas two star-shaped chloroplasts, attached by thin cytoplasmic strands to resemble an animal skin stretched on a frame) but are closely-related, both belonging to the family Zygnemtaceae.
An underwater photograph of the substratum of the River Irt in November 2018 showing patches of filamentous green algae, mosses and (on the right-hand side) diatoms growing on granite boulders.
Filamentous green algae from the River Irt, November 2018. The upper photograph shows cells from a filament of Mougeotiawhilst the lower image shows two filaments of Zygnema. Scale bar: 20 micrometres (= 1/50thof a millimetre).
In between the tufts of filamentous algae were apparently bare patches of rock (they almost certainly had a very thin biofilm that would be hard to sample in isolation from the lusher algal growths that shared their habitat) and some conspicuous orange-brown growths of colonial diatoms. These turned out to be almost pure growths ofGomphonema hebridense, or a close relative (I can’t give a definitive answer until I have examined cleaned material), growing on long mucilaginous, sometimes branched, stalks to create a veritable “bush” of diatoms. There were a few other species of diatom growing within this bush, most notably some cells of Achnanthidium (cf.) caledonicumthat seemed to be growing on short stalks attached to the Gomphonemastalks, but also a few cells of Gomphonema capitatum(which also grows on long stalks) and some chains of Tabellaria flocculosa.
Gomphonema hebridenseis a diatom that I have written about several times before, as it is also common in the River Ehen, and also presents a number of interesting challenges to taxonomists (see “Diatoms and dinosaurs”). Whatever future studies reveal, however, the presence of colonies of this (or these) species that are visible with the naked eye is something I associate with only the cleanest rivers in the country during the cooler times of year. It should not have been a great surprise to me to find it flowing out of one of the most pristine lakes in England (see “The Power of Rock …”).
A close up of cells within a colony of Gomphonemacf hebridense. Several mucilaginous stalks are also visible as well as (top left) a cell of Achnanthidiumcf caledonicum. Scale bar: 10 micrometres (= 100th of a millimetre).
The predominance of boulders over smaller, more easily moved stones, suggests a river that has more energy than the River Ehen, one of my regular Lake District haunts. Both flow out of lakes whose catchments include some of the wildest and most mountainous terrain in the country. Lakes tend to act as shock absorbers in catchments, slowing down the water that pours off the fells after heavy rain. Streams in this part of the world that have no such impediments to flow tend to have rocky, mobile beds and relatively sparse algal communities. By contrast, the Irt and Ehen just below their respective lakes have relatively lush growths of algae. The substrates of the two rivers, however, are very different: the Ehen having very few boulders in comparison to the Irt, due to the presence of a weir at the outfall. This allows Ennerdale Water to be used as a water supply for the towns of north west Cumbria but, at the same time, turns the lake into an even more effective hydrological shock absorber. Yet more of the energy that should be washing smaller stones down the river is no longer available except after the most exceptional storms.
That’s my working hypothesis, then: the Irt is a river that is subject to just enough high energy events to move the rocky substrates around yet no so many that rich algal communities cannot develop between these. The Ehen, by contrast, has fewer events, leading to fewer opportunities for the algae to be scoured away, whilst unregulated streams such as Croasdale Beck (see “What a difference a storm makes …”) have such regular scouring spates that the algal communities are usually sparse. I might be wrong, of course and I might be back in a years time with a better hypothesis. Until then …