Having used Biblical language to encapsulate the floods that immersed much of northern England a week or so ago (see “And the waters prevailed upon the earth …”) I feel I must turn back to Genesis in order to find a title for this follow-up piece (chapter 8 verse 11, if you are interested). My regular fieldwork haunts on the River Ehen in western Cumbria experienced such high flows that at one point they went off the top of the scale, meaning more than 3000 MLD (mega litres per day). To put this in perspective, we regard 500 MLD as the absolute limit for safe field work at this particular location. An additional problem was that our route to the River Ehen passed three of the most flood-affected parts of Cumbria, complicating our travel plans. But that was then, and by Monday morning the hydrograph was indicating flows of about 300 MLD, and I lost no time in heading off down the long and lonesome highway. I get my kicks at the far end of Route 66 …
The differences were apparent as soon as we peered into the river: at this time of the year the Ehen typically supports lush growths of algae (see “How green is my river?”) but today the stones were missing their usual green clothing. This is what the bed of the river usually looks like in the summer, when grazing invertebrates keep the quantities of algae low. It was a surprise to see the river bed looking like this in December.
Cobbles and pebbles on the bed of the River Ehen, Cumbria, just below the outflow of Ennerdale Water, 16 December 2016. The larger stones are about 10 cm across.
A kilometre or so downstream, the bed consists of a greater proportion of boulders and many of the cobbles are more deeply embedded into the river bed. Here, the stones again had lost their usual covering of green algae but, this time, some of the larger stones had conspicuous brown patches, where the diatoms were thriving in the absence of competition from other algae.
A boulder (about 25 cm across) on the bed of the River Ehen, about a kilometre below the outfall from Ennerdale Water. Note the brown growths of diatoms on the upper surface.
The diatoms that I saw when I looked at a sample under the microscope were much the same as those that I had been seeing for much of the time that I have been looking at the River Ehen. I don’t get a sense that this is a community of distinctive “pioneer” species colonising a bare surface, rather that this is an inoculum formed from the scattered remnants of the usual rich community. There were a lot of Achnanthidium minutissimum (which many regard as a typical “pioneer” species) but also long-stalked Gomphonema sp. which we would expect to thrive in thicker biofilms plus two or three species of needle-like Fragilaria species (which are hard to identify with certainty in the live state). One of the Gomphonema sp. is the form that I wrote about in Diatoms and Dinosaurs and which is, again, hard to identify with certainty.
Diatoms in the River Ehen, December 2016: a. Tabellaria flocculosa; b. Gomphonema acuminatum; c. Gomphonema sp. (see note in text); d. and e.: Fragilaria species; g. – i.: Achnanthidium minutissimum. Scale bar: 10 micrometres (= 1/100th of a millimetre).
There were other types of algae in the samples; again, these were the species that I have seen at other times from the River Ehen but this time they were present only in very small quantities. In particular, I noted a few isolated filaments of the cyanobacterium Calothrix sp, one of which had a number of Achnanthidium minutissimum valves growing directly on the sheath.
Achnanthidium minutissimum (and one cell of Gomphonema) growing on Calothrix in the upper River Ehen, December 2016. Scale bar: 10 micrometres (= 1/100th of a millimetre).
I can’t wait now for our next trip to the River Ehen, in the middle of January, in order to see how the river has changed in the month following the floods. Many algae are adapted to growing in winter so it is possible that there will be lush growths again, so long as there are no more big floods. But large climate-driven events can have consequences for rivers that are measured in months (see “How green is my river?”) or years (see “And the waters prevailed on the earth”). We are close to the end of our third year of intensive studies on the River Ehen and there is a pleasure in knowing that there is still more to learn. That said, fieldwork in January does involve plunging your hands into a freezing cold river …