I wrote about the effect of the long period of low flow in the River Wear a few weeks ago (see “Summertime Blues …”) and have, now, completed two dioramas depicting the state of the river in the main channel and in a filamentous algae-dominated backwater. The first of these is dominated by free-living green algae, either single cells or coenobia (see note at end), which is a big contrast to the situation I recorded two months earlier when the assemblage was dominated by diatoms, with patches of filamentous green algae (see “Spring comes slowly up this way” and “A question of scale”).
I sent a small sample of the Wolsingham biofilm to Dave John for his opinion on the green algae, and he sent back a list with twenty one different green algae that he had found. Fortunately, this confirmed my own original list, with Keratococcus bicaudatus, Scenedesmus, Desmodesmus and Monoraphidium all featuring. He also commented that Keratococcus is hard to differentiate from Chlorolobium (which is also in his list) and that most of the green alga on his list are usually considered to be planktonic (Keratococcus and Chlorolobium are exceptions) although, as my earlier post suggested, these definitely formed a distinct biofilm on the surface of stones this year in the River Wear.
A diorama showing the biofilm in the River Wear at Woslingham, July 2018. You can see coenobia of Demodesmuss communis (centre), Scenedesmus sp. (left) and Coelastrum microporum (right – half tucked behind a mineral particle, along with single cells of Keratococcus bicaudatus (upright cells) and Monoraphidium. There are also some cells of Achnanthidium minutissimum on short stalks in the foreground and a cluster of Fragilaria gracilis cells in the background.
There seems to be little hard evidence on the habit of Keratococcus and Chlorobium apart from references to a preference for benthic habitats. I have drawn them as upright cells, drawing on their similarity in form to Characium, for which there is better evidence of an upright habit (although Characium tends to grow on other algae, rather than on hard surfaces). Whereas I often have a strong sense of the three dimensional arrangement of organisms within benthic biofilms, so little has been written about the preferences of these green algae that, apart from the Keratococcus, I have had to show them as a jumble of cells and coenobia across the picture frame.
The second diorama depicts the tangle of filamentous green algae that I found in the pools beside the main channel. As I mentioned in my earlier post, these are species that I do not normally find at this site and are here, I presume, due to the long period of unusually warm weather and low flows. One difference between these communities and that captured in my first diorama is that there is a more obvious organisation of the constituents here: the Cladophora filaments, though appearing as a tangle to us, form the foundation upon which epiphytes grow directly, but also around which Melosira filaments are entangled, rather like the lianas in a tropical rain forest. The quantity of diatoms around the Cladophora is so great that their brown pigments completely mask the Cladophora’s green cells but note how the density of Cocconeis cells reduces towards the tips – the youngest parts of the filaments.
Depiction of filamentous algae growing in the margins of the River Wear at Wolsingham in July 2018, showing epiphytic Cocconeis pediculus and entangled Melosira varians.
There have been some recurring themes in my posts this summer: one is that UK rivers have been behaving quite differently to previous years, due to a combination of low flows (more accurately, a lack of the scour associated with high flows) and warm, well-lit conditions. The low flows have also resulted, to some extent, in rivers becoming more physically heterogeneous, with side-pools and silty areas developing distinct assemblages of algae quite different to those encountered in the main channel. Sometimes, the sum of these effects is for rivers to look less healthy than is usually the case.
The Wear at Wolsingham is one of those sites that I like to think I know well, having visited the location so many times over the past 30 years. It is reassuring, in a rather humbling way, to know that it still has the capacity to surprise me.
Dave’s list of green algae from the Wolsingham biofilm, July 2018
Coelastrum astroideum (very small and atypical)
Coelastrum microporum (very small and atypical)
Desmococcus olivaceum (subaerial species)
A coenobium is a colony in which the cell number is fixed at the time of formation and not augmented subsequently. Coenobia are particularly common in the Chlorococcalees.