September is here. When I visited this site two months ago we were in the midst of the heatwave and the samples I collected from the Wear at Wolsingham were different to any that I have seen at this location before, dominated by small green algae (see “Summertime blues …”). As I drove to Wolsingham this time, I could see the first signs of autumn in the trees and the temperatures are more typical of this time of year. We have had rain, but there has not been a significant spate since April and this means that there has been nothing to scour away these unusual growths and return the river to its more typical state.
That does not mean, however, that there have been no changes in the algae on the submerged stones. Some of these differences are apparent as soon as I pick up a stone. Last month, there was a thin crust on the surface of the stones; that is still here but now there are short algal filaments pushing through, and the whole crust seems to be, if anything, more consolidated than in July, and I can see sand grains amidst the filaments. Biofilms in healthy rivers at this time of year are usually thin, due to intense grazing by invertebrates, so I’m curious to know what is going on here this year.
A cobble from the River Wear at Wolsingham, showing the thick biofilm interspersed with short green filaments. Note, too, the many sand grains embedded in the biofilm. The bare patch at the centre was created when I pulled my finger through it to show how consolidated it had become. The cobble is about 20 centimetres across.
Many of the organisms that I can see when I peer at a drop of my sample through my microscope are the same as those I saw back in July but there are some conspicuous differences too. There are, for example, more desmids, some of which are, by the standards of the other algae in the sample, enormous. We normally associate desmids with soft water, acid habitats but there are enough in this sample to suggest they are more than ephemeral visitors. And, once I had named them, I saw that the scant ecological notes that accompanied the descriptions referred to preferences for neutral and alkaline, as well as nutrient-rich conditions. Even if I have not seen these species here before, others have seen them in similar habitats, and that offers me some reassurance. In addition to the desmids, there were also more coenobia of Pediastrum boryanum and Coelastrum microporum compared to the July sample.
A view of the biofilm from the River Wear at Wolsingham on 1 September 2019.
There were also more diatoms present than in my samples from July – up from about 13 percent of the total in July to just over 40 per cent in September. The most abundant species was Achnanthidium minutissimum, but the zig-zag chains of Diatoma vulgare were conspicuous too. The green filaments turned out to be a species of Oedogonium, not only a different species to the one I described in my previous post but also with a different epiphyte: Cocconeis pediculus this time, rather than Achnanthidium minutissimum. I explained the problems associated with identifying Oedogonium in the previous post but, even though I cannot name the species, I have seen this form before (robust filaments, cells 1.5 to 2 times as long as broad) and associate it with relatively nutrient-rich conditions. That would not normally be my interpretation of the Wear at Wolsingham but this year, as I have already said, confounds our expectations. I did not record any Cladophora in this sample but am sure that, had I mooched around for longer in the pools at the side of the main channel, I would have found some filaments of this species too.
Desmids and other green algae from the River Wear at Wolsingham, 1 September 2019. a. Closterium cf. acerosum; b. Closteriumcf. moniliferum; c. Cosmarium cf. botrysis; d. Closterium cf. ehrenbergii; e. Coelastrum microporum; f. Pediastrum boryanum. Scale bar: 50 micrometres (= 1/20th of a millimetre).
It is not just the differences between months this year that I’m curious about. I did a similar survey back in 2009 and, looking back at those data, I see that my samples from August and September in that year had a very different composition. There was, I remember, a large spate in late July or early August, and my August sample, collected a couple of weeks later had surprised me by having a thick biofilm dominated by the small motile diatom Nitzschia archibaldii. My hypothesis then was that the spate had washed away many of the small invertebrates that grazed on the algae, meaning that there were few left to feed on those algae that survived the storm (or which had recolonised in the aftermath).. As the algae divided and re-divided, so they started to compete for light, handing an advantage to those that could adjust their position within the biofilm. This dominance by motile diatoms was, in my experience of the upper Wear, as uncommon as the assemblages I’m encountering this summer, though probably for different reasons.
Other algae from the River Wear at Wolsingham, September 2018. The upper image shows Diatoma vulgare and the lower image is Oedogonium with epiphytic Cocconeis pediculus. Scale bar: 20 micrometres (= 1/50th of a millimetre).
I suspect that it is the combination of high temperatures and low flows (more specifically, the absence of spates that might scour away the attached algae) that is responsible for the present state of the river. This, along with my theory behind the explosion of Nitzschia archibaldii in August 2009, both highlight the importance of weather and climate in generating some of the variability that we see in algal communities in rivers (see “How green is my river?”). The British have a reputation for talking about the weather. I always scan the weather forecasts in the days leading up to a field trip, mostly to plan my attire and make sure that I will, actually, be able to wade into the river. Perhaps I also need to spend more time thinking about what this weather will be doing to the algae I’m about to sample.