After six bimonthly visits to the River Wear at Wolsingham during 2018, I can now step back and have a look at the complete dataset to see what patterns emerge. Over the course of the year, I have visited the site six times and recorded a total of 107 species: 5 Cyanobacteria, 32 green algae, 69 diatoms and one red alga. The true figure is probably higher than this, as the green algae include a number of “LRGT” (see “Little round green things …”) and certainly did not receive the same level of attention as the diatoms.
This crude enumeration of species, however, disguises some interesting seasonal patterns with, as I described in “Summertime Blues” and “Talking about the weather …”, abundant growths of green algae during the heatwave and associated low flow periods. This can be seen clearly in the bar chart showing the seasonal changes in the river: diatoms predominate in the early part of the year whilst green algae are very scarce. The bloom of the green filamentous alga Ulothrix zonata that I expected to see in March was missing due, I suspected, to the hard weather we experienced in late Feburary (see “The mystery of the alga that wasn’t there …”) but, by the summer, the river had taken on a very different complexion and was dominated by small green algae. The last sample of the year, collected in November, showed a return to diatom dominance with a late autumn showing of Ulothrix zonata(see “The River Wear in November …”).
Relative proportions (by approximate biovolume) of the main groups of algae found in the River Wear at Wolsingham during 2018.
Looking back at records of a similar exercise in 2009, I see that the beginning and end of the year were quite similar, with thick biofilms dominated by diatoms; however, the algae in the summer of 2009 were very different to those I found in 2018. My 2009 exercise involved monthly rather than bimonthly visits and I see that I recorded more Cyanobacteria in June and July than I found in Summer 2018. These were mostly filaments of Phormidium retziiand tufts of Homoeothrix varians, which I assumed to be a consequence of intense grazing (there is evidence that invertebrates find Cyanobacteria to be less palatable than other algae). By July, Cyanobacteria comprised over half the total biovolume of algae; however, there was a major spate soon after my visit. I was surprised to find, when I visited in August, a noticeably thicker biofilm smothering the rocks and, when I looked closely, this was dominated by the small motile diatom Nitzschia archibaldii. The Cyanobacteria had disappeared almost completely. I attributed this change to the invertebrate grazers being washed away by the spate, allowing the algae to grow unhindered. As the biofilm grew in thickness, so the algal cells start to shade each other, and a diatom that can glide through the biofilm has an advantage over any that are stuck to one place. Diatoms remained dominant for the remainder of the year, although my November sample came just after another storm and the stones I sampled were completely bare.
Relative proportions (by approximate biovolume) of the main groups of algae found in the River Wear at Wolsingham during 2009. A sample was collected in November but no living algae were recorded from it.
Overall, however, the similarities between the years outweighed the differences in the summer assemblages, whilst the composition of communities between late autumn and late spring was remarkably similar across the two years. The changes in summer 2018 extended beyond just a shift in the balance of algae in favour of greens: there were also changes in the composition of diatoms too. In fact, the changes in diatoms proved to be quite powerful mirrors of the changes in the community as a whole. I have demonstrated this in datasets spanning a number of sites in the past but it is reassuring to see that they are also reflecting patterns within one site. On the other hand, if I only had examined the diatoms, I would have missed some of the most interesting changes in the river over the course of the year.
Another observation is that no single sample from 2018 contained more than a quarter of the total algal diversity that I recorded over the course of the year. Every month saw some new arrivals and some departures (or, more likely in some cases, a few taxa that were present had dropped below my analytical detection limit). Some of these were expected (the seasonal dynamics of Ulothirx zonata, for example); others not (e.g. dominance by Keratococcus bicaudatusin the summer). I discussed this in “A brief history of time-wasting …” and, in honour of that post, am not going to repeat myself here. In an age when our environmental regulators are cutting back on the amount of data that they gather, I shall go into 2019 reflecting on Yuval Noah Harari’s comment that “the greatest scientific discovery was the discovery of ignorance”.