I was back at the River Wear last week for my final visit of the year. The heatwave that dominated the summer seems like an aeon ago as I plunged my arm into the cold water to find some stones and take some photographs. I’m curious to see what is here, though. The river has surprised me several times already this year. Has it reverted to type as the British climate has regained a semblance of normality, or will the changes that we saw in the summer (see “Summertime blues …” and “Talking about the weather …”) still have consequences for the algae growing on the river bed?
The river bed itself had many patches of green filamentous algae which, on closer examination, turned out to be my old friend Ulothrix zonata, an alga that is common in these parts and which has a distinct preference for early spring conditions (see “Bollihope Bavakakra” and references therein). A closer look showed two types of filament present: the normal vegetative ones with a single chloroplast encircling the cell but also some where the cell contents have divided to produce zoospores which are released and which, if they land on a suitable surface, will produce new vegetative filaments. The “parent” filaments, themselves, are produced as zygotes, produced back in the spring, germinate. The zygotes are the product of sexual reproduction, triggered by lengthening days (see reference in earlier post) and are dormant through the summer, only germinating once day length shortens and temperatures start falling.
The river bed of the River Wear at Wolsingham, November 2018, showing conspicuous growths of Ulothrix zonata.
Magnified views of Ulothrix zonatafilaments from the River Wear at Wolsingham. The upper image shows a vegetative filament and the lower image shows filaments where the cell contents have divided up prior to the release of zoospores. Scale bar: 20 micrometres (= 1/50thof a millimetre).
The areas between the patches of Ulothrix zonatawere covered with a thick film, composed primarily of diatoms, in contrast to the situation on my last two visits when non-filamentous green algae predominated. This time, it was Achnanthidium minutissimumdominated my count (about 70% of cells) although, because they are relatively small, they comprised just under half of the total volume of algae present. Other diatoms bumped this up to about 70 per cent of the total volume, with motile cells of Navicula and Nitzschia, which were so abundant at the start of the year, beginning to appear in numbers again. The green cells that dominated my counts in July and September now only constitute about five per cent of the total. The River Wear, in other words, has shaken off the effects of the summer, just as a healthy human gets over a winter cold, and is now back to its old self.
A view down my microscope whilst examining samples from the River Wear at Wolsingham showing the predominance of Achnanthidium minutissimum with (on the right-hand side) a filament of a narrow Ulothrix (not U. zonata).