Spring officially started on 1 March, as if the officialdom has any say in the natural order. But then officials also say that rivers should stay in their channels and that fallen trees should be removed because they make streams “untidy”. All “official” pronouncements on the seasons (if not the natural order generally) deserve to be treated by aquatic biologists with a pinch of scepticism. A peek below the surface of the Lake District’s streams, for example, suggested that spring was already well underway even before officials had approved its start, with the upper surfaces of cobbles and boulders in streams on my regular beat smothered with thick orange-brown diatom films. But, then, algae have always had a difficult relationship with officialdom.
Back home, after a fieldtrip beset by equipment problems and leaky waders, I had a chance to look at the organisms responsible for these growths. First observation was that whilst the presence of films was almost universal, their composition differed from stream to stream. On the other hand, there were some consistencies that helps pull these differences together to produce a coherent narrative. One final confession before going any further: my fieldwork issues necessitated staying an extra night, and the algae collected on the first day did not look as healthy when examined under the microscope as those collected on my second day.
Broadly speaking, all of the biofilms looked like the photograph below, with a mix of Achnanthidium and Gomphonema species, along with a seasoning of Fragilaria and Tabellaria. They differed, however, in the details. The films in Croasdale Beck, a small stream flowing off the fells around Ennerdale Water (most recently discussed in “The algae that got away …”) were dominated by the diatom Gomphonema parvulum, whereas those from the Rivers Ehen and Irt had one or more relatives. In “Diatoms and dinosaurs” I referred to this as “Gomphonema gracile” but then went on to explain why this name was problematic. Since then, the name G. gracile has been kicked into the long grass for different reasons, so I’ll call it “G. graciledictum” but still retain my original scepticism along with the further proviso that identifying diatoms from live material is not always straightforward.
I don’t think it is a coincidence that these thick biofilms are dominated by species of Gomphonema. You can see stalks emerging from the bases of several of the cells I have photographed. These help the cells exploit space vertically by creating dense “bushes” and, in turn, means that these species are “ecosystem engineers” on a tiny scale, creating habitat and opportunities for other algae too. Just as terrestrial bushes such as hawthorn can act as windbreaks, so these tiny underwater bushes will also slow the flow of water, both reducing physical stress and making nutrients more accessible to the algae. And, just as hawthorn bushes provide branches on which birds can perch, as well as for lichens to grow, other organisms grow in and around Gomphonema. I’ve written about this before (see “The curious life of biofilms …”) but it is easy to overlook because most of us count cells and ignore the stalks that are so important in this process of habitat creation.
The abundance of algae we observed, however, is likely to be due to the low numbers of grazers at this time of year. I did see a few chironomid and simulidae larvae during this trip, but fewer large grazers. From this point of the year onwards, the quantity of algae in these rivers generally declines, probably because invertebrates become more active as the water warms, and the algae are a valuable food resource for them. This period, however, represents a window of opportunity when – cold water aside – they can grow with relatively few constraints. The constraint, as I’ve commented before, lies with the ecologists themselves, relatively few of whom want to venture out to explore these habitats in winter. And, with the right foot of my wader getting gradually damper as streamwater drips in, I’m wondering if they might just have a point.
Reference
Reichardt, E. (2015). Gomphonema gracile Ehrenberg sensu stricto and sensu auct. (Bacillariphyceae): a taxonomic revision. Nova Hedwigia 101: 367-393.
Some other highlights from this week:
Wrote this whilst listening to: Gregorian Chant for Lent and Easter by the Gregorian Chant Schola of Saint Meinrad Archabbey, Indiana, USA.
Currently reading: Chronicles from the Happiest People on Earth by Wole Soyinka.
Cultural highlight: J’ai perdu mon corps (I lost my body). French animated film from 2019. Unique. Also, wonderful new British film Ali and Ava, directed by Clio Barnard and set in Bradford.
Culinary highlight: Sichuan-style fish head and frogs leg soup with plenty of whole chillis, served “hot pot”-style using a laboratory hot plate.
microscopesandmonsters 