Jarrow Bridge, looking downstream from near St Paul’s church, 18 April 2014.
Walking up from the site where I collected the diatoms I wrote about in the previous post, I passed St Paul’s church, dating back to Saxon times. However, as it was Good Friday, the church was busy with worshippers, so I could not go in to look at the ancient stained glass. Just beyond the church is a bridge over the muddy creek that is the mouth of the River Don. The stones in the bed of the creek just upstream of the bridge had a whitish coating of sewage fungus which is, in my experience, never a healthy sign in a stream, tidal or not. Sewage fungus is much less common now in Britain than it was in the past, which is an indication of the gradually improving state of our rivers. The term “sewage fungus” is, in fact, a misnomer as the organisms that are lumped under this term include both fungi and bacteria. The important point is that they are not photosynthetic and so rely upon complex organic compounds in the water in order to obtain the energy that they need to grow. “Complex organic compounds”? Go figure.
Sewage fungus (probably Beggiatoa) smothering stones in the tidal creek just upstream of Jarrow Bridge.
Mixed in amidst the sewage fungus and green algae (Ulva sp.) on the rock there were lots of diatoms. The one that caught my eye was a large sigmoid Nitzschia gliding amidst the Beggiatoa filaments. There were several of these moving around the slide but what was most interesting to me was that each had at least one, and in some cases half a dozen, much smaller diatoms sitting on them as they moved around. Being a relative novice to brackish and marine habitats, I do not know what species these epiphytes were, though I suspect that they were Amphora, possibly A. exigua. The constant motion of the Nitzschia made it impossible to capture this crisply with my stacking software and I lack the hard heart necessary to kill diatoms purely to obtain a better photograph.
A sigmoid Nitzschia with a payload of Amphora spp. collected from the tidal creek near Jarrow Bridge, 18 April 2014.
I wrote “possibly Amphora exigua” based on the flimsiest of evidence. The latest monograph on Amphora, a massive tome, describes the details of the silica frustule in minute of detail but the ecological comments are vague. I should, it tells me, now call this diatom “Halamphora exigua” but, under “Distribution and ecology” all it can tell me is “not precisely known”. It is a sad reflection on the strange world of diatoms where, it seems, we know the shape of everything yet the meaning of nothing.
Levkov, Z. (2009). Amphora sensu lato. In: Diatoms of Europe, Volume 5. (H. Lange-Bertalot, ed). A.R.G. Gantner Verlag K.G. 916pp.