The journey to the location for this post started at the foot of the first “s” in the East Enders title sequence and ended beside the little blue squiggle at the bottom of the television screen, directly under the “a”. I got off the Elizabeth Line at Stratford with no other plan other than that I had some time on my hands and headed roughly south, triangulating on glimpses of the O2 Arena and Canary Wharf as best I could. I passed the splendid Italian Gothic edifice at Abbey Mills that houses the pumping station built as part of Bazelgette’s great sewer, walked alongside the muddy lower reaches of the River Lea, skirted numerous industrial estates and finally arrived, some two hours and nine kilometres later, at Island Gardens, a small area of green amidst the docklands sprawl, with a splendid view across the Thames to Wren’s Royal Navel College. After crossing the river via the Greenwich foot tunnel, I emerged beside the Cutty Sark from where a short walk brought me to the blue squiggle that was my destination.
That blue squiggle is Deptford Creek, scene of some posts at the end of last year (see “Floundering around in Deptford Creek” and “Throwing shapes …”) and I had come back to see how the project was progressing as well as finding some time to poke and scrape at more patches whose golden-yellow hue promised interesting diatoms. The mudflats themselves looked much greener than I remembered, but it is hard to know if this is a genuine biological phenomenon or is just the random chance of two observations being different. Today, however, I was more interested in what was growing around on the training walls at the edge of the creek rather than on the mud itself.
My eye was caught straight away by bright green growths on the shelf on the wall beside the slipway. Their distinctly felty texture made it easy to recognise the genus as Vaucheria although naming the species is a more difficult job. We’ve met Vaucheria several times in freshwater (see “the pros and cons of cell walls …”) but it is also commonly found in brackish habitats. My interest today was not in what species was growing, but in the way that it was helping to build a habitat within which a clump of moss and the shoot of a flowering plant were able to grow. We saw this with happening with Rhizoclonium riparian during my previous visit (see “If only we looked …”); this time it is an alga from a different phylum, but we are looking at essentially the same phenomenon.
A patch of Vaucheria growing on the wooden training wall beside Deptford Creek, March 2024. Note the moss (back right) and the shoot that have established themselves in the algal patch. The patch is about 30 cm across. The photo at the top of the post shows the training wall beside the slipway at Deptford Creek.
Underneath this shelf, there was a distinct yellow-brown patch on the wall whose dominant constituent was Hydrosera triquetra. I commented on its absence in a previous post (see “Deptford’s Heart of Darkness …”) but, in truth, the mudflat is not where I would most expect to find it. It is a chain-forming diatom that seems to be associated with vertical surfaces and which can tolerate exposure. When individual cells are seen end-on (“valve view”), they are very distinctive, resembling a Star of David, with two overlapping triangles; however, the cells attach at these faces and the characteristic shape is not immediately apparent when seen in fresh material. You need to focus carefully to understand the third dimension (see “Seeing with my fingers …”).
Hydrosera triequetra in Deptford Creek, March 2024. The left-hand image shows a patch growing on a wooden support (the central one visible in the image at the top of the post). The right-hand image shows a magnified view. The scale bar is 20 micrometres (= 1/50th of a millimetre) long.
Hydrosera triquetra was first observed in the Thames in the early 1970s, and is widely-assumed to have been accidentally introduced, perhaps by a ship discharging ballast water. Now it is widely-established. I’m often sceptical of claims about “alien” species but H. triquetra is sufficiently large and distinctive that it seems unlikely that it would not have been seen by earlier generations of microscopists at least occasionally. Whether it justifies the term “invasive” is a moot point (see “Pens are too light …”). That would require some evidence of wholesale changes in both the composition of the microscopic flora, and the way it functioned. Personally, I love the diversity of modern London and am prepared to embrace a migrant diatom or two without feeling the need to apply pejorative adjectives.
On the opposite side of the slipway, I noticed a seepage that, again, had a golden-brown hue suggestive of diatoms. However, I could not see any Hydrosera growing amidst this assemblage, instead, there was a mix of Melosira varians and Biddulphia pulchella, a relative of Hydrosera, which lives in brackish water and forms zigzag colonies. Motile diatoms such as Nitzschia sigma glided in and around these filaments. The presence of Melosira varians is interesting. I am fairly sure, based on examination of living cells, that these filaments are not one of the related species of Melosira that are more tolerant of salinity. That a freshwater species with only mild affinity for salt is thriving here supports some observations of higher plants on the slipway made by Nick and Andy from Creekside Discovery Centre. They noticed that these, too, are mostly not especially salt-tolerant plants, leading them to think that the upper slipway represents a freshwater tidal marsh, rather than a salt marsh, and that it is the result of fresh water from the Ravensbourne being lifted above the denser salt water that pushes into Deptford Creek on each tidal cycle. A freshwater species tolerant of some salt rubbing along beside a brackish species that can cope with an occasional dose of freshwater seems like a good metaphor for this part of London where people from every part of the world live side-by-side.
Another seepage beside the slipway at Deptford Creek, this time with a mix of Melosira varians (right) and Biddulphia pulchella (left). The scale bar is 20 micrometres (= 1/50th of a millimetre) long.
Some other highlights from this week:
Wrote this whilst listening to: vintage Jefferson Airplane and, for contrast, Kendrick Lamar’s To Pimp A Butterfly.
Currently reading: The Secret Diaries of Charles Ignatius Sancho by Patterson Joseph, a novel that is, by coincidence, about 18th century migrants in London.
Cultural highlight: Lines, a play at the Crucible in Sheffield exploring injustice in Palestine and Uganda. Rather challenging.
Culinary highlight: Seven course tasting menu at Prashad, an Indian vegetarian restaurant in Drighlington, just south of Leeds