When I was at the Royal Botanic Gardens in Edinburgh last week, I was shown a microscope slide collected in the 19th century from a location recorded as ‘Jarrow Slake’. As Jarrow is just a few miles from where I live, my interest was piqued (which was probably why I was shown the slide in the first place).
Jarrow is a place name that resonates through the history of north-east England for over a thousand years, famous for its links with the Venerable Bede, the Saxon monk who wrote the first history of England. In the twentieth century, it is remembered as the starting point for the Jarrow March, a pivotal moment in the history of the Labour movement. It is located on the south side of the River Tyne, between Gateshead and South Shields and ‘Jarrow Slakes’ was the name for the expanse of intertidal mud to the east of the monastery. Actually, the name ‘Jarrow’ is, itself, derived from the Old English word for mud or marsh, though there is far less mud now as the historical Slake was reclaimed in 1972 and is now part of the Tyne Dock complex (more at “King Ecgfrith’s Port”) . One of the Slakes most gruesome claims to fame is that in 1832 it was the location for the last public gibbeting in England.
Jarrow Slakes, photographed from close to Bede’s World (NZ 338 657) in April 2014.
The Slakes had a local reputation as an area of quicksands which, after a couple of exploratory forays onto the soft mud at low tide, seemed very plausible. As a result, I confined my explorations to the upper part of the remaining mudflats in the area at the mouth of the River Don between St Paul’s church and the Slakes. I soon found what I was looking for: patches on the mud which had a distinct chocolate-brown hue, which I knew from previous experience to be teeming with diatoms.
A patch of diatoms on the upper intertidal mud at Jarrow Slakes, close to the position from which the earlier photograph was taken.
I don’t know enough about brackish and marine diatoms to be able to put names on all the diatoms that I saw in this sample. Many I recognised as species of Navicula, a large genus which is also common in freshwaters but some were less familiar to me. One that particularly entranced me was Bacillaria paxillifer although it’s rapid yet graceful movements were impossible to capture with my camera. The cells are all attached to one another by the raphe and slide along the length of each other in a manner similar to the movement of a slide rule. At one extreme, they form long chains, the cells attached only by the tips (the moment I caught with my photograph). The next moment, they all slide in unison to compress the chain into a ribbon of almost parallel cells. But this happens so quickly that there was no time to refocus and take a photograph before they are sliding along each other again back into an extended chain.
Diatoms from the upper intertidal mud at Jarrow Slakes, April 2014. A., b. and c. are species of Navicula (c. is probably N. phyllepta); d. is Amphora, possibly A. hyalina; e. is a Diploneis sp.; f. is Bacillaria paxillifer and g. is probably Paralia sulcata.
These diatoms are more than just a curiosity. Many of these diatoms produce extracellular polysaccharides for various purposes, including movement. These, in turn, create a sticky matrix which helps to bind the mud and sand together to create a more stable substratum into which other plants can colonise. These plants further consolidate the sediments, making them more resistant to erosion. Slowly, over time, the entire shape of estuaries can change. And all because of these small chocolate-brown patches on the mud.