The story so far: in 2018 I made bi-monthly visits to the River Wear, my local river and tried to capture, in my posts, the changes in the algae that occurred over the course of 12 months (follow the links in “A year in the life of the River Wear” to learn more). It was an interesting exercise, partly because last summer’s exceptional weather led to some intriguing changes over the course of the year. Consequently, as 2019 dawned, I thought I should find a different type of stream within a short drive from my home and try again. So, bearing in mind that Wolsingham is south and west from where I live, I turned in the opposite direction and drove due east instead, stopping on the edge of the brutal concrete housing estates of Peterlee, a most unprepossessing location for a National Nature Reserve.
My journey has brought me right across the Permian limestone that dominates the eastern Durham landscape. Its escarpment rises up close to my home, and I have written about the algae that live in the ponds at the foot of it (see “A hitchhiker’s guide to algae…”). On the other side, however, the limestone ends in a series of cliffs overlooking the North Sea and small streams have cut into the limestone to create a series of wooded valleys, or “denes”. I’ve come to Castle Eden Dene, the largest of these: if you want a cultural reference point, watch the film “Billy Elliott”, set just a few miles further north along the coast, or read Barry Unsworth’s The Quality of Mercy.
We made our way down the footpath into the dene on a crisp and very cold winter morning, past the old yew trees from which the name is derived, and myriad ferns. A deer bounded across the path ahead and disappeared into some scrub, and then we turned a corner and looked into Castle Eden Burn, which runs along the bottom of the dene. To my surprise, the stream was dry. This is a valley that cuts through limestone, so it is common for the stream to be dry in the summer, but I had not expected it to be dry in the middle of winter. Thinking back, however, I realised that there has not been much rain for some weeks, and this may have meant that the water table, still low, perhaps, after last summer’s dry weather, is too low for the stream to flow.
Diatoms and cyanobacterial colonies in Blunt’s Burn, Castle Eden Dene, January 2019. The top photograph shows diatom growths on bedrock; the lower image shows Phormidium retzii colonies, each about two millimetres across. The photograph at the top of the post shows a yew tree overhanging Castle Eden Burn.
A few hundred metres further down the dene, we finally heard the sound of running water where a small tributary stream, Blunt’s Burn, joined the main burn. Judging from my OS map, it drains a good part of Peterlee so it might not have very high water quality. It was, however, a stream and it did, as I could see with the naked eye, have some distinct diatom-rich growths. These, I discovered later, were dominated by the diatoms such as Navicula tripunctataand N. lanceolata which are typical of cold weather conditions (see, for example, “The River Wear in January”). A closer look showed that the orange-brown diatom growths were, in places, flecked with dark brown spots. Somehow, I managed to get my cold fingers to manipulate a pair of forceps and pick up a few of these spots for closer examination.
Diatoms from Blunt’s Burn, January 2019: a. Navicula tripunctata; b. N. lanceolata; c.Gyrosigma cf. acuminatum; d. Nitzschiacf. linearis (girdle view); e. N. linearis(valve view). Scale bar: 10 micrometres (= 1/100thof a millimetre).
I had a good idea, when I first saw these spots, that they were colonies of a filamentous cyanobacterium and, peering through my microscope a few hours later, once I had warmed myself up, I was relieved to see that I was right. I picked out a dark patch and teased it apart before putting it onto a slide with a drop of water. Once I had done this, I could see the tangle of filaments along with a mass of organic and inorganic particles and lots of diatoms. The filaments themselves were simple chains of cells (a “trichome”) of Phormidium retzii, surrounded by a sheath. There were also, however, a few cases, where I could see the sheath without the Phormidium trichome, and in some those I could also see diatom cells.
There are some diatoms that make their own mucilage tubes (see “An excuse for a crab sandwich, really …”) but Nitzschia is not one of those most often associated with tube-formation (there are a few exceptions). On the other hand, there are some references to Nitzschiacells squatting in tubes made by other diatoms. Some of those who have observed this refer to Nitzschia as a “symbiont” but whether there is any formal arrangement or is just a by-product of Nitzschia’s ability to glide and seek out favourable microhabitats, is not clear. There are, as far as I can see, no references, to diatoms inhabiting the sheaths of Cyanobacteria, though Brian Whitton tells me he has occasionally seen this too.
We made our way back along the dry bed of Castle Eden Burn. Many of the rocks here were quite slippery, suggesting that there had been some water flowing along it in the recent past. That encouraged me to scrub at the top surface of one with my toothbrush and I managed to get a sample that certainly contains diatoms though these were mostly smaller than the ones that I found in Blunt’s Burn, and there was also a lot of mineral matter. I’ll need to get that sample prepped and a permanent slide prepared before I can report back on just what diatoms thrive in this tough habitat. Watch this space …
Cyanobacterial filaments from Blunt’s Burn, Co. Durham, January 2019: a. a single trichome of Phormidium retzii; b. and c. empty sheaths colonised by cells of Nitzschia; d. aPhormidiumfilament with a sheath and a trichome but also with epiphytes and adsorbed organic and inorganic matter. Scale bar: 10 micrometres (= 1/100thof a millimetre).
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