For the first time this year, I heard Castle Eden Burn before I saw it. Walking down from the car park, the distant roar of water was apparent almost as soon as the canopy of largely leafless branches closed over me. A few trees still held their leaves – spectacularly golden on beech and birch, in particular, and the Dene’s famous yews were still green, of course – but the forest was dressed for winter now, much as it was on my first visit this year, back in January (see “Castle Eden Dene in January”). Then, I was surprised that there was no water in the Burn. On this trip, however, I wore my chest waders. Back in August, I had compared Castle Eden Burn to a wadi (see “The presence of absence in Castle Eden Dene”) so the heavy rain of the previous few weeks had led me to suspect that today would be different.
The water surging through the Dene was very turbid, so collecting stones to examine involved feeling around on the river bed with my hand until I located one that was not sufficiently bedded into the substratum to remove. That’s not ideal, but needs must and I got the five cobbles I needed, each with a distinct biofilm, slimy to the touch. This is the first time, after eleven months, that Castle Eden Burn’s substratum has looked and felt remotely like the substratum from most of the other rivers I know in this part of the world.
Under the microscope, I see lots of particulate matter but also plenty of algae. Apart from a few filaments of the cyanobacterium Phormidium, these were mostly diatoms. The green algae I described in “When the going gets tough …” back in May were not obvious. The diatoms were mostly largely motile cells of Navicula, with a few sigmoid cells of Nitzschia clausii and some smaller cells whose identity I will need to confirm once I have cleaned the sample and prepared a permanent slide. The Navicula species, in particular, are typical inhabitants of local rivers during winter and early spring, all tolerant to a wide range of conditions. I suspect that the rainfall has washed a lot of fine particulate debris from the industrial estates in the upper catchment into the river, and these diatoms will have the resilience to cope with such types of pollution. A large storm sewer overflow also empties into the burn about a kilometre upstream of where I was standing and this, I suspect, has been flowing over the past month or two.
I also saw a few cells of Achnanthidium minutissimum, which I generally associate with cleaner conditions. I suspect, however, that numbers will be relatively low compared to its more pollution-tolerant brethren. Again, I can give a more authoritative answer once I have cleaned the sample and performed a full analysis.
Diatoms from Castle Eden Burn, November 2019. a., b.: Navicula trpunctata; c. – e.: Navicula lanceolata; f., g.: Rhoicosphenia abbreviata; h., i.: Nitzschia clausii; j., k.: Navicula gregaria; l. Achnanthidium minutissimum. Scale bar: 10 micrometres (= 1/100thof a millimetre). The photograph at the top of the post shows Castle Eden Burn just downstream from the point I sampled.
I originally set out to visit Castle Eden Burn six times during 2019 and this was the last of those. I’ve written about most of these visits already but not about my September visit. There was, on that occasion, little new information to justify a separate post but I will include the sample I collected in my final overview of the algae of Castle Eden Burn, just as soon as I get this final sample cleaned and analysed. Before then, I have one more post to write about the diatoms, based on some more detailed observations of a few of the species, and then it will be time to think about where to focus my observations during 2020.