I spent this afternoon mooching around an old landfill site in Coxhoe, about four kilometres from my house, and a similar distance from the pond at Cassop that I visited in January. The site lies at the foot of the Magnesian limestone escarpment that runs roughly north-south through County Durham. Because of this rock’s value as an aggregate in road-building and the construction industry, this part of Durham is pot-marked with quarries and, as the quarries are exhausted, so the holes left behind become landfill sites, slowly filling up with the detritus of twenty-first century consumer culture. The point where I was standing was a landfill site until about ten years ago, since when it has been capped off and the lorries trundle, instead, to an even larger hole in the ground on the other side of the B6291.
Meanwhile, County Durham’s seemingly incessant rain falls onto the closed landfill site, percolates through the topsoil and then through the layers of waste, gradually becoming polluted as it does so. Some of this water stays underground as groundwater, a portion appears at the surface, either collecting in lagoons dotted around the edge or forming small springs and seepages. I was exploring the site this afternoon with two students who were wondering whether the organisms that lived in these habitats would make a good subject for a dissertation.
We followed a tiny stream down from where it collected the overflow from two settlement lagoons down to the point where it joined a natural stream in the valley bottom. The pebbles on the stream bed here had dark brown, almost black, patches of diatoms that are very common at this time of year. What was interesting was that these patches of diatoms disappeared as soon as the water from the tiny stream bearing the landfill’s effluent joined the natural stream in the valley bottom. And these dark patches were not obvious on the stones in the tiny stream either.
Growths of diatoms – principally Navicula lanceolata – growing on submerged pebbles in a small stream draining Crow Trees Local Nature Reserve, Coxhoe, County Durham in March 2013.
These growths are sufficiently common in streams at this time of year that I could make a fairly confident guess about what to expect when I put a small sample under the microscope. And I was not disappointed: a mass of tiny boat-shaped cells all moving about in irregular patterns across the slide. The majority belong to a species called Navicula lanceolata, though there were also several belonging to a smaller species, Navicula gregaria. I find these particularly in rivers in great abundance at this time of year – so long as the water is neither very soft nor acidic, they will grow across a wide range of conditions, from clean to fairly polluted.
Navicula lanceolata (along with a couple of cells of Navicula gregaria) from the stream draining Crow Trees Local Nature Reserve. The largest cells are about 60 micrometres long.
I was intrigued by the response of the students when they looked down the microscope. Perhaps their surprise is the more natural reaction. Having first approached algae via botany courses, I was conditioned to think of these non-green moving organisms as “plants”. Their brown colour is easy enough to rationalise when you know that they are related to the seaweeds on our coasts. Their movement is less easy to explain, though all of us are familiar with higher plants growing towards the light. The movement we could see under the microscope is just another response to a stimulus, albeit in a single-celled, rather than multicellular, organism. I was actually pleased to be able to share their enthusiasm for a few minutes. I spend too long staring at computer screens where the vitality of the microscopic world is abstracted to numbers and it is therapeutic to reconnect with the living organisms for a few minutes.