River Coquet, Northumberland

coquet_location This series of pictures comes from the River Coquet high in the Pennines, 390 metres above sea level and just one and a half kilometres from the Scottish border. The River Coquet here is only about a metre across. It tumbles off the volcanic rocks that underlie this part of the Pennines – the Cheviots – as a series of rocky riffles and slower-flowing pools, gradually increasing in size as its slope decreases and it meanders through one of the most beautiful and unspoilt valleys in northern England before joining the North Sea at Amble. There is just one small market town, Rothbury, about 30 km downstream from where I stand and, otherwise, just small villages, hamlets and farmland. So many walkers visiting this region have been surprised to see what looks like raw sewage floating down the river at certain times of the year.

Coquet Didymosphenia Crouching down beside a tiny tributary stream, only half a metre or so across, I can see the culprit almost straight away Lifting out one of the larger stones from the stream bed, I see tufts of what look like soggy brown cotton wool. There are points further downstream when these growths almost blanket the river bed in the summer, and you can easily imagine that some of these, sheared from their substrate during a spate, could be mistaken for raw sewage as they drifted down.

Coquet Didymosphenia When I look down the microscope, what I can see is a series of almost colourless, sometimes branched stalks criss-crossing the field of view. Just occasionally, I can see a diatom which, at just over a tenth of a millmetre long, is huge by the standards of most of the diatoms described on these pages. The whole entity is 90 per cent or more stalk, pushing the cells themselves – which belong to a species called Didymosphenia geminata – up towards the sunlight.

Coquet Didymosphenia So the complete colony of Didymosphenia - the tufts of soggy cotton wool mentioned above - looks something like this, except that the stalks have many smaller diatoms living on them as epiphytes. These tufts form dense "copses" on the stone surfaces, with open areas in between, heavily grazed by tiny Ancylidae snails.

Coquet Didymosphenia Finally, a view into a Didymosphenia colony, synthesising all these observations: the forest of branched stalks, with each branch topped by a Didymosphenia cell, and smothered by short-stalked cells of Achnanthidium minutissimum and other epiphytes.