The River Browney does not give up it’s secrets gracefully. To reach the lower stretches of this tributary of the River Wear, just a few kilometres outside Durham City I had to push through thick growths of Himalayan Balsam, stinging nettles, brambles and what looked suspiciously like Giant Hogweed. The bankside luxuriance continued in the river itself, the bed of which is almost completely covered with either submerged water crowfoot or algae. The river has wound its way down from the foothills of the Pennines, collecting the wastewater from small towns and, just a couple of kilometres upstream from where I stand, from a sewage works serving a large village on the outskirts of Durham itself. The algae and plants all thrive in the steady supply of dilute manure that these works provide.
The River Browney at Low Burnhall Nature Reserve, just below the A167 Bridge. Photographed in September 2013.
Many of the stones in the margins were coated with brownish filaments, waving gently in the current. When I pick up one of the stones, these filaments collapsed into amorphous slimy masses but, under the microscope, they resolved themselves into a tangle of chains of algal cells. Two types predominated, both with the yellow-brown colouration typical of diatoms. The most abundant of these was chains of cylinder-shaped cells. This was Melosira varians, a very common diatom in nutrient-rich rivers and which often forms these long brown streamers during periods of low flow during the summer. The other type of cell was cigar-shaped when seen from above (as in fig. d, below) but rectangular when seen from the side (as in fig. c.). These cells were mostly joined at the corners to form zig-zag chains. You can also see, in fig. d., the transverse ribs of silica which are characteristic of this genus of diatoms.
Filamentous growths of diatoms on stones in the River Browney, County Durham, September 2013 and (inset) one of the growths on a stone removed from the stream bed.
Microscopic views of diatoms (and a few desmids) from the River Browney, September 2013. a. low power view of the filaments; b. part of a chain of cells of Melosira varians photographed at high magnification. Note the large number of small brown chloroplasts inside each cell (scale bar: 10 micrometres = 1/100th of a millimetre); c. zig-zag chain of Diatoma vulgare photographed at medium magnification; d. a single cell of D. vulgare at high magnification (scale bar as for b.)
These masses of cells are the microscopic equivalent of the bankside vegetation that I had to push through in order to reach the river in the first place. The surprise was that I did not find a large number of insect larvae feasting on this abundance, as I have described from cleaner rivers such as the River Ehen. The answer, suggested by some recent papers, is that the normal relationship between algae and their grazers breaks down in these enriched rivers. The dense diatom growths can suck the oxygen out of the water at night when there is no sun to generate photosynthesis so the insects that would normally be feeding on the algae cannot survive. This, in turn, will reduce the food supply of fish, as well as smothering the areas where they would normally lay their eggs. We see, in other words, a “domino effect” as the consequences of artificially high nutrients clatter through the different groups of organisms leading, in some cases, to consequences for the way in which we are able to use these ecosystems.