Here’s a link to an interesting article in today’s Independent about the state of England’s chalk streams. They are not, says Environment Editor Michael McCarthy, in a very good condition, pointing to the problems of “diffuse pollution” (a catch-all phrase for all the pollutants that find their way into our rivers across the land, rather than via industrial or sewage effluents). Principal amongst these is the run-off of agricultural chemicals, particularly nutrients. This leads, says McCarthy, quoting members of the Salmon and Trout Association, to the growth of “blanket weed”, filamentous algae such as Cladophora glomerata. Blanket weed smothers the gravel on the river bed which, in turn, makes life difficult for the bugs on which the trout and salmon feed.
The River Wylye at Kingston Deverill: a classic example of an English chalk stream. photographed in May 2010.
All this forms a narrative that has been repeated many times by river users and environmental scientists, including many in the Environment Agency. The condition even has a name: Chalk Stream Malaise. The problem is that there is no “smoking gun” to link the run-off to the decline of salmon and trout beyond reasonable doubt. I had to research the literature on this as part of a study a few years back and was surprised about how little hard evidence I could find. The circumstantial evidence is strong, so long as you cast your net wider than just English chalk streams, but I could not find one paper in a peer-reviewed literature that demonstrated an unambiguous link between nutrients and blanket weed, or between blanket weed and salmon and trout populations, specifically in chalk streams. The reality is that there is much else happening in the surrounding catchments, including over-abstraction by water companies, which also affects stream ecology. Nor are nutrients the sole consequence of increased agricultural production: silt and pesticides also enter the rivers and have their own effects, all of which are difficult to disentangle from one another. The result is a plot of interweaving motives and alibis that would make Hercule Poirot blanch.
See also streams of consciousness, my post from 21 August.