At the bottom of a steep bank beside a dual carriageway in Oxfordshire, a tiny, unnamed stream flows between the trees. The A40 thunders overhead but the water here is clear; among the pebbles and crumbs of golden sand on the bottom, a few freshwater mussels are growing. Sticklebacks flick between strands of green water weed. The stream passes through a culvert, emerges from a concrete pipe and flows into Colwell Brook, where almost everything is dead.
Colwell Brook is perhaps six feet across, and for its entire length only one form of life can be seen: Sphaerotilus natans, a type of bacteria that gathers into grey, hair-like filaments a few centimetres long. Sphaerotilus can thrive in water that is low in oxygen and high in organic matter, hence its colloquial name – sewage fungus. It covers the bottom of the brook for hundreds of metres. Beneath the waterline the fronds sway lazily in the gentle current, but when Peter Hammond pulls up a branch they form into dangling, snotty clumps, the kind of stuff you might expect to find in the guts of an elderly dishwasher.
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