This method has been designed primarily to detect the impact of toxic substances on the distribution of certain species of macroalgae (seaweeds belonging to the group known as 'wracks', and principally members of the genus 'Fucus''). This method monitors the upstream limit of three brown seaweed species; horned wrack, Fucus ceranoides, spiral wrack, Fucus spiralis and bladderwrack, Fucus vesiculosus. It works on the basis that man-made disturbances can limit the upstream colonisation of estuarine habitats by these species of seaweed. It also takes into account variations of fucoid penetration of the estuary owing to natural factors such as the ranges of salinity and turbidity. With little toxic stress they can penetrate almost to the freshwater inflows.
The method statement published in 2008 for the first river basin planning cycle has now been updated for the second cycle, linked below.