The aim of this work is to describe changes in heavy metal concentrations in Ascophyllum nodosum from 1964 to 1994 . Samples were collected from three sites in the Firth of Clyde and analysed for zinc, manganese, iron, copper, lead and nickel . The results were analysed using the multivariate technique Principal Components Analysis (PCA) . At the Wemyss Bay site there was a trend towards increasing lead and nickel over the study period, which could not be accounted for by local industrial activity . At the Hunterston site, two groups were well separated by the PCA ordination, based on manganese and zinc concentrations, which corresponded to land reclamation activities in the area . The separation of samples at the Ardneil Bay site correlated well with copper concentration and this corresponded to the termination of industrial effluent with heavy copper loadings . Other changes in industrial effluent were also reflected in the Hunterston and Ardneil Bay site ordinations . The PCA technique highlighted the interplay between metals . The work demonstrated the potential for using multivariate analysis of seaweed metal concentrations in monitoring a posteriori the environmental impact of industrial change .