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Montrose Alpha oil platform was installed in the North Sea during August 1975. Samples and photographs of marine growth on the platform jacket have been analysed since 1977. Marine growth is depth related, with particular organisms or group of organisms dominant at various depths. Seaweeds dominate the sunlit surfaces extending down to a depth of -10m. On shaded surfaces in the sunlit zone, arborescent bryozoans cover most of the substrate and continue down to -31 m. In the range -31 m to -51 nl calcareous bryozoans cover large areas of the available substrate but hydroids are also common. Between -51 m and -71 m, encrusting bryozoans dominate the community but below -71 m the aggregate tubeworm Filograna implexa and deep-water barnacle Balanus hameriare common. Changes have taken place in the dominant species over the years. There have been relatively rapid changes in the depth range M.L.W. to -31 m and a more gradual change in the mid-water levels. Reasons for these changes are discussed. Marine growth on Montrose is compared with that on contemporary jackets in the Forties field, 40 km to the north-west. Causes for the observed differences are discussed.

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