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1291, the only natural radioactive isotope of iodine (Tu2 = 15.Tx106a), has been artificially produced since the end of the forties by the different nuclear, civil as well as military, activities leading to an increase of the background levels especially in the Northern hemisphere. In the marine environment, seaweed is known to accumulate iodine isotopes at high concentration levels. 16 seaweed species were collected at Goury (France), under the direct influence of discharges of authorised low-level radioactive liquid effluents of La Hague nuclear fuel reprocessing plant, in October 1998 and March 1999. 1291/1271 ratios, computed with stable iodine concentration and 1291 activities measured on each sample, are significantly higher than the background levels. The heterogeneity of 1291 levels with seaweed species is studied as well as temporal variations that are related to the data of the 129I liquid effluents released by the La Hague NRP. Concentration factors, estimated for all seaweed, agree with the recommended values and the 1291 activity in seawater was evaluated and shows a variation factor of 1.7 between the two sampling periods.