In laboratory experiments, copper concentrations in plants of Ascophyllum nodosum (L.) Le Jolis (Fucales, Phaeophyta) increased with the concentrations in the culture media and were highest in younger,meristematic thallus parts. After initial accumulation in high-copper medium and subsequent transfer to clean seawater for 5 days, no release of copper could be detected. Iron concentrations in A. nodosum tissue were not related to concentrations in the culture medium. Differences between copper concentrations in plants from different sites in areas with high yachting activity in Strangford Lough, Northern Ireland, could be explained by differences in water motion and human activity, in particular the application of copper-releasing antifouling paints to leisure boats. Iron concentrations were also highest in plants from the sheltered, polluted site but did not differ significantly between the other two sites. No differences in copper nor iron concentrations were found between different-aged thallus parts of plants from any site. X-ray microanalysis revealed that most of the iron detected was located in epiphytic pennate diatoms on the A. nodosum surface. In thallus areas without diatoms, iron levels were below the detection limit for X-ray microanalysis. Mapping for copper indicated that most of the accumulated copper was located in cells near and immediately below the thallus surface. "Epidermis"-shedding occurred in plants from the culture experiments and also in freshly-collected material and may have resulted in a loss of metal ions accumulated by surface cells and by epiphytic diatoms. The results suggest that A. nodosum could be used as a biological indicator for copper but not for iron, and that young, apical plant parts are most sensitive to changes in metal concentrations in the water. (C) 2000 Elsevier Science BN. All rights reserved.