Pigment contents and photosynthetic parameters of different regions of thalli of Ascophyllum nodosum (L.) Le Jolis collected from Strangford Lough, Irish Sea, and Helgoland, North Sea, were examined on a seasonal basis and after exposure to various irradiance and nutrient treatments in the laboratory. In culture, pigment contents of young plant parts acclimated rapidly to changes in irradiance and increased after nutrient enrichment in all irradiances. Chlorophyll a levels in plant tips in situ increased during autumn and winter and were reduced by 50% during summer. In older plant parts, which were subject to less seasonal variation in irradiance and were more shaded at both high and low tide, chlorophyll a levels were higher than in the tips at all times of the year. Older plant material from the lower shore contained higher pigment levels than corresponding material from the upper shore. Fucoxanthin: chlorophyll a ratios peaked during summer in epical plant parts from the upper shore and increased in culture with irradiance and with length of daily exposure to high irradiance. Variations of pigment concentration within plants and between shore levels were probably due to differences in irradiance between microhabitats caused by water depth and self-shading and enhanced by nutrient limitation in summer. Maximum net photosynthesis (P-max) of young plant parts was higher than that of older plant parts and increased with temperature to a maximum at 20 degrees C, in contrast to older plant parts which had low rates at all temperatures. P-max was highest in tips in all seasons and decreased with distance from the tip along the thallus. Older, basal thallus parts exhibited shade acclimation with high pigment content and high photosynthetic rates at low irradiances, in contrast to apical tips, which were acclimated to high light. The observed decrease in P-max of plants tips collected in summer from the upper shore may have been caused by photoinhibition.