Intertidal seaweeds are exposed to highly variable, rapidly changing, and potentially stressfully environmental conditions like desiccation, solar irradiance, temperatures, osmotic stress and the availability of nutrients. The brown alga Ascophyllum nodosum (L.) Le Jolis (Phaeophyceae) is one of the dominant mid-intertidal species on Ireland's sheltered rocky shores. It is ecologically important as primary producer, and dense stands provide substrate and habitat for other algae and animals. Recent global warming and therefore increasing seawater temperatures are likely to affect A. nodosum beds in composition, distribution and coverage as its growth rate is sensitive to water temperatures and positively correlates with mean annual seawater temperatures. As photosynthetic performance and productivity of A. nodosum are likely to vary during the year due to by photo period, light intensity, temperatures and CO2-availability, which is dependent on the daily tidal cycles, it is important to assess the natural seasonal growth patterns and monitor the growth rate in situ. In culture experiments the growth at different temperatures is observed to determine the optimum environmental conditions. Primary production and photosynthetic efficiency are measured at varied light intensities, CO2-concentrations and temperatures. These data and the field results will allow predictions of the effects of increasing temperatures on the A. nodosum habitats at the west coast of Ireland. The overall aims of this project are to evaluate the potential for A. nodosum to fulfil a role as biological indicator for global change and to consider its possible role as a carbon sink.