| Summary: | The foundation species Ascophyllum nodosum is an important intertidal producer in temperate regions of the North Atlantic. It provides habitat for many species and is an essential facilitator of biodiversity. This study characterised in detail productivity, canopy composition and density, biomass, morphology and recovery from harvesting of different A. nodosum populations on the Irish west coast. Further, potential ecophysiological responses of Irish A. nodosum stands due to predicted environmental change were assessed. Field observations revealed significant high seasonal and spatial variation in environmental parameters in situ and corresponding ecophysiological characteristics of A. nodosum. Even small divergences in environmental factors impacted on growth, canopy, density, biomass, morphology, patchiness and harvesting recovery between and within populations, which had a significant effect on overall A. nodosum productivity. Responses to single and combined environmental stressors were further examined in laboratory experiments; also potential effects of modified environmental parameters (UV-radiation, PAR intensities, nitrate, temperature and CO2) were investigated. Further, photosynthetic characteristics of A. nodosum were assessed measuring PAM-fluorescence and CO2-exchange rates. Results highlight a high spatial and seasonal variability in A. nodosum productivity and indicate acclimatisation of different populations to environmental conditions in situ. Pigment concentrations in A. nodosum were linked to seasonal and spatial variations in photosynthetic performance. Higher photosynthetic efficiency was observed at elevated temperatures and increased CO2-exchange rates at higher CO2-levels; these findings would indicate an increase in A. nodosum productivity under predicted climate change scenarios. Results of this study suggest that productivity of Irish A. nodosum populations is highly variably, both temporally and spatially, and outline population-specific responses to environmental modifications such ...
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