Environmental changes, such as ocean acidification and eutrophication, have created threats to kelp mariculture. In this study, the growth, photosynthesis, respiration and nutrient composition of Saccharina japonica were evaluated at different levels of pCO 2 (400 and 800 µL L –1) and nutrients (nutrient-enriched and non-enriched seawater). Elevated pCO 2 decreased the relative growth rate (RGR), net photosynthetic rate and contents of tissue carbon and tissue nitrogen under non-enriched nutrient conditions, but it had no significant effect on these parameters under nutrient-enriched conditions. The dark respiration rate was positively affected by elevated pCO 2 regardless of the nutrient conditions. However, the C:N was unaffected by elevated pCO 2 at both nutrient levels. These results implied that ocean acidification could reduce the production and nutrient contents in the tissues of S. japonica, which was associated with nutrient conditions.