Ascophyllum nodosum, an abundant Irish brown seaweed, shows significant seasonal variation in chemical composition and biogas production. The polyphenol content is shown to be a more important factor in biogas production than ash content. High polyphenol content in summer months adversely affected biogas production; suggesting two potential harvest dates, March and October. A. nodosum harvested in October showed a relatively low level of polyphenols (2% of TS) and ash (23% of volatile solids), and exhibited a specific methane yield of 215 L CH4 kg VS (1), which was 44% of theoretical yield. The highest yield per wet weight of 47 m(3) CH4 t (1) was achieved in October, which is 2.9 times higher than the lowest value (16 m(3) CH4 t (1)), obtained in December. The gross energy yield of A. nodosum based on the optimal biogas production can achieve 116 GJ ha (1) yr (1) in October. (C) 2016 Elsevier Ltd. All rights reserved.