Seasonal controls on the diet, metabolic activity, tissue reserves and growth of the cold-water coral Lophelia pertusa

Vast cold-water coral (CWC) reefs occur in temperate regions, where strong seasonality in temperature and light leads to a short but highly productive spring period. How CWCs respond physiologically to this strong seasonal forcing remains unclear, due to the remoteness of their deep-sea habitats. In an in situ transplantation study at Nakken reef, Norway, we investigated a full seasonal cycle of (1) temperature and food availability, (2) diet, (3) biomass and tissue reserves, (4) oxygen consumption and (5) linear growth of the reef-building coral Lophelia pertusa. All investigated variables showed a distinct seasonality. An increase in the organic carbon and amino acid content, linear extension and budding rate from February to late May, at a simultaneous increase in phytoplankton and zooplankton fatty acid trophic markers (FATMs), and δ15N-derived trophic level, indicates an efficient exploitation of the spring phytoplankton and the subsequent zooplankton bloom. A pool of neutral-lipid-derived fatty acids, indicative of energy storage and gametogenesis, was formed from May to October, accompanied by increased oxygen consumption, i.e. metabolic activity. In late autumn and early winter (October–December), tissue reserves were maintained, in spite of low sPOM and zooplankton food availability, and the lower tissue δ13C and higher contribution of bacterial FATMs suggest increased reliance on more degraded material. The concurrent reduction in linear growth further suggests a lower energy availability at this time of the year. A large (> 50%) drop of all tissue pools between December and February coincided with the spawning season of L. pertusa and demonstrates a high energetic cost of reproduction. Our results show for the first time a strong seasonal control of critical life history traits such as growth patterns and timing of reproduction in this prominent deep-sea species.