Trophic structure of a cold-water coral mound community (Rockall Bank, NE Atlantic) in relation to the near-bottom particle supply and current regime

On the SE slope of Rockall Bank, cold-water corals form dense aggregations on the top of elevated mounds supposedly because mounds give rise to topographically accelerated currents and thus enhanced particle supply. In 2005, a study was made of the trophic structure of a Rockall coral mound community by means of N-15 stable isotope signatures. Simultaneously near-bottom current speed, turbidity, and temperature were measured on and off the mound to search for links between the predominant feeding mode of the coral community, the particle supply, and the physical factors governing the supply. The range of delta N-15 in the coral community was small in comparison to other deep habitats due to the absence of deposit-feeders. The delta N-15 of corals was very close to those of obligate filter-feeders (tunicate, bivalves) indicating that corals assimilate similar types of particles as these filter-feeders. Benthic lander deployments on the mound and in the adjacent gully and plains showed that currents were highest off mound. No major differences were found in near-bottom turbidity between the habitats at the time of the cruise. An 11-mo lander deployment revealed an extended supply of fluorescent particles to the mound community between early February and July 2005. Particle supply to the corals varied daily with higher concentrations associated with relatively warm and saline water flowing down slope. The collected data point to a simplified food web in the coral community sustained by an advection of fresh particles derived from production higher on the bank. The typical distribution of cold-water corals capping the mounds cannot be explained simply by enhanced currents relative to the adjacent plain and gully.