Particle flux and food supply to a seamount cold-water coral community (Galicia Bank, NW Spain)

Abstract

In a European effort (Atlantic Coral Ecosystem Study [ACES project]) to determine the key conditions for the occurrence of cold-water corals on the NW Atlantic margin, a coral community on a seamount off NW Spain (Galicia Bank) was studied. Cold-water corals (Lophelia pertusa, Madrepora oculata) grow at a depth of ∼800 m as isolated patches amidst small ripples and larger waves of foraminiferal sand. A 17 mo deployment of a near-bottom sediment trap revealed a large seasonal and annual variability in the flux of phytodetritus and carbon. The daily carbon flux in the first 5 mo of 2000 was on average 37 mgC m -2, and in the first 5 mo of 2001 17 mg C m-2. Quantities of faecal pellets and swimmers (copepods, amphipods) were also highly variable. A comparison between the daily carbon flux and the sediment carbon oxidation rate calculated from in situ community oxygen consumption (SCOC), i.e. 17 versus 7 mg C m-2 d-1 respectively, indicated that a surplus of carbon is not oxidised by the sediment community. We argue that the strong tidal currents (max. 30 cm s-1) and the mobile sediment lead to winnowing of the sediment and to near-bed transport of the organic material. The low biomass of the benthic community and the domination of filter-feeders support our arguments. By means of analyses of stable isotopes (δ15N, δ13C) we attempted to find potential food items of the cold-water corals among the particles collected in the sediment trap. The difference between the δ15N signatures of coral tissue (9.5‰) and phytodetritus (2.2‰) was >1 trophic level, indicating that sinking algae are not the sole food source. The δ15N signatures of swimmers caught in the trap, ∼10‰, were on the other hand in the same range as those of the corals. A mixed diet of animals and algae could be one explanation for the observed δ15N of corals. The δ13C value of the coral tissue, -20.55‰, excludes a food source consisting of bacteria supported by methane seepage, as suggested by earlier studies.