Sammendrag
Cold-water corals (CWCs) are important ecosystem engineers in the deep sea that provide habitat for numerous
species and can form large coral mounds. These mounds influence surrounding currents and induce distinct hy-
drodynamic features, such as internal waves and episodic downwelling events that accelerate transport of organic
matter towards the mounds, supplying the corals with food. To date, research on organic matter distribution at coral
mounds has focussed either on seasonal timescales or has provided single point snapshots. Data on food distribution
at the timescale of a diurnal tidal cycle is currently limited. Here, we integrate physical, biogeochemical, and
biological data throughout the water column and along a transect on the south-eastern slope of Rockall Bank,
Northeast Atlantic Ocean. This transect consisted of 24-h sampling stations at four locations: Bank, Upper slope,
Lower slope, and the Oreo coral mound. We investigated how the organic matter distribution in the water column
along the transect is affected by tidal activity. Repeated CTD casts indicated that the water column above Oreo
mound was more dynamic than above other stations in multiple ways. First, the bottom water showed high vari-
ability in physical parameters and nutrient concentrations, possibly due to the interaction of the tide with the
mound topography. Second, in the surface water a diurnal tidal wave replenished nutrients in the photic zone,
supporting new primary production. Third, above the coral mound an internal wave (200 m amplitude) was
recorded at 400 m depth after the turning of the barotropic tide. After this wave passed, high quality organic matter
was recorded in bottom waters on the mound coinciding with shallow water physical characteristics such as high
oxygen concentration and high temperature. Trophic markers in the benthic community suggest feeding on a va-
riety of food sources, including phytodetritus and zooplankton. We suggest that there are three transport mecha-
nisms that supply food to the CWC ecosystem. First, small phytodetritus particles are transported downwards to the
seafloor by advection from internal waves, supplying high quality organic matter to the CWC reef community.
Second, the shoaling of deeper nutrient-rich water into the surface water layer above the coral mound could
stimulate diatom growth, which form fast-sinking aggregates. Third, evidence from lipid analysis indicates that
zooplankton faecal pellets also enhance supply of organic matter to the reef communities. This study is the first to
report organic matter quality and composition over a tidal cycle at a coral mound and provides evidence that fresh
high-quality organic matter is transported towards a coral reef during a tidal cycle.
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