Cabled ocean observatory data reveal food supply mechanisms to a cold-water coral reef

Abstract
We investigated food supply mechanisms to a cold-water coral (CWC) reef at 260 m depth on the
Norwegian continental shelf using data from a cabled ocean observatory equipped with Acoustic
Doppler Current Profilers (ADCPs), an echosounder, and sensors for chlorophyll, turbidity and
hydrography in the benthic boundary layer (BBL). Tidal currents of up to tens of cm s-1 dominated
BBL hydrodynamics while residual currents were weak (~10 cm s-1), emphasizing a supply and high
retention of locally produced phytodetritus within the trough. A direct connection between the reefs
and surface organic matter (OM) was established by turbulent mixing and passive particle settling, but
relative contributions varied seasonally. Fresh OM from a spring-bloom was quickly mixed into the
BBL, but temperature stratification in summer reduced the surface-to-bottom connectivity and reduced
the phytodetritus supply. A qualitative comparison among acoustic backscatter in the ADCPs (600
kHz, 190 kHz) and echosounder (70 kHz) suggests that vertically migrating zooplankton may present
an alternative food source in summer. Nocturnal feeding by zooplankton in the upper water column
sustains downward OM transport independent from water column mixing and may dominate as food
supply pathway over sedimentation of the phytodetritus, especially during stratified conditions. In
addition, it could present a concentrating mechanism for nutritional components as compensation for
the deteriorating phytodetritus quality. Overall, the observed patterns suggest seasonal changes in the
food supply pathways to the reef communities. The moderating role of temperature stratification in
phytodetritus transport suggests stronger dependence of the cold-water corals on zooplankton for their
dietary requirements with increased stratification under future climate scenarios. This study
demonstrates the added value of permanent ocean observatories to research based on dedicated
campaigns and regular monitoring.