van Engeland, Tom
Godø, Olav Rune
Johnsen, Espen
Duineveld, Gerard C. A.
van Oevelen, Dick
2019-02-06
<p>Abstract<br>
We investigated food supply mechanisms to a cold-water coral (CWC) reef at 260 m depth on the<br>
Norwegian continental shelf using data from a cabled ocean observatory equipped with Acoustic<br>
Doppler Current Profilers (ADCPs), an echosounder, and sensors for chlorophyll, turbidity and<br>
hydrography in the benthic boundary layer (BBL). Tidal currents of up to tens of cm s-1 dominated<br>
BBL hydrodynamics while residual currents were weak (~10 cm s-1), emphasizing a supply and high<br>
retention of locally produced phytodetritus within the trough. A direct connection between the reefs<br>
and surface organic matter (OM) was established by turbulent mixing and passive particle settling, but<br>
relative contributions varied seasonally. Fresh OM from a spring-bloom was quickly mixed into the<br>
BBL, but temperature stratification in summer reduced the surface-to-bottom connectivity and reduced<br>
the phytodetritus supply. A qualitative comparison among acoustic backscatter in the ADCPs (600<br>
kHz, 190 kHz) and echosounder (70 kHz) suggests that vertically migrating zooplankton may present<br>
an alternative food source in summer. Nocturnal feeding by zooplankton in the upper water column<br>
sustains downward OM transport independent from water column mixing and may dominate as food<br>
supply pathway over sedimentation of the phytodetritus, especially during stratified conditions. In<br>
addition, it could present a concentrating mechanism for nutritional components as compensation for<br>
the deteriorating phytodetritus quality. Overall, the observed patterns suggest seasonal changes in the<br>
food supply pathways to the reef communities. The moderating role of temperature stratification in<br>
phytodetritus transport suggests stronger dependence of the cold-water corals on zooplankton for their<br>
dietary requirements with increased stratification under future climate scenarios. This study<br>
demonstrates the added value of permanent ocean observatories to research based on dedicated<br>
campaigns and regular monitoring.</p>
https://doi.org/10.1016/j.pocean.2019.01.007
oai:zenodo.org:2558111
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Cabled ocean observatory data reveal food supply mechanisms to a cold-water coral reef
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