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.

Video Image Enhancement and Machine Learning Pipeline for Underwater Animal Detection and Classification at Cabled Observatories

“…The trough has a diverse topography with sand wave fields of up to 7 m high, 10 to 35 m high ridges, and approximately 20 m high CWC mounds [27]. The CWC mounds are predominantly found in the southeastern part of the trough at a depth of ∼260 m just south of the Vesterålsgrunnen bank, being mostly constituted by CWC Desmophyllum pertusum [28].…”

“…The trough has a diverse topography with sand wave fields of up to 7 m high, 10 to 35 m high ridges, and approximately 20 m high CWC mounds [27]. The CWC mounds are predominantly found in the southeastern part of the trough at a depth of ∼260 m just south of the Vesterålsgrunnen bank, being mostly constituted by CWC Desmophyllum pertusum [28]. The following three platforms compose the data collection system of this area: The X-Frame, which measures water current and biomass in water (with an echosounder); Satellite 1, which collects multiple types of data, such as photos, sound, chlorophyll, turbidity, pressure, temperature, conductivity, etc.…”

A Flexible Autonomous Robotic Observatory Infrastructure for Bentho-Pelagic Monitoring

“…The team aims at continuing the development and utilizing the experiences of partners in ongoing projects at existing observatories (Neptune and LoVe). A combination of acoustics with oceanography as done by Engeland and colleagues [43] demonstrates the strength of coordinated multiparametric sampling in time and space with various sensors, with imaging at the center of that development for ecological monitoring (e.g., [44]). The unique contribution of ARIM is to establish technology and routines for combining imaging techniques of the benthic habitat and acoustic sampling of the pelagic habitat.…”

Integrating Diel Vertical Migrations of Bioluminescent Deep Scattering Layers Into Monitoring Programs

“…Even though they are most commonly limited to depths down to the lower mesopelagic zone (∼1,000 m), DVMs can reportedly extend to several km into abyssal waters (e.g., Natantian decapods in the Mediterranean; . Traditionally, they are captured as anomalies in the acoustic backscatter signal (due to different reflective properties attributed to the physical differences of animal tissue and seawater and the presence of the swim bladder in the case of fish; Marshall, 1951) by either upward-or downward-facing Acoustic Doppler Current Profilers-ADCPs (Flagg and Smith, 1989;Plueddemann and Pinkel, 1989;Heywood et al, 1991;Ochoa et al, 2013;Bozzano et al, 2014;De Leo et al, 2018) or sonars/echosounders (Barham, 1966;Opdal et al, 2008;Benoit-Bird et al, 2017;Giorli et al, 2018;Van Engeland et al, 2019), as well as with trawl and plankton net surveys (Roe, 1984;Fock et al, 2002;Steinberg et al, 2002;Drazen et al, 2011;Darnis and Fortier, 2014). Remarkably, trawl avoidance behavior has been reported for some mesopelagic fish species which adapted their vertical migrating patterns (Kaartvedt et al, 2012), while there is a practically inevitable sampling bias favoring size and robustness in the deep pelagic zone (Craig et al, 2015).…”