Z2: The ecological role of gelatinous zooplankton predators. Vertical
distribution, feeding and trophic interactions
Principal investigators: Marsh Youngbluth, Harbor Branch Oceanographic Institute,
Florida; Ulf Båmstedt, University of Bergen, Norway
Other participants: Fransesc Pagès, Jenny Purcell
Study objects: Siphonophores, ctenophores, medusae.
Hypotheses: (1) Gelatinous zooplankton are major predators in pelagic food webs
near the MAR. (2) The swimming behaviour and external morphology defines the
predation efficiency of a given species. (3) Different species of gelatinous predators
utilise different trophic nisches. (4) The metabolic rate of deepwater gelatinous
species is lower than near-surface species at similar temperature.
Aims: Defining the vertical distribution in relation to hydrography, light and biotic
properties of the environment. Describing swimming mode and feeding behaviour,
and quantify swimming speed and feeding intensity of dominant species. Defining
prey selection and predation rate in relation to available prey. Quantifying the weightspecific (carbon-based) respiration rate of dominant species.
Methods: The studies are all based on in situ work, mainly using an ROV with
adequate instrumentation.
Vertical distribution in relation to environmental variables. We will use an UVP
(underwater video profiler) that gives information on the vertical distribution in
physical and biological parameters for documentation of the environmental situation
at each occasion. ROV Aglantha or a simpler submersible video system will then be
used to record the vertical distribution of macroscopic gelatinous organisms. Surface
light will be measured continuously, and the optical transparency measured for each
station, in order to estimate the amount of light penetrating to different depths. More
detailed studies in order to identify specimens will be made by using a high-resolution
digital still camera and also collection of specimens for proper identification in the
laboratory.
Swimming and feeding. ROV Aglantha will be equipped with a B/W high sensitive
video camera with macro and zoom function and a rack of red-light diodes that
provides sufficient illumination for a volume of at least 1 m3 in front of the camera.
Individual specimens will be detected with the main video camera and white light,
illumination will then be switched over to red light, and the target object will be
recorded over time. This will give information on the morphological dimensions of
the animal (e.g. size and tentacle length), behaviour (e.g. swimming and feeding
modes and tentacle configuration) and activities (e.g. swimming speed and feeding
intensity). Documentation of fine details in the swimming and feeding will be made
by high-resolution still photography, using strobe light. Sampling with a suction
sampler mounted on the ROV will provide information on available prey, and
sampling of the gelatinous predators by using a detritus sampler or a suction sampler
mounted on the ROV will provide the necessary material for stomach-content
analysis.
The respiration rate of dominant species. We will use a set of detritus samplers and
a modified suction sampler mounted on ROV Aglantha to collect individual
specimens of dominant species into individual incubation chambers. These are
equipped with a new type of fiber-optic oxygen probes, developed by PreSens
Precision Sensing GmbH in Germany and Anderaa in Norway. The probes are
connected to an eight-channel metering instrument in an underwater housing that
communicates with a PC onboard the research vessel. This brand new technique is
supposed to give more stable and long-lasting readings, with a precision as good as
ordinary Winkler titration. Software makes it possible to follow the changes in
oxygen concentration in the incubation vessels continuously, thereby also getting
information on any effects during reduced pressure when retrieving the ROV. After
returning the ROV onboard, the animal are identified and their size and weight
measured. The material is stored in a proper way for later analysis of carbon content,
in order to express the respiration rate in a standardised way as per unit of carbon.
Specific needs: The program is heavily based on the use of an ROV with video- and
photo equipment and adequate sampling gears. Most of this is already available today
and the sampling gears can be brought from Florida by M.Y. The UVP is presently
not available, but will hopefully be part of the scientific equipment of the new
research vessel “G.O. Sars”. It is the key instrument to get a quick survey of where in
the water column potential “hot-spots” of biomass and activity might occur.
Instrumentation for respiration measurements in situ is essential for getting reliable
estimates of the biological transformation of material through the food web and how
the deep-water gelatinous predators are metabolically adapted to their environment.
Successful results from this technique will be a breakthrough for doing in situ
respirometry of deep-water animals.
Progress plan: Early 2003- Equipping ROV Aglantha for the specific purpose;
March, October 2003- Testing and using some of the equipment during cruises with
R/V Håkon Mosby; September 2003 – Testing and using some of the equipment
during cruises with R/V Seward Johnson. Mid 2004- Main field investigation; Late
2004- end of 2005- Data preparation and writing up results.