Bathymetry data are imported to the OLEX mapping system
EM300 echosounder used for bottommapping
A SCANMAR trawl sensor, specially made for MAR-ECO
Author: Odd Aksel Bergstad (IMR), Thomas de Lange Wenneck (IMR)
Early in the morning we arrived at our first station in the middle MAR-ECO box, located at the Charlie-Gibbs Fracture Zone. Thirteen locations have been pre-selected for comprehensive sampling, 6 south of and 7 north of the main fractures. Today's station is among the deepest, at 3500 m. We towed a bottom trawl at this depth, and that was a record depth for the cruise.
One of the main challenges on Leg 2 of the MAR-ECO expedition is to obtain new knowledge on the identity and distribution of organisms living near or on the bottom. This is no easy task in an area of the ocean where the dominant substrate is rock and the terrain is really rugged. The mid-Atlantic Ridge can best be characterised as a major mountain chain, and it is only in between the rocks and at the bottom of troughs and valleys we find some patches of soft substrate.
We try to meet the challenge by using several methods and technologies, and the work programme for our stations reflects this approach. To complete all tasks scheduled for a 3500m deep station, we need about 25 hours. The programme starts with the deployment of the ROBIO lander, essentially a free-fall tripod with a time-lapse camera that records fishes and invertebrates attracted to a bait. Analyses of the pictures tell us what fishes and invertebrates are present in the area (at least those responding to bait), and indirectly indicates how abundant these organisms are. We carry on with a CTD and ADCP profile to measure characteristics of the abiotic environment; temperature, salinity, and current by depth all the way to a few metres off the bottom. Bottles on the CTD-rosette collect water samples, and the ISIT bioluminescence lander records light flashes from organisms in the water column. After the CTD-profiling we use the SIMRAD EM 300 multibeam echosounder to derive a detailed new map of the bottom where we wish to deploy a bottom trawl and dive with the remotely operated vehicles (ROVs). Generally, the existing maps are unreliable or not sufficiently detailed. We usually spend 2-3 hours mapping an area of approximate size of 5 by 5 km. The multibeam echosounder tells us where suitable conditions are found, and this instrument has proven absolutely essential for our operations.
A bottom trawl can only be deployed where the bottom is even, otherwise the net will be torn or the trawl can get stuck on the bottom. When we deploy a trawl we wish at all times to know its shape and operation characteristics. This is monitored by wireless acoustic SCANMAR sensors. These sensors, attached to the trawl doors (otter boards) and the headline of the trawl, transmits sound pulses from the trawl to microphones on the vessel. The trawl skipper receives interpreted information on the configuration and behaviour of the trawl from these sensors. It is quite remarkable that the SCANMAR sensors can provide reliable information at such large distances from the vessel. A trawl to 3500m requires about 5000m of wire to be paid out, and the total time for a tow is about 4 hours. From the trawl catch we obtain crude density estimates, but most importantly the essential biological samples of fish, cephalopods and other invertebrates living near or on the bottom. We have a video camera on the trawl that films what enters the mouth of the net, but otherwise the trawls only provide indirect information, and only from even bottoms.
We want however new information also from rough ground, preferably direct visual data. This is why we use remotely operated vehicles (ROVs), and the ROBIO lander. An ROV-dive is scheduled for every station, but unfortunately several dives have had to be cancelled or interrupted due to technical problems. We have, however, gathered some remarkable observations from a few stations, and will collect more. Today we made a transect in the water column using the ROV Aglantha. Water column observations are also part of the Leg 2 tasks, and for that we use ROVs, the Underwater Video Profiler (UVP), and a plankton net attached to the bottom trawl.
MS Loran, the longliner that works alongside us, has been doing really well. She is now north of us, exploring and sampling rough ground with longlines, traps and gillnets. The catches from these passive gears supplement the samples we collect on RV G.O.Sars using trawls.