Needle in a haystack?
Today saw our first full science dive at the Northwest station. The Isis ROV left the ship tooled up like a multi-million pound packhorse but once in the water she comes into her own guided by a skilled team of pilots and engineers.
Our targets were the terraces and valleys of the Mid-Atlantic Ridge some 2500m below us. After suffering downtime with the bad weather we have moved into full multi-tasking mode and Isis has several roles to perform.
First up we survey the seabed using a downward-looking HD camera mounted on the front tool tray. This allows us to fly over the seabed at an altitude of 2m making detailed video observations. Much in the same way that you may map the distribution of terrestrial animals from the air we can use these video surveys to calculate the abundance and diversity of animals on the deep-sea floor and then compare those estimates to the data we have collected from the trawls on the previous two ECOMAR cruises.
Although we are sitting over the Mid-Atlantic ridge the seabed is not all hard volcanic rock. There are valleys of soft mud sat between towering cliffs and terraces of basalt. Our survey of these soft valleys showed a wide range of deep-sea species including sea urchins, sea cucumbers and the occasional passing fish. However, one of the most striking features of the deep-sea floor is the huge numbers of tracks and traces visible on the surface of the mud. These give us a tantalising glimpse into those animals that are out and about when we are not around or those who just happened to have already passed by. Burrows of all shapes and sizes can give us an indication of who may be living just below the sediment surface and one of the most spectacular examples of this is the acorn worm or Enteropneust. This animal makes distinctive striking spiral traces on the surface of the seafloor. These traces may spiral clockwise or anti-clockwise, and no it is not different depending on which hemisphere you are in, they can both occur side by side as we have seen.
After completing our survey we then set off for our second task, one which we were unsure as to how successful it may be. Two years ago we dropped off (quite literally) a large whale bone attached to weights and a large sub-surface float. Whenever whales and other large cetaceans die their bodies eventually find their way to the deep-sea floor where the decomposing carcass provides a welcome food parcel to the scavenging animals below. However as the bone begins to break down it releases all its fats into the surrounding sediment and the bone itself then begins to support quite a unique community of animals.
It was the task of Isis to try and locate this relatively small bone (approx. 18kg) in all that water of the North Atlantic. Luckily in this modern age we can accurately mark positions to within 100m using GPS technology so Isis at least had a target to aim for. It would be nice to report that this type of thing happens all the time but many of us have spent too much time at sea to be 100% certain all the time, however on this occasion Isis was able to literally drive right up to the sunken whale bone, which was sitting on the seabed only a ship’s length from where we had thought it may be. The sub-surface float sitting 5m above the bone allowed Isis to detect the bone location on the sonar long before we were able to see it on the cameras. On approaching the bone we saw that it was indeed surrounded by many animals and the surface of the bone was home to lots of little white squat lobsters. After filming the bone we moved on with the intention to return on a subsequent dive when we could recover and sample the bone in more detail.
Leaving the bone and the soft sediments behind us we then made are way into more hostile territory up the slopes and cliffs of the ridge terraces. Using the manipulator arms on Isis we were able to sample many of the diverse arrays of sponges we observed as well as some beautiful stalked crinoids. Crinoids are close relatives of the seastars and these forms use a long stalk to lift themselves up of the bottom so that they can feed by trapping particles in the water.
After several hours of collecting and observing the habitat we left the ridge behind and Isis returned to the surface laden with her new found bounty, ready for delivery into the waiting buckets of eager scientists on the ship.
After a short rest and a bit of TLC from the engineering team Isis will be back in the water tomorrow as we go in search of more and more complex topography and new discoveries amongst the cliffs, gullies pinnacles and overhangs of the Mid-Atlantic Ridge.