In this video clip NIWA scientist Malcolm Clark, explains how seamounts offer habitats to communities of organisms that would otherwise find it very difficult to survive in the abyssal depths of the Antarctic waters. Malcolm explains that two specific sleds are used for sampling the benthic life forms; the epibenthic sled which is quite robust, and the slightly bigger beam trawl.
Points of interest for teachers:
- When watching the underwater footage, students may want think about the terrain these sleds have to be pulled over
- What are the advantages of the two sleds? Note the beam trawl being pulled on board at the end of the clip.
Seamount biodiversity is widely recognised as being very high for the size of the seamounts, and that’s due to three main reasons. The first being that they cover such a very wide depth range, the ocean sea floor is over most of its area is about four to five thousand metres deep. So if you’re an animal that can’t live that deep you’ve got a problem unless you can find a ridge or a seamount. So the wide range of depths the seamount covers enables a lot of animal groups to - to live there.
Secondly because of the volcanic nature of seamounts they can be very, very rocky and hard while most of the ocean floor is very soft sediment so animals that need a hard surface to attach to; corals, sponges, those sorts of animal groups they find a home where otherwise they wouldn’t.
The third thing with - with seamount ecology is that seamounts can affect the local oceanographic conditions. The water current flow around and over the top of the seamount, water moving across the ocean hits the seamount, it accelerates, and comes up and over the top and around the flanks. That brings a lot of nutrients from down deep up to the animals living on the sides of the seamounts, so they can be very productive sights and can sustain very large aggregations of benthic invertebrates, sea – fish, marine mammals and birds.
We use photographic images as well but we need to obtain actual samples of the animals to verify what we see in the photographic record. So we have a a small seamount sled which is a metre wide which gives us quite a good sample but it’s a very robust and rugged piece of sampling equipment that one way or another we generally get it back. It can bounce over rocks, even if it gets jammed we can pull it out backwards, that gives us the ability to capture some animals to quite a small size and therefore verify what we’re seeing in the photographs. The second piece of equipment we’ve used not quite so successfully is the beam trawl which is a four metre wide rigid opening towed trawl and it’s more designed for flat surfaces but on seamounts it can be very effective for catching small fish which our seamount sled doesn’t do very well, because it’s a bit too small, a bit too slow. The beam trawl can work well on - on the tops of seamounts but where it strikes hard rugged patches, as we have on a couple of occasions, it doesn’t stand up to – to that type of – of terrain and so we – we occasionally break the bean trawl, but it’s designed to do that, we get the bits back even though the beam itself has - has broken.