Day 13: Friday 3 July 2020
A day full of mapping! We have put behind us 20 hours of transit to our next survey area, located just east off the Icelandic continental shelf, at the very beginning of Aegir Ridge. That’s where we expect to find reefs of cold water corals – the ‘Lophelia’. Corals are, as any other hard substrate on the seafloor, the solid base to most marine life. They build vast reef areas and are home to the most delicate biodiversity. Although being totally different in appearance and motion, medusae (or jellyfish) and corals have something in common: both animals belong to the group Cnidaria. They use nettles to capture their prey and to defend themselves against predators. However, unlike jellyfish, the corals’ harpoon-like nettles sit in cnidocytes and can be expelled whenever something is passing by.
Depending on the local water depth, corals can be seen in the multibeam bathymetry. Hence doing a careful map survey in advance to a ROV dive can save a lot of time as the location of the dive and where to take samples can be decided a lot more precisely. However, it’s also important to know the local water depth, which can usually be derived sufficiently enough from satellite altimetry (down to a resolution of about 1km, the seafloor topography can very roughly be obtained from satellite gravity measurement). In this special case however, at the beginning of Aegir Ridge, the difference in depth between the altimetry model and the true multibeam bathymetry was up to 1000m – and that makes a huge difference if the water depth is between 700 and 2600m!
