Exploring seamounts south of the Polar Front
Vysokaya Bank by Alex Rogers
Seamounts are underwater mountains with an elevation of more than 1000 m from the seafloor to the summit which lies beneath the ocean waves. Most of them are volcanic in origin which is why we investigated the Vysokaya Bank, also known as Kemp Seamount, for hydrothermal vents. Seamounts are, however, also hotspots of biological activity in the oceans and concentrations of large fish, such as tuna, as well as sharks and whales are found around them. Seamounts may also host rich seabed communities formed by animals such as corals, sponges, sea pens, crinoids and stranger denizens of the deep such as the giant single celled animals, xenophyophores.
Vysokaya Bank was no exception and when we surveyed the seamount with the towed camera SHRIMP spectacular sights met our eyes. The seabed was formed of volcanic ash and gravel and covering it were millions of white brittlestars, in some places so dense that they were touching each other arm to arm. Here and there were larger purple brittlestars and also large black sunstars, some with their arms raised into the water to catch particles of food. There were also large orange and yellow predatory seastars and giant yellow sea anemones and whip corals, some trembling in the strong currents sweeping across the seamount. Deeper on the slopes of the volcano we saw large yellow bushes of primnoid octocorals covered in brittlestars, using the corals as platforms to get at richer food supplies in the water above the seabed.
Why seamounts are such hotspots of biological diversity and abundance is still not understood by scientists. The movement of water past seamounts may generate strong currents and upwelling of mineral nutrients in to the surface of the oceans. This can stimulate the growth of microscopic plants above seamounts and enrich the local environment. Alternatively, seamounts may act as giant traps for the planktonic and small swimming animals of the oceans as they drift past in the currents. Everyday these animals move up in the water column at dusk and down in the morning hundreds of meters in the world’s greatest but unseen migration. When they drift over a seamount at night their passage into the ocean depths in the morning is blocked and they are consumed by the animals living on or around the seamount. Whatever the reason, as we saw on Vysokaya Bank, seamounts can be tremendously rich ocean habitats teaming with spectacular marine life.