Bobbing along at the bottom of the sea

By Martin Foley

The Extended Ellett Line, our cruise track, runs over Rockall Trough, a cradle of deep sea biology. In Rockall Trough, we go to a place about 2200m deep, just off Anton Dohrn seamount, named Station M (pictured). By reaching down and bringing a load of sediment up, we can look at the animals that live at the bottom and see how their numbers change in time. It is not a static environment by any stretch of the imagination!

As I write this, I am sitting in the belly of the RRS Discovery half-way through a three week cruise and our epibenthic sleds are strapped to the deck and ready to be deployed when we reach Station M. The sled is basically a metal cage with a closable door in which we lash a net to the inside. It is is lowered off the back of the ship until it reaches the bottom of the Trough and dragged for around 2km along the seabed.

As it skims the bottom, the sled disturbs the top layer (the epi in epibenthic) of the sediment and collects the creatures that live there. All manner of beasts live there, from bivalve molluscs to tube dwelling crustaceans, polychaete worms to tiny starfish, and bundles of nematode worms. These are all then hauled up on deck, sediment and all, and are sieved.

Everything bigger than 4mm is classed as megafauna, this gets caught in the first sieve; next comes macrofauna, everything between 0.5 and 4mm; and then finally the meiofauna, anything that has eluded the first two sieves but cant squeeze through a 420m gap.

Finally, the samples get fixed in formalin to stop the soft tissue from turning to mush and they are ready for their journey back to dry land where it is sifted and sorted into taxonomic groups by anyone up to the challenge.

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