Thursday 10th June 2010

Walking the line….?

If have ever been involved in sampling intertidal areas you will no doubt, at some stage, have resorted to the use of a transect line.  Usually laid out with a length of string and/or a measuring tape, it would have guided you across or along the shore while at set intervals you laid quadrats, square frames that delineated a fixed area along your line.  By recording, or even collecting, the occupants of these quadrats you would have been able to build up a picture of any variations that may have existed in the distribution of species along your line.  For example, were individuals clumped together or were they more widely and randomly spaced? Were population levels high or low? Which species were common, which rare?  If you laid your line along the top of the shore and another along the lower shore you could check for similarities and differences in these and other parameters that might be attributable to differences in the living conditions in the two areas.  Great – easy isn’t it?  All you need is a wire frame and a length of string.

Sampling a rocky shore

Now do the same exercise at 2500m below the Atlantic Ocean.  Not so easy, but that’s what we are doing on the mid-Atlantic Ridge.  Using swath bathymetry to produce maps of the sea bed, Dan has randomly selected four 500m transect lines in each of three areas that differ in slope from flat to almost vertical. Using the co-ordinates provided by Dan, the ROV then flies the length of each transect at a known height (around 2.5m) above the sea floor or, for steeper areas, a known distance from the cliff face.  A pair of lasers, place a 10cm marker on the surface, from which accurate measurements of the transect width can then be made.  This ‘belt transect’ can later be divided into quadrats as required, avoiding bias by randomising the process.

Red lasers on flat sea bed pick out the holothurian, Paelopadites

Green lasers enter bottom left, striking the rock face below the white sponge

The ROV flight records the passing surface using a high definition video camera, mounted vertically for flat terrain and looking forward for steep slopes.  The flight is slow; a very slow walking pace (0.4 knots) on level bottoms, even slower when we negotiate the more difficult, steeper terrain.  Each transect line produces between one and two hours of video.  We have a total of 48 transects planned for the cruise for some lucky sole to analyse when the tapes are back in the UK.  These are usually viewed at half speed so that amounts to around 144 hours of viewing.  Let’s hope that the story that unfolds is an exciting one.

Today we scheduled an ROV dive to complete the 12 transect lines for our NE Station.  We plan to make a lazy decent to allow Marsh to examine pelagic animals on the way down (descent time is usually in the order of 80 minutes but we plan to dawdle and take an extra 60 minutes).  After a false start arising from issues with the HD camera, the dive is now under way and, with a scheduled duration of 32 hours, will not be completed until late on Friday night.  Walking the line takes time!

As if looking at the sea bed was not enough to occupy our time, we have also deployed the Megacorer today to collect sediment samples.  This works by pushing plastic tubes into the sediment then sealing the tubes with sliding shutters until we have the sediment safely on board.

It takes almost 3 hours to sample the sea bed at 2500m and as much to sieve and store the samples.

Debbie is now the proud owner of a tube of mud.....

....and Mark looks pretty happy too!

They were still smiling after 3 hours of sieving mud – in cold water!  Now I’m off to a warm control cabin to see how our ROV dive is going.

Geoff Wigham

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