So Much Water – So Little Time

Great Belt Cruise Track v1.0

We just got an email from Barney on sampling logistics for the cruise.  You would think that researchers on an oceanographic research vessel floating in the middle of the ocean could have all the water they wanted whenever they wanted it.   However, we have limited sampling opportunities while the ship is moving and we only have 36 days to make it to Cape Town.  The map to the right shows the current cruse track.   The dots on the line are the sampling locations.  We are scheduled to sample 120 stations 52 nautical miles apart (6240 nautical miles, 24 days of steaming).  We must keep our average wire time (sampling time) to less than 8 hours for us to be able to get to Cape Town on 16 Feb.  Barney divided up the 8 hours to match the sampling need of all the research groups on the ship.   At this point, this is what a typical day will look like:

CTD Sampler

1) Productivity casts (growth rate measurements) will always be pre-dawn, between midnight and 0400h.  A CTD sampler similar to the one shown right is lowered to depth at a rate of  1 meter per second.   A cast to 500 meters can take up to an hour depending on the time to trip (close) the bottles.

2) Trace metal casts, Snatcher and McClane pump casts will always follow the CTD so folks know where to target the sample bottle depths of subsequent casts.

a) The trace metal casts will go to 1000m and involve hanging individual Niskin bottles so this will take 1.5 hours provided the spool rate of that winch can reach 60m/minute.

b) Another sampler, called the Snow Catcher (aka “Snatcher”), will be deployed to ~75m, triggered, and recovered, taking another 30 minutes.

McClane Pump

c) McClane pumps will follow Snatcher casts.  These pumps collect the particles from a large volume of seawater by pumping water through a filter.   It takes time to lower the pump to the sampling depth and more time to collect the sample.  Total time 4 to 5 hours.  This task is scheduled for every other day.

3) Optics casts must happen as close to local apparent noon as possible and never outside the 1000h-1400h window in order to get maximum solar elevation.  This task will take at least 30 minutes.

4) Finally, additional CTD casts will be taken later in the afternoon.   Time for deployment 1 hour.

Total time for all the sampling is 7 to 8 hours per day.  This assumes that the ocean is accommodating.   If seas get too rough to deploy gear we will simply have to keep moving. Cape Town is a long way at an average ship speed of 11 knots.

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