Sampling the Ocean and Not the Ship

Metal-free Niskin Sampler

In my last post I described our workhorse sampler, a rosette holding twelve Niskin bottles.   This is a great sampler for many analytes like dissolved oxygen, salinity, and major nutrients (nitrate, phosphate, silica, and ammonia) because the sampler is not made of these things.   However, the rosette and hydrowire are made of iron.   If you look carefully at the surface water as you deploy the rosette you can see the rust streaming off the hydrowire that connects the rosette to the ship.   This is can be a real problem if you job is to measure iron.   It wasn’t until oceanographers like John Martin (Colby class of 1962) developed metal free sampling techniques that collecting uncontaminated samples for metal analysis was possible.

Thirty-liter GoFlow Sampler

The photo in the upper right shows Ben and Sarah recovering a special metal-free Niskin bottle.  The spring that holds the ends of the bottle closed is located on the outside of the bottle.   Also, notice that the bottle is not mounted in a rosette, but is attached to a metal-free Kevlar cable.   Each bottle is attached to the cable by hand and deployed in a string from the deepest bottle to the shallowest.  Once the bottles are in place, the white Teflon messenger is sent down the cable.   The messenger hits a trigger on the first bottle that closes the bottle caps and releases another messenger.  The second messenger triggers the second bottle, and the process continues down the string of bottles until all bottles are closed.  It is a slow process, but if done correctly, it is very clean.   These small bottles are easy to deploy because they are light and can be lifted by one person.  Ben also has a thirty-liter version of these bottles that his group uses for collecting a large volume of water from a single depth.   It takes several people to recover this bottle and today I was given the honor of helping to carry the full bottle to the lab.  I don’t think they wanted me for my mind!

Clearly, bottles have size limitations.   Some measurements, like the analysis of metals adsorbed on particles, require sampling much more seawater than could ever be contained in a bottle.   The Lam group from WHOI uses submersible McClane pumps to collect hundreds to thousands of liters of water for particulate analysis.   They deploy the pumps on the same metal-free wire to avoid contamination.   Each pump is an autonomous sampler programmed to turn on and off after being lowered to their sampling depth.   The pump pulls water through filters where particles are trapped. After recovery, the filters are removed for analysis on the ship or lab.

Luke (yellow hat) deploying his surface camera

Other samples simply can’t be collected for analysis on the ship.   Try to collect a photon in a bottle.  Luke Logan from Miami and Barney’s group from Bigelow use in situ optical sensors to measure the light levels in the surface ocean.  Luke’s sensors are designed to float just below the surface and measure the upwelling light being scattered from depth.

Optical "Lawn Dart"

The Bigelow sensors measure both the upward and downward light flux using sensors mounted on the top and bottom of an “oceanographic lawn dart”.   The dart is allowed to sink behind the ship and measures light levels as it goes.

Finally, the ship’s crew have their own sampling systems.   We have started to deploy these “samplers” behind the ship whenever we move between stations.   Stay tuned for Annie’s post tomorrow for more details on what we collect with this sampling gear.  It was delicious!

More photos and details of other sampling gear used on the ship are posted on Rebecca’s blog.

– Whitney

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