14.8.2019 – “Eyes and ears” on site: The mooring goes into the water

One notorious problem in ocean sciences is that on a cruise like ours we can only take samples at certain points in time. The ocean, however, is constantly changing over the course of weeks and years. To get an idea of the scale of these changes we need some “eyes and ears” on site while we are not there. One method of doing this is to use moorings. They consist of a bottom weight, holding the mooring in place, a line with attached instruments that record data and buoyancy equipment to keep the line upright in the water column.

The mooring, which we exposed today in the middle of the South China Sea, is to remain there for two years, and on recovery will tell us about the changes in currents, salinity, temperature, nutrients and material sinking down through the water column. In the morning, the tension of everyone involved in the mooring work was tangible. No wonder, because such a mooring is about 1.5 kilometres long and it takes several hours until everything is in place. Therefore all components were neatly laid out on deck to be checked and double-checked: The stack of railway wheels used as the bottom weight, the bright orange top buoy, the packages of yellow buoyancy spheres, an optical sensor for measuring the nitrate concentration, several current sensors with CTDs, the sediment trap, the acoustic releasers as well as various lengths of rope and all the shackles and rings needed to put them together.

At first, the weather was not ideal: A strong breeze created waves with whitecaps. However, as our meteorologist on board had promised decreasing waves for later, the mooring work was scheduled to start after lunch. At 12:19, with the top buoy, the first piece of the mooring went to water. Next up was the optical nitrate sensor, a pack of buoyancy spheres and the first current meter with CTD. A lot of helping hands are needed to get all the components ready and to lay out the ropes, so that everything can be swiftly and smoothly put together. Such a well-choreographed activity always draws an interested audience, which today followed the activities from one deck up. About one hour and 850 meters of mooring line after the top buoy went in the sediment trap was launched. It will collect material sinking through the water column in its funnel in sample bottles. There are 40 bottles that are put below the funnel one after the other by a rotation mechanism. This will allow to identify temporal changes in quality and quantity of the sinking material over a longer period of time.

Another 550 metres of mooring line later, the acoustic releasers were lowered into the water. They are a crucial part of the mooring as they are needed to retrieve the measuring devices in two years’ time: After receiving an acoustic signal from the ship, they disconnect the line with the instruments from the bottom weight and allow it to float to the surface. To increase the chances of a successful release, we have installed two independent releasers. At 3:13 p.m., almost three hours after the top buoy, the bottom weight finally went in. It has a parachute so that it doesn’t plummet down too quickly. This is important, because this way, all components of the mooring are pulled into the depth evenly without tangling. And so we looked for the buoyancy packs and the top buoy with some tension to see if everything goes as planned. Reassuringly, the buoyancy packs disappeared from the water surface one after the other and after about ten minutes the top buoy also vanished from sight. It will float at a depth of about 150 to 200 metres below the surface, hopefully deep enough not to be caught by fishing activities.

The last step of this tricky operation was to determine the exact position of the mooring, because after all you have to be able to find it again: To do this, the distance between ship and releasers was checked with an acoustic transducer from three different ship positions. Then the releasers were disabled by a command and hopefully will wake up again when we come back for the recovery of the mooring. A final safety check of the determined position was carried out with the echo sounder that shows a clear signal from the buoyancy packs in the water column. After all: Not only the instruments are too valuable to loose, but also the data treasure they collect. But as everything worked out so well today, (almost) everyone could enjoy the traditional “mooring beer”.

 

(Photos: IOW / R. Prien, please click for a larger view)