By Kevin Saw
In her earlier blog post, Corinne talked about the sediment trap moorings and described the sediment traps that are attached to it. The PELAGRA sediment traps perform a similar function by collecting sinking marine snow but rather than being statically located at great depth, they are neutrally buoyant drifting traps that can be set to drift freely at any required depth in the upper ocean, up to 1000 m deep.
Typically, a number of PELAGRA traps will be deployed from the same location but at different depths for periods of a few days. The idea is that each trap will collect a different amount of marine snow with the shallower traps collecting most and each successively deeper trap collecting less. By doing this, a picture can be built up of the particle flux which is a measure of how much of the marine snow is being removed from the upper ocean by foraging zooplankton and therefore never reaches the seafloor.
The name “PELAGRA” is an acronym derived from the more precise title “Particle Export measurement using a LAGRAngian trap” with “Lagrangian” being a reference to the fact that the traps drift with zero velocity relative to the water they are drifting in. This latter fact is important; it is considered that statically moored sediment traps may over or under sample at times because of turbulence around the trap openings caused by cross-flow of water currents. The PELAGRA traps avoid this by drifting with the currents and therefore experiencing little or no turbulence.
The PELAGRA traps consist of a central float surrounded by four particle collection funnels. Each funnel has a collection cup beneath it that can be rotated into position directly under the funnel so that each funnel can collect for precise periods of time whilst drifting at depth. The central float has a built-in ‘buoyancy engine’ that can make fine adjustments to the trap’s buoyancy in order to maintain the required depth and compensate for small changes in water temperature and salinity. At the end of a mission, a 2 kg weight is released which makes the trap positively buoyant so it rises to the surface. Once at the surface it acquires a GPS position and relays this to the ship via the Iridium satellite telephone system so we can locate and recover it.
Building a device that weighs around 130 kg that can achieve neutral buoyancy at some depth in the ocean is a challenge. It means we have to adjust the trap’s mass to within a few grams and its volume to within a few cubic centimetres. Each time we make a change to the trap we need to go through a rigorous ballasting process back at NOC before we come out to sea. Recently we have made some engineering improvements to the sample cup rotation mechanism so we have had to go through the ballasting process again. For engineers Rob Brown and me, this cruise to the PAP site is therefore an engineering trials exercise rather than for pure scientific research. However, any samples of marine snow that we collect during our trials are readily snapped up by the scientists on board for further study.