D341: Porcupine Abyssal Plain cruise 2009


 D341



Cruise blog

Monday 3 August 2009

Mastering the oxygen dynamics in the open ocean : IODA6000.

Oxygen maintains life on earth as we know it. In the ocean the oxygen is present at various concentrations at all depths and there is few oceanic areas deprived of oxygen completely. Oxygen dynamics mostly depends on the thermodynamics parameters (temperature and salinity of the seawater) and oceanic circulation, and to a lesser extent, on biological activity (photosynthesis and respiration). This is the reason why we are measuring O2 concentration and oxygen dynamics in the surface ocean, where light is available, as well as the whole deep water column (the dark side of the ocean).

On board the ship, we are studying the biological oxygen fluxes linked to marine micro-organisms activities. We are sampling from the CTD rosette for in situ oxygen concentration determination, but also we are taking samples in little glass bottles to incubate at sea on a drifting line (“The drifter”) for 24 hours, equipped with an Argos® transmitter and a flashing light and a bright pink flag for day and night positioning and visibility. This mooring line allows us to reproduce the in situ temperature and light conditions to replicate the physiological requirement of the sample taken from the CTD-Rosette.

Then we measure the difference of oxygen concentration between the reference sample (pickled at the start of the incubation) and either dark incubated sample, to derive respiration rate, or the “light” incubated sample to derive photosynthetic rate of the marine micro-organisms. This approach, although very accurate, it is time consuming and fastidious, constraining the number of samples which can be analysed.

The great novelty for us of this cruise is to deploy a new “toy”, the so called IODA6000 (In Situ Oxygen Dynamic Autosampler). This equipment has been developed through a close collaboration between our laboratory (Laboratoire de Microbiologie Geochimie Ecologie Marine de Marseille) and the CPPM (Centre de physique des Particules de Marseille). The IODA6000 is made of a 5 litres incubation chamber, auto sampling water at its depth of immersion. Every 3 minutes, temperature and oxygen concentration are recorded in and out the chamber. After each cycle (incubation time), the chamber opens for an hour and then close again for a new incubation. The incubation time varies according to the immersion depth, i.e. the expected oxygen drift related to the in situ biological activity. The deeper we are the lower rate we expect, which is a first order assumption. Some of the IODA6000 are equipped with a light sensor to discriminate day and night period at the surface ocean; nevertheless the IODA6000 is able to reach 6000m. The combination of these measured parameters enables us to derive both the in situ photosynthetic and respiration rates in the upper ocean and the respiration rates in the dark ocean.

During the D341 PAP cruise, 5 IODA6000s are being deployed at 25, 150, 500, 1000 and 2000m depth, for 20 days, focusing our effort on the “twighlight” zone of the Ocean. When the mooring will be retrieved, it will have remained at sea for 20 days, representing 18 cycles of 24 hours for the surface IODA6000, opening and closing time being set in relation to the sunset, 9 cycles of 48 hours at 150 m, 7 cycles of 60 hours at 500 m, 6 cycles of 72 hours at 1000m, and 4 cycles of 96 hours at 2000m. In parallel of this in situ work some work on microbial activity respecting the in situ hydrostatic pressure is being carried out to enable us constraining the effect of this controlling factor may have on the oxygen dynamics.

Energy provider for trophic network, Oxygen is...
The dark side, Carbon is...

Master IODA6000, his Jedi and young Padawan.


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