The currents and the biological activity near the coast of Peru are such that nearly all the oxygen disappears between 50 and 500m depth. This affects life from microbes to large fishes.
We are currently onboard the FS METEOR to try to understand why and how this occurs. Carolina Cisternas-Novoa and myself, Frédéric Le Moigne both oceanographers from the biological oceanography group at GEOMAR, Kiel, are working on this question. We are specifically looking at an important function of the ocean called the “biological carbon pump”. The biological carbon pump is the amount of particles that sink by gravity from the surface ocean (100m) to the ocean interior (>1000m). Those particles (see Figure 1) are formed mostly by biological processes for example when the phytoplankton and zooplankton die and coagulate in the water. Sinking particles contains of lot of organic carbon that originates from photosynthetic plankton. This mechanism is essential for the ocean because it “pumps” carbon from the surface ocean to transfer it to the ocean interior for long periods of time. One central question to our current expedition is to know how low oxygen affect the magnitude of the biological carbon pump, little information is currently available on the topic.
To collect sinking particles (Figure 1) and thereby measure how strong is the biological carbon pump off the coast of Peru, we deploy free drifting sediment trap (Figure 2). This instrument consists in a vertical line of 600m with 8 particle collectors attached to it. The particle collectors are made of 12 long tubes attached to a cross (Figure 2). The line is ballasted by a weight at the bottom of line so the line stays vertical in the ocean. In surface, the line is connected to a series of small buoys (white and oranges, see Figure 2) that ensures floatability and compensates for wave motion.
At the surface end of the line, our satellite tracking system is attached (Argos and Iridium). We track its positions every 30 mins so the trap is not lost on the ocean. The sediment trap stays in the water for about a week which gives enough time to collect a sufficient amount of particles to perform our measurements. We are deploying four trap in this expedition which should give us enough data to assess the influence of low oxygen level on the magnitude of the biological carbon pump.
Frederic Le Moigne, GEOMAR, Kiel.