Wednesday 9 January 2008 à 15:55

The oceans "skin"

Par Sophie Mouge. Correspondent aboard the Aurora Australis

Jamie, the Third Officer, profits from the day full of sunshine to practice using the sextant.

Jamie uses a sextant to calculate the position of the ship.

Created in the 18th century, this instrument was used by sailors of yesteryear to take their bearings at sea. The sextant comprises a fixed monocular and mirror that a navigator uses to measure the angle between the sun and the horizon; today, at 1300h, it is 49°. Jamie then calculates the position of the ship using mathematical tables provided for each day of the year.

Since the 1990s, the GPS (Global Positioning System), a system for navigating by satellite, has replaced the manual instruments. But since any machine is subject to events, young officers are always trained in the use of such devices to mitigate any equipment failure.

The thermometer is showing 2°C on deck. The thermometer in Harvey Marchant’s laboratory shows just 3°C. Why work in such cold? Simply because the organisms that Harvey is working on have to be maintained at a temperature close to that of their natural environment (the Southern Ocean) in order to be kept alive.

Harvey Marchant is a researcher who specializes in marine plankton biology. He has worked in the Australian Antarctic Division for many years and has participated in many polar oceanographic missions. He was the scientific Voyage Leader on board the Aurora Australis in 2001. Here he explains to us how to succeed in observing microscopic organisms dispersed in sea water.

Harvey filters sea water and retrieves the microorganisms that are deposited on the filter.

Harvey profits from the samples of water that are brought up by the CTDs. He analyzes the microorganisms that proliferate in the water samples. He is studying organisms called planktonic, that is, carried passively on ocean currents.

The filters are placed under the plate and object-slide to be observed with the microscope.

Examples of filters obtained after microorganisms are deposited. These filters appear to be rich in phytoplankton.

Planktonic plant life, or phytoplankton, is composed mainly of unicellular microscopic organisms: dinophytes et diatoms. These organisms have a siliceous or calcareous envelope that settles to the ocean bottom when the organism dies. Phytoplankton is most often photosynthetic, that is, it uses the sun to convert water and the CO2 present in the water into organic matter (sugars).

This characetristic puts it at the base of the food chain of the living ocean world.

Legend: The black arrow indicate "who is eaten by".
In the first 100 meters of the water column, the concentration of plankton is estimated at more than one million cells per liter of sea water! Antarctic waters are among the richest in the world in minerals (nitrates, phosphates, etc.). This wealth allows plankton to flourish and raises its biomass to 95% of the total ocean biomass. That means that all the other marine organisms (whales, penguins, blennies, sea stars, antedons, etc.) represent only 5% of the biological mass of the oceans.

We know that 95% of the mass is concentrated in the top 50 meters of the ocean. Now the world ocean has an average depth of 4,000 meters. So 95% of the mass that lives in the oceans occupies only 1.2% of their volume!

That is why Harvey compares all the marine microorganisms to a “peridermis” covering the surface of the ocean: an oceanic skin of sorts.

Yet another splendid sunset, just to please the eyes!





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