25 October 2011
On Saturday, October 22nd, we begin the last sampling station of our leg between Honolulu and San Diego, 200 nautical miles from the American coast.
The sky is grey, water temperature is 16º C, and on deck, the crew is bustling about in heavy gear. The sailors and scientists are all nostalgic, having left behind beautiful Hawaiian weather, and hope that the famed California sunshine will appear. What we are sampling, so near the coast, is a thin upwelling created by a Californian current.
An upwelling is water from the depths that rises to the surface, much colder and rich in nutrient minerals (nitrates, phosphates, etc.) This richness generates an increase in plankton growth, resulting in higher chlorophyll concentrations detected by satellite imaging. However, the currents evolve rapidly and the richest zone can be difficult to find. A couple of cloudy days prevent us from determining precise temperature and chlorophyll distribution, and the daily oceanographic charts can’t keep the pace. Satellite images provide only fragmentary information.
Thanks to onboard temperature, salinity and chlorophyll sensors, the drylab computers indicate areas where chlorophyll is concentrated at the surface: the color-coded line oscillates from dark blue (poor) to red (rich), suggesting relative abundance along our route. This colored mark, a thin slice on the map, pushes us to imagine the form and structure of the upwelling, and to plan a 48-hour station.
At noon, the scientists realize that the morning’s sampling is relatively poor compared to the maximal chlorophyll observations carried out during the night. After consulting with captain Hervé Bourmaud and the team, our head scientist Isabelle Taupier Letage makes the decision to reposition the boat 40 nautical miles back, and to start a new long station the following day. It’s better to retrace our steps in a familiar sector rather than take a risk in unknown zones.
On Sunday, October 23, the nets and pumps are hard at work. Jérémie Capoulade observes an important concentration of diatoms, types of phytoplankton that absorb large quantities of mineral nutrients to synthesize organic material via photosynthesis. We’ve definitely managed to put Tara’s hull into the California current.
And finally on Monday, October 24, the California sun rises over a slick-surfaced sea. We can see the reflection of the boat in the water, smooth as a mirror. Below the surface are colonies of salps and medusas swimming in rich, green-tinted water sparkling with prisms of light. At 16:30 we’re ready to continue towards San Diego with no regrets.
Some comments from the lab:
Jérémie Capoulade: “Observing plankton onboard Tara is not an easy job. You have to be patient to capture images of these small animals that are constantly moving with the incessant rolling of the boat. But all in all it’s totally rewarding to observe the beauty and elegance of these specimens.”