GEP&CO uses the voyages of a merchant ship in order to ensure the frequency of cruises on a long track from le Havre to Nouméa via Panama. A laboratory has been installed onboard of this ship. The list of measurements and sampling that are routinely made is detailed here below.
Pigments determination (HPLC)
Filtrations (2 to 4 liters, on Whatman GF/F filter) are made every 4 hours. Then, the filters are stored at – 80°C. The measurements are made at the Station Marine d'Arcachon. The following pigments are measured :
- Chlorophyll a, b, c, divinyl-chlorophyll a and pheopigments
- 19' hexanoyloxyfucoxanthin
- 19' butanoyloxyfucoxanthin
Pigments determination ( spectrofluorimetry)
Filtrations (0,5 liter, on Whatman GF/F filters) are made every 4 hours. The small filtered volume presents two advantages : a gain of time on one hand, and also avoiding the pigments degradation that can occur when filtration last longer than reasonable. Filters are stored at – 80°C, and measurements are made at LODyC using the project’s spectrofluorimeter. These measurements are intended to measure the pigments concentration of the chlorophyll group : chlorophyll a, b and c, pheophytin a, b and c, divinyl chlorophyll a and b, and divinyl pheophytin a and b (Neveux et Lantoine, 1993). We also measure phycoerythrin, and the ratio of phycoerythrobilin to phycourobilin on separate membrane filters (filtered volume : 0.1 liter), using a new method (Han and Dandonneau,in preparation).
by flow cytometry
Analyses of the picophytoplankton by flow cytometry need only a small water volume, but quality of the results strongly depends on sampling conditions and storage, because picoplankton cells are fragile. Sampling is made at the intake of the ship’s engine cooling system. Glutaraldehyde is added for fixation, and storage is at – 80°C until analysis in the laboratory. Routine counts give the numbers of Prochlorococcus, Synechococcus, Picoeucaryotes (according to Partensky et al., 1996), and, after addition of SYBR Green dye, heterotrophic bacteria (Marie et al., 1997).
Reflectances are measured using the SIMBADA radiometer developed at LOA by Pierre-Yves Deschamps. It has been designed to be operated from ships of opportunity. The new SIMBADA measures at the standard wavelengths of satellite-borne sea color sensors, and also have a channel un the UV, as red tide phenomena seem to have a strong signature in the UV. The measurements must avoid the glitter of the water, the white caps raised by the ship’s motion, and the very vicinity of the ship where the irradiance field is modified. These conditions are not easy to fulfill in all occasions. Moving the radiometer at angles between 30° and 60° seems to give good results.
Nutrients measurements are made after the cruises at the Station Marine d’Arcachon on samples fixed with mercury chloride, and stored at -80°C in plastic tubes, 20 cc. Nitrate, nitrite, silicate, and phosphate are measured, using an autoanalyser.
Colored dissolved organic matter(CDOM)
Light absorption spectra (200 to 800 nm) by CDOM are made onboard using a spectrophotometer. Measurements are made immediately after filtration on 0.2 µ Nuclepore filters. A blank measured on Milli-Q water is subtracted to the absorption spectra.
Total inorganic carbon and alkalinity
Samples are taken every 4 hours in glass bottles, 0.5 l, poisoned with mercury chloride, and stopped with Apiezon grease. Measurements of TCO2 and alkalinity are made at the Laboratoire de Biogéochimie et de Chimie Marines in Paris, using a potentiometric technique developed at LBCM.
CO2 partial Pressure
A CARIOCA sensor (Lefèvre et al., 1993) will be installed onboard on the thermosalinograph circuit, allowing to measure pCO2 every hour along track. The adaptation of the sensor is being made in collaboration with the Technical Division of INSU. A validation of the results will be made by comparison with pCO2 estimates made using TCO2, alkalinity, salinity and temperature data.
As the main objective of GeP&CO is to describe and understand the variability of phytoplankton populations, a general methodology has to be developed to deduce the composition of the phytoplankton from the above measurements. There is no straightforward method for this. Mackey et al. (1996) proposed an automatic computing procedure, CHEMTAX, that is intended to convert pigments inventories into phytoplankton classes abundance. While we can use this technique, we think that it cannot give unambiguous results given the high number of degrees of freedom, and an approach based on comparison with published results will be favored.
measurements and pCO2 : A thermosalinograph is working permanently
onboard of Conbtship London. This instrument was bought and is managed
by the physical oceanographers at the Centre IRD de Nouméa,
as a part of french Programme National d’Etude du Climat (PNEDC).
A Carioca pCO2 sensor is being modified by DT INSU and will be placed
onboard of Contship London just after the thermosalinograph.
Nutrients : measurements are made using the Technicon autoanalyser at the Station Marine d’Arcachon.
Filtration devices: partly purchased for GeP&CO, permanently onboard of Contship London
Spectrofluorimetry : A Hitachi spectrofluorimeter was bought by IRD in 1999.
HPLC : existing equipment at the Station Marine d’Arcachon has been completed (automatic refrigerated sampler, ternary pump).
Flow cytometry : cytometer bought by IRD in 1993, actually at University of La Réunion.
Colored dissolved organic matter : Beckman spectrophotometer financed by IRD
Light absorption by particles and phytoplankton : Beckman spectrophotometer financed by IPSL.
Marine reflectances: SIMBADA radiometers of the Laboratoire d’Optique Atmosphérique in Lille.
Refrigeration : two deep freezers (-80°C) : one, 30 l, purchased on PROOF financement, onboard of Contship London, and one, 90 l, purchased by IPSL.
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