|In this section, brief descriptions
are provided of the instrument we deploy from the ship
during monthly cruises.
|CTD and water sampling
The CTD/rosette package
is based on a Sea-Bird Electronics (SBE) 911 plus, equipped
with the following sensors:
- Pressure (Digiquartz Paroscientific,
- Temperature (SBE 3),
- Conductivity (SBE 4),
- Dissolved oxygen (SBE 13 until January 2003, and
then an SBE 43), and
- Altimeter (Datasonics PSA 900).
The sensors are mounted on a SBE 32 rosette with
11 12L Niskin bottles (the 12th being replaced by
an AC-91 as discussed below).
The pressure sensor is not regularly calibrated (last
calibration in 1995), it is known from past experience
with this type of sensors that the uncertainty is on
the order of 0.5 dbar. It is indeed acknowledged that
this type of sensor only experience a drift of the zero,
which is easy to control and to correct for when the
sensor is at the surface.
The temperature sensor is sent once a year for calibration
to SBE, which has the agreement from the NIST (National
Institute of Standards and Technology) for this type
of work. Fig. 11 provides an example of the calibrations
performed in January 2002 and January 2003 (was about
the same between January of 2001 and January of 2002).
The drift is of the order of 0.0025oC over one year, and
it is linear. Therefore, any temperature value computed
between two of these calibrations is provided with a
The conductivity sensor is sent once a year to SBE
for calibration. Additional calibration against values
determined from water samples and subsequent lab analyses
(with a lab salinometer) are not performed. Fig. 12
provides an example of the calibration performed in 2001.
The change of the calibration slope is due to biological
fouling that grow inside the sensor, modifying its geometry.
The influence of this fouling is larger as the conductivity
increases. Typical values of the BOUSSOLE site are of
the order of 5Siemens m-1. It is therefore estimated that
the sensor drift over one year is of the order of 0.0005
in conductivity units, which is about 0.005 in salinity
units. The drift was a bit higher in 2002, around 0.001
in conductivity units, which is about 0.01 in salinity
From July 2001 to December 2002, a SBE 13 sensor
was used, with a Beckman membrane. This sensor showed
a large drift. Starting in January of 2003, a new sensor
is used (SBE 43), which is provided with a nominal
5% uncertainty and a less than 2% drift. No Winkler-based
calibration are performed on these sensors.
At each cast, a first round trip down to 50m is performed
with the CTD, in order to rinse all instruments and equilibrate
them in temperature. Then the package is lowered down
to 400m at a descent rate of about 0.5ms-1. The data are
collected at a 24Hz frequency, and then processed through
the SeaSoft software until binned values are produced
at a 1m resolution. To each data file is associated a
header file that contains the necessary ancillary information
(latitude, longitude, date and time of acquisition, meteorological
Some of the sampling depths are fixed (i.e., 5 and
10m for surface analyses) and others are determined at
the end of the rosette descent, based on the phytoplankton
fluorescence profile, in order to best represent any
specific features of the vertical profile. No sampling
is done deeper than 200m. Bottles are closed during the
ascent of the rosette. When back on deck, 2.8L plastic
bottles are used to collect water that is used for subsequent
filtration. Filters are then stored in Petri slides,
wrapped in aluminum and stored first in liquid nitrogen
on board then in a -80oC freezer in the laboratory. The
material collected on filters is then used for HPLC-based
analyses of the phytoplankton pigments (Sect. 7.1)
and filter pad absorption (Sect. 7.2). For the surface
samples, triplicate filtration are performed for the
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Calibration drift of the temperature
sensor mounted onto the CTD.
Calibration drift of the conductivity
sensor mounted onto the CTD.
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A Chelsea MK III Acquatracka fluorometer is installed
onto one of the analog channels of the CTD SBE 911. This
instrument is equipped with an excitation filter at 430nm
and an emission filter at 680nm. The data are provided
both as voltages and chlorophyll concentrations, the
latter being computed with the initial calibration coefficients
provided by the manufacturer. These concentrations are
just indicators. Another calibration of the fluorometer
signal can be performed against the HPLC-derived discrete
chlorophyll concentration values.
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One of the rosette bottles is replaced by a 25cm path
AC-9 plus sensor. This instrument measures the absorption
and attenuation coefficient of the water at nine wavelengths
(i.e., xxx). In order to visualize and collect the data
in real time, this AC-9 is linked to the surface through
a 19,200 baud serial connection that as well provides
power to the instrument. Although the two sensors can
be independently controlled, only one computer is used
in order to provide the same time frame and depth measurements
to both the CTD and the optics data. The connection is
only working in the computer-to-AC-9 direction, although
theoretically both ways are possible. This is due to
the different transmission speeds (300 baud from the
computer to the SBE 911 and 19,200 from the SBE 911 to
Figure 13 displays the cabling of the complete
system, including the AC-9 and the other optical sensors.
A SBE 25 pump is installed at the entrance of
the AC-9 water circuitry, and it is coupled with the
pump that provides water to the conductivity sensor.
Both are only functioning under water, when a certain
conductivity threshold is reached.
The AC-9 is programmed as to start the following
acquisition sequence immediately after being powered
by the CTD sensor: 15s time lag before starting acquisition,
then a 60s warming period, then a 60s period allowing
for cleaning of the water circulation, and then a 35min
data collection period. The latter is enough for the
round-trip down to 400m, including the stops for bottle
Note that the starting sequence of 135s is occurring
during the initial round trip down to 50m. This procedure
ensures a progressive and full temperature rise of the
electronics and a correct water circulation into the
Data collection is performed by the WetView software
v5.0a, as provided by the manufacturer.
Cabling of the AC9 – CTD
A WETLabs ECO-BB3 backscattering
meter is connected onto one of the auxiliary ports of
the AC-9 plus (serial port with a 19,200 baud speed).
This sensor emits light at 440, 532, and 650nm, and measures
the backscattered light at the same three wavelengths.
Power is also provided by the CTD. The data cannot however
be visualized in real time. They are stored into the
AC-9 plus memory, and then collected once the rosette
is back on the ship's deck and the 35min acquisition
sequence of the AC-9 is closed.
|WetStar CDOM fluorometer
A WETStar CDOM fluorometer is plugged
into one of the analog ports of the CTD. This instrument
has an excitation wavelength at 370nm and an emission
filter at 460nm. It is supposed to provide a qualitative
measure of the Colored Dissolved Organic Matter (CDOM).
It is connected to the CTD water circulation; the water
goes from the conductivity sensor to the dissolved oxygen
sensor, then to the CDOM fluorometer and to the pump.
The data are collected in parallel to the CTD data and
processed in real time through the SeaSoft software.
The raw signal is provided as well as a CDOM concentration
computed with the calibration coefficients provided by
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The Satlantic SPMR (SeaWiFS Profiling
Multichannel Radiometer) free-fall radiometer is used
at LOV since 1995, and in many other laboratories around
the World for now 8 years. The LOV version of this profiling
instrument is equipped with two irradiance sensors, collecting
the upwelling and downwelling plane irradiances at the
following 13 wavelengths: 412, 443, 456, 490, 510, 532,
560, 620, 665, 683, 705, 779, and 865nm (the latter was
replaced by 380nm after July 2003).
The SPMR Profiler is made of a long pressure case (1.2m,
9cm diameter) that contains the majority of the system
electronics, while the optical sensors are located and
separately housed at either end of the case and facing
in opposite directions (i.e., up and down). The top end
of the instrument has buoyant fins to stabilize the instrument's
underwater free fall deployment, and the bottom end has
a small annular lead ballast to further stabilize the
orientation and provide for fine tuning of the free-fall
The light sensors used for measuring irradiance (in
units of milliwatts per square centimeter per nanometer),
have a black Delrin plate on the end. The plate contains
13 specially-designed, diffuser-based, cosine collectors.
Tilt and pressure are recorded at the same frequency
as the irradiance measurements (6Hz).
The SPMR is accompanied by a deck reference sensor,
called the SeaWiFS Multichannel Surface Reference (SMSR).
This sensor is equipped with the same 13 wavelengths
as the SPMR, and is based on the same electronics. Data
acquisition is synchronized between the SPMR and the
SMSR and it is performed again at the same 6Hz frequency.
The absolute calibration (Sect. 6) of the SPMR
and SMSR with respect to NIST-traceable standards is
performed every six months in the Satlantic optics calibration
laboratory, and it is tracked between these absolute
calibrations using at the LOV the ultra-stable portable
light source developed for that purpose by Stalantic,
i.e., the SeaWiFS Quality Monitor, 1=SQM-II1 (Hooker
and Aiken 1998). Combining these two elements allows
a 3% maximum uncertainty to be maintained on the calibration
of the SPMR and SMSR.
An SPMR profile starts when the instrument has reached
a distance of 50m off the ship stern (the ship is 25m
long and a mark on the cable indicates the 50 distance).
The instrument is then released and falls at approximately
0.5ms in the water column, collecting data at a 6Hz frequency.
The descent is stopped when the pressure sensor indicates
a depth of about 150m, except in extremely clear waters
where the profile is performed down to 200m. This technique
allows to steer clear of the ship shadow, and to get
measurements with tilt angles less than 2. The sun is
usually on port side of the ship, which is anyway not
so important precisely because the ship shadow is not
affecting the measurements.
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The SIMBADA instrument is an above-water radiometer
designed and manufactured by the University of Lille,
France. It is an upgraded version of the SIMBAD (reference
?) above-water radiometer. It measures both water-leaving
radiance and aerosol optical thickness in 11 spectral
bands centered at 350, 380, 412, 443, 490, 510, 565,
620, 670, 750, and 870nm by viewing the ocean surface
(ocean-viewing mode) and the sun (sun-viewing mode) sequentially.
The same optics (field-of-view of 3o), interference filters,
and detectors are used in both ocean-viewing and sun-viewing
mode. Different electronic gains are used for each mode.
The optics are fitted with a vertical polarizer, to reduce
reflected skylight when the instrument is operated in
ocean-viewing mode. A small GPS is fixed in the front
panel of the SIMBADA for automatic acquisition of geographic
location at the time of measurement. Viewing angles are
Viewing of the ocean must be made in clear sky conditions
(3/4 of sky cloudless, and no clouds obscuring the sun),
outside the sun glint region (relative angle between
solar and viewing directions of 45-90o), and at a nadir
angle of about 45o. For those angles, reflected skylight
is minimized as well as residual ocean polarization effects.
The measurements can be made on a vessel underway, so
there is no need to stop the ship to make measurements.
To normalize water-leaving radiance, incident solar irradiance
is not measured, but computed using the aerosol optical
The operator can select, in addition to ocean-viewing
and sun-viewing modes, dark current and calibration modes.
Each series of measurements lasts 10s. Frequency of measurements
is about 8Hz. Data is stored internally and downloaded
onto diskette at the end of the day or a cruise. The
instrument is powered by batteries that can be charged
using a main supply of 110-240V, 50-60Hz. About 2h of
charging should be enough for one day of measurements.
The SIMBADA has been used from the bow of the ship
or from its upper superstructure when the weather was
allowing to do so, which were the two more convenient
locations where we were able to be sufficiently high
above the sea surface. This is important, because any
ship shadow effect or perturbation from reflection on
the ship superstructure is minimized when increasing
the vertical distance between the operator and the sea
surface, and then the horizontal distance between the
point of the sea surface which is aim at and the ship
The full sequence of measurements was as follows:
- Three dark current recordings, each 10s.
- Three sun
viewing, each of 10s (for derivation of the aerosol
- Three or four sea viewing, each
- Three sun viewing, each of 10s (for derivation
of the aerosol optical thickness).
dark current recordings, each 10s.