RV Pelagia Cruise
Report
Cruise 64PE156,
Project ACSEX-I,
Mozambique Channel,
20 March –13 April
2000
H. Ridderinkhof
Chief Scientist
NIOZ, Texel, 2000
Table of contents
nr. Chapter page
1 Cruise
Narrative 5
Highlights
5
Cruise
Summary Information 5
Scientific
Programme and Methods 14
Major
Problems Encountered during the Cruise 19
List of
Cruise Participants 20
2 Underway
Measurements 21
Navigation
21
Echo
Sounding 21
Thermo-Salinograph
Measurements 21
Vessel
mounted ADCP measurements 21
Meteorological
measurements 21
3 Hydrographic
Measurements -Descriptions, Techniques, and Calibrations 23
Rosette
Sampler and Sampler Bottles 23
Salinity
Measurements 23
Oxygen
Measurements 23
Nutrient
Measurements 24
CTD Data
Collection and Processing 25
4 Acknowledgements
25
Appendix A (cruise summary file) 26
Appendix B (mooring information file) 38
The research reported here was funded by the Institute
for Marine and Atmospheric Research of Utrecht University (IMAU, partly via
COACH) and the Netherlands Institute for Sea Research (NIOZ).
1 Cruise Narrative
1.1 Highlights
a: ACSEX-I, RV Pelagia cruise PE156 in Mozambique Channel
b: Expedition Designation (EXPOCODE): 64PE156
c: Chief Scientist: Dr. ir. Herman Ridderinkhof
Netherlands Institute for Sea Research (NIOZ)
P.O. Box 59
1790AB Den Burg/Texel
The Netherlands
Telephone: 31(0)222-369423
Telefax: 31(0)222-319674
e-mail: rid@nioz.nl
d: Ship: RV Pelagia, Call Sign: PGRQ
length 66 m.
beam 12.8 m.
draft 4 m.
maximum speed 12.5 knots
e: Ports of Call: Cape Town (South Africa) to Victoria
(Seychelles)
f: Cruise dates: March 20, 2000 to April 13, 2000
1.2 Cruise Summary Information
Summary
In the afternoon of 20 March RV Pelagia left the quay in
Cape Town, South Africa, and headed for Mozambique Channel to perform
observations in the framework of the Agulhas Current Sources Experiment –I (ACSEX-I).
During transit from 21 to 23 March an XBT line was carried out on the South
African Shelf, roughly following the 100 m isobath from Port Elisabeth to the
edge of the shelf, to the south east of the continent. Along the south-eastern
part of South Africa RV Pelagia followed a route close the coast in order to
bypass the strong southward Agulhas Current. On 24 March an XBT line was
carried out during transit on and from another part of the African shelf, near
Durban. On 26 March the first hydrographic station was occupied, mainly for
testing of the instrumentation.
Here the observations in the framework of the ACSEX-I
program started. Steered by the latest satellite-altimetry observations the
original cruise plan was adapted slightly. The general strategy of the
hydrographic part of the program was to obtain some sections from the African
shelf to the centre of a relatively large anomaly in Mozambique Channel, as
suggested by altimetry. At representative locations on these sections drifters
were launched. During transit between the hydrographic sections an XBT line was
carried out. The first hydrographic section (1) at 15 S started in the late
afternoon on March 26 and was finished in the early morning of March 28,
immediately followed by an XBT line towards the first station of the second
section. This section (2) at 20 S was carried out from early morning March 29
to early morning March 30, again followed by an XBT line towards the first
station of section (3).
This section, at the narrowest part of Mozambique Channel,
was the main section of the ACSEX-I cruise. At the Mozambique side of the
channel moorings with fast recording instruments, ACS01, ACS02 and ACS03, were
deployed for a period of 5-6 days at a waterdepth of 500 m, 1600 m and 2200 m
on March 31 and April 1. These deployments were combined with the first
stations of the hydrographic section across the entire Channel, between the
shelf at the Mozambique side and the shelf at the Madagascar side. This section
could not be finished because the tropical cyclone ‘Hudah’ was expected to
arrive in the area. Therefore work on this section was stopped late in the
evening on April 1 and subsequently RV Pelagia sailed in a north-westerly
direction to hide for Hudah. The regular launching of XBT’s continued while
sailing in this direction. On April 3 early morning RV Pelagia arrived at 12 S,
roughly 300 nm to the north of the main section. Here a short hydrographic
section started at the Mozambique shelf. RV Pelagia sailed back to the research
area on April 3, late evening, sailing at a safe position relative to the
centre of Hudah (roughly 250 nm to the north –east of the cyclone). On April 6,
early morning, RV Pelagia arrived at the Mozambique side of the Channel. The
three short – term moorings were recovered succesfully. From a first inspection
of the data it was concluded that the original design of the moorings did not
need to be adjusted. While servicing the instruments from the moorings a 12.5
hours CTD yo-yo station at a waterdepth of 1200 m, in between mooring ACS01 and
ACS02, was performed. On April 7 the deployment of the long – term moorings was
started at the Mozambique side of the Channel. Five moorings, ACS03,
ACS04,…..,ACS07 were deployed on that part of the section that had been
surveyed previously, before the tropical cyclone entered the area. On April 8
the hydrographic and bathymetric survey of the main section was first completed
before moorings ACS08, ACS09 and ACS10 were deployed on the Madagascar side of
the Channel. The cable length of these moorings needed to be adapted because at
this side of the Channel the bottom slope appeared to be substantially steeper
than predicted from nautical charts. After finishing the deployment of the
moorings on April 9, early morning, RV Pelagia headed for the port of Victoria,
Seychelles. Underway, from April 10 to April 12, while crossing the South
Equitorial Current, an XBT line was performed. RV Pelagia arrived in port
Victoria on April 13, late afternoon, where the scientific crew debarked.
Cruise Track
The cruise was carried out from Cape Town, South Africa to
Victoria, Seychelles. The main research area was the Mozambique Channel where
hydrographic sections were performed and moorings deployed. The complete cruise
track is shown in figure 1.
Figure 1.
Cruise track of RV Pelagia cruise 64PE156, ACSEX-I
Hydrographic Stations
A total of 36 CTD casts was recorded, including one 12.5 hours
CTD yo-yo cast. On 34 of these casts, water samples were taken for the
determinations of nutrient, dissolved oxygen and, less frequent, salinity. A
lowered Acoustic Doppler Curent Profiles (LADCP) was attached to the CTD frame
to measure vertical profiles of the current speed and direction. The positions
of the hydrographic stations are indicated in figure 2.
At the hydrographic stations the SBE9/11+ CTD was lowered
with a speed of about 1 m/s. Due to the use of a bottom indicator switch
we were able to sample to within quite a short distance from the bottom (5 m).
Figure 2. Distribution of hydrographic stations and
sections. The isobaths at 200, 1000, 2000 and 3000 m are indicated.
Hydrographic Sampling
During the up-cast of each CTD/rosette station water up to
25 samples were taken at regular depth intervals. The samples were analysed for
nutrients and oxygen. For calibration purposes also regularly, but less
frequent, samples were analysed for salinity. The vertical distribution of the
sampling locations is indicated in table 1.
|
Depth (m)
|
Samples
|
|
Bottom
|
Salinity, oxygen, nutrients
|
|
3000
|
Nutrients
|
|
2500
|
Salinity, oxygen, nutrients
|
|
2000
|
Nutrients
|
|
1500
|
Salinity, oxygen, nutrients
|
|
1250
|
Nutrients
|
|
1000
|
Salinity, oxygen, nutrients
|
|
900
|
Nutrients
|
|
800
|
Oxygen,
nutrients
|
|
700
|
Nutrients
|
|
600
|
Oxygen,
nutrients
|
|
500
|
Nutrients
|
|
400
|
Oxygen,
nutrients
|
|
300
|
Nutrients
|
|
200
|
Oxygen, nutrients
|
|
150
|
Nutrients
|
|
125
|
Nutrients
|
|
100
|
Oxygen, nutrients
|
|
75
|
Nutrients
|
|
50
|
Oxygen, nutrients
|
|
25
|
Nutrients
|
|
10
|
Nutrients
|
Table 1. Depths at which samples were collected
XBT casts
XBT’s, type T7, where launched both during transit to and
from the Mozambique Channel and in between the hydrographic sections. XBT’s
were launched every 2 hours on each XBT section. 153 XBT’s were used covering
141 locations (12 failed). The position of the XBT casts is indicated on the
map in Figure 3.
Figure 3. Location of the stations where XBT’s have been launched.
Moorings
The moorings ACS01, ACS02 and ACS03 were deployed during the
cruise for a period of 5-6 days at the Mozambique side of the channel on the
main section (3). The position of the moorings and the location and type of instruments
in the cross-section is shown in figure 4. The measuring interval of the
instruments ranged from 0.5 minutes (current meters), 2.5 and 5 minutes
(ADCP’s) to 20 minutes (tiltmeters). All short-term moorings were recovered
succesfully.
The long-term moorings ACS04, ACS05,……, ACS10 were deployed
on the main section, at the narrowest part of the Mozambique Channel, more or
less evenly distributed over the entire section. The position of the moorings
and the location and type of instruments in the cross-section is shown in
figure 5. The measuring interval of the instruments ranges from 20 minutes
(current meters) to 60 minutes (ADCP’s and tiltmeters). These moorings are
scheduled to be recovered during the ACSEX-II cruise in March-April 2001.
Detailed information on the moorings is given in the list in
appendix B.
Figure 4. Position (top) and vertical distribution of instruments in the
short-term moorings (bottom) that have been deployed and recovered during the
ACSEX-I cruise.
Figure 5. Position of long-term moorings in Mozambique Channel (top) and
vertical distribution of instruments in the long-term moorings (bottom).
ARGOS drifters
During the cruise eight ARGOS drifters were deployed. The
drifters used were standard spherical WOCE/TOGA mixed layer drifters (diameter
30 cm), fitted with a holey sock drogue at 15 m. The drogues had a length of 7
m, and a diameter of 1 m. In figure 6 the deployment positions are indicated
with dots.
Figure 6. Positions where ARGOS drifters have been deployed.
*.SUM file
A hard copy of the *.SUM file describing all stations is
added in the appendix A.
1.3 Scientific Programme and Methods
The purpose of the ACSEX-I cruise was 1) to identify the
location of the Mozambique current along the African coast and to determine
it’s strength, 2) to test the existence of large scale eddies in the area as
suggested by altimetry data, 3) to deploy short-term moorings for a period of
one week and 4) to deploy moorings for a period of one year, to be recovered in
March-April 2001. Given the logistic limitations these goals could only be
reached if the available ship’s time was used very efficiently. Therefore,
optimal use had to be made from the latest information, ship borne or from
satellites. This information had to be available more or less real-time. In
practice this information led to slight but essential adaptations of the
original cruise plan, e.g. adaptions of the location of a CTD section for a
better fit on the latest altimetry data.
The general plan to address the scientific goals 1) and 2) was
to obtain some hydrographic CTD sections from the African shelf towards the
center of an eddy (by that time still ‘artificial’) in the Mozambique channel,
the exact latitude (these sections always ran from west to east), length and
distance between stations to be determined by firstly the altimetry and
secondly, for further ‘tuning’, the ‘local conditions’ in the eddy. These local
conditions were determined from upgrading the temperature and salinity contour
plots of the hydrographic cross-sections immediately after a station had been
finished. Other information came from the officers at the bridge: the drift of
the ship proved to be an excellent parameter to determine the local current
conditions. In practice information of the ship’s drift was used not only to
have a ‘check’ on the analyzed CTD data but also to adapt the distance between
successive CTD stations on a CTD section.
In between successive CTD sections an XBT line was carried
out to have additional information on the vertical structure of the temperature
in the area, in order to be able to reconstruct the location and size of the
eddies from two different lines with observations (CTD and XBT) and similarly
for the location and size of the boundary current near the African continental
slope.
To adress the goals 3), the deployment of 3 moorings with
instruments sampling at a high-frequency near the African continental slope for
a period of one week during the cruise and 4) the deployment of 7 moorings
across the narrowest part of Mozambique channel, accurate information on the
depth and on the vertical current structure had to be obtained. Therefore the
deployment of the long-term moorings was done after a CTD section along the
planned ‘mooring-line’ had been completed and first results of the short-term
moorings had become available .
Preliminary Results
All observations that have been analyzed thus far suggest
that no intense, coherent western boundary layer-type current exists in the
Mozambique Channel, but that the current field merely consists of a train of
passing anti-cyclonic eddies that intermittently cause a net poleward flow
along the shelf slope. Moreover, the observations have revealed a significant
Mozambique Undercurrent against the continental slope carrying intermediate and
deep waters of Atlantic origin equatorward into the Channel. Below these
preliminary results are illustrated briefly.
Figure 7. Surface current vectors, averaged between 40-200 m waterdepth, as
observed with the LADCP at the hydrographic stations.
Figure 8. Movement of 2 drifters released at 24 S, and 1 drifter released at 20
S.
Figure 7 shows surface currents, averaged between 40 – 200 m
waterdepth, as observed with the LADCP at the hydrographic sections. The
relatively strong currents, O(1 m/s), in Mozambique Channel (sections 1,2 and
3) form part of anti-cyclonic eddies. Note that the strongest currents are
found (far) away from the continental slope, above the deep part of the
Channel. Near the continental slope no western boundary layer-type current was
found. The suggestion that these strong surface currents are caused by the
presence of anti-cyclonic eddies is confirmed by the movement of the drifters
that have been deployed during the cruise. Figure 8 shows the track of the
drifters that were deployed in the eddies at 20 S (1) and 25 S (2). Strong
anti-cyclonic motions can clearly be recognized. The other drifters show less
regular motion. In general the movement of these drifters is relatively slow
near the continental slope. When their movement is influenced by an eddy strong
cross-channel and/or anti-cyclonic motions are found.
Figure 9 shows the vertical distribution of the northward
component of the currents (from the LADCP observations) at section 1 and
section 3. The eddies are over 300 km wide and penetrate all the way to the
bottom where swirl velocities are still around .10 m/s. These eddies propagate
more or less through the centre of the Channel and produce an intermittent
southward current at the Mozambique side and a northward current on the
Madagascar side. The LADCP observations have revealed also that there is a
remarkable (under)current structure along the Mozambican continental slope.
Inshore of the eddies an undercurrent flows generally northward. At 23 S
(section 1) there is a core at around 1000 m which seems to weaken northward,
but still exists at section 3. At 23 S, around 2500 m deep, a very strong deep
western boundary current is observed with speeds over 60 cm/s. This jet has
weakened considerably at section 3. Probably the main part of this jet is
steered by bottom topography across the Channel.
Figure 10 shows the vertical distribution of salinity and
oxygen at 23 S, section 1. The eddies carry very salty (and warm) tropical
upper layer and thermocline water southward. In their cores, at intermediate
level, they contain relatively salty, low oxygen, Red Sea Water (RSW). At the
same intermediate depth level, the undercurrent core against the Mozambique
continental slope carries Antarctic Intermediate Water (AAIW) with its marked
salinity minimum of less than 34.5 ppt, northwards. Results from the other
hydrographic sections show that strong isopycnal mixing occurs between the AAIW
and RSW in the Channel, northwards of section 1, suggesting that the eddies
eventually provide a shortcut for the AAIW that enters the Indian ocean. The
deep narrow jet at around 2500m depth against the Mozambique slope carries
North Atlantic Deep Water (NADW) northward as can be deduced from its
relatively high salinity and oxygen concentrations.
All preliminary results available from this cruise lead to
the conclusion that a coherent and persistent Mozambique Current along the
shelf edge off Mozambique does not exist. Instead, a regular train of
anti-cyclonic Mozambique Channel eddies, might form a significant link in the
global ocean circulation system.
Figure 9. Vertical distribution of the north-ward component of the current
velocity as observed with the LADCP at hydrographic section 1 (top) and 3
(bottom)
Figure 10. Vertical distribution of salinity (top) and oxygen (bottom) at
hydrographic section 1.
1.4 Major Problems Encountered during the Cruise>
While preparing a Long Ranger ADCP for deployment in a
short-term mooring a short-circuit caused damage to the batteries, the ADCP
housing and the wires connecting the batteries to the instrument. The
instrument could be repaired and functioned well during the short-term
deployment. However, due to a lack of sufficient batteries the sampling rate
during the short-term deployment had to be reduced.
Surface currents in the area appeared to be very strong,
often exceeding 1 m/s. This appeared to give major problems during the
deployment of the moorings. Initially this was done ‘traditionally’ by first
deploying the weights from the side of the vessel and subsequently lowering the
entire cable of the mooring, sailing to the right location and launching the
mooring. However, the tension on the cable appeared to be very strong and
chaving of the cables while sailing could not be prevented. Therefore a
different technique was used to deploy all long-term moorings. These were
deployed from the stern of RV Pelagia, first launching the floats (deployment
‘upside down’) and sailing to the right location before launching the weights.
This method of deployment appeared to work very well in this area with strong
surface currents.
1.5 Lists of Cruise Participants
Scientific crew
|
Name
|
Institute
|
Nationality
|
Function/
Speciality
|
|
Herman Ridderinkhof
|
NIOZ
|
NL
|
Cruise leader
|
|
Theo Hillebrand
|
NIOZ
|
NL
|
Current meters
– CTD
|
|
Margriet Hiehle
|
NIOZ
|
NL
|
Datamanagement – CTD
|
|
Bas Groot
|
NIOZ
|
NL
|
Oxygen
|
|
Marcel Bakker
|
NIOZ
|
NL
|
Nutrients
|
|
Leon Wuis
|
NIOZ
|
NL
|
Moorings
|
|
Karel Bakker
|
NIOZ
|
NL
|
Moorings
|
|
Bob Koster
|
NIOZ
|
NL
|
Electronics
|
|
Caroline Katsman
|
University Utrecht
|
NL
|
PhD student –
CTD
|
|
Mathijs
Schouten
|
University
Utrecht
|
NL
|
PhD student –
CTD
|
|
Johann Lutjeharms
|
University Cape Town
|
South Africa
|
General hydrography
|
|
Marjolaine Krug
|
University Cape Town
|
South Africa
|
Air-sea interaction
|
|
Jamaloodien Shaheen
|
University Cape Town
|
South Africa
|
Student UCT hydrography
|
|
Daan van
Schooneveld
|
|
NL
|
Physician
|
|
John Bemiasa
|
|
Madagascar
|
Observer
|
2 Underway Measurements
2.1 Navigation
Differential GPS receiver for the determination of the
position. The data from the receiver were recorded every ten seconds in the
underway data logging system. After removal of a few spikes these data were
sub-sampled every minute.
2.2 Echo Sounding
The 3.5 kHz echo sounder was used on board to determine the water
depth. The uncorrected depths from this echo sounder were recorded in the
underway data logging system. Over the steepest parts of the continental slope
and the ridges in the Channel, the depth digitizer of the echo sounder was
occasionally not able to find a reliable depth.
Preceding the deployment of the current meter moorings one 2
or 3 lines were surveyed to determine the mooring sites. During these surveys
the echo sounder data were also recorded on paper chart in order to allow hand
digitizing over the parts of the slope where the automatic digitizing failed.
2.3 Thermo-Salinograph Measurements
The Sea Surface Temperature, Salinity, and Fluorescence were
measured continuously with an AQUAFLOW thermo-salinograph system with the water
intake at a depth of about 3 m. For the calibration of the salinity sensor
water samples were taken regularly.
2.4 Vessel mounted ADCP measurements
A 75 kHz vessel mounted ADCP (RDI) recorded the current
field continuously, starting at the first hydrographic (test) station and
ending at the last XBT cast. First inspection of the raw dataset suggests that
the quality of the dataset is good. However, these data require considerable
effort to become a coherent set. The post-processing includes calibration with
data from the lowered ADCP.
2.5 Meteorological measurements
Air temperature and humidity, relative wind velocity and
direction as well as air pressure were measured and recorded by the underway
logging system. In addition specific sensors were installed by participants
from University of Cape Town which are described below:
Radiation measurements:
To calculate the net heat budget at the surface, measurement
of the incoming short wave and long wave radiation was required. The outgoing
long wave radiation is calculated as a function of the sea surface temperature
and the outgoing short wave radiation is taken as a fraction of the incoming
short wave radiation (7%). The incoming short wave radiation was measured with
an Eppley precision spectral pyranometer. It has a thermopile detector with a
long term stability. The pyranometer has two hemispheres, with the inner
hemisphere blocking the infrared radiation from the outer one. The incoming
long wave radiation was measured with an Eppley precision infrared radiometer.
It measures the exchange of radiation between a horizontal blackened surface
and the sky. Both devices were sampled with a Campbell CR10 datalogger system.
Backup relative humidity and temperature measurements:
Relative humidity and temperature measurements were supplied
by a Vaisala HMP35D relative humidity sensor, housed in a radiation shield.
This instrument utilises a HUMICAP, a thin film capacitive sensor that has
become a research standard (Meisä, 1993). The humidity and temperature
were sampled every 10s with the datalogger system.
CR10 Datalogger:
The CR10 is a measurement and control system protected in a
sealed and rugged canister. It provides sensor measurement, timekeeping,
communication, data reduction and data and program storage. The datalogger
interfaces with a wiring panel where the slow response sensors (pyranometer and
radiometer) can be powered and sampled. Communication was also maintained with
a laptop computer so that the data stored on the datalogger could be backed up
to magnetic tape daily.
3 Hydrographic measurements - Descriptions, Techniques,
and Calibrations
3.1 Rosette Sampler and Sampler Bottles
A 25 position rosette sampler was used, fitted with NOEX
sampler bottles. A multi-valve system, developed at NIOZ, allowed closing the
sampler bottles by computer command from the CTD operator. The general
behaviour of the samplers was good. Only a few samples are considered to be
suspect because of sampler failure. No errors in the functioning of the rosette
sampler itself were detected.
3.2 Salinity Measurements
Water was drawn from the samplers into a 0.5 litre glass
sample bottle for the salinity determination after 3 times rinsing. The sample
bottles had a massive rubber stopper as well as a screw lid. Salinity of water
samples (SALNTY) was determined on board by means of an Guildline Autosal 8400A
salinometer. The salinometer was used in a laboratory container, fitted with an
air conditioning system. This kept the surrounding air temperature constant
within 1°C. The readings of the instrument were performed by computer, giving
the average and statistics of 10 consecutive readings. For each sample 3
salinity determinations were carried out. From each deep CTD/rosette cast an
extra duplicate sample was drawn. Salinity determinations from the duplicate
samples obtained from independent runs were used to determine the
reproducibility of the salinity determination.
SALNTY was compared with the salinity reading from the CTD
(CTDSAL), for samples obtained below 2000m depth. The mean difference
SALNTY-CTDSAL was less than 0.001 and similar to the differences obtained
during the previous cruise, RV Pelagia cruise 64PE155 (MARE 1). It was decided
to apply no offset correction to CTDSAL (-0.0009).
3.3 Oxygen Measurements
For the oxygen determination water samples were drawn in
volume calibrated 120 ml pyrex glass bottles. Before drawing the sample each
bottle was flushed with at least 3 times its volume. When the samples were
drawn the temperature of the sample was determined. The determination of the
volumetric dissolved oxygen concentration of water samples was carried out by
means of a spectro-photometer Winkler technique, recently developed at NIOZ
[see Su-Chen Pai et al., Marine Chemistry 41 (1993), 343-351]. Before and after
the cruise the spectro-photometer were inter-calibrated with a automatic end
point determination Winkler method. The stock solution of KJO3 used
in the analysis was prepared and calibrated in the laboratory by using
gravimetric methods. The stock solutions were stored at low temperature (~4°
C).
At each cast duplicate samples were taken from the deepest
and shallowest sampler, and occasionally from a sampler at an intermediate
level in order to determine the precision of the analysis. From the volumetric oxygen
concentration in mmol/dm3 the densimetric oxygen concentration in
mmol/kg (OXYGEN) was determined by dividing by the sample density at sample
temperature and salinity.
3.4 Nutrient Measurements
From all sampler bottles samples were drawn for the determination
of the nutrients silica, nitrite, nitrate and phosphate. The samples were
collected in polyethylene sample bottles after three times rinsing. The samples
were stored dark and cool at 4° C. All samples were analysed for the nutrients
silicate, phosphate, nitrate and nitrite within 10 hours with an autoanalyzer
based on colorimetry. The lab container was equipped with a Technicon TRAACS
800 autoanalyzer. The samples, taken from the refrigerator, were directly pored
in open polyethylene vials (6ml) and put in the auto sampler-trays. A maximum
of 60 samples in each run was analysed.
The different nutrients were measured colorimetrical as
described by Grashoff (1983);
Silicate reacts with ammoniummolybdate to a
yellow complex, after reduction with ascorbic acid the obtained blue
silica-molybdenum complex was measured at 800nm (oxalic acid was used to
prevent formation of the blue phosphate-molybdenum).
Phosphate reacts with ammoniummolybdate at pH
1.0, and potassiumantimonyltartrate was used as an inhibitor. The yellow
phosphate-molybdenum complex was reduced by ascorbic acid to blue and measured
at 880nm.
Nitrate was mixed with a buffer imidazole at pH
7.5 and reduced by a copperized-cadmium coil (efficiency> 98%) to nitrite,
and measured as nitrite (see nitrite). The reduction-efficiency of the
cadmium-column was measured in each run.
Nitrite was diazotated with sulphanilamide and
naftylethylenediamine to a pink coloured complex and measured at 550nm.
The difference of the last two measurements gave
the nitrate content
Calibration standards were prepared by diluting stock
solutions of the different nutrients in the same nutrient depleted surface
ocean water as used for the baseline water. The standards were kept dark and
cool in the same refrigerator as the samples. Standards were prepared fresh
every two days. Each run of the system had a correlation coefficient for the
standards off at least 0.9998. The samples were measured from the surface to
the bottom to get the smallest possible carry-over-effects. In every run a
mixed control nutrient standard containing silicate, phosphate and nitrate in a
constant and well known ratio, a so-called nutrient-cocktail, was measured, as
well as control standards, sterilized in an autoclave or gamma radiation. These
standards were used as a guide to check the performance of the analysis and the
gain factor of the autoanalyzer channels. The reduction-efficiency of the
cadmium-column in the nitrate lane was measured in each run.
The autoanalyzer determined the volumetric concentration
(mmol/dm3) at a temperature of 20° C. In order to obtain the
densimetric concentration in mmol/kg the volumetric concentrations were divided
by the density of sea water at 20° C, sample salinity, and zero sea pressure.
3.5 CTD Data Collection and Processing
The SBE 9/11+ CTD was fitted with temperature sensor SN1219
and conductivity sensor SN1046. For the data collection SEASAVE software,
version 4.218, supplied by SBE, was used. The CTD data were recorded with a
frequency of 24 data cycles per second. On-line a correction was applied for
the sampling time difference due to the forced flushing through a tube system
between temperature and salinity sensor. After each CTD cast the data were
copied to a hard disk of the ship's computer network, and a daily back-up copy
was made on tape. Back on Texel these data have been downloaded into the NIOZ
computer network. Separate copies of the back up were taken directly from
Victoria to Texel.
The up-cast data files were sub-sampled to produce files
with CTD data corresponding to each water sample, taken with the rosette
sampler. After the determination of the final calibration of the CTD system
these values were corrected accordingly.
After the cruise the raw down-cast CTD data were processed
with the SEASOFT software supplied by SBE. A correction was applied for the
temperature change between the temperature and conductivity sensor due to heat
exchange with the flushing tube and conductivity sensor, and for different
response times of both sensors. The correction factors for these corrections
were determined empirically from CTD stations over the continental shelf. At
these stations the vertical temperature gradient in the seasonal thermocline
were largest, and so were the resulting salinity spikes. The correction
settings, determined for the 1997 TripleB data and confirmed during Pelagia
cruise 155 appeared to be adequate for the corrections. Mean values of the
readings were produced for 1 dbar pressure intervals. Consecutively the parameter
values in physical units were determined using the final calibration constants.
4. Acknowledgements
The research reported here was funded by the Institute for
Marine and Atmospheric Research of Utrecht University (IMAU, partly via COACH)
and the Netherlands Institute for Sea Research (NIOZ). I thank the ships crew
and the personnel of the supporting technical departments of NIOZ for their
professional support and active participation in the preparation and execution
of the ACSEX-I cruise reported here. The contributions from IMAU (prof. De
Ruijter and collegues) are highly appreciated, firstly because of their supply
of the latest satellite-altimetry data during the cruise and secondly because
of the scientific discussions before, during and after the cruise. Also, the
active participation of the University of Cape Town (prof. Lutjeharms and
students) is highly acknowledged. Finally collegues from the NIOZ department of
Physical Oceanography (especially dr. Hendrik van Akern) are thanked for their advice.
Appendix A
cruise summary (*.SUM file) of Pelagia cruise 64PE156
Appendix B
Mooring information file of Pelagia cruise 64PE156
