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The CCAMLR krill synoptic survey
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CTD sampling protocol
Introduction
These draft protocols are posted prior to agreement in order that
their development will attract constructive criticism and input from qualified
individuals.
It is recognized that the survey should collect data to World Ocean
Circulation Experiment (WOCE) standards, however it is acknowledged that
this may not be feasible for all participants. Notwithstanding this standard
is recommended where possible. Participants are therefore directed towards
the WOCE internet web site http://whpo.ucsd.edu/manuals.htm,
in particular the pointers to CTD methods.
Objectives
The main objective of sampling with a conductivity-temperature-depth (CTD)
package is to identify environmental characteristics of the survey area,
in particular the water masses which influence krill distribution and transport.
A further objective of the survey is to identify the approximate geographic
location of important fronts and to estimate geostrophic currents.
Equipment
Standard CTD
The preferred CTD unit for the CCAMLR Synoptic Survey is a Sea-Bird 911
plus. The nations (Japan, UK and the USA) currently participating
in the survey each possess CTDs of this type. This CTD has three sensors;
a series 410K-105 Digiquartz pressure transducer, a SBE 3 plus temperature
sensor, and a SBE 4C conductivity sensor. In the standard configuration
of the Sea-Bird 911 plus the SBE 3 plus and the SBE 4C sensors are connected
to a SBE 5 T submersible high-speed pump.
The preferred mounting orientation of the CTD is horizontal, this is
so that the flow of water within the rosette frame is as good as possible
on both the down-cast and the up-cast.
Accessory instrumentation
It is highly desirable that the CTD package carry a suitable means
of independently verifying water temperatures at standard water bottle
sampling depths, for example reversing thermometers or an SBE 35 Reference
Temperature Sensor. For stations where the bottom of the cast is likely
to be near the sea bed, a suitable means of detecting the bottom is required,
for example an acoustic pinger (Japan and UK), or an altimeter (USA).
Such additional equipment, together with any other ancillary equipment
mounted on the frame should not compromise water flow over the CTD sensors.
This CTD package should be fitted with adequate ballast to ease deployment
in heavy seas.
Water sampling equipment
The CTD should be mounted within a suitable carousel water sampler capable
of carrying sufficient suitably sized sampling bottles. The bottles themselves
should have external closure mechanisms, or internal Teflon-coated springs.
Data acquisition
The CTD data should be logged via the SBE 11 plus deck unit to a suitable
PC (Pentium or high-level 486) running Seasoft Data Acquisition Software
(Sea-Bird Electronics Inc.).The preferred version is Seasoft version 4.221,
or later, for DOS. Problems have been reported with Seasoft for Windows,
though these may now have been fixed.
It is recognised that it is not possible to standardise the post processing
software. Japan currently uses CONT3D (NRIFSF, Shimizu), the USA uses Ocean
Data View version 4 (AWI, Bremerhaven) and the UK uses PEXEC (IOS, Southampton).
Japan is currently considering changing to Ocean Data View.
Auxiliary equipment
A suitable laboratory salinometer is required for determining water bottle
salinities. Each of the participating nations (Japan, UK and USA) currently
possesses a Guildline Autosal 8400B.
Pre-cruise sensor calibration
All sensors on the CTD and on accessary, or auxiliary equipment should
be laboratory calibrated both prior to, and following the cruise. The calibrations
should be carried out by a professional laboratory appropriate to the task.
Details of the calibrations should be archived for future reference. Any
spare sensors should also be calibrated.
Pre-survey software setup
Prior to the survey the current sensor calibration parameters should be
set within the Seasoft SEASAVE module. This will only need to be carried
out once, unless the sensors on the CTD are changed during the survey.
Various other SEASAVE options should also be set prior to data acquisition.
It is recommended that the recording rate should be set to the maximum
24 scans for each channel per second. The header data should be kept to
a minimum, but should include vessel name and cruise number. It is also
suggested that the PC status line variables should be set to display pump
status, pressure, temperature (IPTS-90), salinity, and density (?0), and
that the display type should be set to be overlaid X-Y plots. It is recommended
that the display should include pressure (Y-axis) versus each of temperature
(IPTS-90), salinity, and density (?0) (X-axis). It is likely that the scale
for these X-Y plots will require alteration throughout the cruise.
Test deployment
A test deployment should be carried out prior to the main survey. This
cast should be treated on a par with all survey stations. The test deployment
should be to a depth equivalent to the deepest planned deployment
in the survey. Water samples should be taken at pre-planned depths.
Station protocol
Pre-station preparations
Immediately prior to arriving on station, the water sampling bottles should
be cocked and the CTD readied for launch. The depth to the sea bed
should be monitored to see if it is necessary to use an acoustic pinger
or altimeter. Communication should be established between the winch driver,
the deck operators and the CTD laboratory.
Prior to data acquisition the PC clock should be synchronised with GMT.
Local times and the time zone should also be noted.
The Seasoft module SEASAVE should be executed in preparation for data
acquisition. Appropriate files should be setup in readiness for the cast.
Although a common file naming convention is desirable, it is unlikely to
be achieved, given the existing protocols on board each vessel. It is therefore
paramount that station identification is possible from each national convention.
On-station deployment
When the ship is settled on the station the CTD may be deployed. The exact
latitude and longitude should be noted. The water depth and time (GMT)
should also be recorded. As the CTD is deployed the SBE 11 deck unit should
be turned on and the rosette firing mechanism should be armed. As the package
enters the water the winch cable metering system should be zeroed, or the
amount of cable out recorded. The CTD package should be halted at
approximately 8 metres depth, where it should be left to equilibrate. During
this period the status of the SBE 5 T pump should be monitored to ensure
that it switches on. After the equilibration period (dependent upon instrument
load) the CTD package should be hauled to the surface after which the main
deployment should commence. The depth and time (GMT) at the start of the
main descent should again be recorded.
The down-cast
The CTD package should be veered at a rate not exceeding 70 m min-1. Near
the surface, this rate may be less to reduce problems associated with the
heave of the ship. The CTD should be lowered to the desires depth
and for stations where the cast will approach near-bottom the acoustic
pinger or altimeter should be monitored. The CTD package should be lowered
at the full speed until the sensors (wire-out, ships sounder, CTD pressure
sensor, altimeter, acoustic pinger) indicate that the package is approximately
30 m above the bottom. At this point the veering speed should be reduced
and the winch finally stopped with the CTD a safe height above bottom.
The distance above the sea-bed will depend upon bottom topography, current
speed, weather conditions, wire out, and a range of other factors specific
to each deployment and platform. The maximum depth of each cast will depend
upon one of two criteria: firstly, the available ocean depth, and secondly
the default depth agreed for the survey (1000 m).
At maximum depth
At the maximum depth the exact latitude and longitude should be noted,
as should the time (GMT), depth and the amount of wire out. The first bottle
should also be triggered. The package should remain stationary before and
after the bottle is triggered, depending upon the thermal response time
and the sampling period of any accessory temperature recording equipment.
Whilst at maximum depth any wire-washing equipment should be switched
on.
The up-cast
The CTD package should be hauled at a rate not exceeding 70 m min-1. All
subsequent water sampling bottles are tripped during the upcast. Water
should be sampled from pre-determined depths in order to allow independent
salinity determination. Water should be sampled in regions of little vertical
gradient, yet the samples should span the depth and salinity range of the
water column. As other protocols (primary production, nutrient analysis
etc.) may dictate that water is sampled from particular levels, care should
be taken to ensure adequate samples are taken for salinity determination;
these salinity samples should not be compromised.
End of cast
After the last bottle is tripped the CTD should be brought on deck without
stopping at the surface (although conditions may dictate that the winch
operators require to stop at some point). The package should not be re-covered
with bottles still cocked as they may trip accidentally during recovery.
As the package leaves the water the exact latitude and longitude should
be noted, as should the time (GMT), depth and the amount of wire out.
The SBE 11 deckunit should be turned off after noting instrument status.
Post processing
Salinity Samples
Salinity samples should be taken from all CTD casts at the bottle depths
prescribed. The salinity samples should be taken in 200 ml glass medicine
bottles (or suitable equivalent), with each bottle being rinsed three times
before being filled to just below the neck. The rim of the bottle should
be wiped dry with chlorine-free tissue, a plastic seal inserted and the
screw cap replaced. The salinity samples should then be placed near to
the salinometer for at least 24 hours before measurement. This will allow
the sample temperatures to equalise with that of the salinometer.
The salinometer should be located in a room where through traffic will
not cause disturbance to the ambient temperature, thereby ensuring a stable
room temperature. The temperature of the room should be monitored and recorded.
The salinity samples should be analysed using standard seawater; the
batch number should be recorded. One vial of standard seawater should be
run through the salinometer at the beginning and end of each set of samples
to enable a calibration offset to be derived and to check the stability
of the salinometer.
Reference temperatures
Where reference temperatures are available, for example from reversing
thermometers or from a SBE 35 Reference Temperature Sensor, they should
be used in the post-processing of the CTD data.
The quality of the conductivity calibration
After applying the calibration coefficients and adjusting for the residual
offset, the salinity of the bottle sample should be differenced with the
derived CTD salinity. After rejecting outlying samples, the mean of the
remaining samples and the standard deviation should be documented The residual
offset applied to each cast after calibration should be documented.
Data processing
It is recognised that CTD data are processed differently on each vessel.
The downcast should be used as the principal data record. Initial processing
will make use of Seasoft, however more detailed analysis is likely to require
access to other software such as CONT3D (Japan), Ocean Data View (USA)
pr PEXEC(UK). However, CTD data should be bin-averaged to 1 dbar. Data
files should include all raw data such as conductivity, temperature (IPTS-90)
and pressure, as well as derived data.
Hard copy data (log sheets, downcast plots and salinometer sample details)
should be archived.
Station sampling
Proposed station sampling depths
It is proposed that CTD casts over continental self areas should sample
to the near-bottom, that is within 10 m of the sea bed (depending upon
conditions). A shelf station is arbitrarily defined as any station where
the water depth is less than 1000 m.
It is proposed that CTD casts over the open ocean should sample to 1000
m. An open ocean station is arbitrarily defined as any station where the
water depth is greater than 1000 m.
Proposed depths for water bottle salinity sampling
In order to sample the water column for salinity calibration, the water
sampling rosette should be triggered at the bottom of the cast, at 800
m, 600, 400 m, 200 m, 150 m, 100 m and at 30 m. Where additional depths
are sampled (for accessory sensors) salinity samples may also be taken.
Not all of the sampling depths will be possible over shelf areas.
Proposed bottom depth
One of the principal water masses of the Antarctic Circumpolar Current
(ACC) is Circumpolar Deep Water (CDW). This is divided into Upper CDW (UCDW)
and Lower CDW (LCDW). UCDW has recently been shown to be an important defining
factor in the southern extent of the ACC (Orsi et al., 1995). Consequently,
sampling to encompass UCDW would be advantageous. It is suggested that
the climatological level of UCDW should therefore be examined, so that
the time overhead can be determined should sampling to these levels be
agreed. A preliminary literature search (Peterson and Whitworth, 1989;
Orsi et al., 1995) suggests that this overhead will be minimal.
References
Peterson, R. G., and T. Whitworth III (1989). The Subantarctic and
Polar Fronts in relation to deep water masses through the southwestern
Atlantic. J. Geophys. Res., 94, 10817-10838.
Orsi, A. H., T. Whitworth III, and W. D. Nowlin (1995). On the meridional
extent and fronts of the Antarctic Circumpolar Current. Deep-Sea Res.,
42, 641-673.
Page last updated on 28 April 1999