PROCEDURES FOR WATER-COLUMN Profiling during the SOUTHERN CALIFORNIA BIGHT'98 Regional survey
Nine different organizations are collecting data for the BIGHT'98 water quality survey using a standard set of protocols and sampling methods described in this Workplan and appendix. The Southern California Bight Pilot Project (SCBPP) Field Coordination Team and the CTD Users Group in Southern California initially developed the protocols for water column profiling during the 1994 regional survey (SCBPP 1994). The Marine Monitoring Methods Committee slightly modified these protocols in developing a field operations manual (SCBPP 1995) for ocean compliance monitoring programs of the four largest publicly owned treatment works (POTWs). The BIGHT'98 Water Quality Committee has modified these protocols for use in the 1998 regional study. This appendix describes those procedures, which were selected to ensure consistency in equipment operations and intercomparability of the acquired data sets.
A. Equipment
All participating organizations must sample using a conductivity-temperature-depth (CTD) profiler with additional sensors (Table C-1) to provide continuous water-column profiles of temperature, salinity, dissolved oxygen (DO), hydrogen ion content (pH), optical beam transmission at 660 nm, and chlorophyll fluorescence with depth. Sea-Bird SBE 9 or SBE 25 underwater CTD units are used by seven of the program participants; a SeaBird SeaCat (SBE 19) and an Idronaut CTD are used by two other participants. CTD units must meet the program performance specifications described earlier in this Workplan for temperature, salinity, DO, pH, beam transmission, and pressure before participating in the regional survey. The following sensors are currently being used with Sea Bird CTDs: temperature, SBE 3; conductivity, SBE 4; pH, SBE 18; DO, SBE 13; beam transmission, Sea-Tech or WETLabs 25 cm path-length; chlorophyll fluorescence, WETLabs Wet Star fluorometer or Chelsea Instruments fluorometer; and pressure (for depth), Sensor-Metrics strain-gauge interfacing pressure sensor or Paroscientific Digiquartz pressure sensor model #2300A-110. The data acquisition software for the Sea-Bird instruments will be Seasoft version 4.3 or higher.
Table C-1. Equipment used by participating agencies during the Bight'98 regional survey.
B. CTD Deployment
The CTD must be deployed with a means of data collection, such as a SBE 11 deck
unit, SBE 17 RAM (random access memory — an internal recording instrument) unit, or equivalent.
The instrument should have a scan rate of no fewer than 8 scans/sec, except for the SBE19 and Idronaut.
The SBE 9 and SBE 25 units measure data at 24 or 8 scans (data)/sec, respectively. The user
must designate the average rate that the data will be recorded to memory or the computer. For
example, SBE 9 set at 24 scans/sec and averaged at 3 scans/bin means 8 data lines in memory per second.
In the configuration file, the scan rate is set at 24 scans/sec for SBE 9 or 8 scans/sec for SBE 25.
To preserve as much data as possible we recommend that data be logged at 8 scans per second for
both the SBE 9 and SBE 25 (set NAVG = 3 for SBE 9 and NAVG = 1 for SBE 25). If either the
instrument is incapable of this data rate or the internal memory is not sufficient to log at this rate,
data logging should be set at the highest rate possible.
The CTD descent rate must not exceed 1 m/sec (the recommended optimum speed is
0.25-0.50 m/sec). If deploying real-time, some manufacturer software allows this rate to be monitored
by displaying and viewing the lowering rate variable. If RAM is used during deployment, the rate
must be monitored with a meter wheel and timer. Descent rates must always be slower than 1 m/sec
to minimize spiking of sensor output. Additionally, during data processing, a cast whose
average descent rate is found to exceed 1 m/sec may be considered for omission.
Before beginning any cast, position the vessel on station using DGPS. Record the required
information for each station on CTD data sheets. Enter a new file name into the computer software on
which the CTD data will be stored.
Begin a cast by initiating the instrument to start recording data. Lower the CTD to a depth
of about 5 meters and, if possible, monitor salinity and DO values to ensure stable readings.
(NOTE: deployment with the SBE 17 RAM unit precludes monitoring salinity and DO values). This step
is important because CTD sensors will be brought to thermal equilibration with the ambient
sea-water and the pump (if applicable) will be activated so bubbles are purged from any tubing. A
three minute equilibration upon initial power-up at the first station of the day and 90 sec at each
station thereafter is the minimum soak time for thermal equilibration and sensor stabilization. After
sensor stabilization, the CTD should be raised slowly so that the sensors are near the water surface but
not breaking the air-water interface (intrudes air bubbles to sensors). One goal of this survey is
to resolve the nearsurface one-meter layer as well as possible. This is important because
freshwater runoff plumes tend to float near the surface of the water and may be very thin. A line may be
employed to raise the bottom of the instrument to a horizontal position or it may be useful to lift
the package so that the top of the CTD is out of the water, but without raising the intake for the
various sensors above the sea surface. Once the sensors have been brought near the surface the
downward profile will begin. The CTD is lowered at the recommended descent rate to within 2-3 meters of
the bottom or 100 meters, whichever comes first.
Use of an onboard water bath is recommended to prevent excessive heating of the
sensors while the CTD is on the ship's deck. If an onboard water bath is not used, a wet towel will
be wrapped around the instrument to prevent the sensors from heating excessively. If a water bath
is not used, rinse the lenses of the transmissometer with deionized water to remove any crystallized
salt prior to each cast.
C. Quality Assurance/Quality Control
Six types of quality assurance and quality control will be implemented to ensure the highest possible data quality during Bight'98:
· All participating organizations must have regular factory calibration and mainte- nance service on their instruments.
· All personnel participating in Bight'98 sampling will undergo training to ensue their familiarity with the instrumentation.
· Prior to the conduct of Bight'98, all sampling organizations will participate in an intercomparison exercise designed to assess instrument comparability.
· Each organization will complete a series of pre-cruise checks and calibrations
prior to each sampling day
· The CTD data will be checked in the field prior to leaving each sampling station.
· Each organization will undergo a series of post-cruise checks at the end of each
sampling day.
1. Factory Calibration and Maintenance
Maintenance and calibration of the CTD must be documented, including dates of most recent servicing. The temperature and conductivity sensor must be sent to the manufacturer every 6-12 months for calibration. Preventative maintenance must be conducted on the CTD unit periodically, according to manufacturer's recommendations (i.e., 3 yr.). All "O" rings, attached sensors, and seals are checked at this time and the unit is pressure tested. The pressure sensor is calibrated at this time. Upon return from the factory, enter any new factory calibration coefficients into the configuration file.
2. Training
Any individual who is maintaining, calibrating, or operating the CTD must be trained in each of these operations. Prior to performing these operations unsupervised, an individual must demonstrate proficiency in that operation to a senior, CTD experienced staff member. Proficiency is evaluated on the basis of successfully completing the operation following written procedures and demonstrating an understanding of the equipment. Additionally, the individual should demonstrate his/her ability to evaluate and troubleshoot common problems. All training and demonstration of proficiency must be documented.
3. CTD Intercomparison Exercise
A CTD intercomparison exercise will be conducted to evaluate the precision, accuracy,
and comparability of the CTDs used by each participating organization. All of the CTDs are placed in
a common sea-water tank. The water temperature is controlled with a chiller to a few degrees
centigrade below ambient temperature and aerated to achieve oxygen saturation. DO values measured
by the CTDs are compared against expected saturation table values and Winkler titrations.
Salinity values are compared to Association Internationale d' Oceanographie Physique (IAPSO)
salinity standards; temperature values are compared to measurements made with a National Institute
of Standards and Testing (NIST) certified thermometer; and pH values are compared to
measurements made with NIST certified buffers. Prior to the exercise all CTDs must be within
manufacturer calibration specifications and be calibrated as if user were preparing to deploy the instrument in a survey on the following day.
4. CTD Precruise Checkout and Calibration
Precruise Equipment Checkout
A precruise equipment checkout must be conducted within 24 hr prior to starting the cruise and prior to calibration. This inspection must include the following:
1) A visual inspection of the CTD for any obvious defects;
2) A check of all metal components for corrosion, cleaning or replacing as necessary;
3) An inspection and cleaning of all connections with contact cleaner, as necessary;
4) Verification that the plugs are secure, waterproof and lubricated with silicone grease;
5) An inspection of all cables for nicks, cuts, abrasions, or other signs of physical damage;
6) A test of the CTD to see if connections and software work properly;
7) Cleaning and/or replacement of all accessory tubing as necessary; and
8) Checking battery status for all units using RAM data storage.
Precruise Calibration
A precruise calibration must be conducted within 24 hr prior to starting the cruise for
pH, DO, beam transmission, and pressure. There is no lab calibration for conductivity and temperature.
Verification that the proper sensor coefficients are in the configuration file must be made
before proceeding. A CTD calibration data sheet is prepared at that time, with all required
information entered on that sheet.
Prior to the cruise, a calibration tank is filled with fresh tap water, which is aerated by
placing an air-stone no more than 10 cm below the water surface and mixed with a pump. This
aeration should last for 12 hr or sufficiently long to reach saturation. Moderate aeration should be
maintained to avoid supersaturation. The water temperature is controlled with a chiller/heater or in a
temperature controlled room, if possible. Water volume should be large enough to resist significant (
±1°C) thermal change during calibration.
Hydrogen Ion Content (pH). The pH sensor should be calibrated by using
commercially available buffer solutions. When sampling in the ocean, it is best to use three buffers of pH 7, 8, and 9. The manufacturer's specifications must always be followed during calibration of the probe.
For example, when calibrating the sensor, it may be necessary to make an electrical connection
between the body of the pH sensor module and the buffer solution. This connection may be made using
any convenient piece of wire. One end of the wire is attached to one of the screws attaching the
zinc anode. The other end of the wire is immersed in the solution. It is important that the buffer
is thermally equilibrated with the water bath; this is best accomplished by keeping the CTD in
the water bath and using a holding bracket for the cup of buffer. The water temperature, pH, and
voltage output for each of the three buffers is then recorded. These values are entered into the
Seasoft (SeaBird software) module PHFIT following the prompts. The residuals for each buffer, pH
slope, and pH offset values are then recorded. The slope and offset values are entered into SEACON.
SEASAVE will be reentered and checked against three buffers again, recording the pH values.
Agreement between sensor output and known values should be within 0.1 pH unit. If agreement
is outside this range, the buffers must be rerun and the procedure repeated. Corrected values are
added to the CTD's software. When complete, the pH electrode is stored in pH buffer 4 saturated
KCl solution.
Dissolved Oxygen (DO). With the power to the CTD off, the CTD is placed in the
calibration tank. If a pump other than the CTD mounted pump is used, the flow rate over the sensor
must be between 15 and 40 mL/sec. The CTD is calibrated according to Sea-Bird Application Note
13-1, Revision B (April 1993) (Appendix F). After new coefficients are calculated and entered into
the configuration file, the DO sensor value is compared with saturation table values (Standard
Methods, 18th., 1992, table 4500-O:1) and it must match to within 0.1 mL/L (0.143 mg/L). Sensor
performance is monitored and the membrane or sensor module must be replaced if results are
unreliable and if
1) the slope of oxygen current (SOC) 4.0;
2) SOC is less than 4.0 but increases by more than 0.3 between calibrations; or
3) response to anoxic conditions is slow (i.e., the sensor output will reach the asymptotic zero value within 2 min and remain relatively stable while fluctuating around a zero value). If this response is slower or a zero value cannot be reached, the membrane may be reaching the end of its life.
Transmissometer. This calibration is performed in air and the CTD software must be modified to reflect any changes that are made during the calibrations. The transmissometer is calibrated according to procedures in the Sea Tech transmissometer manual and Sea-Bird Electronics Application Note No. 7 (Appendix F). If the unblocked percent transmittance value is below 93.5%, the M and B coefficients are recalculated according to Sea-Bird Application Note No. 7, entered in SEACON, and reevaluated. If the value remains below 93.5%, the manufacturer should be consulted and the instrument must be sent in for servicing.
Pressure Offset. This determination must also be performed in air. The pressure sensor is checked before use, recording air sea-level values. The pressure reading in air at sea level should be a negative number between 0.0 and -0.60 db. If out of this range, adjustments must be made if possible and the manufacturer's software (i.e., SEACON) should be run to achieve a value between 0.0 and -0.60 db. The value 0.5 and the opposite sign of that shown on the SEASAVE display are entered when prompted for "offset = ...new value = ...". If this does not correct the displayed pressure value, the offset value must be changed until pressure is within range. NOTE: If the offset value exceeds ±1.5, the sensor must be serviced. The pressure output and any changes to the offset value are recorded on the calibration sheet.
Following calibration, the sensors and equipment should be disturbed as little as possible. The CTD should always be transported in its original shipping case or a comparably secure unit.
5. CTD Cast Acceptability
Cast acceptability is determined immediately following the first cast of the day (it is
recommended that this be done following each cast for real time data) in one of two ways:
1) All parameters can be displayed graphically to determine if any grossly anomalous
readings occurred. Graphs can be scaled to illustrate obviously anomalous values that
lie outside the control limit range for each parameter (Table C-2); or
2) A range-checking computer program can be used to evaluate the presence of anomalous values on the basis of predetermined criteria (i.e. range acceptability checks).
3) Casts should also be evaluated by comparison of values obtained at previous or nearby stations.
If anomalous values are present, the cause must be investigated and remedied before proceeding. If damage to the CTD (due to striking the bottom or some other event) is suspected, that cast should be reviewed as described above to ensure acceptability. Further review of the subsequent cast in a like manner will ensure that all sensors are functioning properly. If a sensor is replaced during the day, a replicate cast must be made with the new sensor, at the last station at which the malfunctioning sensor was known to have been working properly. If a sensor is replaced, all coefficients for that sensor must be entered and saved in the configuration file. All activities relating to the occurrence of these types of events (e.g., repeated casts, damaged equipment and remedies, replaced sensors, etc.) are noted in the a field logbook. If feasible, a station should be resurveyed when an unacceptable profile is obtained.
Table C-2. Range of reasonable measurements from waters of the mainland shelf of Southern California.
6. Postcruise Calibration
Postcalibration must be conducted within 24 hr of cruise completion. The Chief Scientist is responsible for deciding whether postcruise calibrations are within acceptable limits.
Hydrogen Ion Content (pH). The only similarity between the postcalibration and precruise calibration of the pH sensor is that the sensor is checked against the three buffers with no adjustments being made to the software. The water temperature is recorded, as well as the pH and voltage output for each buffer on data sheets. Agreement should occur between each sensor value and the known buffer value, and must be within 0.15 pH units. If agreement is out of this range, the unit is recalibrated and the stations resampled if feasible.
Dissolved Oxygen (DO). Postcalibration of the DO sensor must follow the same procedures as those for precalibration. However, the DO concentration must be evaluated and recorded on data sheets prior to adjusting the coefficients. If the preadjusted value is equal to or less than + 0.3 mg/L of the expected saturation table value, all DO data for the survey are tentatively acceptable until further review. If this preadjusted value is greater than ± 0.3 mg/L different than the saturation table value, all the DO data for the survey must be reviewed for consistency. If these data are deemed unreliable, the survey must be done again as soon as possible (if feasible). If stations cannot be resampled, data are flagged or qualified in the data base. Following postcalibration, the oxygen sensor is cleaned and stored following manufacturer procedures given in Sea-Bird Application Note 13-1, Revision B (April 1993) (Appendix F).
Beam transmission and Pressure. Postcalibration of the transmissometer and pressure sensors are the same as those performed in the precalibration. If the pressure reading in air at sea level is not a negative number between 0 and -0.60 db, record the pressure output and any changes to the offset value on the calibration sheet.
Southern California Bight Pilot Project (SCBPP). 1994. Field operations manual for the Southern California Bight Pilot Project. Southern California Coastal Water Research Project. Westminster, CA.
Southern California Bight Pilot Project. 1995. Field operations manual for marine water-column, benthic, and trawl monitoring in southern California. Southern California Coastal Water Research Project. Westminster, CA.