Metadata Report for BODC Series Reference Number 805467

• Metadata Summary

• Problem Reports

• Data Access Policy

• Narrative Documents

• Project Information

• Data Activity or Cruise Information

• Fixed Station Information

• BODC Quality Flags

• SeaDataNet Quality Flags

Metadata Summary

Problem Reports

No Problem Report Found in the Database

Data Access Policy

Public domain data

These data have no specific confidentiality restrictions for users. However, users must acknowledge data sources as it is not ethical to publish data without proper attribution. Any publication or other output resulting from usage of the data should include an acknowledgment.

The recommended acknowledgment is

"This study uses data from the data source/organisation/programme, provided by the British Oceanographic Data Centre and funded by the funding body."

Narrative Documents

Sea-Bird Electronics SBE 911 and SBE 917 series CTD profilers

The SBE 911 and SBE 917 series of conductivity-temperature-depth (CTD) units are used to collect hydrographic profiles, including temperature, conductivity and pressure as standard. Each profiler consists of an underwater unit and deck unit or SEARAM. Auxiliary sensors, such as fluorometers, dissolved oxygen sensors and transmissometers, and carousel water samplers are commonly added to the underwater unit.

Underwater unit

The CTD underwater unit (SBE 9 or SBE 9 plus) comprises a protective cage (usually with a carousel water sampler), including a main pressure housing containing power supplies, acquisition electronics, telemetry circuitry, and a suite of modular sensors. The original SBE 9 incorporated Sea-Bird's standard modular SBE 3 temperature sensor and SBE 4 conductivity sensor, and a Paroscientific Digiquartz pressure sensor. The conductivity cell was connected to a pump-fed plastic tubing circuit that could include auxiliary sensors. Each SBE 9 unit was custom built to individual specification. The SBE 9 was replaced in 1997 by an off-the-shelf version, termed the SBE 9 plus, that incorporated the SBE 3 plus (or SBE 3P) temperature sensor, SBE 4C conductivity sensor and a Paroscientific Digiquartz pressure sensor. Sensors could be connected to a pump-fed plastic tubing circuit or stand-alone.

Temperature, conductivity and pressure sensors

The conductivity, temperature, and pressure sensors supplied with Sea-Bird CTD systems have outputs in the form of variable frequencies, which are measured using high-speed parallel counters. The resulting count totals are converted to numeric representations of the original frequencies, which bear a direct relationship to temperature, conductivity or pressure. Sampling frequencies for these sensors are typically set at 24 Hz.

The temperature sensing element is a glass-coated thermistor bead, pressure-protected inside a stainless steel tube, while the conductivity sensing element is a cylindrical, flow-through, borosilicate glass cell with three internal platinum electrodes. Thermistor resistance or conductivity cell resistance, respectively, is the controlling element in an optimized Wien Bridge oscillator circuit, which produces a frequency output that can be converted to a temperature or conductivity reading. These sensors are available with depth ratings of 6800 m (aluminium housing) or 10500 m (titanium housing). The Paroscientific Digiquartz pressure sensor comprises a quartz crystal resonator that responds to pressure-induced stress, and temperature is measured for thermal compensation of the calculated pressure.

Additional sensors

Optional sensors for dissolved oxygen, pH, light transmission, fluorescence and others do not require the very high levels of resolution needed in the primary CTD channels, nor do these sensors generally offer variable frequency outputs. Accordingly, signals from the auxiliary sensors are acquired using a conventional voltage-input multiplexed A/D converter (optional). Some Sea-Bird CTDs use a strain gauge pressure sensor (Senso-Metrics) in which case their pressure output data is in the same form as that from the auxiliary sensors as described above.

Deck unit or SEARAM

Each underwater unit is connected to a power supply and data logging system: the SBE 11 (or SBE 11 plus) deck unit allows real-time interfacing between the deck and the underwater unit via a conductive wire, while the submersible SBE 17 (or SBE 17 plus) SEARAM plugs directly into the underwater unit and data are downloaded on recovery of the CTD. The combination of SBE 9 and SBE 17 or SBE 11 are termed SBE 917 or SBE 911, respectively, while the combinations of SBE 9 plus and SBE 17 plus or SBE 11 plus are termed SBE 917 plus or SBE 911 plus.

Specifications

Specifications for the SBE 9 plus underwater unit are listed below:

Further details can be found in the manufacturer's specification sheet.

Aquatracka fluorometer

The Chelsea Instruments Aquatracka is a logarithmic response fluorometer. It uses a pulsed (5.5 Hz) xenon light source discharging between 320 and 800 nm through a blue filter with a peak transmission of 420 nm and a bandwidth at half maximum of 100 nm. A red filter with sharp cut off, 10% transmission at 664 nm and 678 nm, is used to pass chlorophyll-a fluorescence to the sample photodiode.

The instrument may be deployed either in a through-flow tank, on a CTD frame or moored with a data logging package.

Further details can be found in the manufacturer's specification sheet.

SeaTech Transmissometer

Introduction

The transmissometer is designed to accurately measure the the amount of light transmitted by a modulated Light Emitting Diode (LED) through a fixed-length in-situ water column to a synchronous detector.

Specifications

• Water path length: 5 cm (for use in turbid waters) to 1 m (for use in clear ocean waters).
• Beam diameter: 15 mm
• Transmitted beam collimation: <3 milliradians
• Receiver acceptance angle (in water): <18 milliradians
• Light source wavelength: usually (but not exclusively) 660 nm (red light)

Notes

The instrument can be interfaced to Aanderaa RCM7 current meters. This is achieved by fitting the transmissometer in a slot cut into a customized RCM4-type vane.

A red LED (660 nm) is used for general applications looking at water column sediment load. However, green or blue LEDs can be fitted for specilised optics applications. The light source used is identified by the BODC parameter code.

Further details can be found in the manufacturer's Manual.

RV Pelagia 125 CTD Data Documentation

Cruise Principal Scientist and Data Originator

Dr. Hans van Haren, Nederlands Instituut voor Onderzoek der Zee (NIOZ), Texel, The Netherlands.

Content of data series

(*) the total number of profiles include 31 yoyo profiles between 30 and 80 m carried out as part of cast 039 over a period of 1.5 hours on 23 October 1998 (for all parameters but oxygen and irradiance).

Instrumentation and data processing by originator

CTD unit and auxiliary sensors

Sea-Bird Electronics 911 Plus system fitted with the following additional sensors: 25 cm pathlength transmissometer (SeaTech SN160), Aquatracka III fluorometer (Chelsea Instrument SN 88/725/042), oxygen sensor and PAR sensor (K-meter SN4410) for downwelling irradiance.

Changes of sensors during the cruise: none reported.

Data were logged on a PC running Seabird Seasave data acquisition software v. 4.224 and manufacturer's calibration coefficients were applied to the raw data.

Data were supplied to BODC as downcasts only, binned to 0.5 db. Data from the yoyo cast were at a higher resolution.

Sampling device

Rosette sampling system equipped with 24 x 12L NOEX sampling bottles. The pressure sensor was located close to the base of the bottles, at a distance of 1m from the top aperture of the bottles. The calculation of bottle sampling depth by BODC takes into account the geometry of the CTD frame.

No reversible thermometer was used.

BODC post-cruise processing and screening

Reformatting

The data were converted into BODC internal format (PXF) to allow use of in-house software tools notably the workstation graphics editor SERPLO. In addition to reformatting, the transfer program applied the following modifications to the data:

• transmissometer readings were converted from percent transmission to attenuance using the algorithm:

where PL is the transmissometer pathlength in m (0.25 m).

• oxygen was converted from µmol kg^-1 to µmol l^-1 as follows:

Screening

Reformatted CTD data were transferred onto a high-speed graphics workstation. Downcast channels were screened graphically using custom in-house graphics editors. If present, spikes and suspicious values were manually flagged. No data values were edited or deleted; flagging was achieved by modification of the associated quality control flag.

For the yoyo cast, turning points were visually identified and tagged.

Banking

Once screened on the workstation, the CTD downcasts were loaded into a database under the ORACLE Relational Database Management System. In the process the yoyo cast was split into 32 individual profiles (the full depth profile and 31 up and down sections between 30 and 80 m).

Calibration and correction

• Attenuance: on-board conversion from volts to percent transmission was carried out using a calibration coefficient of 20 instead of 22.55. The resulting error on attenuance (calculated as: offset = -1 / 0.25 * log_e (22.55 / 20) = -0.48 per m) was significant. The offset correction was applied to the data directly in the database at BODC according to the following formula:

It was later noted that the attenuance values during this cruise were consistently higher than those measured during the two other concomitant PROVESS NNS cruises Dana D1198 and Challenger CH140. A correction was attempted based on comparison of attenuance values in the upper 40 m (<mixed layer depth) between CTD casts from these three cruises. Only casts that had been made within 0.5 hours and within 0.1 degree of latitude and longitude of each other were selected for the intercomparison. Difference between CH140 and D1198 CTD attenuance values were not significant so the two were merged for comparison with PE125. The offset was very consistent between depths and stations (n=13) and averaged 0.36 ±0.03 m-1 . The following correction was applied to the attenuance channel:

• Fluorescence: chlorophyll fluorescence (µg Chl a l^-1) was calibrated against extracted chlorophyll concentrations measured on samples collected during the cruise using the following equation determined by K. Wild-Allen (Napier University, Edinburgh, UK):

where Chl a (µg l^-1) is the chlorophyll concentration of samples filtered through GF/F filters, extracted in 90% acetone and determined by spectrophotometry using the trichromatic method and Fluor is the chlorophyll concentration (µg l^-1) calculated from the CTD fluorometer output and the manufacturer's calibration coefficients.

This calibration equation resulted in a number of negative chlorophyll concentration values (up to -0.24 µg l^-1) being observed occasionally.

• With the exception of the oxygen channel, data from the other channels had already been calibrated by the data originator and no further calibration/correction was applied.

Comments on data quality

Oxygen analyses were not performed during this cruise. The oxygen channel is therefore uncalibrated and absolute values should be used with caution.

Project Information

PROcesses of Vertical Exchange in Shelf Seas (PROVESS)

Introduction

PROVESS was an interdisciplinary study of the vertical fluxes of properties through the water column and the surface and bottom boundary layers. The project was funded by the European Community MAST-III programme (MAS3-CT97- 0159) and ran from March 1998 to May 2001.

Scientific Rationale

PROVESS was based on the integration of experimental, theoretical and modelling studies with the aim of improving understanding and quantification of vertical exchange processes in the water column, in particular in the surface and benthic boundary layers and across the> pycnocline. PROVESS also explored mechanisms of physical-biological coupling in which vertical exchanges and turbulence significantly affect the environmental conditions experienced by the biota with particular reference to aggregation, flocculation, sedimentation and trophic interactions.

Fieldwork

The experimental phase of the project was carried out at two contrasting sites in the North Sea: the northern North Sea site (NNS) and the southern North Sea site (SNS).

The two sites had the following characteristics:

At both locations measurements were concentrated at a central position with additional measurements being made to estimate horizontal gradients. Moored instruments (including current meters, temperature and pressure sensors, fluorometers, transmissometers, nutrient analysers and meteorological sensors) were deployed between 7 September and 5 November 1998 at the NNS and between 29 March and 25 May 1999 at the SNS. Each experiment was supported by intensive measurement series made from oceanographic ships and involving turbulence dissipation profiler CTD, particle size profilers, optical profilers, benthic sampling and water bottle sampling.

Details of the cruises were as follows:

Data Activity or Cruise Information

Cruise

Complete Cruise Metadata Report is available here

Fixed Station Information

No Fixed Station Information held for the Series

BODC Quality Control Flags

The following single character qualifying flags may be associated with one or more individual parameters with a data cycle:

SeaDataNet Quality Control Flags

The following single character qualifying flags may be associated with one or more individual parameters with a data cycle: