Metadata Report for BODC Series Reference Number 841388

• 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

Open Data supplied by Natural Environment Research Council (NERC)

You must always use the following attribution statement to acknowledge the source of the information: "Contains data supplied by Natural Environment Research Council."

Narrative Documents

Neil Brown MK3 CTD

The Neil Brown MK3 conductivity-temperature-depth (CTD) profiler consists of an integral unit containing pressure, temperature and conductivity sensors with an optional dissolved oxygen sensor in a pressure-hardened casing. The most widely used variant in the 1980s and 1990s was the MK3B. An upgrade to this, the MK3C, was developed to meet the requirements of the WOCE project.

The MK3C includes a low hysteresis, titanium strain gauge pressure transducer. The transducer temperature is measured separately, allowing correction for the effects of temperature on pressure measurements. The MK3C conductivity cell features a free flow, internal field design that eliminates ducted pumping and is not affected by external metallic objects such as guard cages and external sensors.

Additional optional sensors include pH and a pressure-temperature fluorometer. The instrument is no longer in production, but is supported (repair and calibration) by General Oceanics.

Specifications

These specification apply to the MK3C version.

Further details can be found in the 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.

Chelsea Technologies Photosynthetically Active Radiation (PAR) Irradiance Sensor

This sensor was originally designed to assist the study of marine photosynthesis. With the use of logarithmic amplication, the sensor covers a range of 6 orders of magnitude, which avoids setting up the sensor range for the expected signal level for different ambient conditions.

The sensor consists of a hollow PTFE 2-pi collector supported by a clear acetal dome diverting light to a filter and photodiode from which a cosine response is obtained. The sensor can be used in moorings, profiling or deployed in towed vehicles and can measure both upwelling and downwelling light.

Specifications

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.

RRS Challenger 119A CTD Data Documentation

Instrumentation

The CTD profiles were taken with an RVS Neil Brown Systems Mk3B CTD incorporating a pressure sensor, conductivity cell, platinum resistance thermometer and a Beckmann dissolved oxygen sensor. The CTD unit was mounted vertically in the centre of a protective cage approximately 1.5 m square. Attached to the bars of the frame were a Chelsea Instruments Aquatracka fluorometer and a SeaTech red light (661 nm) transmissometer with a 25 cm path length.

Above the frame was a General Oceanics rosette sampler fitted with twelve 10-litre water bottles. These comprised a mixture of Niskin, general purpose Go-Flo and ultra-clean teflon lined Go-Flo bottles as dictated by sampling requirements. The bases of the bottles were 0.75 m above the pressure head and their tops 1.55 m above it. One bottle was fitted with a holder for twin digital reversing thermometers mounted 1.38 m above the CTD temperature sensor.

Above the rosette was a PML 2pi PAR (photosynthetically available radiation) sensor pointing upwards to measure downwelling irradiance. A second 2pi PAR sensor, pointing downwards, was fitted to the bottom of the cage to measure upwelling irradiance. It should be noted that these sensors were vertically separated by 2 metres with the upwelling sensor 0.2 metres below the pressure head and the downwelling sensor 1.75 metres above it.

No account has been taken of rig geometry in the compilation of the CTD data set. However, all water bottle sampling depths have been corrected for rig geometry and represent the true position of the midpoint of the water bottle in the water column.

Data Acquisition

On each cast the CTD was lowered to a depth of approximately 5 metres and held until the oxygen reading stabilised. It was then raised to the surface and lowered continuously at 0.5 to 1.0 m s^-1 to the closest comfortable proximity to the sea floor. The upcast was done in stages between the bottle firing depths.

Data were logged by the Research Vessel Services ABC data logging system. Output channels from the deck unit were logged at 16 Hz by a microprocessor interface (the Level A) which passed time stamped averaged cycles at 1Hz to a Sun workstation (the Level C) via a buffering system (the Level B).

On-Board Data Processing

The raw data comprised ADC counts. These were converted into engineering units (Volts for PAR meters, fluorometer and transmissometer; ml l^-1for oxygen; mohms cm^-1 for conductivity; °C for temperature; decibars for pressure) by the application of laboratory determined calibrations. Salinity (Practical Salinity Units as defined in Fofonoff and Millard, 1983) was calculated from the conductivity ratios (conductivity / 42.914) and a time lagged temperature using the function described in UNESCO Report 37 (1981).

The data were submitted to BODC in this form on Quarter Inch Cartridge tapes in RVS internal format for post-cruise processing and data banking.

Post-Cruise Processing

Reformatting

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

• Dissolved oxygen was converted from ml l^-1 to µM by multiplying the values by 44.66.
• Transmissometer voltages were corrected to the manufacturer's specified voltage (4.758V) by ratio using transmissometer air readings recorded during the cruise (4.687V).
• Transmissometer voltages were converted to percentage transmission by multiplying them by a factor of 20.
• The transmissometer data were converted to attenuance using the algorithm:-

Editing

Reformatted CTD data were transferred onto a high-speed graphics workstation. Using custom in-house graphics editors, downcasts and upcasts were differentiated and the limits of the downcasts and upcasts were manually flagged.

Secondly, spikes on all the downcast channels were manually flagged. No data values were edited or deleted; quality control was achieved by modification of the associated quality control flag.

The pressure ranges over which the bottle samples were being collected were logged by manual interaction with the software. Usually, the marked reaction of the oxygen sensor to the bottle firing sequence was used to determine this. These pressure ranges were subsequently used, in conjunction with a geometrical correction for the position of the water bottles with respect to the CTD pressure transducer, to determine the pressure range of data to be averaged for calibration values.

Once screened on the workstation, the CTD downcasts were loaded into a database under the ORACLE Relational Database Management System.

Calibration

With the exception of pressure, calibrations were done by comparison of CTD data against measurements made on water bottle samples or, for temperature, from the reversing thermometers mounted on the water bottles. In general, values were averaged from the CTD downcasts but where inspection on a graphics workstation showed significant hysteresis, values were manually extracted from the CTD upcasts.

All calibrations described here have been applied to the data.

Pressure

The pressure offset was determined by looking at the pressures recorded when the CTD was clearly logging in air (readily apparent from the conductivity channel). The offset was acceptably stable throughout the cruise and the following correction has been applied:

Temperature

The CTD temperature was compared with the digital reversing thermometers attached to the water bottles and was found to agree closely. Consequently, no correction has been applied to the temperature data.

Salinity

Salinity was calibrated against water bottle samples measured on the Guildline 55358 Autolab Salinometer during the cruise. Samples were taken from the bottles, usually the bottom bottle, fired on 42 casts.

Samples were collected in glass bottles filled to just below the neck and sealed with plastic stoppers. Batches of samples were left for at least 24 hours to reach thermal equilibrium in the constant temperature laboratory containing the salinometer before analysis.

The corrections determined for this cruise were:

Dissolved Oxygen

The CTD dissolved oxygen sensor was calibrated using calibrated surface underway dissolved oxygen data, collected using an Endeco pulsed electrode system. Regression of the CTD data against the surface data produced the following correction equation for the CTD oxygen data:

This is not regarded as a 'good' calibration, evidenced by the low R² value and an estimated accuracy of between 20 and 30 M. However, it is the best that could be achieved with the data set available. Efforts to improve the calibration by subsetting the calibration data failed to achieve any improvement.

Chlorophyll

Extracted chlorophyll data were collected by filtering water taken from the CTD bottle through 0.2-micron pore filters. The pigments were extracted in acetone and assayed fluorometrically.

The fluorometer was calibrated by regression of the fluorometer voltage against the natural log of the extracted chlorophyll concentration.

The resulting calibration was:

PAR

The light meter voltages were converted to µE/m²/s using the following equations determined by RVS in February 1990.

Note that the scaling constant of 0.0375 is an empirical conversion from µW/cm². The data may therefore be converted to W/m² by dividing by 3.75 if required.

Attenuance and Suspended Particulate Matter

he attenuance data were calibrated in terms of suspended particulate material by regressing attenuance values at the bottle firing depths against gravimetric determinations of sediment load. The data from all three legs of the cruise were pooled to produce a single calibration.

The calibration obtained was:

On the third leg of this cruise (CH119C) a scientist from University of Wales, Bangor also measured sediment load by gravimetry at the same depths as the PML team using a newly implemented protocol. This involved the use of small (2.5 litre) CTD rosette bottles. The total contents of these were used for the UWB SPM sample whereas the PML sample was collected as a sub-sample from a 10-litre bottle risking sediment loss through settling.

Comparing the two data sets showed the UWB data to be systematically significantly higher than the PML data. The calibration has been based on the PML data for the sake of consistency throughout the project. However, it should be borne in mind that the result might represent an underestimate of the actual suspended load.

Data Reduction

Once all screening and calibration procedures were completed, the data set was binned to 2 db (casts deeper than 100 db) or 1 db (casts shallower than 100 db). The binning algorithm excluded any data points flagged suspect and attempted linear interpolation over gaps up to 3 bins wide. If any gaps larger than this were encountered, the data in the gaps were set null.

Oxygen saturation was computed using the algorithm of Benson and Krause (1984).

Data Warnings

There is some evidence that the calibrated transmissometer data might underestimate the true suspended load.

The dissolved oxygen calibration exhibited a large degree of scatter and consequently the absolute dissolved oxygen values should be used with care.

References

Benson B.B. and Krause D. jnr. 1984. The concentration and isotopic fractionation of oxygen dissolved in fresh water and sea water in equilibrium with the atmosphere. Limnol. Oceanogr. 29 pp.620.632.

Fofonoff N.P. and Millard Jr., R.C. 1982. Algorithms for Computation of Fundamental Properties of Seawater. UNESCO Technical Papers in Marine Science 44.

Project Information

Land Ocean Interaction Study (LOIS)

Introduction

The Land Ocean Interaction Study (LOIS) was a Community Research Project of the Natural Environment Research Council (NERC). The broad aim of LOIS was to gain an understanding of, and an ability to predict, the nature of environmental change in the coastal zone around the UK through an integrated study from the river catchments through to the shelf break.

LOIS was a collaborative, multidisciplinary study undertaken by scientists from NERC research laboratories and Higher Education institutions. The LOIS project was managed from NERC's Plymouth Marine Laboratory.

The project ran for six years from April 1992 until April 1998 with a further modelling and synthesis phase beginning in April 1998 and ending in April 2000.

Project Structure

LOIS consisted of the following components:

• River-Atmosphere-Coast Study (RACS)
  • RACS(A) - Atmospheric sub-component
  • RACS(C) - Coasts sub-component
  • RACS(R) - Rivers sub-component
  • BIOTA - Terrestrial salt marsh study
• Land Ocean Evolution Perspective Study (LOEPS)
• Shelf-Edge Study (SES)
• North Sea Modelling Study (NORMS)
• Data Management (DATA)

Marine Fieldwork

Marine field data were collected between September 1993 and September 1997 as part of RACS(C) and SES. The RACS data were collected throughout this period from the estuaries and coastal waters of the UK North Sea coast from Great Yarmouth to the Tweed. The SES data were collected between March 1995 and September 1996 from the Hebridean slope. Both the RACS and SES data sets incorporate a broad spectrum of measurements collected using moored instruments and research vessel surveys.

Data Activity or Cruise Information

Cruise

Complete Cruise Metadata Report is available here

Fixed Station Information

Fixed Station Information

LOIS (RACS) Humber-Tweed Grid HT

The Humber-Tweed grid was one of two areas sampled during the LOIS(RACS) project. The measurements collected lie within an area with its centre point at 54.7912N 0.8693W and which stretches from the Humber Estuary to Berwick upon Tweed.

CTD casts were performed by several cruises between December 1992 and July 1995.

Related Fixed Station activities are detailed in Appendix 1

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:

Appendix 1: LOIS (RACS) Humber-Tweed Grid HT

Related series for this Fixed Station are presented in the table below. Further information can be found by following the appropriate links.

If you are interested in these series, please be aware we offer a multiple file download service. Should your credentials be insufficient for automatic download, the service also offers a referral to our Enquiries Officer who may be able to negotiate access.