Metadata Report for BODC Series Reference Number 949853

• 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

CH119_c Hydrography Data Quality Report

There is some evidence that the sediment load data derived by transmissometer calibration may be low by up to a factor of 1.5.

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

Alpkem RFA-2, RFA-300 and Astoria colorimetric autoanalysers

The Alpkem rapid flow analysers (RFA) and Astoria analyser are continuous flow analysers consisting of a sampler, peristaltic pump, analytical cartridge, heating bath, colorimeter, data station, and printer. The sample and reagant are pushed into the reaction cartridge by the pump, where air-segmentation bubbles are introduced to facilitate mixing. The solution is then analysed colorimetrically, with the absorbance of a specific wavelength of light being indicative of the concentration of the solution. These autoanalysers may be used to measure nutrient concentrations in seawater.

The Alpkem rapid flow analysers were manufactured by OI Analytical, with the RFA-300 being a precursor to the smaller RFA-2 (also known as the RFA II or RFA/2). Astoria Pacific Inc. bought the RFA line, upgraded the instrument and renamed it the Astoria analyser.

The Astoria analyser incorporates a six-channel digital detector, which allows analysis of four to six parameters from a single sample. The components of the system are the 311 XYZ Sampler, 322 Auxiliary Pump (for sampler wash), 302D Micropump (peristaltic pump), 303A Cartridge Base, with room for up to six chemistries, and 305D Digital Detector. The six channels of the 305D allow for extra channels that can be used for reference signals, and optional detectors (fluorometer, UV, and Flame) can be plugged into the system as well.

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.

Global Positioning Satellite System

A location system of unspecified make or model that determines location on the Earth's surface using the Global Positioning Satellite Network. Angular co-ordinates are given relative to WGS84 CRS. Other parameters such as platform velocity may be derived from this.

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.

CH119_c Sea surface hydrography instrument details

Underway hydrography was recorded by a suite of instruments in the ship's flow-through system. Instrument details are given in the table below.

CH119_c Sea surface Hydrography Series

Hydrography Processing Notes

Dissolved Oxygen

Dissolved oxygen was measured using two independent systems: an Endeco type 1125 pulsed dissolved oxygen controller multiplexed to two Endeco type 1128 dissolved oxygen probes and a self-logging YSI dissolved oxygen probe.

An initial inspection of the data showed that the Endeco system worked very well on this cruise, particularly Probe 1. Consequently, data from this probe have been taken as the cruise dissolved oxygen data set.

Calibration of the Probe 1 data (logged assuming zero salinity) against Winkler titration data computed at zero salinity and cycle number (to correct for linear drift detected in the data) gave the following result:

Corrected O₂ = 1.3755 * Raw O₂ - 0.00496 * cycle - 14.215 (n=40, R²=88.1%)

This calibration was applied to the data and the corrected data were recomputed to in-situ temperature and salinity on the basis of calibrated thermosalinograph data.

Nutrients

The nutrients were measured using an AlpKem auto-analyser connected to the pumped seawater supply by a continuous filter block (Morris et al, 1978). The chemistries used were:

The colorimeter outputs were logged by a Level A connected to the chart recorder inputs. The system was calibrated by running nutrient-depleted seawater washes to determine baseline and sets of four standards. Standards were generally run at least once per day.

The following processing procedures were adopted by BODC to convert the raw voltage streams into nutrient concentrations.

The data were then segmented into internally consistent subsets (i.e. taking account of any instrumental change such as gain or baseline alterations documented by the analyst), with constant or linearly drifting baselines. For each segment, the baseline as a function of time was fitted to a linear regression, which was applied to standardise the segment to a baseline of zero.

The standard voltages were used to construct calibration curves to convert raw voltages into nutrient concentrations. Note that nutrient concentrations in the low-nutrient seawater matrix were determined manually from the autoanalyser chart rolls and added to the spike values for calibration curve generation.

The calibration equations were applied on a segment by segment basis to the data stream. All baseline and calibration equations were checked manually before they were used. Once concentrations had been calculated, checked and checked again, the custom baseline and standard flags were changed to suspect flags.

The data were subsequently quality controlled by inspection on a graphics workstation with particular attention paid to the concentrations obtained for the standards. Any data identified as suspect were flagged. These checks included comparative screening between the nutrient channels.

The nutrient time channels were adjusted to correct for the delay incurred whilst the sample passed through the complex plumbing of the auto-analyser. No salt corrections (corrections for difference in optical density between sample and standards) were applied as the standards were prepared in nutrient-depleted seawater.

The time corrections were determined by first synchronising nitrate with salinity using data from sharp gradients going into plumes where nitrate may be considered a conservative fresh water tracer. The other nutrients were then synchronised to nitrate using data when standards were running.

The time corrections applied were:

Due to the chemistry used for the nitrate analysis, interference from nitrite is inevitable. The nitrate channel has been processed throughout as nitrate+nitrite and no attempt has been made to correct for nitrite.

Dissolved sulphur

Dissolved sulphur (both as sulphide and thiols) was measured continuously by stripping voltammetry (Al-Farawati and van den Berg, 1997) using an automated system based on flow analysis. The sulphide measurements were calibrated using thiourea rather than sulphide.

Chlorophyll

Chlorophyll was measured by a Chelsea Instruments Aquatracka immersed in a light-tight pond. The fluorometer was calibrated by regressing the natural log of extracted chlorophyll concentration against corresponding fluorometer voltage. The following relationship was hence derived and used to compute the chlorophyll concentrations:

Chlorophyll (mg m^-3) = exp (3.2513*fluorometer voltage - 2.7926) (n=129, R²=75.7%)

Sea surface temperature and salinity

Temperature and salinity were measured using an autoranging TSG103 thermosalinograph. The temperature sensor was a thermistor in the non-toxic supply inlet manifold. Conductivity was measured by a unit in the ship's wet laboratory, which included a second thermistor to provide temperature for the computation of salinity.

The raw ADC counts were calibrated to give conductivity and two temperature channels based upon laboratory calculations by RVS. Salinity was computed from the housing temperature and conductivity using the UNESCO 1978 Practical Salinity Scale (Fofonoff and Millard, 1983).

The thermosalinograph was calibrated against surface values taken from the calibrated CTD. In the case of salinity, bottle samples analyzed by a Guildline Autosal bench salinometer also contributed to the calibration data set. A constant offset of -0.120 was found for temperature. The salinity calibration was subdivided into segments as follows to correct for variable linear drift.

The linear drift model used was:

Corrected salinity = Raw salinity + A*cycle number + B

The values of the coefficients A and B were as follows:

Optical Attenuance and Sediment Load

Optical attenuance was measured using a SeaTech red light (661nm) transmissometer with a 25cm optical path length mounted in a light-tight box on the starboard deck continuously flushed by the non-toxic supply.

Transmissometer air readings made during the cruise were used to correct the transmissometer voltage to the manufacturer's specified voltage by ratio. The air reading used was 4.729 V. The manufacturer's voltage for the instrument was 4.789 V. The voltages were then converted to attenuance to eliminate the influence of instrument path length using the equation:

Attenuance = -4.0 * log_e (voltage/5.0)

The transmissometer was calibrated in terms of sediment load using gravimetric data obtained by filtering water through tared GF/F filters. The following calibration was obtained by pooling the data obtained by the LOIS Core Team from PML on the three legs of Challenger 119:

SPM (mg/l) = (Attenuance - 0.3595) / 0.4468 (n=266, R²=91.8%)

However, a scientist from University of Wales, Bangor also measured sediment load by gravimetry at the same depths as the LOIS team using a newly implemented protocol. This involved the use of small (2.5 litre) CTD rosette bottles. The total content of these was used as 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. If these data are used to calibrate the underway transmissometer, the resulting calibration equation is:

SPM (mg/l) = (Attenuance - 0.192) / 0.3 (n=90, R²=86.2%)

The feature to note here is the slope of the UWB calibration is over 1.5 times that of the PML calibration. The PML calibration has been applied to the data for the sake of consistency throughout the project. However, it should be borne in mind that the result may represent an underestimate of the actual suspended load.

References

Al-Farawati, R. and van den Berg, C.M.G., 1997. The determination of sulphide in sea water by flow analysis with voltammetric detection. Mar. Chem., 57, 277-286.

Fofonoff, N.P. and Millard Jr., R.C. (1983). Algorithms for computation of fundamental properties of seawater. UNESCO Technical Papers in Marine Science 44.

Morris, A.W., Howland, R.J.M., and Bale, A.J. (1978). A filtration unit for use with continuous autoanalytical systems applied to highly turbid waters. Est. Coast. Mar. Sci. 6, 105-109.

CH119_c Sea surface Hydrography, Meteorology and Navigation Series

Instrumentation

Seawater was continuously pumped from the hull of the ship at a depth of approximately 4 m from the ship's non-toxic supply. The thermosalinograph was fed through a small (100-litre) header tank. A large plastic tank on the starboard deck was fed directly from the non-toxic manifold and contained the transmissometer and fluorometer. Baffles in the tank ensured that a continuous flow was maintained over the instruments. A selflogging YSI dissolved oxygen probe was also placed in this tank. The nutrient autoanalyser was fed from plastic tubing connected to taps on the non-toxic supply. The electrochemical sulphur analyser was fed from an all-plastic supply fed from an intake on the port echo sounder fish at approximately 2m depth and driven by a perisaltic pump.

The Endeco dissolved oxygen system electrodes were placed in a Perspex cell (approx. 1 litre volume) supplied from the general non-toxic supply via a constant head device at 2 litres/minute.

Calibration samples were either taken from the non-toxic tap in the wet laboratory or the thermosalinograph outlet.

Data Acquisition

For most parameters, data logging and initial processing was handled by the RVS ABC system. The Level A sampling microcomputer digitised an input voltage, applied a time stamp and transferred the data via the Level B disk buffer onto the Level C where the data records were assembled into files. Note that the autoanalyser was included in this arrangement by logging voltages supplied to the instrument chart recorder.

Exceptions to this rule were the electrochemical sulphur analyser and Endeco dissolved oxygen system which were logged by dedicated PCs and the YSI dissolved oxygen which was logged by the probe's internal logger. Careful attention was paid to the synchronisation of the PC and logger clocks to the master clock used by the ABC system.

Sampling rates varied from 10 seconds to 15 minutes.

The Level C included a suite of calibration software, which was used to apply initial calibrations to convert raw counts into engineering units. At the end of the cruise the Level C disk base was transferred to BODC for further processing.

BODC Data Processing Procedures

Data from the underway files were merged into a common file (the binary merge file) using time as the primary linking key with a sampling interval of 30 seconds. Data sampled at higher frequencies were reduced by averaging. Data logged as voltages (e.g.PAR, nutrients) were converted to engineering units.

The sulphur and dissolved oxygen data were merged with the data logged by the ABC system using small bespoke programs.

Each data channel was inspected on a graphics workstation and any spikes or periods of dubious data were flagged. The power of the workstation software was used to undertake all possible comparative screening checks between channels. Note that the software used provided a window displaying the current position along cruise track to allow the spatial context of the data to be taken into account during screening.

Project Information

LOIS River-Atmosphere-Coast Study (LOIS - RACS)

Introduction

The Land-Ocean Interaction Study (LOIS) was a NERC research programme designed to study processes in the coastal zone. The Rivers, Atmosphere and Coasts Study (RACS) was a major component of LOIS that looked at land-sea interactions in the coastal zone and the major exchanges (physical, chemical and biological) between rivers and estuaries and the atmosphere. The study focused on the east coast of the UK from the Wash to the Tweed.

RACS included several sub-components

• BIOTA - A study of salt marshes of the Humber and Wash
• RACS (A) - An atmospheric chemistry study looking at air mass changes from the Wash into East Anglia
• RACS (C) - A study of the estuaries, coasts and coastal waters between Great Yarmouth and Berwick upon Tweed.
  1. The coastal oceangraphic survey
  2. The Humber estuarine study
  3. The Tweed estuarine study
  4. The Holderness experiment
• RACS (R) - A study of rivers that drain into the North Sea

RACS (A) was coordinated by the University of East Anglia and RACS (C) by the Plymouth Marine Laboratory.

RACS (A)

The bulk of the RACS (A) data set was collected during two field campaigns in the winter (October/November) of 1994 and the summer (May/June) of 1995. During these campaigns data were collected continuously from the University of East Anglia Atmospheric Observatory at Weybourne on the north Norfolk coast. An instrumented vessel was stationed offshore to provide a second sampling site to allow changes in a given air mass to be monitored. The Imperial College Jetstream research aircraft made one flight during each campaign to provide a link between the two surface stations. The Jetstream made four additional flights in 1996 and 1997.

RACS (C)

The coastal oceanographic survey

The coastal oceanographic data set was collected during a series of 17 RRS Challenger cruise legs. Most cruises covered two survey grids. One from Great Yarmouth to the Humber designed around the distribution of the sandbanks and a second simple zig-zag grid from the Humber to Berwick on Tweed. A large number of anchor stations, usually over one or two tidal cycles, were worked in the area of the Humber mouth or the Holderness coast.

The Humber estuarine study

The Humber estuarine data set was collected during a series of 33 campaigns on the Environment Agency vessels Sea Vigil and Water Guardian in the Humber, Trent and Ouse river systems at approximately monthly intervals between June 1993 and December 1996. Each campaign consisted of two or three one-day cruises. The tracks covered the estuary from the tidal limits of both Trent and Ouse to Spurn Point. Instrumental and sample data are available from a series of fixed stations that were sampled during every campaign.

The Tweed estuarine study

The Tweed estuarine data set was collected during a series of 13 campaigns using RV Tamaris in association with a rigid inflatable vessel at approximately monthly intervals between July 1996 and July 1997. Each campaign covered the tidal reaches of the River Tweed.

The Holderness experiment

The Holderness Experiment was designed to monitor the process of sediment transport along the Holderness coastline. It consisted of three moored instrument deployments during the winters of 1993-1994, 1994-1995 and 1995-1996. Mooring platforms were deployed at eight stations along two lines off the Holderness coast. A northerly and a southerly line of four stations each were used (N1 - N4 and S1 to S4) with the lowest numbers being inshore. Both lines were approximately perpendicular to the coast, although the S4 station lay to the south of the S line, off Spurn Head.

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: