AIMS Vertical Profiles of Conductivity Temperature and Depth (CTD)

Brief description

Biological oceanographers from the Australian Institute of Marine Science (AIMS) have studied the physical, chemical, and biological properties of seawater around northern Australia using a variety of methods including in situ sampling, moored sensors, and vertical profiles. This metadata record describes vertical profiles of physico-chemical data collected by researchers from the from 1974 until the present. Vertical profiles are important tools that oceanographers routinely use to understand ocean structure. These profiles are collected using a Conductivity, Temperature and Depth (CTD) profiler, which is lowered on a steel cable from the water’s surface to close to the seafloor. Niskin bottles can also be attached to the CTD (in a rosette) or used independently to collect water samples from different depths for analysis. AIMS uses Sea-Bird Electronics CTD profilers, which are typically fitted with additional sensors to measure parameters such as fluorescence (a proxy for chlorophyll a concentration), turbidity, beam attenuation, dissolved oxygen concentration, and photosynthetically active radiation (downwelling light) sensors. Instrument models SBE 19plus and SBE 19plusV2 are used currently, while older records include data from SBE 25, SBE 25plus, and SBE 9plus instrument models. Annual calibrations of profilers are carried out in Sea-Bird Electronics laboratories in the USA. These calibration values are included within the instrument configuration file. Pre-trip CTD checks are carried out before each field trip, which include checking the physical status of the sensors and cables and battery voltage. Prior to conducting a cast, the CTD is secured to the hydrographic wire, tubing is removed to allow flush water to drain from the conductivity-temperature cell, and any protective caps are removed from the other sensors. The CTD is lowered into the water sitting ~3 m below the surface, and a three minute "soak" allows sensors to equilibrate and air bubbles to be flushed by the pump. The CTD is then raised to ~0.2 below the surface and the profile is commenced at a rate of 0.5 – 1 m s-1. The CTD is sent to near-bottom, ensuring it does not touch the seafloor, and retrieved to the surface. Casts are done on the sunny side of the boat to avoid the boat's shadow interfering with the measured light profiles. Data processing is conducted using Sea-Bird proprietary software and includes: conversion of raw instrumental records to measurement units, alignment, removal of ship roll, outlier removal, and bin averaging the down-cast at 1 m increments. Further details of CTD data handling can be found in the Quality Assurance and Quality Control Manual from the Marine Monitoring Program (see link below in Related Information). CTD data can be downloaded from this record (see link below in Data Downloads). Each CTD cast occurs at a unique combination of geographic location, time, and date. In the AIMS database, each cast is assigned a unique alphanumeric identifier (called a ‘station’), which is comprised of a 3-letter area code and a 3-digit station number (e.g. WQM324). Using this code, water chemistry measurements taken at the time of the CTD cast can be retrieved from AIMS’ water quality database (see link below in Related Information). Some data in this record were collected as part of the Great Barrier Reef Marine Monitoring Program for Inshore Water Quality, which has its own metadata record (see link below in Related Information).

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Maintenance and Update Frequency: asNeeded

Statement: A detailed description of data processing and quality assurance/quality control information can be found in the Marine Monitoring Program's annual QA/QC report (see link below in Related Information).

Notes

Credit
Zagorskis, Irena

Credit
Skuza, Michele

Credit
Furnas, Miles