Guidelines for determination of salinity and temperature using CTD

Abstract

Salinity Measurements based on electrical conductivity have since the 1960s replaced measurements of chlorinity. The Practical Salinity Scale of 1978 (PSS-78) presently used, has been defined to maintain a continuity with older scales and methods. The scale is based on conductivity of a reference solution prepared from potassium chloride. Practical Salinity (SP) is calculated from the ratio of conductivity between sample and reference solution. Since the scale is based on a ratio, no unit is assigned to it. Despite this, salinity data are sometimes presented with the units ‰ or psu. The equations used in calculation of Practical Salinity from conductivity are valid for practical salinity ranging from 2 to 42. A new standard for the properties of seawater was introduced in 2010; the thermodynamic equation of seawater 2010 (TEOS-10). This standard also includes a new scale, called the Absolute Salinity scale. Absolute Salinity (SA) is expressed as a mass fraction, in grams per kilogram of solution. While Absolute Salinity is the variable needed to calculate density and other properties of seawater, Practical Salinity is still the variable measured, reported and archived in marine environmental monitoring. Temperature Temperature sensors are calibrated to the ITS 90scale. 1.2 Purpose and aims Although not HELCOM Core Indicators, salinity and temperature are essential supporting parameters in marine monitoring. CTD profiles should always be recorded when water is sampled for monitoring purposes. A CTD cast gives vital information about the present characteristics of the water column. The data and information obtained from a CTD cast can be used for identifying water masses by its salinity and temperature, finding the depth for the onset of hypoxia and anoxia, view the phytoplankton distribution and other oceanographic phenomena depending on which additional sensors that are used together with the CTD. If the CTD data undergoes basic quality controls onboard it can also be made available in near-real time for example to be assimilated into ocean models (Baltic Operational Oceanographic System, www.boos.org). The parameters obtained from the CTD could be used for assessment of water quality and/or as supporting parameters. To ensure that CTD data of high quality is collected there is a need to follow standard protocols for CTD sampling, data processing, documentation, quality control, sensor control, calibration and maintenance, data reporting and data storage.