Offshore CTD Monitoring
King County collects monthly in situ water column profiles at its offshore monitoring stations using a CTD.
A CTD is a multi-sensor array that measures dissolved oxygen, salinity, temperature, density (calculated), transmissivity, photosynthetically active radiation, and fluorescence (as a measure of chlorophyll).
The sensor array is deployed on a rosette; a round metal frame that also holds 12 Niskin sampling bottles.
The CTD collects water quality measurements 8 times a second (8 Hertz) as it is lowered through the water column from the surface to the
bottom (downcast) and back up again (upcast). 
The CTD also triggers the closure of the 5-liter Niskin bottles on the upcast at
pre-programmed depths to collect discrete water-column samples for laboratory analysis of additional parameters.
The water column profile data for each cast are averaged, or binned, over each 0.5 meters of the total depth.
Please click on the station map to the left to download CTD data for both current and historic sampling stations. Detection limits and methods are described
in the following table.
| Parameter |
Sensor |
Reporting units |
Detection Limit |
|
Chlorophyll |
WET Labs WETStar Fluorometer |
µg/L |
0.06 |
|
Conductivity |
Seabird SBE 4 Conductivity Sensor |
S/m |
-- |
|
Depth |
Seabird SBE 29 Pressure Sensor |
m |
-- |
|
Dissolved Oxygen |
Seabird SBE 43 Dissolved Oxygen Sensor |
mg/L |
0.5 |
|
Density |
Calculated from conductivity, temperature, and depth sensors |
kg/m3 |
-- |
|
Light Transmission |
WET Labs C-Star Transmissometer |
% Light |
0.01 |
|
PAR (Surface) |
LI-COR Surface Light Intensity (PAR) LI-190-SA-50 |
µmol.sm3 |
-- |
|
PAR (Water Column) |
LI-COR Light Intensity (PAR) LI-193-SA-50 |
µmol.sm3 |
-- |
|
Salinity |
Calculated from conductivity and temperature sensors |
PSS |
-- |
|
Temperature |
Seabird SBE 3 Temperature Sensor |
°C |
-- |
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