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Offshore Water Quality

Monitoring offshore water quality in King County is essential for understanding naturally occurring conditions across Puget Sound and how human and climate impacts are affecting this valuable marine ecosystem.
Two marine scientists deploying a CTD, an oceanographic measurement device.

King County’s marine environment supports a rich ecosystem that ranges from microscopic plankton to iconic orcas. These creatures depend on an ecological balance that is threatened by environmental challenges like pollution, habitat degradation, and climate change. To better understand and safeguard the health of Puget Sound, King County monitors offshore waters by conducting laboratory analysis of water quality. We look at parameters such as bacteria and nutrients and take physical, chemical, and biological measurements with a multi-sensor device called a CTD (conductivity, temperature, and depth) at 14 offshore locations in the Central Basin and 4 locations in the marine water-influenced area of the Duwamish River. These locations include sampling near wastewater outfalls as well as sampling located further away from outfalls. We collect these samples twice monthly during most of the year (monthly in December and January).

The laboratory and CTD data we collect offer insights into Puget Sound’s ecosystems, such as the distribution of nutrients and amount of dissolved oxygen in the water, the influence of regional climate and freshwater input on water properties, and the influence of incoming water from the Pacific Ocean. These data allow scientists to track changes and make informed decisions for ecosystem management and conservation.

More Information

The goals of this monitoring are to: 

  • Collect and maintain a long-term dataset of conditions within Puget Sound’s Central Basin. 

  • Characterize water quality within King County’s marine ecosystem. 

  • Evaluate spatial and seasonal variation of water quality parameters from year-to-year. 

  • Determine physical, chemical, and biological dynamics that influence water quality. 

  • Assess long-term trends in water quality parameters. 

  • Monitor water quality near King County marine wastewater outfalls. 

  • Support regional marine monitoring and modeling efforts. 

  • Evaluate data with regard to Washington State water quality guidelines. 

View the full-size map (better display for mobile browsers).
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 --
Sigma-Theta Density Potential density (minus 1000) from conductivity, temperature, and depth 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 --
Nitrite + Nitrate Seabird SUNA Nitrate Sensor (10 mm pathlength) mgN/L 0.034
Parameter Units Method Detection Limit Reporting Detection Limit Method
Ammonia Nitrogen mg/L 0.002 0.01 KEROUEL & AMINOT 1997
Chlorophyll a ug/L 0.05 0.1 EPA 445.0
Enterococcus CFU/100ml 1 SM 9230C2A+B 23RD
Fecal Coliform CFU/100ml SM 9222D 23RD
Nitrite + Nitrate Nitrogen mg/L 0.01 0.04 SM4500-NO3-F
Orthophosphate Phosphorus mg/L 0.005 0.01 SM4500-P-F S
Pheophytin a ug/L 2.3 4.66 EPA445.0
Secchi Transparency m
Silica mg/L 0.05 0.1 WHITLEDGE 1981
Total Suspended Solids mg/L 0.5 10 SM2540-D
Total Nitrogen mg/L 0.05 0.1 SM4500-N-C S
Dissolved Oxygen mg/L 0.1 0.5 SM4500-O-C
Salinity PSS 2 3 SM2520-B

Download Data

Download data here.

Note: some data may be provisional. Contact MarineWQ@kingcounty.gov for information about the most recent quality control of data. 

Documentation

Learn more about our program by reading documents such as the latest sampling and analysis plan and read relevant reports and presentations.

Contact us with any questions or access additional resources in the Science Section Library.

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