The Marine Water Condition Index (MWCI) measures changes in water quality against a baseline reference period (1999-2008). The index integrates 12 variables that each describe an important aspect of water quality, such as temperature, salinity, nutrient balance, algae biomass, and dissolved oxygen, to monitor improvements or declines in Puget Sound marine water quality.
Green shades show improved marine water quality relative to the baseline reference, 1999-2008. Orange to red shades indicate worse conditions. The maximum range of the index scores spans from -50 to +50, indicating that monthly measured variables averaged over a year are falling either above or below the expected baseline.
Key Vital Sign Indicator Results
Marine Water Condition Index scores have generally declined over the last 20 years, indicating that marine water quality continues to diminish.
Index scores in Georgia Basin, Bellingham Bay, and Whidbey Basin have declined continuously from 1999 to 2019 relative to the baseline time period from 1999-2008. Other regions decreased, but not as consistently.
Unfavorable ocean conditions in the Salish Sea and off the west coast of Washington, with effects on water renewal have contributed to the apparent decline in water quality across all regions.