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Marine Water Condition Index

The Marine Water Condition Index (MWCI) provides a way to assess the "vital signs" and conditions in saltwater harbors, bays, and Puget Sound. The information helps us understand the health of saltwater resources as measured against seasonal and regional trends for marine waters.

A tool to understand changing marine systems

We measure changing conditions in the many estuaries of Puget Sound, coastal Willapa Bay, and Grays Harbor against prior years' data using the Marine Water Condition Index.

Marine water conditions and water quality differ greatly from one estuary to another. Natural conditions fluctuate from year to year, and respond to climatic and oceanographic influences. This makes it challenging to apply a single water quality criterion in these diverse marine water systems.

We take larger natural oceanic influences and climatic variability into account to tell whether a change has affected marine water quality positively or negatively. When marine conditions change significantly in a particular region, we focus our attention on identifying and understanding these changes.

From 1999-2008, we gathered baseline data using standard water quality gradients that track minute changes in marine waters. Conditions identified between 1999 and 2008 are not pristine, but provide an informed baseline against which we can measure change. Because we have very good data, use consistent methods, and incorporate information on larger oceanographic and climatic patterns, we are able to identify how water quality is changing over time.

As a result, the Marine Water Condition Index can quickly detect changes at each station.

To describe continuing changes with high confidence, we statistically test whether yearly index scores follow a trend over a 10-year window. To test if changes or trends continue or stabilize at a new equilibrium we use statistical testing, specifically, non-parametric test statistics using the Spearman Rank Correlations (N=10, p=0.05).

The Marine Water Condition Index in the 2015 State of the Sound report

How to read the Marine Water Condition Index

We present significant trends for the Marine Water Condition Index (MWCI) and the Eutrophication sub-index on maps. Each estuarine area is given an MWCI score that ranges from -50 to +50, a range of 100 points.

  • Positive change, score greater than 0 (green)
  • Unchanged, score = 0 (yellow)
  • Negative change, score less than 0 (red)

Color-coded charts can display water quality over a complex estuarine environment. The detailed reports show how conditions at each station have changed relative to a historic snapshot in time. Maps show regions with significant negative (red) or positive (dark green) change in the MWCI score.

Maps show if a change in the MWCI occurs in a region of general water quality concern. Red and orange colors depict regions that had persistently water quality concerns in the last decade.

A specialized index measures nutrient pollution

To show significant changes specific to nutrients, we use a subset of the MWC — the Eutrophication Sub-index — which is displayed on the chart with negative changes displayed as orange and positive changes displayed as green.