Marine Sediment Quality

A functioning, resilient Puget Sound ecosystem is defined to include marine sediment quality that supports functioning, healthy communities of sediment-dwelling invertebrates. The sediment (gravel, sand, silt, and clay) on the floor of Puget Sound forms a unique habitat that is home to clams, marine worms, burrowing shrimp, bottom-dwelling fish, and thousands of other unique species. In turn, these animals form a critical part of the marine food web, are instrumental in mixing and oxygenating the sediments, and they process the sediments allowing nutrients to cycle between the sediments and the overlying waters.

The Marine Sediment Quality Vital Sign tells us about the level and toxicity of chemical contaminants in Puget Sound sediment and the ability of this bottom habitat to support aquatic life. The Vital Sign also helps understand whether efforts to reduce pollutants to marine waters are effective.

While there are sediments burdened with chemical contaminants in some locations in Puget Sound, we know that other environmental pressures, including those associated with climate change, ocean acidification, and nutrient loading, are affecting the composition of sediment-dwelling biota. Declines occurring in invertebrate community condition despite low and declining sediment contaminant levels point to the influence from these other pressures. Revisions have been made to the Puget Sound Sediment Monitoring Program to better understand these relationships, and revisions to the Marine Sediment Quality Vital Signs that reflect these additional sediment quality stressors are currently under development.


Key Vital Sign Messages

      Benthic marine organisms. Dept. of Ecology

Recent measurements from an expanded suite of Puget Sound Sediment Monitoring Program parameters indicate:

  • Organic matter matters – There is strong spatial concordance between locations with high levels of organic matter in sediments and adversely affected benthic communities. 
  • Terminal inlets accumulate organic matter - Spatial patterns observed for sedimentary biogenic silica, total organic carbon, total nitrogen, and particle size indicate that organic matter accumulates more in terminal inlets than in other areas of Puget Sound.
  • Oxygen matters – There is also strong spatial concordance between areas of low dissolved oxygen in sediments, as seen in the Salish Sea Model, and adversely affected benthic communities. These areas again include terminal inlets.
  • Benthic communities are impacted in terminal inlets - The benthic invertebrates in Puget Sound's weakly flushed depositional terminal inlets have lower taxa richness, impacted biomass, and are characterized as adversely affected more frequently than in other areas of the Sound.
  • Pressures projected to intensify - Projected effects of climate change and nutrient loading pressure on Puget Sound include longer water residence time, increased water temperature, and increased phytoplankton growth and die-off in the water column.  All have the potential to concentrate organic matter and harmful decomposition by-products and lower dissolved oxygen in the bottom water and sediments.  Terminal inlets and other areas already influenced by these pressures will likely see further decline in water and sediment quality, negatively affecting benthic communities. 

Three decades of monitoring sediment chemistry, toxicity, and benthic invertebrates indicate: 

  • Sediment chemistry - The majority of Puget Sound sediments sampled from 1997-2015 do not have elevated levels of measured chemical contaminants. Highest concentrations were near population and industrial centers, with improvements seen in these areas over time.
  • Sediment toxicity – 88% of Puget Sound sediments measured from 1997-2015 were non-toxic. Sediments with low, moderate, or high toxicity were located near urban and non-urban terminal inlets and areas with poor circulation, low dissolved oxygen, and high sulfides.
  • Benthic invertebrates – Significant declines in total abundance and taxa richness occurred in sediment-dwelling invertebrate assemblages in Puget Sound regions and bays sampled from 1997-2015. Declines were also noted for these stations grouped by harbor, urban, passage, and rural characteristics.
  • Little correspondence between measures – Statistical analyses revealed low correspondence between the sediment chemistry, toxicity, and invertebrate community characteristics measured in sediments throughout Puget Sound from 1997-2015.
  • Alternative environmental pressures – Declines occurring in invertebrate community condition despite low and declining sediment contaminant levels point to stresses from environmental pressures other than priority pollutant chemical contamination – for example, climate change, ocean acidification, and nutrient loading. Those other pressures should now be examined.

Moving forward with sediment monitoring, Vital Signs, and reporting:

  • Monitoring program redesign – Beginning in 2016, sediment toxicity testing was eliminated from the Puget Sound Sediment Monitoring Program. New parameters were added to better understand other environmental pressures not previously measured. 
  • Vital Sign revisions – Without future toxicity testing, the Sediment Quality Triad Index was also eliminated from the monitoring program. A revised Benthic Index has been recommended and will be developed as a new Vital Sign.
  • Reporting changes – Sediment monitoring data is now being reported in an engaging, user-friendly, interactive story map format. Visit the Department of Ecology’s Sediment Quality in Puget Sound story map collection to see Ecology’s current findings.

Contributing Partners

Margaret Dutch, Marine Sediment Monitoring Team, Washington State Department of Ecology.
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