Healthy Water Quality
Marine Water
Marine Benthic Index
Vital Sign Indicator
Index Value (index value)

No targets are currently set for this indicator.

Sandra Weakland
Contributing Partners
Last Updated
2/1/2024 1:58:55 PM
Puget Sound and urban bays (outlined in blue) sampled for the marine Sediment Benthic Index (SBI). Map shows SBI categories, “adversely affected” and “unaffected “, based on most recent measurements.

Scientists are developing a new Marine Benthic Index to better assess and communicate status and trends of benthic invertebrate community health. While this work is being conducted and evaluated, Ecology will continue to report on the Sediment Benthic Index (SBI) to track changes in the benthos. The SBI tells us whether marine areas are either adversely affected or unaffected by natural or human stressors based on the composition of the benthic community.

Vital Sign Indicator Chart

Percent of study area with unaffected benthic invertebrate communities, as determined by our Sediment Benthic Index for each sampling event including Puget Sound-wide and urban bays monitoring.

Invertebrates that live in the sediment, or benthos, are key components of the Puget Sound ecosystem. Many species provide important services such as nutrient processing, food for other species (including humans), and modifying the sea floor. Benthos have limited capacity for movement, which makes them vulnerable to natural and human-caused pressures, including chemical contamination, carbon and nutrient loading, and climate change. Benthic communities change in response to multiple shifting environmental conditions, making them excellent biological indicators.

Key Vital Sign Indicator Results
  • The Sediment Benthic Index (SBI) indicates that the area of Puget Sound with unaffected benthic communities is gradually declining over the nearly three decades sampled.
  • The SBI indicates that benthic condition is improving in the East Possession Sound and Elliott Bay study areas, and declining or holding steady in Bainbridge Basin, Commencement Bay, and Budd Inlet.
  • The entire Bellingham Bay study area was categorized as having adversely affected benthic communities.
Monitoring Program

Department of Ecology’s Puget Sound Sediment Monitoring Program

Data Source

Department of Ecology’s Environmental Information Management (EIM) System: Marine Sediments

The Sediment Benthic Index (SBI) is based on species counts and characterizes sampling sites as either adversely affected or unaffected by natural or anthropogenic stressors utilizing a multiple lines of evidence approach. The Department of Ecology's Marine Sediment Monitoring Team (MSMT) samples the sediment and organisms living within it once a year, in either April or June, depending on the survey. Fifty sites throughout Puget Sound (see map) are sampled annually to characterize the entire Puget Sound. In addition, each year the MSMT samples 30 or more sites in one of six urban bays on a rotational basis. Monitoring details are available in the Quality Assurance Monitoring Plan (MSMT, 2023).

The SBI designations of “adversely affected” and “unaffected” are achieved by examining multiple lines-of-evidence including:

  • Levels of nine calculated benthic indices (abundance; taxa richness; evenness; species dominance; and annelid, arthropod, mollusk, echinoderm, and miscellaneous taxa abundance)
  • Presence, absence, and abundance of stress sensitive and tolerant species
  • Canonical analysis of principal coordinates (CAP) on benthic community structure
  • Best Professional Judgment

The SBI designations

  • Adversely Affected – typically defined by low taxa richness, evenness, and dominance; low abundance of stress sensitive species such as arthropods and echinoderms; high abundance of stress tolerant species of annelids and mollusks.
  • Unaffected – typically defined by high abundance, taxa richness, evenness, and dominance index values; high abundance of stress sensitive species such as arthropods and echinoderms; low abundance of stress tolerant species of annelids and mollusks.

Critical Definitions
Interpretation of Results

Spatial patterns in benthic condition as measured with the Sediment Benthic Index indicate that adversely affected communities are found primarily in the deep basins of Puget Sound and the poorly flushed terminal inlets of Whidbey and Bainbridge Basins, Hood Canal, and South Puget Sound (see map at top of this page).

Natural and anthropogenic drivers place pressure on Puget Sound’s pelagic environment, influencing the state of the sediments and ultimately the composition and ecological functioning of the benthos. We monitor the condition of the benthic habitat and associated invertebrate assemblages in order to track change over time in response to pressures such as inputs of carbon, nutrients, and chemicals to the system, and in response to climate-related pressures.

The terminal inlets of Puget Sound are areas with a higher incidence of adversely affected benthic communities, possibly attributable to pressure from organic matter (OM) deposition. Spatial patterns observed for sediment biogenic silica, total organic carbon, total nitrogen, and particle size indicate that OM accumulates preferentially in terminal inlets. Carbon and nitrogen content increases with fraction of fines (silt+clay), which also tends to be high in low-energy environments such as terminal inlets. Sediment carbon-to-nitrogen ratios and measurements of stable carbon and nitrogen isotopes suggest that the OM accumulating in these areas is primarily from marine algal production. Spatial patterns in sediment and water column OM also correspond with visual observations of large algal blooms made by the Eyes Over Puget Sound marine water monitoring program. In addition, estuarine circulation may transport and preferentially deposit OM to areas with higher residence times, often terminal inlets.


Sediment Quality in Puget Sound story map collection

While informative, the binary Sediment Benthic Index has some limitations, including that benthic communities of intermediate condition are forced into the categories of the extremes (either adversely affected or not). We recognize that finer resolution is needed to better assess and communicate the status and trends of benthic invertebrate community health.

The Puget Sound Partnership has funded a project to develop two new tools to help us understand how human disturbance affects life at the bottom of Puget Sound. The project has developed a Marine Benthic Index and a graphical causal model (Schoolmaster and Partridge, 2024).

Another potential tool the AZTI-Marine Biotic Index (AMBI; Borja et al., 2000) and its multivariate extension M-AMBI (Muxika et al., 2007). M-AMBI has been used throughout the world to assess estuarine and coastal health and has been shown to respond to multiple pressures and stressors (Pelletier and Charpentier, 2023). This index has been adopted by EPA’s coastal monitoring program, the National Coastal Condition Assessment (NCCA), to assess the condition of U.S. waters.

Additional work is being conducted to assess adaptation of these tools for use by the MSMT.

References cited:

Borja, A., J. Franco, V. Pérez. 2000. A marine biotic index to establish the ecological quality of soft-bottom benthos within European estuarine and coastal environments. Mar. Pollut. Bull.,40 (2000), pp. 1100-1114, 10.1016/S0025-326X(00)00061-8

Marine Sediment Monitoring Team (MSMT). 2023. Quality Assurance Monitoring Plan: The 2023-2028 Puget Sound Sediment Monitoring Program. Publication 23-03-104. Washington State Department of Ecology, Olympia.

Muxika, I., Á. Borja, J. Bald. 2007. Using historical data, expert judgement, and multivariate analysis in assessing reference conditions and benthic ecological status, according to the European water framework directive. Mar. Pollut. Bull., 55 (2007), pp. 16-29, 0.1016/j.marpolbul.2006.05.025

Pelletier, Peg and Mike Charpentier. 2023. Assessing the relative importance of estuarine stressors to the benthic index, M-AMBI in U.S. waters. 13th National Monitoring Conference, Virginia Beach, VA, April 24 - 28.

Schoolmaster, D.R. and V.A. Partridge. 2024. A new method for bioassessment of ecosystems with complex communities and environmental gradients. Ecological Indicators, 158.


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