View in Vital Signs- Functioning Habitat
- Beaches and Marine Vegetation
- Indicator
- Eelgrass Area
- Vital Sign Indicator
- Acre (acres)
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No targets are currently set for this indicator.
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Bart Christiaen
- Contributing Partners
- Last Updated
- 12/30/2024 17:37:51
Soundwide eelgrass area is a metric for the overall health of native seagrass beds in greater Puget Sound. Seagrass is an important component of nearshore habitats and is sensitive to human disturbance and declines in water quality.
Annual estimates of soundwide eelgrass area (acres) in greater Puget Sound. The dashed line is the long-term average of annual estimates from 2004 to 2023. Error bars are standard error.
Eelgrass and other seagrass species play a key role in nearshore ecosystems. They provide food, shelter, and nursery habitat for a wide range of organisms, from small invertebrates to commercially important fish species and wading birds. Eelgrass also helps prevent erosion and maintains shoreline stability by anchoring seafloor sediment with its spreading roots and rhizomes.
· The current best estimate of eelgrass area in greater Puget Sound is approximately 51,700 acres (pooled 3-year average 2021-2023).
· Soundwide eelgrass area showed an increasing trend between 2004 and 2016, and a declining trend between 2016 and 2023. The magnitude of these changes was relatively small compared to the total amount of eelgrass present in greater Puget Sound. The relative stability is reassuring and sets Puget Sound apart from many other developed areas, where substantial system-wide declines are ongoing.
· While most sites appear stable, the spatial pattern in site level trends suggests that eelgrass is susceptible to declines in certain areas of greater Puget Sound. For more information on regional patterns, see the short and long-term change at eelgrass sites indicator.
- Monitoring Program
Washington State Department of Natural Resources Nearshore Habitat and Eelgrass Monitoring: Submerged Vegetation Monitoring Program
- Data Source
The Washington State Department of Natural Resources (DNR) monitors status and trends of native seagrass species (Zostera marina and Phyllospadix spp.) in greater Puget Sound through the Submerged Vegetation Monitoring Program (SVMP). This program uses towed underwater videography to generate estimates of eelgrass area and depth distribution. Observations of the non-native seagrass Zostera japonica are recorded as part of monitoring but are not included in the estimates. Field sampling is generally conducted from May to September.
The soundwide eelgrass area indicator is based on a stratified random sample of 214 sites, selected from 2,467 potential sample sites in greater Puget Sound. These sites are sampled based on a 3-year rotating panel design: every three years DNR resamples 78-80 individual sites throughout greater Puget Sound. These sites belong to different habitat strata: narrow fringe, wide fringe, rotational flats, and core and persistent flats. Fringe sites are 1,000-meter sections of shoreline, measured along the -6m bathymetry line (MLLW). Flats sites include embayments and river deltas and are delineated based on the geomorphology of the shoreline. The core and persistent flats strata include some of the largest eelgrass beds in the study area and are comprehensively sampled on an annual basis.
DNR samples sites by collecting towed underwater video footage along transects that are oriented perpendicular to shore. The way these transects are selected has changed over time. From 2000 to 2015 sites were sampled with a new simple random draw of transects. Beginning in 2016, sites have been sampled using stratified random transects that are resampled over time. From the towed underwater video footage, we calculate a weighted mean fraction of the total transect length that is covered by native seagrass species. This value is multiplied by the area of a sample polygon to generate annual estimates of seagrass area at individual sites.
We extrapolate site area estimates based on the total length of the fringe stratum or the total area of the flats stratum to generate estimates of soundwide eelgrass area, as well as a measure of uncertainty associated with these estimates. We generate both annual estimates (n~80) or estimates based on data pooled over the most recent 3 years (n = 214). These data are compared to the long-term average of annual estimates from 2004 to 2023.
For more information on methods, see the latest iteration of DNR's Puget Sound Seagrass Monitoring Report and the Eelgrass Monitoring GIS Database Manual.
- Critical Definitions
Native seagrasses in greater Puget Sound include eelgrass (Zostera marina) and surfgrass (Phyllospadix spp.). Eelgrass is by far the most abundant native seagrass. It grows on sandy and muddy substrates between 1.4 meters and -12.5 meters relative to the low tide line (MLLW). Surfgrass is mostly found on hard substrates along the exposed rocky coasts of the San Juan Islands and the Strait of Juan de Fuca. The non-native seagrass Zostera japonica is not included in the area estimates for the indicator.
DISTRIBUTION OF EELGRASS AND SURFGRASS IN GREATER PUGET SOUND
There are two genera that include native seagrasses in greater Puget Sound: Zostera (with species Zostera marina, eelgrass) and Phyllospadix (with multiple species of surfgrass). Eelgrass is by far the most abundant native seagrass. It grows on sandy and muddy substrates between +1.4 meters and -12.5 meters relative to mean lower low water (MLLW). Eelgrass is widespread in Puget Sound but does not occur in the extreme reaches of southern Puget Sound and Liberty Bay and is relatively sparse in Sinclair Inlet and Dyes Inlet. Surfgrass is mostly found on hard substrates along the exposed rocky coasts of the San Juan Islands and the Strait of Juan de Fuca.
Eelgrass distribution patterns vary across greater Puget Sound. Approximately half of the area of all eelgrass is found on tidal flats, such as Samish Bay, Padilla Bay, and Skagit Bay. The other half grows in many small fringing beds spread throughout the Sound. In Northern Puget Sound and the Saratoga Whidbey Basin most eelgrass grows on tidal flats. In the other two regions, Central Puget Sound/Hood Canal and San Juan Islands/Strait of Juan de Fuca, most eelgrass grows along fringes.
While eelgrass has been observed as shallow as +1.4 meters and as deep as -12.5 meters, the majority occurs between 0 and -4 meters relative to MLLW. There is a large-scale spatial pattern in seagrass depth distribution in greater Puget Sound. Eelgrass tends to grow deepest near the Strait of Juan de Fuca, the San Juan Islands, and the northern portion of Central Puget Sound. It does not grow as deep in South Central Puget Sound, the Saratoga Whidbey Basin, and in bays and inlets with limited water exchange.
TRENDS IN SOUNDWIDE EELGRASS AREA
Based on data collected by DNR between 2021 and 2023, Puget Sound supports approximately 51,700 acres of eelgrass (~20,900 hectares). The annual estimates of soundwide eelgrass area show some variability, but do not exhibit a consistent trend over the entire monitoring period (2004-2023). On shorter timescales there were changes: soundwide eelgrass area increased between 2004 and 2016 and declined between 2016 and 2023.
The above figure shows soundwide estimates based on pooled 3-year samples between 2014 and 2023. According to the most recent estimate (2021-2023), there is approximately 51,700 ± 2,700 acres (mean ± standard error) of eelgrass in greater Puget Sound. Since 2014, the 3-year average has gradually declined by approximately 6,000 acres. The current estimate is slightly below the long-term average for ~55,900 acres between 2004 and 2023.
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It is unclear what drives increases and declines in soundwide eelgrass area, but the large-scale nature of this indicator points to a regional stressor, such as variability in marine water quality on larger spatial scales, regional climate, and tides. The overall magnitude of increases/declines is relatively small as compared to the long-term average, which suggests that on a soundwide scale, eelgrass populations are resilient to change.
Eelgrass beds show significant variability on smaller spatial scales, and experience different stressors depending on where they grow. For more information, see the change at eelgrass sites indicator.
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- Reporting Instructions
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