Eelgrass and other seagrass species play a key role in the nearshore ecosystem. They provide food, shelter and nursery habitat for a variety of organisms, ranging from small invertebrates to commercially important fish species and wading birds. Eelgrass also helps prevent erosion and maintain shoreline stability by anchoring seafloor sediment with its spreading roots and rhizomes.

• Eelgrass area was stable at most sites surveyed by the Department of Natural Resources. Over half of all sites (soundwide and within regions) showed no trend in eelgrass area, both over the long-term (2000-2020) and in recent years (2015-2020). 
• Out of 214 sites surveyed soundwide, there was a similar number of sites with long-term increases and declines in eelgrass area (33 vs. 38). In recent years, sites with declines in eelgrass area (32) outnumbered sites with increases (9). This is consistent with the results from the soundwide eelgrass area indicator.
• The Northern Puget Sound and the Saratoga Whidbey Basin region and the Central Puget Sound and Hood Canal region are assumed to be in stable condition because there was no significant difference between the number sites with increases and declines.
• The San Juan Islands and Strait of Juan de Fuca has been identified as a region of concern because sites with declines in eelgrass area significantly outnumber sites with increases, both over the long-term and in recent years. Most sites with declines were located in the San Juan Islands.
• Some of the largest eelgrass losses in the San Juan Islands have occurred in embayments. The most notable examples are Westcott Bay on San Juan Island (near total loss), Reef Net Bay on Shaw Island (over 60% loss), Shallow Bay on Sucia Island (about 75% loss), and Swifts Bay on Lopez Island (about 50% loss).
• Losses in embayments and at the end of inlets have also been observed in other regions of greater Puget Sound.
• Eelgrass beds near river deltas are highly dynamic. In both the Skokomish and the Nisqually deltas, eelgrass populations fluctuated by over 50% between 2000 and 2020. At a site in Skagit Bay, nearly 500 acres of eelgrass was lost between 2004 and 2020. The overall loss of eelgrass in Skagit Bay is more extensive, as adjacent sites were also impacted.

While eelgrass in greater Puget Sound appears stable on a regional scale, there is significant variability at smaller spatial scales. Approximately 40% of vegetated sites from the 3-year rotating panel had a significant long-term trend, while 23% of vegetated sites had a significant trend in recent years. Out of these changing sites, there was a similar number of long-term increases and declines. However, in recent years declines outnumbered increases. This pattern is consistent with the results of the soundwide eelgrass area indicator, which shows declines in total eelgrass area between 2016 and 2020.

There is a regional pattern in the number of sites with increases and declines. More sites have declined around the San Juan Islands and Strait of Juan de Fuca over the long-term and in recent years. However, in Northern Puget Sound and the Saratoga Whidbey Basin and in Central Puget Sound and Hood Canal, there was no significant difference between sites with increases and declines. Both regions are assumed to be in stable condition. Overall, eelgrass beds at ends of inlets and in enclosed embayments seem particularly vulnerable to declines.

SAN JUAN ISLANDS AND THE STRAIT OF JUAN DE FUCA

Based on a random sample of sites, there is a higher number of declines than increases near the San Juan Islands than would be expected by chance. Site-level trends in the larger SVMP dataset seem to confirm this pattern. Of the 89 sites with enough data for trend analysis near the San Juan Islands, four sites showed long-term increases and 16 sites had long-term declines. This pattern becomes more pronounced when you look at data between 2015 and 2020 (no sites with increases, 15 sites with declines). Eelgrass losses occurred across the potential depth range. Depending on the site, eelgrass losses were observed along the shallow edge, the deep edge, or throughout the entire eelgrass bed. This suggests that declines were likely due to a variety of stressors.

Some of the largest eelgrass losses in the San Juan Islands (both in terms of the relative and the absolute amount of eelgrass lost) have occurred in enclosed embayments. However, the timing of these declines has varied. At some embayments, such as Westcott Bay and Watmough Bay, declines happened relatively early in the time series. In Swifts Bay the declines were gradual over time, while in Reef Net Bay and Shallow Bay the loss was most pronounced after 2016.

While the absolute amount of eelgrass lost at individual sites in the San Juan Islands is relatively small compared to the large losses in, for example, Skagit Bay, the cumulative loss in the region could be substantial and have implications for the ecosystem as a whole. The San Juan Islands are an important feeding and rearing environment for out-migrating juvenile salmon from northern Puget Sound and British Columbia. Local eelgrass beds provide significant spawning and rearing area for forage fish, an important source of prey for juvenile salmon.

EELGRASS NEAR RIVER DELTAS

River deltas in the Pacific Northwest are often home to expansive eelgrass beds. The gentle sloping sand flats create large amounts of potential habitat, but are also subject to a range of environmental stressors such as variable salinity, desiccation at low tide, and deposition and erosion due to wave action and river flow. We have documented high variability in eelgrass area at three river deltas that have been extensively sampled.

In both the Skokomish and the Nisqually deltas, eelgrass populations fluctuated by over 50% throughout the course of our monitoring program. Near the Skagit delta, eelgrass populations have declined by more than 200 hectares over the last 15 years. At the Skokomish and the Nisqually deltas, the variability in eelgrass area may be due to complex interactions between environmental drivers. At the Skagit delta, the large loss in eelgrass is likely due to erosion, after an avulsion at the North Fork of the Skagit River. Since 2014 a series of newly formed drainage channels has progressively cut into a contiguous eelgrass bed at the center of Skagit Bay.

These changes could have an impact on local salmon populations. Juvenile Chinook and chum salmon make extensive use of nearshore and estuarine environments during their early marine rearing phase. These species are often found in high abundances in eelgrass beds at the outer edge of river deltas during their out-migration period. A large loss, such as in Skagit Bay, could limit available habitat for out-migrants which rely on eelgrass for forage habitat and refuge from predation.

Puget Sound Seagrass Monitoring Report: Monitoring Year 2018-2020

Puget Sound Eelgrass Recovery Strategy March 2015

Nearshore Habitat Eelgrass Monitoring | WA - DNR