Newly-published DNR study finds declining eelgrass in San Juan Islands

Eelgrass in the San Juan Islands is declining, according to a newly-published DNR report and story map, prompting concern for the health of one of Washington’s most diverse and pristine marine areas.

Widespread local declines

Based on a random sample, DNR documented 4 times more sites with long-term declines than increases in eelgrass cover in the San Juan Islands over the last 2 decades. In recent years, eelgrass losses have been even more pronounced. One third of all eelgrass beds sampled near the San Juan Islands between 2015 and 2020 showed signs of declines, and there were no increases. Additional sites sampled as part of targeted studies confirm this pattern. The long-term losses in the San Juan Islands contrast starkly with monitoring results from the rest of greater Puget Sound, where long-term site trends are more balanced between increases and declines.

Trends in eelgrass area at randomly selected sites in the San Juan Islands and Cypress Island. Losses, shown in red, greatly outnumber gains. Long-term trends span the period between 2000 and 2020, short term trends are based on data collected between 2015 and 2020.

Eelgrass losses occurred in a wide variety of habitat types, spread throughout the San Juan Islands. Spatial patterns of loss varied at sites; along the shallow edge, the deep edge or throughout the entire eelgrass bed. The timing of losses also varied among sites. At some sites, eelgrass was lost relatively early in the monitoring period, while at other sites declines occurred later on.

Change in eelgrass area at all sites sampled in the San Juan Islands, both on longer term (left) or in recent years (right).

Potential Causes

The pattern of eelgrass declines in the San Juan Islands is surprising because this location is often seen as one of the pristine areas within greater Puget Sound. While we are not sure what caused these declines, there are some indications to what may have happened.

One potential stressor is eelgrass wasting disease. This disease causes dark lesions on the leaves, and lowers how much carbon is stored in the rhizomes of the plants, which limits plant growth and survival. Dr. Drew Harvell and other researchers from the University of Washington’s Friday Harbor Laboratories have found high levels of eelgrass wasting disease near the San Juan Islands. After the 2015-2016 marine heat wave, an increase in prevalence and severity of disease coincided with declines in eelgrass shoot density at their study sites.

Losses could be due in part to human activities. DNR documented declines at several embayments that are popular destinations for recreation. Eelgrass can be damaged through trampling, anchoring, prop scars or water quality degradation. Recent efforts by local resource managers to create voluntary no-anchor zones in sensitive embayments may reduce some of the impact on eelgrass at these locations.

Environmental conditions could have exacerbated the loss of eelgrass in recent years. Puget Sound experienced two marine heat waves in 2015-16 and 2019. Warmer water temperatures can change the metabolic balance in seagrasses, increasing their light requirements. This could have contributed to loss inside embayments, which are often warmer than the surrounding waters.

Eelgrass in greater Puget Sound

Eelgrass meadows grow along many of Washington State’s shorelines, where they provide essential food, nursery and shelter for a rich community of animals, including forage fish, Dungeness crab, and salmon. In addition to providing critical habitat, these meadows are a good indicator of the health of Puget Sound.

Since 2000, DNR has monitored eelgrass populations in greater Puget Sound as part of its stewardship responsibilities. Every year, a research vessel collects hundreds of hours of underwater footage at sites throughout the region to see how eelgrass meadows are changing. This footage is scored at one second intervals for presence of eelgrass, and combined with location and depth data to estimate how much eelgrass was present at each of these sites. Over time, this dataset has grown to almost 40,000 transects, spread over 867 sites in greater Puget Sound. A subset of 214 randomly selected sites is sampled regularly to assess regional changes in eelgrass area. Other sites were visited as part of targeted studies, often in collaboration with local governments and Tribes.

Monitoring results are used to calculate soundwide eelgrass area, which is one of the indicators for the health of nearshore ecosystems by the Puget Sound Partnership. Soundwide eelgrass area has remained relatively stable between 2000 and 2020. This suggests that eelgrass populations in greater Puget Sound are in good condition.

Eelgrass beds are dynamic on smaller spatial scales, and often change in response to local and regional stressors. Some changes are likely due to human activities, while others are driven by natural processes. One dramatic example is the recent decline in Skagit Bay. In recent years, the North Fork of the Skagit River diverted from its established channel, and formed a major new outflow channel into the bay. The new channel is progressively eroding a large eelgrass bed in the center of the bay, causing a loss of more than 200 hectares since 2004.

Eelgrass beds provide habitat for Dungeness crabs and many other animals.

DNR will further explore losses in the San Juans and compare results to other areas

DNR’s findings raise a red flag – widespread eelgrass losses have occurred in the San Juan Islands, raising concerns about both the causes of decline and the ramifications for species that rely on this critical habitat. The study also reports good news – soundwide eelgrass area did not change significantly between 2000 and 2020. To further explore these findings, DNR is seeking to expand its surveys in the San Juan Islands. DNR will also continue to monitor eelgrass throughout greater Puget Sound in order to track this critical resource.

For more information: see our story map or download the report