Eelgrass grows in dense beds in the shallow waters of Puget Sound. This important marine plant serves as food source, nursery, and haven for birds, fish, crabs, shellfish, and other marine organisms. Eelgrass also filters sediments and nutrients, improving water clarity, and stabilizes the sea floor, which protects shorelines from erosion.
Eelgrass is valuable to the health of Puget Sound not only for the ecosystem functions it provides, but because it is sensitive to environmental stressors. Eelgrass health is an indicator of changing conditions in our watersheds and estuaries.
Although some larger Puget Sound eelgrass beds are stable, many of the smaller, fringing beds throughout the Sound are in decline. The reasons for this decline are not fully understood, but nitrogen pollution entering Puget Sound from human sources is likely having major impacts in many locations, while in other areas increases in sediment inputs and direct physical damage are stressing eelgrass beds.
Eelgrass (Zostera marina) is an important submerged marine plant growing throughout Puget Sound. Changes in the abundance or distribution of this resource reflect changes in environmental conditions.
Eelgrass and other seagrass species are used as indicators of ecosystem health throughout the world because they respond sensitively to many natural and human-caused environmental factors that affect water quality and shoreline conditions. These factors are also likely to affect many other species that depend on eelgrass habitat.
For example, excess nutrients, sewage, and algae can reduce water clarity, while storms, runoff, and dredging can stir up sediment, preventing light from penetrating the water and reaching the eelgrass. Boat wakes, propellers, and docks can also disturb eelgrass beds.
Also, since eelgrass is protected by many regulations, its condition reflects, in part, the success of management actions. The Washington Department of Natural Resources assesses status and trends in eelgrass by evaluating eelgrass area and depth range at more than 100 sites throughout Puget Sound annually, using a statistical sampling framework.
Two measures are used to demonstrate eelgrass status and trends in Puget Sound:
A 20% increase in the area of eelgrass in Puget Sound relative to the 2000-2008 baseline reference by the year 2020.
The Soundwide area of eelgrass measured in 2011 has not changed relative to the 2000-2008 baseline reference, and thus there has been no progress toward the 2020 target. In fact, we may be losing ground as there are more eelgrass sites that are getting smaller than sites that are getting bigger.
A total of 205 sampling sites across Puget Sound were classified for eelgrass area trends in 2012. The majority of these sites showed no changes in eelgrass area since monitoring began; they were stable (172 sites).
Furthermore, sites with decreasing trends in eelgrass area greatly outnumbered those with increasing trends. Only in the Saratoga Whidbey Basin has the number of sites that are getting bigger surpassed the number of sites that are shrinking. Of all sites analyzed, there were five cases of total eelgrass loss.
The regions where there is the most concern for eelgrass is Hood Canal, San Juan and South Sound. Although the exact causes of eelgrass decline there are not known, scientists suspect that nitrogen is the most important factor.
Monitoring information indicates that the goal to achieve a 20% increase in eelgrass area by 2020 cannot be met with current management practices: the stresses on eelgrass in Puget Sound must be significantly reduced to see gains in eelgrass area and health.
There was no change in eelgrass area in 2011 relative to the 2000-2008 baseline, or even compared to 2009 and 2010.
Puget Sound supports roughly 22,600 hectares of eelgrass beds (Figure 1). Eelgrass distribution patterns vary by sub-basin, with two main types of eelgrass beds: narrow fringing beds and broad beds on shallow flats. Approximately 25% of the total eelgrass area occurs in only two embayments: Padilla and Samish bays.
There was no significant increasing or decreasing trend in eelgrass area in 2011 relative to the 2000-2008 baseline, calculated as the weighted mean of eelgrass area in that time period (Figure 1).
A total of 205 sites were classified for eelgrass area trends in 2012. The majority of these sites were eelgrass beds where no change or trend in the size of the bed was detected since monitoring began (172 sites; Figure 2).
However, of all sites where a change was detected, more than twice as many sites had decreasing eelgrass than sites with increasing eelgrass. Of all sites analyzed, there were five cases of total eelgrass loss. In no region did improving eelgrass sites outnumber declining eelgrass sites, except in the Saratoga Whidbey Basin.
Concerns about Hood Canal
Hood Canal is the region that had the greatest number of sites where the amount of eelgrass decreased (Figure 3), including two sites where eelgrass beds completely disappeared. The Hood Canal region is a major concern, particularly because 83% of changing sites are in decline. However, the eelgrass bed at the mouth of the Skokomish river grew bigger, perhaps due to restoration actions and less nitrogen.
The eelgrass in Hood Canal has been indicating signs of eutrophication, where excess nitrogen loading from human sources contributes to the formation of seaweed blooms in the nearshore which accumulate and grow in eelgrass beds, stress the plants and contribute to the observed decline. Although not related to human nitrogen loading and its impacts to eelgrass, stratification and low dissolved oxygen have been seen in this deep, fjord-like basin. The localized eutrophic conditions in Hood Canal are evident throughout Puget Sound and pose a major threat to eelgrass and its health throughout Puget Sound.
Two other regions of concern for the size of eelgrass are the San Juan Islands and South Sound.
The Saratoga Whidbey Basin is notable for having many sites with growing eelgrass beds; none lost ground. Scientists plan to investigate whether restoration activities at the mouth of the Snohomish River mouth may have contributed to this change.
A total of 205 sites were classified for eelgrass area trends. The majority of these sites were eelgrass beds where no change or trend in the size of the bed was detected. Hood Canal has the greatest number of sites where the amount of eelgrass decreased, including two sites where eelgrass beds completely disappeared.