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Eelgrass is threatened by human activities that increase water turbidity (such as agriculture or road building), block light (construction of docks), or disturb the bottom (anchoring or dredging). These impacts have caused significant declines in eelgrass in many U.S. coastal waters. The Washington State Department of Natural Resources, which monitors the distribution of eelgrass in Puget Sound, has not yet found evidence of large-scale loss; however, losses have occurred in some areas, and the increasing urbanization of the region poses threats to eelgrass and the entire nearshore ecosystem.
As part of the Coastal Habitats in Puget Sound project, U.S. Geological Survey (USGS) scientists are investigating the dynamics of seawater flow over and through eelgrass meadows to quantify the influence of eelgrass on waves and currents and to determine whether it retains sediment. Our first study site is in Indian Cove, Shaw Island, in the San Juan Islands. Eelgrass grows taller (greater than 1 m) and in deeper water in the San Juan Islands than elsewhere in the sound because the water is clearer there.
Jessie Lacy, Guy Gelfenbaum, Jodi Harney, Kevin O'Toole, Hal Williams, Jodi Eshleman, and Dave Gonzales deployed two instrumented frames in Indian Cove from the research vessel Karluk for a two-week experiment in July. One frame was deployed within the eelgrass meadow, which extends to a depth of 5 m MLLW (mean lower low water); the other was deployed outside the meadow. Our goal was to measure velocity and suspended-sediment concentration above and within the eelgrass meadow, and to determine the influence of eelgrass on currents and waves by comparing velocities measured at the two stations. Underwater video cameras were mounted on the frames to record how the posture of the eelgrass varies with current strength. This information is important in defining the vertical extent of the velocity measurements above the meadow, because the vegetation interferes with the acoustic velocimeters. The video also captured images of many fish in the eelgrass.
The frame within the meadow was in very shallow water (2.5 m MLLW), and the top half of the 2.2-m-high frame was exposed at low tide during spring tides. The Karluk was able to access the site at high tide because of the large tidal range in Puget Sound (about 3 m in the San Juan Islands).
We took high-resolution digital photographs of bed sediment within and outside the eelgrass meadow to determine grain size, using technology developed by Dave Rubin, Hank Chezar, and Jodi Harney. Jodi Harney took images of the sea floor by deploying an underwater camera from the Karluk. With these images, we will gauge the influence of eelgrass on bed-sediment grain-size distribution.
Before the deployment, Jessie and seagrass ecologist Sandy Wyllie-Echeverria of the University of Washington mapped the seaward edge of the eelgrass meadow, using boat-mounted underwater video and a global positioning system (GPS). Jessie mapped the landward edge of the meadow from shore at low tide.
Sandy and his family, who have been monitoring eelgrass colonization in Picnic Cove on Shaw Island for more than 10 years, showed Jessie their techniques for estimating eelgrass density. We counted stems in 20 quadrats, each 1 m2, during an extreme low tide. Both plant density and plant morphology are expected to be important factors in the influence of eelgrass on waves and currents.
Not only does eelgrass influence waves and currents, but waves and currents also influence eelgrass. Current strength and wave energy play a role in limiting the distribution of eelgrass and may govern the patchiness of meadows. The role of storms in disturbing or destroying eelgrass is poorly understood, as is the relation between meadow density and the ability to survive storms. In future experiments, we plan to use similar techniques to investigate these interactions between waves, currents, and eelgrass. Measurements of flow in and above eelgrass meadows can help to distinguish between natural and human-induced stresses on eelgrass populations.
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