Non-Native Eelgrass: Zostera japonica

Photo <i>Zostera Japonica</i> taken by Dr. Kim Patten
Z. japonica and Z. marina mosaic. Z. japonica is the shorter and darker eelgrass in the middle. Photo courtesy of Jeff Gaeckle, WA Department of Natural Resources.

Dwarf eelgrass (Zostera japonica) is a non-native, grass-like marine plant that is believed to have been introduced to Washington from Japan. It exists in Puget Sound, Willapa Bay, Grays Harbor, and along the outer coast. It has spread from Willapa Bay as far south as California and as far north as Alaska.

Zostera japonica affects important shellfish growing areas and appears to displace important ecological functions of mudflats and other habitats. However, research has found that japonica also provides many similar basic ecosystem functions as the native eelgrass – Zostera marina. Given Z. japonica and Z. marina occur in close proximity or in mixed beds, additional research is needed to learn more about the risks to native habitats and shellfish beds from Z. japonica, the beneficial ecological services it provides, and potential management options that are protective of Z. marina. Ecology participates in state agency discussions regarding study and responses to this non-native invasive plant.

Ecology also:

  • Administers a water quality permit for the use of the aquatic herbicide imazamox on commercial clam beds in Willapa Bay.
  • Provides Shoreline Master Program guidance to local governments. Ecology's current advice to local governments is that projects should be reviewed on a case-by-case basis to determine if: 1) the project site contains native and/or non-native eelgrass, 2) the project will cause a loss of ecological functions, and 3) what mitigation or project changes are needed to avoid impacts. Local governments are encouraged to consult with state natural resource agencies and base their project review on the most current scientific information and studies. (See the Shoreline Master Programs Handbook - Aquaculture Chapter.)

Meeting: The Science and Management of Zostera japonica, June 2013

To better understand the state’s role in managing this invasive aquatic plant, the Departments of Ecology, Natural Resources, and Fish and Wildlife, and the Washington State Invasive Species Council co-sponsored a state meeting on June 18-19, 2013: The Science and Management of Zostera japonica in Washington. These state resource agencies wanted to better understand the detrimental and beneficial effects of Z. japonica – and what its continued growth means for Washington’s future. The meeting included presentations from eelgrass experts and an analysis of the data and science by an independent panel of scientists.

Some general conclusions that Ecology drew from the meeting are:

  • Z. japonica is a non-native species.
  • It exhibits many characteristics of an invasive plant (higher seed production, vegetative reproduction, greater tolerance to a range of environmental conditions).
  • It is expanding its range north and south along the Pacific coast, and local populations are expanding in place.
  • It is an ecosystem engineer and modifies habitats.
  • Due to its non-native, invasive characteristics, Z. japonica should not be protected as “critical saltwater habitat” as defined in the shoreline master program guidelines [WAC 173-26-241(2)(c)]
  • It provides many similar, basic ecosystem functions as native eelgrass, including carbon sequestration.
  • There should be a multi-agency, collaborative approach to determine next steps in studying and responding to this invasive aquatic plant.

Provided below are meeting materials and other resources related to the meeting. Included is a summary report from a japonica science workshop sponsored by the Department of Natural Resources and Washington Sea Grant in September 2010. The science portion of this June 2013 meeting built on the 2010 workshop.

Meeting materials

Staff contact: Cedar Bouta, Shorelands and Environmental Assistance Program, 360-407-6406





Shoreline Master Programs

Willapa Bay water quality permit

Noxious weed listing

Nearshore habitat eelgrass monitoring

Hydraulic project approval rulemaking

Invasive Species Council

University of Washington Eelgrass Research

Global Seagrass Monitoring Network

Humboldt Bay eelgrass project