Actually a different species than the infamous floating denizen of the Sargasso Sea, Sargassum muticum originated from the Sea of Japan but now occupies most areas of North America's Pacific Coast and Europe's Atlantic coast. Riding on the shells of introduced Pacific oysters in the early 1900's, Sargassum was first documented in Washington waters in the 1950's. By 1997 the Washington Department of Natural Resources ShoreZone Inventory found Sargassum inhabiting 34% of Whatcom County's shoreline.
In the Whatcom County maps (depicted above), brown algae (including sargassum) is indicated by brown shading. This data was provided by Washington State DNR and Whatcom County PDS. The maps were created by Anchor Environmental.
Sargassum's vigorous spread upon arrival in the Pacific Northwest may be due to simple but effective methods of reproduction and dispersal. Each individual plant contains both male and female reproductive parts. Once fertilized, eggs remain stuck to the parent frond but immediately begin rapid development. After several days the heavy offspring fall off, usually settling within a few feet of the parent. Alternatively, if the alga breaks, its holdfast not only grows new fronds, but the broken fronds may survive for months, dropping young algae in new locations. Fronds die off in September, but holdfasts over-winter and rapid growth renews in March.
Sargassum muticum can be found colonizing cobble and rocky substrates in lower intertidal and shallow subtidal habitats. The rapid growth of this algae, along with its ability to reproduce in a single season allows it to establish itself quickly, particularly in disturbed areas of sheltered bays that provide open substrate for offspring to settle and mature. Once established, Sargassum reduces the abundance of native algae by shading.
The ecological impact of Sargassum is not fully known. On one hand, the complex branching of the fronds provides habitat for large numbers of grazing amphipods and other small creatures that are in turn fed upon by other species. As with eelgrass and kelp, Sargassum provides spawning surface habitat for Pacific herring, which lay eggs on the blades. However, where habitats overlap. On the other hand, aggressive colonization by Sargassum shades out eelgrass, kelp, and other native algae - vital habitat for juvenile salmon, forage fish, and other marine species.
The harvest of sea-weeds is not allowed in the Puget Sound because of marine vegetation's vital role in providing habitat to important species. The commercially harvested green urchin feeds on kelp and thus is displaced by Sargassum, which it dislikes. Impacts on fish and other species are uncertain.
Since introduction to Whatcom County waters less a half-century ago, the presence of Sargassum muticum now exceeds one-third of the county's shoreline. Observations in the Birch Point and Cherry Point areas have shown continued expansion. Research has shown that disturbance of native algae beds strongly aids invasion by Sargassum. The influence of human activity is uncertain, however possible factors include:
- Dredging removes native plants and buries those nearby, leaves substrate open for colonization by Sargassum.
- Construction of docks and other nearshore structures disturbs eelgrass and algae beds, which may allow Sargassum to takeover.
Research is being conducted to learn the impacts of Sargassum on the Puget Sound ecosystem and how its spread can be controlled. Although removal of Sargassum can allow native species to re-establish themselves, eradicating Sargassum can be extremely difficult, as it requires intensive manual labor and holdfasts left behind will regenerate new fronds. Prevention of disturbance to native habitats may be the key to their protection.
- O'Clair & Lindstrom, North Pacific Seaweeds (2000)
- Washington Department of Natural Resources. Shorezone Inventory. (1997)
- Tom Mumford (DNR), personal communication.
- Kevin Britton-Simmons, Friday Harbor Lab and personal communication.
- Kurt Stick and Dan Penttila (WDFW), personal communication.