Rock Crab on Sargassum, Crina Hoyer
Description: While the name Sargassum may conjure up mythic images
of ancient sailing ships engulfed in floating seaweed, seashore explorers
may see fronds of Sargassum muticum draped upon intertidal rocks
like a mermaid's locks. A disc-shaped holdfast glues the bushy alga
to the substrate. From this base grow several stalks, or fronds,
often over three feet long with alternating leafy branches. Among
the golden-brown, leaf-like blades are pea-size floats, or vesicles,
and similarly shaped reproductive organs.
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.
Distribution: 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.
Reproduction: 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
dropping young algae in new locations. Fronds die off in September,
but holdfasts over-winter and rapid growth renews in March.
Subtidal Sargassum, Britton-Simmons
Ecology: 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 repro-duce
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 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 surfaces for Pacific herring, which lay eggs on the blades.
However, where habitats overlap, aggressive colonization by Sargassum
shades out eelgrass, kelp and other native algae - vital habitat
for juvenile salmon, forage fish, and other marine species.
Economic Impact: 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.