Sargassam muticum is a non-native brown seaweed species associated with kelp forests. It was introduced by the oyster mariculture industry to the Northwest in the 1930s (Anderson 1998). Sargassum can occupy space on rocks normally used by bull kelp if bull kelp is damaged or lost for some reason (Thom and Hallum 1990).
Kelp was mapped early in the 20th century, because it was considered an important source of potash. The WDNR ShoreZone Inventory (Washington State Department of Natural Resources 2001) indicates that 11% of the Puget Sound shoreline is bordered by floating kelp (Puget Sound Water Quality Action Team 2002). Subsequent mapping has shown that kelp is either similar to historic levels or has increased in area since that time. The partial explanation may be that armoring of shorelines has increased erosion and exposed more rocky substrata. Kelp forests form refuge habitat for a number of fish species, especially rockfish. Juvenile and subadult salmon have also been noted in kelp forests. Because kelp attains a size in excess of 15 m between late winter and midsummer, it is considered one of the fastest growing organisms in the world. Adequate light, temperature, and nutrients are required for this growth. Kelp is effective at reducing wave energies and thereby reducing the erosion of beaches. Herring are known to spawn on kelp blades. Sea urchins can graze extensively on kelp in some areas. Massive floating mats of kelp and other species begin to deposit on beaches in the fall and winter, where amphipods and other “shredders” break up the material, which makes the plant material available to other animals. Many rockfish species are commonly found associated with kelp forests. Besides providing a three-dimensional structure that affords some protection from predators, food abundance is high. The structure of the forest alters currents and may help concentrate planktonic food used by these fish and invertebrates in the forest.
Factors affecting water clarity or light can adversely impact kelp. Competition with other understory species can affect kelp abundance. In particular, once Sargassum becomes established it may be hard for bull kelp to recolonize an area (Thom 1978). Kelp in shallow waters has been subject to increasing harvest pressure, which may be reducing its distribution near highly populated areas (Thom and Hallum 1990). Oil was effective in bleaching and killing plants during the Tenyo Maru oil spill in 1991 (Antrim et al. 1995).
According to the WDNR Shorezone Inventory, several kelp beds have been observed at Wing Point on the eastern shore of Bainbridge Island and Point White along the southwestern shore (Eelgrass and Kelp Occurrence Map, Appendix A). |
