Kelps are large seaweeds in the order Laminariales that form dense canopies in temperate rocky intertidal and subtidal habitats less than 30 m in depth. The kelp flora of the Pacific Northwest is one of the most diverse in the world.

Giant Kelp (Macrocystis pyrifera). Photo by Claire Fackler. Courtesy of NOAA.
Giant Kelp (Macrocystis pyrifera). Photo by Claire Fackler. Courtesy of NOAA.


Kelps are characterized by a highly dimorphic lifecycle consisting of a large diploid sporophytic (bed-forming) phase and a microscopic haploid gametophytic phase. In the Puget Sound region, bull kelp (Nereocystis luetkeana) occurs throughout Puget Sound and the Strait of Juan de Fuca, while the distribution of giant kelp (Macrocystis integrifolia) is restricted to the Strait of Fuca (Berry et al. 2005, Mumford 2007). Both form conspicuous floating canopies, or kelp beds. Sporophytes of Nereocystis are annual or semi-annual, whereas sporophytes of Macrocystis are perennial, persisting for several years. In addition to these dominant bed-forming taxa, numerous species of understory (non-floating) kelp occur in subtidal habitats, many of which are present in southern and central Puget Sound (Mumford 2007).

Kelps are important primary producers. They contribute to Puget Sound food webs by providing food for herbivores and detritivores, and by releasing dissolved organic carbon (Duggins et al. 1989). In addition, kelps create important biogenic habitat that is utilized by fish, invertebrates, marine mammals, and birds (e.g., Ojeda and Santelices 1984, Graham 2004). Kelp can significantly alter the physical environment by modifying current and wave energy (Eckman et al. 1989) and this buffering capacity can influence the ecology of other organisms that utilize kelp environments for larval dispersal and settlement, for example rockfish (Carr 1991).

The extent and composition of kelp beds varies through time in response to natural and human-induced influences. In general, the distribution of kelp is determined by the amount of light available for photosynthesis, nutrient levels, grazers, physical disturbances, and toxic contaminants (reviewed in Mumford 2007). In addition to these external factors, demographic structure may play an important role in driving temporal dynamics of Macrocystis kelp beds through decreased fitness of older, more inbred populations (Raimondi et al. 2004, Reed et al. 2006).

Sea Otter (Enhydra lutris)Sea Otter (Enhydra lutris)

Sea Otter (Enhydra lutris). Photo by Mike Boylan. Courtesy U.S. Fish and Wildlife Service. 

Sea otters have been shown to be keystone predators in kelp forest ecosystem through their consumption of sea urchins, a major grazer of kelps (Estes and Palmisano 1974). In Washington state, otter populations have been slowly increasing since their reintroduction in 1969 and 1970 (Lance et al. 2004) following their extirpation through hunting in the 1900s. While they are more abundant on the open coast, otters have been observed as far east as Pillar Point in the Strait of Juan de Fuca (Lance et al. 2004, Laidre and Jameson 2006) where they have been shown to consumes a high proportion of urchins (Laidre and Jameson 2006). The potential for sea otters to expand into further into Puget Sound could affect kelp populations through trophic interactions. Furthermore, harvest of urchins by humans may be an important indirect driver of kelp populations in the Strait of Juan de Fuca; Berry et al. (2005) anecdotally observed that historic increases in urchin harvest rates were positively associated with increases in kelp abundances. However, in an experimental study, neither simulated fisheries removals nor simulated otter predation significantly affected the abundance of kelps in the San Juan Archipelago (Carter et al. 2007).

In addition to trophic interactions, climate changes associated with El Nino are known to cause short-term declines in kelp populations (e.g., Dayton and Tegner 1984), while the Pacific Decadal Oscillation could be driving changes over longer time periods. Substrate movement, as a result of altered nearshore hydrology and geomorphology, may also influence the amount of available habitat for attachment of kelps (Mumford 2007).

Due to their proximity to shore, kelps are likely to be subjected to anthropogenic impacts such as pollutiondischarge, nutrient influxes from urban and agricultural sources, increased turbidity, and increased rates of sedimentation (Dayton 1985, Mumford 2007). These can alter photosynthetic performance and growth of sporophytes and prevent settlement, growth, and reproduction of microscopic gametophytes. Toxic contaminantssuch as petroleum products are known to damage kelp by lowering photosynthetic and respiratory rates in meristematic tissue (Antrim et al. 1995).


Bull Kelp (Nereocystis luetkeana)Bull Kelp (Nereocystis luetkeana)

Bull Kelp (Nereocystis luetkeana). Image courtesy of NOAA.

The Washington Department of Natural Resources (WDNR) conducts an annual inventory of canopy-forming kelp beds along the outer coast of Washington and the Strait of Juan de Fuca (approximately 360 km of shoreline). Inventories have been conducted annually since 1989 (with the exception of 1993) using aerial color-infrared photography (Van Wagenen 2004). In 2005, Berry et al. (2005) reported a total of approximately 1,700 hectares of floating kelp (Nereocystis and Macrocystis) on Washington’s outer coast and the Strait of Juan de Fuca.


See: Kelp crisis? Decline of underwater forests raises alarms, Salish Sea Current, November 22, 2019.


Kelps are important primary producers and create important biogenic habitat in Puget Sound ecosystems. Annual aerial surveys of floating kelp canopies conducted by WDNR show that between 1898 and 2004 floating canopies increased in outer coastal areas an in the western Strait of Juan de Fuca. Floating kelp canopies in the eastern Strait of Juan de Fuca showed no statistical change over the same period. Anecdotal evidence indicates sharp local declines in kelp abundance in southern and central Puget Sound and the San Juan Archipelago and calls for new investigations and expansion of kelp surveys.

Literature Cited

Antrim, L. D., R. D. Thom, W. W. Gardiner, V. W. Cullinan, D. K. Shreffler, and R. W. Bienert. 1995. Effects of petroleum products on bull kelp (Nereocystis luetkeana). Marine Biology 122:23-31.

Berry, H. D., T. F. Mumford, and P. Dowty. 2005. Using historical data to estimate changes in floating kelp (Nereocystis luetkeana and Macrocystis integrifolia) in Puget Sound, Washington. Puget Sound Georgia Basin Research Conference. Nearshore Habitat Program, Washington Department of Natural Resources, Olympia, WA.

Britton-Simmons, K., J. E. Eckman, and D. O. Duggins. 2008. Effect of tidal currents and tidal stage on estimates of bed size in the kelp Nereocystis luetkeana. Marine Ecology Progress Series. 355:95.

Carr, M. H. 1991. Habitat selection and recruitment of an assemblage of temperate zone reef fishes. Journal of Experimental Marine Biology and Ecology 146:113-137.

Carter, S. K., G. R. VanBlaricom, and B. L. Allen. 2007. Testing the generality of the trophic cascade paradigm for sea otters: a case study with kelp forests in northern Washington, USA. Hydrobiologia 579:233-249.

Dayton, P. K. 1985. Ecology of Kelp Communities. Annual Review of Ecology and Systematics 16:215-245.

Dayton, P. K., and M. J. Tegner. 1984. Catastrophic storms, El Nino, and patch stability in a southern California kelp community. Science 224:283-285.

Druehl, L. D. 1969. The northeast Pacific rim distribution of Laminariales. Pages 161-170 in International Seaweed Symposium.

Duggins, D. O., C. A. Simenstad, and J. A. Estes. 1989. Magnification of secondary production by kelp detritus in coastal marine ecosystems. Science 245:170-173.

Eckman, J. E., D. O. Duggins, and A. T. Sewell. 1989. Ecology of under story kelp environments. I. Effects of kelps on flow and particle transport near the bottom. Journal of Experimental Marine Biology and Ecology 129:173-187.

Estes, J. A., and J. F. Palmisano. 1974. Sea Otters: Their role in structuring nearshore communities. Science 185:1058-1060.

Graham, M. H. 2004. Effects of Local Deforestation on the Diversity and Structure of Southern California Giant Kelp Forest Food Webs. Ecosystems 7:341-357.

Laidre, K. L., and R. J. Jameson. 2006. Foraging Patterns and Prey Selection in an Increasing and Expanding Sea Otter Population. Journal of Mammalogy 87:799-807.

Lance, M. M., S. A. Richardson, and H. L. Allen. 2004. Washington State Recovery Plan for the Sea Otter. Washington Dept. of Fish and Wildlife, Wildlife Program, Olympia, WA.

Mumford, T. F. 2007. Kelp and eelgrass in Puget Sound. Seattle District, U.S. Army Corps of Engineers, Puget Sound Nearshore Partnership, Seattle, WA.

Ojeda, F. P., and B. Santelices. 1984. Invertebrate communities in holdfasts of kelp Macrocystis pyrifera from Southern Chile. Marine Ecology Progress Series 16:65-73.

Raimondi, P. T., D. C. Reed, B. Gaylord, and L. Washburn. 2004. Effects of self-fertilization in the giant kelp, Macrocystis pyrifera. Ecology 85:3267-3276.

Reed, D. C., P. T. Raimondi, L. Washburn, B. Gaylord, B. P. Kinlan, and P. T. Drake. 2006. A metapopulation perspective on patch dynamics and connectivity in giant J. P. Kritzer and P. F. Sale, editors. Marine Metapopulations. Elsevier Academic Press, Amsterdam; Boston.

Thom, R. M., and L. Hallum. 1990. Long-term changes in the areal extent of tidal marshes, eelgrass meadows and kelp forests in Puget Sound, Wetland Ecosystem Team. United States Environmental Protection Agency.

Van Wagenen, R. 2004. Washington coastal kelp resources, Summer 2004. Nearshore Habitat Program, Washington Department of Natural Resources, Olympia, WA.

About the Author: 
The Puget Sound Science Review builds upon and adapts work completed as part of the Puget Sound Partnership’s April 2011 Puget Sound Science Update, a document "synthesizing existing, peer-reviewed scientific information on specific topics identified by policy leaders [Puget Sound Science Update Synthesis 2010]."