Incredible plants: Nereocystis

I wrote previously about a fascinating intertidal kelp, the sea palm, found in the northeastern Pacific Ocean. A very close relative of this species is the bull kelp, Nereocystis luetkeana. This very large marine seaweed is a favorite beach find of kids (and more playful adults) since the flexible stipes of the plant make excellent ropes or whips during beach adventures.

Bull kelp forest at Deception Pass State Park, northern Washington, July 2013.

Despite the genetic similarity between Postelsia and Nereocystis, the bull kelp occupies a different habitat than Postelsia and has a rather different morphology. Nereocystis is principally a subtidal species with morphological features that are adapted for maximizing photosynthesis in deeper water. At the ocean bottom, plants are attached to a hard substratum with conical holdfasts that support long, thin, flexible stipes. Each stipe gradually expands into a semi-spherical pneumatocyst, the air bladder of the species. Several smooth strap-shaped blades emerge from the pneumatocyst at the top of each plant. Filled with mostly carbon monoxide (yes, the poison!), the function of the pneumatocyst is to help the blades stay afloat near the water surface.

A Nereocystis blade with sori (dark brown
patches) at Trinidad, Humboldt County,
California, August 2002.

Like all kelp species, the large plants seen along the coast actually only represent half of the kelp life cycle. The macroscopic plants are called sporophytes. The other, microscopic stage of the life cycle begins with the production of spores in tissues (called sori) that develop in the center of the blades of the sporophytes. The sori fall to the sea floor and release spores which develop into gametophytes - multicellular, but microscopic, filaments of cells on rock surfaces (Druehl 2000). Separate male and female gametophytes produce eggs and sperm and after fertilization of the egg, a new sporophyte generation is born.

 Nereocystis is one of only a few kelps worldwide that grows large enough to form subtidal kelp forests. Ecologically, such species are known as “foundation species” (Dayton 1972) because they literally provide the habitat structure upon which an entire ecosystem is built. Kelp forests are home to other numerous other species of macroalgae, invertebrates, fish and marine mammals. In the northern Pacific Ocean (the global hotspot of kelp diversity), other kelp species that are large enough to form forests include Macrocystis (giant kelp), Pelagophycus (elk kelp), and to some degree, Eualaria fistulosa and Egregia menziesii. Nereocystis is the main canopy-forming kelp species north of about Santa Cruz, California. From about Santa Cruz south to Baja California, Macrocystis is the main forest-forming species. In certain areas of the central California coastline, such as Big Sur, Macrocystis and Nereocystis can co-occur.

An intertidal bull kelp sporophyte with Costaria costata, Egregia
menziesii and other seaweeds, Carmel, Monterey County,
California, May 1999.

Like its relative Postelsia, Nereocystis is usually an annual species (Abbott and Hollenberg 1976). Most adult plants last no more than one growing season. With such a short life span and the capacity to attain a length of up to 36 meters, the bull kelp sporophyte has prodigious growth rates – estimated to be as much as 6-17 cm per day (Druehl 2000, Springer et al. 2010)! Though usually subtidal, bull kelps also occasionally grow in the intertidal and can be seen on a calm day at low tide. I love finding small sporophytes anywhere from a few inches to a few feet long during a low tide excursion.

To dive in a Nereocystis kelp forest is a delightful experience. One of my most memorable dives occurred during college at the Big Creek Ecological Reserve south of Monterey on the rugged Big Sur coast. There, in the frigid water I was able to observe the graceful stipes of bull kelps rising like kites up in the sea. The bull kelp is a stout but graceful plant, perfectly depicting the dual beauty and wildness of the Pacific coast.

References

Abbott IA and Hollenberg GJ. 1976. Marine Algae of California. Stanford University Press.

Dayton PK. 1972. Toward an understanding of community resilience and the potential effects of enrichment to the benthos at McMurdo Sound, Antarctica. In: Proceedings of the Colloquium on Conservation Problems in Antarctica.

Druehl LD. 2000. Pacific Seaweeds. Harbour Publishing.

Springer Y et al. 2010. Ecosystem based management of Nereocystis. Oceanography and Marine Biology: An Annual Review.