E-Flora BC: Electronic Atlas of the Flora of British Columbia

Species description:
The holdfast of the sporophyte can reach 40 cm (about 16 in) in diameter (but is usually much smaller) and is richly branched to enhance its adhesion to rocks. New holdfast haptera can grow where the stipe is joined to the holdfast. The stipe grows upwards and is usually about 10 m (33 feet) long, but there are records of specimens with stipes up to 36 m (118 feet) in length. This stipe is hollow, about 1 cm (0.5 in) in diameter basally, but wider above. Fibrils of cellulose in the walls of stipe cells are oriented mostly at a 60 degree angle to the long axis of the stipe, allowing the stipe to stretch, especially near the base, when it is stressed by water movements. Individuals with undamaged stipes can withstand impressive amounts of drag.

The upper end of the stipe is enlarged to form a pneumatocyst, or float, and the gas in the stipe and pneumatocyst can contain up to 10% carbon monoxide (carbon monoxide is a poison of hemoglobin, but kelps do not, of course, contain hemoglobin). The stipe elongates by growing at a region at its upper end.

On the end opposite where it tapers into the stipe, the pneumatocyst carries two bunches of up to 64 blades, and since the pneumatocyst often reaches the surface of the water, the blades stream out along the surface in strong currents or hang down in the upper, sunlit layers where photosynthetic rates are higher. The blades form by splitting along predetermined weakened lines. They can reach up to 10 m (about 33 feet) long and 15 cm (6 in) wide. They grow out continuously from a meristem located at their base, and slough off at their older, outer tips. The detritus formed by the sloughing tips has recently been shown to be an important source of carbon for inshore intertidal communities. This detritus feeds many of the filter feeders, such as Pacific Blue Mussels (Mytilus trossulus) in the intertidal zone, then other organisms feed on the mussels, and so on.

This kelp grows rapidly; in fact, this is an annual that reaches its mature size in just one growing season, and in Washington State it can grow a phenomenal 14 cm (5.5 in) per day. Half of all the linear growth occurs in the basal 1/10 of the blade and 95% within the basal half. Growth at night is only slightly slower than growth during the day, and individuals held in continuous darkness had sustained growth for up to 12 days.

Studies done in central California have shown that removal of some blades inhibits growth of the rest of the kelp, indicating that materials used in growth are derived from the blades. A single harvest in early summer cut blade growth almost in half; workers noted that the blades would have to be harvested quite distally in order to avoid a subsequent reduction in growth.

On the west coast of Vancouver Island, British Columbia, photosynthetic pigments (chlorophylls and fucoxanthin) are low in late summer but reach high levels in fall and winter, possibly in response to declining light levels. Despite the low levels of photosynthetic pigments in late summer, that is the time of maximum photosynthesis, thanks to abundant light. The minimum photosynthetic rate occurs in April at the time of the phytoplankton bloom.

Short days (8 hours of light and 16 hours of dark), which occur in the fall, inhibit maturation. At maturity, entire spore-forming patches (called sori) are released from the blades, starting near the older, outer tips. The large holes left by these abscissions give the blades a ladderlike appearance before being sloughed. In central California, 80% of the abscission occurs from two hours before sunrise to four hours after sunrise, with an individual releasing spores for two to four days. In British Columbia, abscission occurs only at the beginning of the lowest spring tides each month. Since the sori are produced at or near the sea surface, they have the potential to be widely dispersed, but many sink to the bottom near the parent.

When the spore-forming organs settle, they release their spores (probably billions from each individual Bull Kelp), which can germinate and develop into microscopic filamentous gametophytes. The release of relatively heavy spore-forming organs insures that final spore release occurs near the sea floor, hence this helps maximize the reproductive potential of this species.

Another species famous for forming large kelp forests, Bull Kelp provides important habitat for invertebrates, fish, and Sea Otters (Enhydra lutris). Urchins relish it, so where otters are abundant and their urchin prey rare, Bull Kelp flourishes. In one study, 51% of the beached Bull Kelps had telltale urchin grazing marks at the point where the stipe had broken. These sporophytes are low in caloric value, however (just 2.10 Calories per gram of dry weight). Because Bull Kelp is an opportunist, it rapidly colonizes areas stripped by urchins. In Washington State, if urchins are either naturally rare or are removed, Bull Kelp establishes extensive beds and excludes species of Agarum. Where urchins are present, however, they preferentially eat Bull Kelp, allowing Agarum to invade and become established.

Bull Kelp does not grow west of Umnak Island in the eastern Aleutians, but individuals torn off the substratum by storms are frequently found throughout the Aleutian chain, and even as far west as Hokkaido, Japan, and as far north as St. Lawrence Island, just south of Bering Strait, showing that reproductive spores must surely also be spread among these islands. It has been suggested that dense fogs in the Aleutian summer prevent growth of Bull Kelp in the central and western part of the chain. Fog eliminates the far-red wavelengths from sunlight, and these wavelengths are necessary for stipe elongation in Bull Kelp.

In Washington State, researchers have examined the biophysics of Bull Kelp architecture. They discovered that some Bull Kelps had narrower and flatter blades than others. The narrow, flat blades tend to collapse into a streamlined bundle that enable these individuals to live in areas of stronger current. Individuals with wider, more ruffled blades would be torn from the rocks by strong currents. But ruffled blades tend to remain more spread out, which decreases self-shading, so individuals with ruffled blades have an advantage in areas with lower currents. They found that individuals with flat blades became more ruffled when transplanted to places where the currents were slower, and conversely, ruffled individuals became flatter when experimentally placed where currents were stronger. Blade shape is thus a tradeoff between streamlining to reduce drag and avoiding self-shading. The same researchers think that structural features like blade ruffling, perforations, marginal spines and floats increase turbulence and enhance photosynthesis (and hence growth).

Other work done in Washington State has shown that Bull Kelp is vulnerable to exposure to petroleum products. Weathered diesel was found to be most toxic, with unweathered intermediate fuel oil, unweathered diesel, weathered intermediate fuel oil, unweathered crude oil, and weathered crude showing decreasing amounts of toxicity.

Bull Kelps from southern British Columbia/northern Washington can survive water temperatures of 18°C (65°F) for at least a week, although they succumb to higher temperatures.