Sometimes it's difficult to see the "forest" through the kelp fronds. For several months I've been writing about the ecology of the kelp forests, but haven't actually talked about the main component... the kelp itself. It's time I remedy that with this week's article on Macrocystis pyrifera or giant bladder kelp, the focus of my scientific research ever since I arrived on Catalina.
Giant kelp is found from central California south to Bahia de Magdalena, Baja California and also in South America, New Zealand, Tasmania and the Sub-Antarctic islands. Its unusual distribution in both the northern and southern hemispheres may reflect oceanographic events occuring during the Ice Ages. It is a temperate water alga, preferring water 68 degrees or less, and is therefore not found in the subtropics and tropics where corals dominate.
Naturalist Charles Darwin (yes, the evolution guy) was impressed with the diversity of life he observed when he first saw kelp forests. Kelp serves as both food and habitat for at least 800 species of marine animals and 300 marine algae. A number of fish such as halfmoon and opaleye, as well as invertebrates like snails and sea urchins, feed directly on the kelp. Many others feed on drifting pieces of kelp or particles from decaying organic matter including kelp known as leptopel. Kelp itself produces its own food through photosynthesis, similar to land plants. Many fish live within the protection of the kelp forest while invertebrates such as bryozoa, hydroids and worms actually attach to the blades of this alga once they stop growing. Kelp may be attacked by diseases like black rot.>
Giant kelp has an interesting natural history involving two different types of plants. Most people are familiar only with the giant sporophyte plants forming the kelp forests, but there is a second generation involving tiny gametophyte plants found on the bottom. The giant sporophytes may produce as many as 500,000 spores an hour. Most settle to the bottom near the parent plants, but some may drift in the ocean currents for a brief time. The spores develop into the male and female gametophytes which produce sperm and eggs that fertilize and develop into the giant sporophytes. Of course only a small percentage of the spores and young gametophytes actually survive to reproduce or we'd be overrun by giant kelp!
There are several different structures on a mature sporophyte. These include a root-like holdfast which serves to attach the plant to the bottom substrate (usually rock). The holdfast is not a true root since it does not absorb water or nutrients from the soil as in land plants. The stipe is the vine- or stem-like strand that originates at the holdfast level and reaches towards the surface. Stipes are covered with leaf-like blades which are where nutrients are absorbed and photosynthesis largely takes place. At the point of attachment of the blade to the stipe is a floatation bulb known as a pneumatocyst which buoys the alga towards the surface where sunlight is abundant. Although some have mistakenly said these float bulbs contain mainly methane, there are actually a number of different gasses inside them. A single stipe with its blades and pneumatocysts is called a frond.
Under optimum conditions of water temperature, nutrients and light, giant kelp can grow as much as 24" a day and reach lengths of 200 feet. Growth starts near the base of the holdfast where the individual stipes form. Since these growth regions are at the base, they are not directly affected by kelp harvesting which is limited to the top four feet of the water column. Growth regions known as apical meristems can be seen at the end of each stipe where the individual blades are splitting to form new blades and pneumatocysts. Individual fronds survive for about six months but the kelp holdfast, and therefore the alga itself, may survive several years.
Giant kelp has not been harvested commercially off Catalina for many years. Historically kelp has been harvested by man for a variety of uses. Prior to our involvement in World War I, the United States conducted a major survey of the kelp resources on the West Coast. The reason was to find a replacement for the potash that came primarily from German mines prior to the Great War. Why? It is a primary ingredient of gunpowder. Some say that kelp won the war!
Beginning in the 1930's kelp was harvested for the sodium alginate in it. Alginate was an important gelling and colloidal agent. It has been used in a wide range of products including beer (you can see where my priorities are), toothpaste, ice cream, cake mixes and frostings, shampoos, health foods, lubricating oils and welding rods. Due in part to the ecological value of giant kelp, synthetic chemicals have been developed to reduce the need for alginate and kelp harvesting.
This week's article barely covers the topic of giant kelp. Future articles will discuss its ecological importance, why kelp forests may fluctuate over time, and how kelp serves as an important mechanism for dispersing marine life to isolated habitats like those of Catalina. Stay tuned. Now it's Miller (I mean kelp) time. Cannery Row's "Doc" Ricketts would appreciate that.
© 2003 Dr. Bill Bushing. Watch the "Dive Dry with Dr. Bill" underwater videos on Catalina Cable TV channel 49, 10:00 AM and 5:00 PM weekdays.
Image caption: Giant kelp holdfast; sunbursts through
two apical meristems or growth regions
This document maintained by
Dr. Bill Bushing.
Material © 2003 Star Thrower Educational Multimedia