Dive Dry with Dr. Bill

#336: Giant Kelp: A Worthy Subject for a Lifetime

Macrocystis pyrifera, the giant kelp that forms the beautiful forests in southern California waters, has been the focus of my professional life for almost 45 years. Even before I arrived here on Catalina in 1969, I worked with a noted Harvard marine biologist from New Zealand, Dr. H. Barraclough "Barry" Fell. Barry triggered my interest in kelp during a freshman seminar in college, and I conducted research on it with him both during my later undergraduate years, and for the first seven years I taught at Toyon Bay. Despite this, it has been under represented in my newspaper columns... until recently. I'm currently editing two episodes on giant kelp for my proposed "Munching & Mating in the Macrocystis" cable TV show, and want to share the story of kelp with my readers.

Charles Darwin was one of the first to bring it to the attention of the western world. In 1834 while anchored off Chile on the famous voyage of the H.M.S. Beagle, he wrote "The number of living creatures of all orders whose existence intimately depends on the kelp is wonderful... if in any country a forest was destroyed, I do not believe nearly so many species of animals would perish as would here, from the destruction of kelp."

Kelp is believed to have first appeared on Earth during the Miocene, five to 23 million years ago. Coincidentally this was the period during which the geologic processes of plate tectonics and volcanism were at work creating the southern California region... including Catalina. It is a relatively well studied species, especially for a marine alga or "seaweed." Despite this, there is considerable confusion regarding the classification of kelp in the genus Macrocystis, as well as other closely related ones like the elk and bull kelp. Traditional classification relied on their morphology or external appearance. However, more recent studies using ribosomal RNA suggest that their genetic structure does not agree with these earlier classifications.

Because of this, the distribution of Macrocystis has been variously reported throughout the world. Some have asserted three different species, Macrocystis pyrifera, M. integrifolia and M. angustifolia are found along the West Coast from as far north as Sitka, Alaska, to southern Baja. Other scientists recognize only two species off North America. In the southern part of this range off Baja, kelp is largely limited to locations of cold water upwelling with rich nutrients. Members of the genus are known from cold, temperate waters off South America, South Africa, the Sub-Antarctic islands, southern Australia, Tasmania and New Zealand.

Kelp normally attaches only to well-consolidated rocky substrate. However, off Santa Barbara there are kelp forests attached to worm tubes on protected sandy bottoms. I have seen it attached in similar habitats at a few locations on Catalina's protected leeward coast. Soft bottoms pose another problem in that the sediments may bury the younger stages of kelp or the holdfasts of the mature ones on the bottom. Areas with moderate water motion are preferred since currents and other water movement brings fresh nutrients in.

The maximum depth for kelp on Catalina's windward side and the mainland is generally about 50-60 feet. This is due to the more turbid waters caused by suspended sediments, which restrict the amount of light reaching the kelp for photosynthesis. In the clearer waters off our leeward coast, one can find kelp attached in depths to 130 feet. Because the red and violet ends of the spectrum get filtered out quickly in water, at depth the light field is primarily composed of blue and green wavelengths. If kelp used chlorophyll alone to capture sunlight, it would reflect back the wavelengths (or colors) that are most abundant at depth. Therefore kelps use brown accessory pigments such as fucoxanthin to capture light and transfer its energy to the chlorophyll for photosynthesis.

This cooler water alga prefers temperatures in the 50 to 68 degree Fahrenheit range. During warm water episodes such as El Niños, the kelp may completely die off if water temperatures exceed the upper limit for more than a week or two. Three summers ago we had an exceptionally warm period with water temperatures of 79 degrees at the surface, and warm water down to depths of 150 feet. The kelp almost died out completely, leaving the poor kelp bass and other species with few hiding places. However, temperature is not always the direct cause of death. Warmer water contains fewer nutrients and kelp may become starved for nitrogen and other necessary substances.

Morphologically, kelp is divided into three structural elements: the leaf-like blades, the stem-like stipe and the root-like holdfast. The broad, flat blades are responsible for most of the photosynthesis. They are corrugated (known technically as rugosity) and have spines along the edges. It is believed these structures slow down water flow and allow higher rates of transfer for nutrients and gases. To keep the blades near the surface where sunlight is most intense, each one has a hollow float bulb known as a pneumatocyst at the base. Contrary to popular opinion, these are not filled with methane, but with a combination of several gases.

The blades are attached to the stipes which may number one to several hundred on an individual kelp. Together, a stipe with its blades and pneumatocysts is called a frond. Each frond lives about six months, and new ones are created at the top of the holdfast to replace the ones that die and slough off. They grow upward towards the surface by having the end blade divide and form many new blades. This region of growth is called the apical meristem. All blades have another growth region at their base, which has cells that divide in three directions causing each blade to grow longer and thicker as it ages.

Although the holdfast looks like a root on land plants, it does not serve to collect water and nutrients since the alga is surrounded by both in the water column. It's primary function is to attach the kelp securely to the bottom and keep it there in the face of storms, surge and wave action. It attaches by forming root-like structures known as hapterae (singular haptera). Initially purplish to red in color, these turn yellow or brown once they actually contact and adhere to the rocks. The holdfasts are perennial, and may live as long as six years.

Giant kelp has been a worthy subject for my studies over the past five decades. I will continue to research it even if I am forced to stop diving, since it can be detected in satellite images, mapped and the distributions and persistence analyzed within a geographic information system using a computer. Of course I've already done that, but each year new kelp maps can be created and added to the existing ones to extend the database. This information can be very useful in designating new marine protected areas. I guess I'm just not cut out for bridge, or golf or collecting postage stamps (getting too expensive to do that!) as I age. Perhaps it is because of all the kelp I've eaten in salads, soups, cheesecake, ice cream, cakes, brownies and... oh, yes... I can't forget the beer!

© 2009 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 and on Charter Communications Cable channel 33 at 7:30 PM on Tuesdays in the Riverside/Norco area. Please help me climb out of self-imposed poverty... buy my DVD's (see this link). Yes, take Dr. Bill home with you... we'll both be glad you did!

Giant kelp blades and pneumatocysts, the growth region or apical meristem,
the stipes and the holdfast showing the hapterae.

This document maintained by Dr. Bill Bushing.
Material and images © 2008 Star Thrower Educational Multimedia