STUDIES ON THE GIANT KELP, MACROCYSTIS. I. GROWTH OF YOUNG PLANTS

NEUSHUL, M., and F. T. HAXO. (Scripps Inst. Oceanography, La Jolla, Calif.) Studies on the giant kelp, Macrocystis. I. Growth of young plants. Amer. Jour. Bot. 50(4): 349-353. Illus. 1963.The growth of entire young plants of Macrocystis pyrifera was studied in the sea and in the laboratory. Measurements were made of plants in natural stands and of plants transplanted to submerged rafts. Macrocystis was raised in laboratory tanks under controlled conditions of light and temperature. Maximum growth rates in the laboratory paralleled those observed in the sea, plants doubling in area every 16-20 days and in length every 20-30 days under optimum conditions. The transplantation experiments and diving observations demonstrated the important influence of biological activity on the establishment and growth of young plants in the sea. MACROCYSTIS, the largest known marine plant, is a conspicuous constituent of the sublittoral plant communities in many locations along the western coast of North America. Plants are attached to the sea floor by a large holdfast, and their many long stipes are floated to the surface of the sea by numerous pyriform pneumatocysts, each subtending a lanceolate blade. The stipe, pneumatocysts, and blades constitute a frond; these extend from the sea floor to the surface in a compact, tangled bundle. A canopy is formed when the fronds float out just under the water surface. Such surface vegetation in a good year may occupy 250 km2 of coastal waters in southern California alone. Plants sometimes grow in the intertidal zone but more often are subtidal, occurring at depths down to, but usually not exceeding, 30 m. Brandt (1923) first studied the growth of an entire intertidal Macrocystis plant. Later studies were made in Tasmania by Cribb (1954), in New Zealand by Moore (1943), and Rapson, Moore, and Elliot (1943). These workers investigated mainly the surface portions of large mature plants. Scagel (1948), working in British Columbia with intertidal Macrocystis integrifolia, measured the stipe growth of naturally occurring young plants as well as that of plants transplanted from the intertidal zone into deeper water. North (1961) has recently studied stipe growth and longevity of mature Macrocystis in California. The present investigation deals with the effects of light and depth on the growth of young sporophytes of Macrocystis pyrifera. Naturally occurring and transplanted individuals were studied in the sea. Smaller plants were cultivated in laboratory tanks under controlled conditions. MATERIALS AND METHODS-Underwater work was carried out using scuba-diving techniques like those described elsewhere (Neushul, 1961). 1 Received for publication August 27, 1962. 2 Present address: Department of Botany, University of Washington, Seattle 5, Washington. Macrocystis sporophytes were removed from natural kelp beds to submerged rafts made of plastic pipe, rope and brass fittings (Fig. 1). Plants were marked with tags of mat-surface, white plastic. They were attached by holders made from thick-walled rubber tubing (Fig. 2). Four experiments on growth were initiated on the undersea rafts during the following periods: (1) Sept. 17 to Oct. 18, 1956; (2) Oct. 18, 1956 to Jan. 10, 1957; (3) May 24 to June 7, 1957; (4) Oct. 18, 1957 to Jan. 6, 1958. Growth of plants on the rafts was measured photographically to avoid excessive handling and prolonged exposure of the plants to the air. Direct measurements of length and area were taken from the photographs, the area being measured with a polar planimeter. A large number of small Macrocystis plants were found during exploratory dives in July 1957. These were growing at a depth of 15 m in a submarine canyon head situated to the north of the Scripps Institution pier at La Jolla, California. It is assumed that a submarine landslide, similar to that described for this same spot by Shepard (1951), exposed the rocks, thus providing a substrate for growth of Macrocystis. The plants were measured under water with a steel measuring tape by 2 divers working together. Plants were observed for a period of 10 months. Macrocystis plants were grown in the laboratory in 2 cement tanks that held 548 and 430 liters, respectively, and in 4 small, 12 liter aquaria. Fresh seawater, 5-10 liters/min, flowed into the large tanks, while less than 5 liters/min entered the aquaria. The water was cooled to maintain a temperature of 15 C, and overhead "cool-white" fluorescent lights provided constant illuminiation. The intensity was regulated either by changing the height of the lamps or by introducing neutral density filters into the light path. Layers of waxed paper placed between transparent plastic sheets served as filters over the large tanks. Sheets of exposed photographic film were used over the smaller aquaria. Light measurements were made