Growth of juvenile Macrocystis pyrifera (Laminariales) in relation to environmental factors

Instantaneous relative growth rates, (d^-1), were measured for juveniles of the giant kelp Macrocystis pyrifera transplanted to study sites in Southern California kelp forests between 1978 and 1982. Growth rates ranged from negative values (indicating loss of tissue) to 0.03 (doubling of total frond length every 19 d). Multiple regression analysis of growth versus irradiation, temperature, nitrogen concentration and amount of fouling revealed that all these factors had significant effects, together accounting for about 50% of the total variance. Elevated irradiation and nitrogen levels had strongly stimulatory effects (tissue nitrogen may have been more critical than ambient nitrogen for growth), while high temperature and fouling had strongly inhibitory effects. Irradiation was the most important factor influencing growth in 6 of the 8 transplant experiments. During these 6 experiments, the compensating irradiation level (below which there was no growth) was between 0.4 and 0.7 E m^-2 d^-1, and saturating irradiation was between 2 and 3 E m^-2 d^-1. During two of the experiments, growth was apparently limited by extremely high temperatures or low nitrogen levels. Quantum irradiation levels in the kelp forest were generally between the compensation and saturation levels. However, irradiation levels occasionally dropped below the compensation point for several months. Irradiation was occasionally low enough to limit the distribution of juvenile kelp by inhibiting growth, especially in the deeper portions of the kelp forest and under dense canopies formed by adult plants.