ABSTRACT: Two catastrophic-scale storm disturbances of a giant kelp forest community were followed by very different oceanographic conditions, the warm, nutrient-stressed period of the 1982-1984 El Niño and the cold, nutrient-rich La Niña of 1988-1989. Here we compare the fates of the 2 post-disturbance algal communities, or '2 cohorts', under conditions determined by large-scale, low-frequency oceanographic events. Succession and population dynamics of the competitive dominant kelp, Macrocystispyrifera, and understory kelps, Pterygophoracalifornica and Laminariafarlowii, were followed at 5 permanent sites in the Point Loma kelp forest near San Diego, California, USA, where kelps have been mapped quarterly since 1983. There was intense kelp recruitment after both disturbances. The different oceanographic conditions, however, strongly affected the population dynamics of M. pyrifera and its competitive interactions with the lower standing species. Poor M. pyrifera growth, canopy formation, and survival during the El Niño apparently allowed the persistence of understory populations. Extraordinary conditions for M. pyrifera growth during the La Niña were associated with the near extinction of understory populations. The number of stipes per plant and stipe density are indices of M. pyrifera growth and carrying capacity, respectively, which appear to be very sensitive to environmental conditions. The 2 cohorts exhibited very different stipe patterns. In both cases, the anomalous oceanographic conditions lasted for about 2 yr after the disturbances, but the effects on kelp community structure persisted for the lives of the M. pyrifera cohorts, despite average or relatively poor conditions later. In summary, these data suggest that large-scale, low-frequency oceanographic phenomena are important to kelp forest successional processes, population dynamics, and competitive interactions among kelp guilds.

KEY WORDS: El Niño · Kelp forest · Kelp harvest · La Niña · Macrocystis · Nutrients · Succession · Temperature · Understory kelps

Published in MEPS Vol. 146 (1997) on January 30
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 1997