LARGE-SCALE, LOW-FREQUENCY OCEANOGRAPHIC EFFECTS ON KELP FOREST SUCCESSION - A TALE OF 2 COHORTS

Two catastrophic-scale storm disturbances of a giant kelp forest commu nity were followed by very different oceanographic conditions, the war m, nutrient-stressed period of the 1982-1984 El Nino and the cold, nut rient-rich La Nina of 1988-1989. Here we compare the fates of the 2 po st-disturbance algal communities, or '2 cohorts', under conditions det ermined by large-scale, low-frequency oceanographic events. Succession and population dynamics of the competitive dominant kelp, Macrocystis pyrifera, and understory kelps, Pterygophora californica and Laminari a farlowii, were followed at 5 permanent sites in the Point Loma kelp forest near San Diego, California, USA, where kelps have been mapped q uarterly since 1983. There was intense kelp recruitment after both dis turbances. The different oceanographic conditions, however, strongly a ffected the population dynamics of M. pyrifera and its competitive int eractions with the lower standing species. Poor M. pyrifera growth, ca nopy formation, and survival during the El Nino apparently allowed the persistence of understory populations. Extraordinary conditions for M . pyrifera growth during the La Nina were associated with the near ext inction 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 cond itions. The 2 cohorts exhibited very different stipe patterns. In both cases, the anomalous oceanographic conditions lasted for about 2 yr a fter the disturbances, but the effects on kelp community structure per sisted for the Lives of the M. pyrifera cohorts, despite average or re latively poor conditions later. In summary, these data suggest that la rge-scale, low-frequency oceanographic phenomena are important to kelp forest successional processes, population dynamics, and competitive i nteractions among kelp guilds.