This paper describes the population biology of Lessonia trabeculata and Macrocystis, integrifolia during and after the 1997-1998 El Nino in an area of permanent coastal upwelling in northern Chile. Spatial and temporal pattern- of distribution were evaluated seasonally for adult and juvenile sporophytes of both species between 1996 and 2003. These two kelp form an assemblage distributed between 2 and 15 m depth, with disjunct patterns along a bathymetric gradient, including two morphs of L. trabeculata, the occurrence of which depends on t he presence or absence of M. integrifolia. During the 1997-1998 El Nino the spatial -temporal patterns of abundance of the kelp assemblage were maintained by the continuity of coastal upwelling, which buffered and moderated superficial warming of the sea and depiction of nutrients. In this context, localities associated with coastal upwelling areas Could function as "sources" of reproductive propagules after passage of El Nino. thus increasing kelp recolonization rates in "sink" localities, which suffered local kelp extinctions. Intensification of upwelling processes after the 1998-2000 La Nina increased nutrient inputs into subtidal habitats. favoring the productivity of the kelp assemblage. However, an abrupt change in the spatial-temporal patterns of abundance of the black sea urchin Tetrapygus niger, the most conspicuous benthic grazer in northern Chile. produced local extinctions of M. integrifolia and compression of the range of bathymetric distribution of L. trabeculata. Top-down (mortality of benthic carnivores during the 1997-1998 El Nino) and bottom-up effects (intensity and frequency of upwelling) in this subtidal coastal ecosystem appear to regulate the kelp-herbivore interactions in the study area. The main sources of reproductive propagules for the establishment of the assemblage kelp were fertile sporophytes which included isolated, low density patches of M.integrifolia located within the bed of L. trabeculata, although drifting kelp rafts and "seed banks" of microscopic dormant stages may provide supplementary recruitment. In the temperate SE Pacific, oceanographic events that act on different spatial -temporal Scales plus low Frequency biological processes (changes in grazer abundance), which act on local scales, produce inter-annual variability in the long-term dymanics of kelp populations. Furthermore, the interactive effects between centers of permanent upwelling and the oscillating temporal periodicity of oceanographic events that produce positive (El Nino) and negative (La Nino) thermal anomalies modify the spatial arrangement of subtidal kelp populations oil a latitudinal gradient.
