Abstract

Annual populations of Macrocystis pyrifera in Southern Chile have been the main focus of studies intending to understand how these populations can couple consecutive sporophytic generations. Research has included studying the population dynamics and gametophytic responses to environmental conditions and the role of recruitment, grazing, and the use of benthic filter feeders as secondary substrate. Adult sporophytes undergo senescence due to changes in abiotic factors during summer and autumn producing 100 % mortality. This study provides evidence about the environmental factors driving the decline in sporophyte populations occurring in summer and fall by monitoring two independent kelp populations and also by running experiments using 400 L tubular photobioreactors with semicontrolled environmental factors for testing the capacity for new recruits to recover population levels under winter conditions. The study of natural populations of giant kelp indicates that high temperatures (> 15-17 A degrees C) explain the high mortality of adult plants in summer. On the other hand, the sporophytes established in late winter/early spring are able, under high nitrogen availability, to increase their chlorophyll content significantly, allowing the individuals to reduce their light saturation point and thus allow a higher productivity under the low light conditions that exist in late winter and early spring. These results, in addition to the recruitment facilitation produced by filter feeders, help to explain how giant kelp can deal with, and couple sporophytic generations, in variable environments. These results also emphasize the highly plastic physiology of giant kelp that enables this species to colonize diverse habitats across its large distributional range.