Delayed recovery of giant kelp near its southern range limit in the North Pacific following El Nino

The northern distribution limit of giant kelp, Macrocystis pyrifera (Agardh), along the Pacific coast of North America is relatively stable near Point Ano Nuevo, California, but its southern limit has varied over hundreds of kilometers along the Baja California peninsula during the past 20 years. The factors that drive this variability in the southern limit were examined by: (1) quantifying the abundances of M. pyrifera and the subsurface stipitate kelp, Eisenia arborea (Areschoug), near M. pyrifera's then southern limit at Punta San Roque (27° 15'N; 114° 42'W) on ten occasions between August 1997 and June 2002 (prior to and following the 1997/1998 El Nino Southern Oscillation); (2) initiating a field manipulation at Punta San Roque in June 2000 to examine competition between these two kelp species; and (3) surveying 20 additional sites near Punta San Hipolito (27° 01'N; 114° 00'W) in June 2002 to determine whether M. pyrifera populations had recovered to their pre-1982/1983 El Nino southern range limit after ∼ 20 years of absence. These observations and the experimental results suggest that M. pyrifera is removed from its southern limit by the high temperatures and large waves associated with El Ninos, while E. arborea is able to survive and recruit in high densities and thereby delay or even prevent M. pyrifera's recovery. An inverse relationship was observed between these two species over the 5-year study. The field experiment showed that at both 4 months and 1 year after areas occupied by E. arborea were cleared, substantial recruitment of M. pyrifera occurred, while recruitment was negligible in uncleared areas. This study suggests that the southern range limit of M. pyrifera is determined by increased mortality and recruitment failure following the warm temperatures and low nutrient conditions associated with El Ninos, limited substrate availability, and by competition with the more persistent populations of E. arborea.