Predicting the Effects of El Niño on
Seaweeds from Baja California, Mexico

Lydia Ladah¹, Ricardo Scrosati²

¹CICESE, Div. Oceanology, Dept. Ecology, Km. 107
Carr. Tijuana a Ensenada, Ensenada, Baja California,
22860 México; or P.O. Box 434844, San Diego,
CA 92143-4844 USA, E-mail: lladah@cicese.mx) &
²University of British Columbia, Department of
Botany, Vancouver, BC V6T 1Z4, Canada,
E-mail: scrosati@axion.net

The Baja California Peninsula extends for about 1300 km across the Pacific coast of Mexico and entails a large biogeographic transition zone between warm-temperate and tropical waters with several temperate and tropical species encountering their southern or northern limits in the Northern Hemisphere in this region. El Niño events are known to dramatically alter the abundance of several marine organisms, although not always in a negative way, as traditionally assumed. Our aim was to document the differential impacts of El Niño events on seaweeds from Baja California, depending on their biogeographic affinity. For this, we chose one common temperate and one common tropical species as models, namely Macrocystis pyrifera (Phaeophyceae, Laminariales) and Caulerpa sertularioides (Chlorophyta, Bryopsidales), respectively.

During the 1997-98 El Niño event, one of the strongest in recorded history, macrothalli of M. pyrifera (giant kelp) disappeared completely from its southern limit of distribution, located in the central part of the peninsula's coast, where it is relatively isolated from the more continuous populations of the north and is normally abundant. Subsequent subtidal surveys demonstrated that macrothalli appeared surprisingly again a few months after the event, and recovery was attributed to seed bank formation of microscopic stage over-summering during the warming. In the following two years, which included La Niña (cooler) conditions, forests of M. pyrifera reached their highest carrying capacity during a 5 yr. study. On the contrary, thalli of the tropical species C. sertularioides were unusually abundant during the same El Niño event in the southern part of the peninsula. In the following two years of La Niña, thalli were much less abundant, being even absent in spring, opposite to what occurred during the El Niño spring, when biomass was particularly high.

Baja California is a relatively unexplored region in terms of the effects of El Niño on seaweed populations; there is only one other published example available for comparison. The agarophyte Gelidium robustum (Rhodophyta, Gelidiales) is a temperate species that has its southern limit of distribution in Baja California. During the 1982-84 El Niño, another strong event, researchers noted that its population biomass dramatically declined, compared with normal periods, in the central part of the peninsula.

Therefore, the biogeographic affinity of seaweeds could be used as a tool to predict their changes in abundance in Baja California when El Niño is predicted. This could be useful to improve resource management in this region, as many seaweeds are harvested themselves or form important habitat for other important commercial species. Ultimately, this information, together with that of temperature effects on the intensity of interactions with competitors and herbivores, could be useful to predict latitudinal changes in the abundance of seaweed species as affected by long-term changes of seawater temperature.