Carmen Franco-Gordo

Zooplankton Biomass Variability in the Mexican Eastern Tropical Pacific

The time and space distribution of zooplankton biomass recorded during a year cycle (December 1995-1996) off the Pacific coast of central México is analyzed. Samples were obtained by surface (42-86 m) oblique hauls at 12 sampling sites using a Bongo net. The overall average displacement volume biomass of zooplankton during the surveyed period was 1138 cm3 / 1000 m3 . Principal component analysis indicated that highest biomass concentrations occurred at coastal stations. The months with highest biomass values were those in which the lowest sea surface temperature values occurred (January-May). This was the same period in which the California Current was strongest and clearly influenced the hydrological conditions of the surveyed area. In these months, advective processes are active along the outer shelf, favoring upwelling of colder, relatively nutrient-richer waters that promote an overall local increase of zooplankton activity and populations. The high variability of biomass values is indicative of episodic, localized processes that enhance productivity in the area.The time and space distribution of zooplankton biomass recorded during a year cycle (December 1995±1996) off the Paci®c coast of central MeÂxico is analyzed. Samples were obtained by surface (42±86 m) oblique hauls at 12 sampling sites using a Bongo net. The overall average displacement volume biomass of zooplankton during the surveyed period was 1138 cm/ 1000 m. Principal component analysis indicated that highest biomass concentrations occurred at coastal stations. The months with highest biomass values were those in which the lowest sea surface temperature values occurred ( January±May). This was the same period in which the California Current was strongest and clearly in ̄uenced the hydrological conditions of the surveyed area. In these months, advective processes are active along the outer shelf, favoring upwelling of colder, relatively nutrient-richer waters that promote an overall local increase of zooplankton activity and populations. The high variability of biomass values is indicative of episodic, localized processes that enhance productivity in the area. Resumen: Se analiza la distribucioÂn espacial y temporal de la biomasa zooplaÂnctica obtenida durante 11 meses en las costas de Jalisco y Colima, en el Pacõ®co Central Mexicano, desde diciembre de 1995 a diciembre de 1996. Las muestras fueron obtenidas por arrastres doble oblicuos en 12 estaciones, con una red bongo con mangas de 0.333 mm y 0.505 mm, y una profundidad de arrastre entre los 42 y 86 m. En este estudio so lo se analiza la malla de 0.505 mm. El valor medio de la biomasa obtenido para todo el periodo fue de 1138 cm estandarizado a 1000 m. A traveÂs del anaÂlisis de componentes principales, se observo que las mayores concentraciones de biomasa estuvieron en las estaciones cercanas de la costa, y el periodo de mayores concentraciones coincide con los valores maÂs bajos de temperatura, bajo la in ̄uencia de la Corriente de California (Enero±Mayo). Durante ese periodo el aÂrea de estudio se encuentra bajo la in ̄uencia de procesos advectivos, lo que propicia a ̄oramientos de aguas profundas maÂs frõÂas y ricas en nutrientes. La alta variabilidad mostrada por los datos indica que los procesos que controlan los incrementos de la productividad son episoÂdicos y locales. The zooplankton community represents a relevant link that transfers energy from primary producers to higher trophic levels in pelagic trophic webs (Baduini 1997). The patterns of distribution and abundance of zooplankton are affected by physical phenomena on very different spatial scales (Haury et al. 1978, Denman and Powell 1984). Physical factors include hydrographic events such as currents, wind stress, eddies, and upwelling and strati®cation of the water column (Haury et al. 1978, Owen 1981). One of the parameters most frequently 191 1 Manuscript accepted 16 August 2000. 2 Centro de Ecologõ Âa Costera, Universidad de Guadalajara, GoÂmez Farõ Âas no. 82 San Patricio-Melaque, Jalisco CP 48980, MeÂxico. Current address: Universidad de La CorunÄ a, Departamento de Biologõ Âa Animal, Vegetal, y Ecologõ Âa, Campus Zapateira s/n, La CorunÄ a, EspanÄ a 15071 (cfranco@mail2.udc.es). 3 El Colegio de la Frontera Sur (ECOSUR), Zona Industrial No. 2 Carretera Chetumal-Bacalar, A.P. 424, Chetumal, Quintana Roo 77000, MeÂxico. Paci®c Science (2001), vol. 55, no. 2:191±202 : 2001 by University of Hawai`i Press.