Trophic conditions and zooplankton distribution in the entrance of the Sea of Cortés during summer

The mouth of the Gulf of California has a complex hydrographic structure, with cool California Current waters, nutrient-rich Gulf of California water, and comparatively poor tropical waters arriving from the south. During the PATO II cruise to this region in June 1993, samples of zooplankton (between 0 and 200 m) and particulate matter from three depths, (1%, 10% and 100% light attenuation level (LAL)) were obtained. In the frontal areas near Capes San Lucas and Corrientes, particulate organic matter (POM=protein+carbohydrates) and zooplankton biomass increased. Particulate protein, carbohydrates and POM were significantly different between the three depths. Among the four transects investigated at the vicinity of the gulf mouth, only the northern one located across the entrance (109–118 transect) showed highly significant multiple linear regressions between zooplankton biomass and POM at surface and 10% LAL, and also between zooplankton biomass and POM at surface and the 1% LAL. Three water masses were individualized at the north, the south and the east of the area. Significant differences were found among them for zooplankton biomass, and for POM and protein, at the 1% LAL. At the northern sector, significant linear regressions between zooplankton biomass and POM for the surface and 10% LAL were calculated, and significant multiple linear regressions were found between zooplankton biomass and POM for surface and the 10% LAL, respectively. For the eastern sector, other less significant multiple linear regressions were obtained among zooplankton biomass and POM at surface and the 1% LAL. In the southern sector, a significant multiple linear regression between zooplankton biomass and protein at the three studied LALʹs was found.