Longitudinal distribution of zooplankton at 10.5°N in the Northeastern Pacific

Abstract

We investigated the longitudinal variations in zooplankton abundances and their related physicochemical properties at nine stations located between 136°W and 128°W at 10.5°N in the northeastern Pacific in summer 2004. Temperature, salinity, inorganic nutrients, chlorophyll-a (hereafter chl-a) and zooplankton (>200 μm) were sampled within the depth from the surface to 200 m depth at 1° longitude intervals. Zooplankton (>200 μm) samples were vertically collected at two depth intervals from surface to 200 m, consisting of surface mixed and lower layers (thermocline∼200 m). Longitudinal distributional pattern of hydrological parameters (especially salinity) was physically influenced by the intensity of westward geostrophic current passage relating to the NEC (North Equatorial Current). Data from the longitudinal survey showed clear zonal distributions in the hydrological parameters (temperature, salinity and nutrients). However, spatial patterns of the chl-a concentrations and zooplankton abundances were mostly independent of the zonal distributions of hydrological parameters. The two peaks of zooplankton abundance in the surface mixed layer were characterized by different controlling factors such as bottom-up control from nutrients to zooplankton (129°W) and accumulation by increment of friction force and taxonomic interrelationship (133° and 134°W). Divergence-related upwelling caused introduction of nutrients into surface waters leading to the increment of chl-a concentration and zooplankton abundances (129°W). Increased friction force in relation to reduced flow rates of geostrophic currents caused accumulation of zooplankton drifting from eastern stations of study area (133° and 134°W). Besides, high correlation between immature copepods and carnivorous groups such as chaetognaths and cyclopoids also possibly contributed to the enhanced total abundance of zooplankton in the surface mixed layer (p<0.05). Zooplankton community was divided into three groups (A, B, C) which consecutively included the eastern peak of zooplankton (129°W), the western peak (133° and 134°W) and high nutrient but low chl-a concentration and zooplankton abundance (136°W). Moreover, Group B corresponded to the westward movement of low saline waters (<33.6 psu) from 128 to 132°W. In summary, longitudinal distributions of zooplankton community was characterized by three different controlling factors: bottom-up control (129°W), accumulation by increased friction force and relationships among zooplankton. groups (133° and 134°W), and mismatch between hydrological parameters and zooplankton in the high nutrient low chlorophyll area (136°W) during the study period.