Interannual, Seasonal, and Diurnal Variations in Vertical and Horizontal Distribution Patterns of 6 Oithona spp. (Copepoda: Cyclopoida) in the South China Sea

Abstract

Temporal changes in abundance and distribution patterns of cyclopoid copepods belonging to the genus Oithona were analyzed from a 3 yr survey in the South China Sea comprising 9 sampling cruises, during spring, summer, and autumn of 2000-2002. Plankton samples were collected by oblique tows of a North Pacific zooplankton net of 100 μm mesh size including 3 different depth strata (50-0, 100-0, and 150-0 m) and provide the 1st multiscale depth study for an assemblage of Oithona spp. worldwide. The mean occurrence rate of Oithona spp. was > 85% with an average density of 45.86 ± 206.97 individuals (ind.)/m3. Among the 6 most abundant species, including O. attenuata, O. setigera, O. fallax, O. similis, O. rigida, and O. brevicornis, O. attenuata was dominant, with an average density of 45.9 ± 133.61 ind./m3 (with an occurrence ratio of 47.59%) throughout the study, followed by O. setigera with an average abundance of 9.49 ± 34.11 ind./m3. The average densities of these 2 highly abundant species were higher in samples collected from 100-0 and 150-0 m depths than those from surface samples. The intermediatesized oceanic warm-water species, O. setigera, was most abundant during 2000 and 2001. During 2002, the smaller (< 1000 μm) species, O. attenuata, became dominant. The occurrence rate of O. attenuata was higher at coastal stations than at offshore stations, whereas O. setigera showed no significant differences in occurrence between coastal and oceanic stations. The average oithonid abundance recorded during the spring was higher than in either summer or autumn. In spring, the oithonid assemblage was dominated by O. setigera in 2000 and 2001, whereas O. attenuata was the most dominant species in 2002. Oithona fallax appeared in summer and autumn; in contrast, O. similis was not recorded in summer samples. Total oithonid densities did not significantly differ in relation to the depth layer sampled.