How coastal upwelling influences spatial patterns of size-structured diversity of copepods off central-southern Chile (summer 2009)

This study assessed the structure of the copepod community in the upper 200 m of the coastal upwelling region off central-southern Chile in late summer 2009. Vertically stratified zooplankton samples and hydrographic variables were obtained from 42 stations over the continental shelf and oceanic areas. The survey took place during active upwelling, reflected by a cold upwelling plume extending out to 150 km offshore. A total of 62 copepod species were found. Of these, Oithona similis and Paracalanus indicus accounted for ca. 60% of the whole community. Species richness (R) and the Shannon–Wiener diversity index (H′) were estimated, and the latter was additionally modified to incorporate the effect of copepod size on diversity (H′s). Samples were analyzed for two depth strata (0–50, 50–200 m) and for day vs. night conditions. Significant effects of day vs. night and strata on R, H′ and H′s indicated that diel vertical migration between these two layers was an important source of variation in the zooplankton community. H′s seemed to represent copepod diversity better than R and H′ over the spatial scale. H′s was also closely linked to colder upwelled water and the depth of the oxygen minimum zone following a principal component analysis. A positive relationship was even detected between depth of the oxygen minimum zone and H′s when strata and day/night effects were excluded. Our findings suggested that the coastal upwelling process could be an important driver of copepod diversity in this region. Upwelling leads to changes in the depth of the oxygen minimum zone and these changes impact the community composition due to species-dependent tolerances to low oxygen water.