Mesoscale structure of copepod assemblages in the coastal transition zone and oceanic waters off central-southern Chile

Abstract

In the Humboldt Current System, the region off central-southern Chile has relatively high eddy kinetic energy, generating an extensive coastal transition zone (∼600 km offshore) in which coastally derived eddies are recurrent features. This energy might promote strong exchanges of water, biogeochemical properties, and plankton between the coastal upwelling band and the adjacent oceanic zone. In this study, the mesoscale structure of epipelagic copepod assemblages and its relationship to environmental variability and the eddy field in the coastal transition zone and oceanic areas off Concepción (34–39°S, 73–84°W) were investigated. Zooplankton samples were collected during cruises of opportunity carried out during the 2006 coastal upwelling season and the oceanographic conditions were derived from satellite data on sea surface height, temperature, and chlorophyll a. The use of cluster analyses and indicator species revealed two main copepod assemblages: (i) species with a mainly coastal distribution, the maximum in total abundance being found nearshore and (ii) species with a mainly oceanic distribution (beyond ∼500 km from the coast), where a secondary maximum was observed. Both types of assemblages, however, included species widely distributed in the coastal transition zone. An ordination analysis identified sea surface temperature and chlorophyll a as the main factors affecting the distribution of most species and the clustering of stations resembled the distribution of these variables. Some of the large calanoid species that are common in shelf waters were more abundant within nearshore eddies rich in chlorophyll a and other cyclonic eddies far offshore; however, the eddy field alone did not explain the copepod mesoscale distribution. Altogether, the wide distribution of shelf/slope copepod species in this region suggests that physical and biological mechanisms might be acting to extend the productive area of the coastal upwelling zone.