Spatial distribution of copepods in the north of the Humboldt Current region off Chile during coastal upwelling

During the austral spring (December 1996), numerical abundance of copepod s pecies (Crustacea: Copepoda) was estimated from 53 sampling stations scatte red over the coastal waters off Mejillones Peninsula, northern Chile. Sampl es were obtained from two depth strata; 0-50 m and 50-200 m depth. Oceanogr aphic data over the three-day cruise indicated that coastal upwelling was a ctively occurring during the survey. A total of 19 species was identified, of which four of them, Paracalanus pa rvus, Oncaea conifera, Oithona similis and Calanus chilensis, accounted for more than 80% of the whole copepod assemblage in terms of numerical abunda nce. Using the K-means exploratory analysis the 53 locations were grouped i nto two clusters: an upwelling type and a non-upwelling type. The former ch aracterized by locations nearshore, with low temperature (<17 degrees C) an d high chlorophyll-a (>13.4 mg m(-3)), while the latter had warmer waters ( >18 degrees C) and low chlorophyll-a (<4.8 mg m(-3)). Chlorophyll-a (chl-a) , distance to shoreline and temperature at 10 m depth (T10) were all signif icant variables (P<0.05) for clustering analysis. There was a strong negati ve correlation between T10 and chl-a (P<0.01). The same clustering techniqu e, based on species abundance, suggested the presence of a unique cluster, whose composition was dominated by P. parvus, O. similis, Acartia tonsa and Centropages brachiatus. Stepwise multiple regression showed that these spe cies were also strongly correlated to chi-a and T10. Altogether these analy ses suggest that water mass circulation during upwelling is the driving for ce for structuring spatial patterns of copepod distribution. In addition, a vertical distribution parameter suggested that copepods, independently of time of the day, tended to remain in a large proportion in near surface wat ers. possibly constrained by a shallow oxygen minimum layer. This implies t hat advection during active upwelling may cause large fractions of populati ons being transported offshore, allowing zooplankton export to more oceanic waters. Rapid turnover rates of copepods growing continuously year-round, may act as a mechanism to compensate population losses during persistent co astal upwelling.