Gelatinous zooplankton assemblages associated with water masses in the Humboldt Current System, and potential predatory impact by Bassia bassensis (Siphonophora : Calycophorae)

Large numbers of gelatinous zooplankton were collected off Mejillones Penin sula, Chile (Humboldt Current System) in January 1997 during an oceanograph ic cruiser The area was characterized by the mixing of 3 water masses and t he development of coastal upwelling. Siphonophores were the predominant gro up at most of the stations and the calycophoran Bassia bassensis was overwh elmingly the most abundant species. Five group associations were distinguis hable in relation to the water masses identified. Siphonophores were associ ated with Subtropical Surface Water, the ctenophore Pleurobrachia sp, with Subantarctic Water, the pelagic tunicate Salpa fusiformis with Equatorial S ubsurface Water, an assemblage of all gelatinous groups with mixed waters, and a low occurrence of gelatinous groups with upwelled Equatorial Subsurfa ce Water. Molluscs were the group least associated with any water mass. The potential percentage of small copepods removed by B. bassensis ranged betw een 2.9 and 69.3%. Our results indicate that B. bassensis was the most impo rtant secondary predator in the top 50 m of the water column, and could the refore have had a significant trophic impact on the population of small cop epods off the Mejillones Peninsula during the sampling period, where small copepods constituted 80.6% of the total mesozooplankton community. This sip honophore potentially ingested an average of 17.3% of the total copepod bio mass.