Feeding of Sagitta enflata and vertical distribution of chaetognaths in relation to low oxygen concentrations

Zooplankton samples were collected in Mejillones Bay, northern Chile (23degrees00'15''S, 70degrees26'43''W ). Sampling was conducted at 4 h intervals, for 24 h during three seasons, austral spring (October 2000), summer ( January 2001) and winter (August 2001) at three different strata (0-25, 25-50 and 50-100 m). Five species of chaetognaths were collected. Sagitta enflata was the most abundant species, representing up to 65% of all chaetognaths in total numbers, followed by Sagitta bierii, making up 34% of the total abundance of chaetognaths. S. enflata was distributed mainly above the Oxygen Minimum Zone, while S. bierii remained below this zone. Feeding rates were relatively constant within the upper layer (0-25 m depth), for each sampling date, averaging 1.2 prey S. enflata day(-1), and decreasing with depth. Gut content analyses demonstrated that predation was principally focused on small copepods (<1500 mum), with greatest feeding activity occurring at night. The daily predation impact on the total standing stock of small copepods varied seasonally between 6% in spring and 0.4% in winter. This percentage may represent a negligible impact on the entire copepod community, but it is relevant at the species or genus level, since S. enflata removed more than 20% of the standing stock of Centropages brachiatus and Corycaeus sp. Thus, during some periods of the year, chaetognaths may strongly influence the abundance and size distribution of copepods in coastal upwelling ecosystems.