Downward fluxes of faecal material and microplankton at 2300 m depth in the oceanic area off Coquimbo (30°S), Chile, during 1993–1995

Vertical fluxes of biogenic elements (carbon, carbonate) and their main sources (faecal material, foraminifers, and coccolithophorids) were studied during 1993–1995 with a sediment trap deployed at a depth of 2300 m in the oceanic area off the coastal upwelling system off Coquimbo, Chile (30°S). The average flux of particulate organic carbon (POC) was 6.8 mg m−2 d−1. Faecal material made the main contribution to the total POC flux, averaging 2.1 mg C m−2 d−1 (35%), followed by carbon-based tintinnids, with an average flux of 0.7 mg C m−2 d−1 (9.3%). Zooplankton faecal material made up 17% of the vertical POC flux in spring and 44% in the other seasons. Faeces from euphausiids and appendicularians contributed the most to total faecal carbon flux (65%), whereas the faeces of copepods and protists were relatively minor (10%). The vertical flux of microzooplankton was dominated by tintinnids, foraminifers, and coccolithophorids, which, respectively, averaged 61×103, 68×103, and 26×103 individuals m−2 d−1. Intra-annual variability in carbon flux seems to result mostly from seasonal changes in oceanographic conditions, such as Ekman transport offshore, which was highly correlated with the POC flux ( r 2 = 0.59 ). Inter-annual variability, on the other hand, might result partly from remote forcing. Other factors, such as surface pigment distribution and the composition and abundance of both zooplankton and phytoplankton in the study area, also are discussed. e carbonate fluxes of foraminifers and coccosphere+coccolith were 56 and 7 mg m−2 d−1, respectively, or 73% and 11% of the total carbonate flux. The sinking flux of CaCO3 was significantly higher than that of POC throughout the year, averaging 90% of the total carbon and suggesting deep-water delivery of biogenic CaCO3 to be the main pathway for removing carbon from the upper-ocean biosphere in the study area.