Abstract

Sedimentation rates of faecal material, phytoplankton and microzooplankton and production rates of faecal material from crustaceans and pelagic tunicates were estimated during the austral summer and winter 1997, and summer 1998, in the northern Humboldt Current (23 degrees S, off Antofagasta, Chile). Sampling periods covered pre-El Nino (January 1997) and El Nino 1997-98 (July 1997 and January 1998). Samples were collected using floating sediment traps deployed at 65, 100, 200 and 300 m depth in oceanic and coastal areas. Sedimentation rates during January 1997 were, on average, 152 +/- 23 and 85 +/- 57 mg C m(-2) day(-1) at 65 and 300 m depth, respectively. During July, these rates averaged 93 +/- 56 mg C m(-2) day(-1) at 65 m depth and 35 +/- 12 mg C m(-2) day(-1) at 300 m depth, while in January 1998 they were 98 and 109 +/- 37 mg C m(-2) day(-1) at 65 and 200 m depth, respectively. Recog nizable faecal material made up the bulk of the sedimenting matter, accounting for 8 +/- 5% (n = 14), 31 +/- 26% (n = 16) and 8 +/- 5% (n = 5) of the average total organic carbon recorded from all sediment trap samples collected during January and July 1997 and January 1998, respectively. However, at 300 m depth, the contribution of recognizable faecal material to total sedimented organic carbon increased to 43 +/- 33% (n = 4) during July 1997. The remaining sedimenting particles consisted mainly of tintinnids, crustacean exuviae, heterotrophic dinoflagellates (both thecated and athecated) and diatom cells. During this study, we estimated that only a minor fraction (average +/- SD = 5 +/- 8%) of the copepod faecal material produced within the photic zone sedimented down to 300 m depth, suggesting an efficient recycling within the overlaying water column. On the other band, an important fraction (47 +/- 30%) of the euphausiid faecal strings was collected in the 300 m depth trap, suggesting that this material would enhance the downward flux of particulate organic matter (POC). POC fluxes to 65 and 300 m depth traps were in the range of 4-20% and 3-8% of the estimated primary production during the whole study period. It is postulated that the overall vertical flux of particulates and, in particular, faecal pellets was determined by a combination of three factors. The first was the composition of the zooplankton assemblages in the study area. When the dominant group was calanoid copepods, their faeces seemed to contribute poorly to the vertical flux of particulates. On the other hand, when the dominant group was euphausiids, a significant proportion of their faecal material was collected in the sediment trap located at 300 m depth. The second was the relatively high abundance of cyclopoid copepods from the genera Oncaea, Corycaeus and Oithona, which are reported to feed on aggregates of phytodetritus and faecal pellets produced by calanoid copepods, suggesting that they may act as a natural filter to sedimenting particulates. The third was the composition and size spectrum of the phyto- and microzooplankton assemblages which are potential food sources for the meso- and macrozooplankton. These factors were partially modulated by both the 1997-1998 El Nino and seasonality.