Temporal and spatial variability in export production in the SE Pacific Ocean: evidence from siliceous plankton fluxes and surface sediment assemblages

Flux of siliceous plankton and taxonomic composition of diatom and silicofl agellate assemblages were determined from sediment trap samples collected i n coastal upwelling-influenced waters off northern Chile (30 degreesS, CH s ite) under "normal" or non-El Nino (1993-94) and El Nino conditions (1997-9 8). In addition, concentration of biogenic opal and siliceous plankton, and diatom and silicoflagellate assemblages preserved in surface sediments are provided for a wide area between 27 degrees and 43 degreesS off Chile. Reg ardless of the year, winter upwelling determines the maximum production pat tern of siliceous microorganisms, with diatoms numerically dominating the b iogenic opal flux. During the El Nino year the export is markedly lower: on an annual basis, total mass flux diminished by 60%, and diatom and silicof lagellate export by 75%. Major components of the diatom flora maintain much of their regular seasonal cycle of flux maxima and minima during both samp ling periods. Neritic resting spores (RS) of Chaetoceros dominate the diato m flux, mirroring the influence of coastal-upwelled waters at the CH trap s ite. Occurrence of pelagic diatoms species Fragilariopsis doliolus, members of the Rhizosoleniaceae, Azpeitia spp. and Nitzschia interruptestriata, se condary components of the assemblage, reflects the intermingling of warmer waters of the Subtropical Gyre. Dictyocha messanensis dominates the silicof lagellate association almost year-around, but Distephanus pulchra delivers ca. 60% of its annual production in less than three weeks during the winter peak. The siliceous thanatocoenosis is largely dominated by diatoms, whose assemblage shows significant qualitative and quantitative variations from north to south. Between 27 degrees and 35 degreesS, the dominance of RS Cha etoceros, Thalassionema nitzschioides var. nitzschioides and Skeletonema co statum reflects strong export production associated with occurrence of coas tal upwelling. Both highest biogenic opal content and diatom concentration at 35 degrees and 41 degrees -43 degreesS coincide with highest pigment con centrations along the Chilean coast. Predominance of the diatom species Tha lassiosira pacifica and T. poro-irregulata, and higher relative contributio n of the silicoflagellate Distephanus speculum at 41 degrees -43 degreesS s uggest the influence of more nutrient-rich waters and low sea surface tempe ratures, probably associated with the Antarctic Circumpolar Water. (C) 2001 Elsevier Science Ltd. All rights reserved.