Impact of ENSO variability on the flux and composition of sinking POM in the western equatorial Pacific Ocean: Amino acids and hexosamines

Sediment trap moorings deployed along the equator at three distinct oceanographic zones within the western Pacific Ocean during 1991–93, 1994–95, and 1999 provided data on total mass and amino acid fluxes and composition of particulate organic matter (POM) sinking in this part of the Pacific. An intercomparison of annual averages of various parameters revealed discrete patterns in flux and composition of POM under the El Niño (1991–93, 1994–95) and La Niña (1999) conditions that prevailed over the equatorial Pacific during the sampling periods. In the hemipelagic zone of the far western equatorial Pacific, where terrestrial deposition of particles larger than 5 μm accounts for >25% of the total sediment, average total mass and amino acid fluxes were relatively higher during El Niño than during La Niña. However, at the oligotrophic warm pool and upwelling sites, total mass and amino acid fluxes were higher during La Niña. Influence of ENSO-related changes in the sinking particle flux was much clearer in the hemipelagic zone compared to that in the warm pool. Average values of biogeochemical parameters such as glucosamine/galactosamine and amino acids/hexosamines, and bulk parameters like amino acid carbon and nitrogen contents relative to organic carbon and total nitrogen (THAA-C% and THAA-N%, respectively), and organic carbon normalised amino acid concentrations indicated that sinking POM was more labile during La Niña at all the sites. The ratios Aspartic acid/β-alanine, Glutamic acid/γ-aminobutyric acid, and relative concentration of non-protein amino acids (β-alanine+γ-aminobutyric acid) suggested that POM degradation was more intense during La Niña than during El Niño conditions at all sites. >