Ecological significance of salp fecal pellets collected by sediment traps in the eastern North Pacific

Abstract

Five sediment traps were moored at depths of 740, 940, 1 440, 3 440 and 4 240 m for 7 d in December 1982 at Station 5 in the eastern North Pacific about 400 km from San Francisco. Dark green sinking particles enclosed in tough membrane consisted of mostly coccolithophores with some diatoms, dinoflagellates and chrysophytes. The average size of the particles was 10x5x2 mm. These characteristics indicate that the particles were fecal pellets of salp inhabiting the surface waters. Vertical fluxes of the organic carbon and nitrogen through sinking of the salp fecal pellets ranged from 6.7 to 23 mgC m-2 d-1 and from 0.88 to 3.2 mgN m-2 d-1, respectively. These values were several times higher than those determined in other oceanic areas by sediment trap experiments. Hydrocarbons consisting of short-chain n-alkanes (n-C15-C20) with n-C17, the most predominant component, heneicosa-hexaene (n-C21:6), br-C25 alkenes and long-chain n-alkanes (n-C21-C30), without any odd or even carbon number predominance, were found from five depths. The presence of short-chain n-alkanes and n-C21:6 indicated that phytoplankton in the surface waters was a primary source of organic matter in the sinking particles. Two isomers of br-C25:3 and br-C25:4 alkenes found in these particles also indicated that br-C25 alkenes were the important biological marker of fecal pellet of zooplankton. The distribution pattern of the long-chain n-alkanes suggested that the sinking particles may be affected by bacteria to some extent. Fatty acids of the sinking particles were separated into free, triglyceride and wax ester fractions consisting of mono- and poly-unsaturated, and saturated fatty acids, with a range from C14 to C30. Concentrations of mono- and poly-unsaturated fatty acids decreased more rapidly toward the deep than those of saturated fatty acids, which cause low ratios of mono- and poly-unsaturated fatty acids/saturated fatty acids. This indicates that unsaturated fatty acids were more rapidly decayed by marine microbes than saturated fatty acids in the deep water, despite the fact that a significant amount of unsaturated fatty acids still remained in the sinking particles collected from the deep waters. Our results revealed that the salp fecal pellet plays an important role in supplying foods to organisms in intermediate and deep seas.