Seasonality and composition of downward particulate fluxes at the continental shelf and Okinawa Trough in the East China Sea

Abstract

Abstract As part of the Marginal Sea Flux Experiment in the West Pacific (MASFLEX) program, conventional sediment traps were deployed at three mooring sites on the East China Sea shelf for 1–2 days during 4 seasonal cruises. Time-series sediment traps also were deployed in the East China Sea, one at the northern part of the Okinawa Trough (Stn. SST 1) and the other to the north-west of the Okinawa Island (SST 2), for 1 year from March 1993 to February 1994. The settling particles were analyzed for total mass flux and the abundance of major biogenic constituents (organic carbon, nitrogen, inorganic carbon and opal). On the inner shelf, particulate fluxes were large and rapidly increased near the bottom throughout the year, probably due to resuspended bottom sediment and suspended particles from the Changjiang, forming a bottom turbid layer (BTL). On the central shelf, particulate fluxes in the BTL were extremely high in summer when Changjiang discharge peaks, possibly indicating a seasonal supply of the Changjiang particles to the central shelf. At the shelf edge, the fluxes in the BTL were significantly higher in autumn and winter than in spring and summer. At the Okinawa Trough stations (ca. 1100 m ), particulate fluxes at 600 m depth were extremely low but increased noticeably with depth (800 and 50 m above the bottom). The flux generally peaked from late autumn to early spring and was lowest during summer, with an unusually large peak in September at SST1. Considering that there is no strong seasonality of the primary productivity and phytoplankton biomass in the surface layers at the Okinawa Trough, we hypothesize that the seasonality of particle fluxes at the deep waters of the Trough is linked to seasonal events in shelf waters. Downwelling and seaward bottom flow induced by the northeast monsoon in winter favor the offshore transport of particles in the BTL, while in summer a transverse circulation pattern may inhibit it (Hu, 1994; Yanagi et al., 1996). Near-bottom transport may be a key process for shelf-to-deep sea export of biogenic/lithogenic particles. As a result, the continental slope and the Okinawa Trough may play an important role as a carbon sink for the East China Sea.