Coccolithophore fluxes from sediment traps in the northeastern Arabian Sea off Pakistan

Abstract

We investigated coccolithophore fluxes from four sediment traps located in the northeastern Arabian Sea at the continental slope of Pakistan. Two moorings, WPT and EPT (Western/isastern PAKOMIN Trap), consisting of one and two sediment traps were deployed at water depths of about 500 m and 1500 m. They collected settling particles from October 1993 to February 1994 and from May 1995 to February 1996 at intervals of 22 and 24 days, respectively. A longer sampling was prevented probably due to high sedimentation events which occluded the trap cones. Both areas were characterised by a strong seasonality in coccolith fluxes. High fluxes were recorded with up to 2000 × 10 6 coccoliths m –2 d –1 in the early SW (summer) monsoon and up to 5400 × 10 6 coccoliths m –2 d –1 in the early NE (winter) monsoon. Generally, coccosphere fluxes exhibited a similar trend but were some orders of magnitude lower than coccolith fluxes. Twenty-seven taxa were identified in this study, all well to moderately well preserved and with a small range of diversity. The species composition was dominated by Gephyrocapsa oceanica, Florisphaera profunda, and Emiliania huxleyi making up about 70% of the assemblages. During the SW (summer) monsoon high productivity in the surface waters of the eastern Arabian Sea may be linked to the upwelling system off Oman. Apparently, nutrient-rich surface water derived from the southwestern upwelling areas off Oman with high standing stocks were carried into this area by northeast flowing currents. In contrast, during the NE (winter) monsoon the locally observed high coccolithophore production is hypothesised to respond to local injections of nutrient-rich deep water into the surface water due to sea-surface cooling leading to convection. Consequently, fluxes during the winter monsoon were in the same range as during the summer monsoon.