||This study investigates the exports , deposition rates and budgets of particulate carbon and nitrogen in the Gaoping river-sea system. Concentrations of dissolved materials in the Gaoping River ( GPR ) downstream were generally lower in the wet season than in the dry season due to the dilution effect of runoff. However, concentrations of particulate matters were higher in the wet season than in the dry season, arisen largely from high physical and chemical weathering rates in the wet season. Total suspended matters ( TSM ), particulate organic carbon ( POC ) and particulate nitrogen ( PN ) in the GPR existed mainly in 10-63 μm particles during the wet season and in 3-10 μm particles during the dry season. Particulate inorganic carbon ( PIC ), however, was associated with different particle sizes and its pattern was no significant difference between dry and wet seasons. The GPR carried about 1.88 × 1010 mol C yr-1 POC, 1.04 × 1010 mol C yr-1 PIC and 1.07 × 109 mol N yr-1 PN into the Gaoping coastal sea during the study period. The total particulate carbon was approximately consisted of 64 % POC and 36 % PIC.|
Distributions of particle sizes in Gaoping coastal sediments were largely < 63 μm as fine particles were generally carried by the coastal current, wave, tide and deposited on places away from the coastal line. The mean burial flux was 2.25 mg cm-2 yr-1 for particulate carbon ( PC ) and 0.27 mg cm-2 yr-1 for PN, equivalent to 5.7 ×109 mol C yr-1 ( 6.84 ×104 ton C yr-1 ) for PC and 1.0 × 108 mol N yr-1 ( 1.41 × 103 ton N yr-1 ) for PN in the study area. The buried PC was consisted of 58 % POC and 42 % PIC.
The geochemical features of core sediments in the Gaoping Submarine Canyon ( GPSC ) show that the sedimentation was not steady in places near the canyon head affected obviously by extreme events and those cores were not used for determining sedimentation rates. In addition to GPSC, the shelf on the northern side of GPSC was apparently prominent in receiving river borne sediments. The southern shelf sediment of GPSC, however, was significantly influenced by Liuchiu Islet and showed relatively high concentrations of PIC. In general, concentrations of particulate organic matters ( POM ) in sediments decreased as the core depth increased, but dissolved organic matters ( DOM ) and dissolved inorganic matters ( DIM ) in pore water increased as the core depth increased. The contents of clay, POM and POC/PN in sediments and concentrations of dissolved organic and inorganic carbon in pore water increased as the distance of sampling station increased from the coastal line.
The burial efficiency ( BE ) of carbon and nitrogen was estimated from the burial fluxes of particulate matters in core sediments and the diffusion fluxes of dissolved materials across the sediment-water interface. The BE of carbon and nitrogen ranged from 50 % to 85 % ( ave. 84 % ) and from 30 % to 95 % ( ave. 45 % ), respectively in the Gaoping coastal sea. The deposited carbon and nitrogen account for only 23.4 % total PC and 20 % total PN derived from the river loads or 3.6 % total PC and 0.9 % total PN derived from the river loads and the net ecosystem production ( NEP ). The results imply that most particulate carbon and nitrogen derived either from the Gaoping River or NEP may be partly recycled in the water column or largely moved off the study area to the deeper ocean.