Spatial and seasonal variations of nutrients in sediment profiles and their sediment-water fluxes in the Pearl River Estuary, Southern China

Three cruises were launched in the Pearl River Estuary (PRE) in 2005 to investigate the biogeochemical cycling of nutrients associated with early diagenesis related to degradation of organic matter. Seasonal and spatial variations of pore water nutrient concentrations and profile patterns in sediments were studied. Nutrient fluxes at the sediment-water interface (SWI) were measured by incubation experiments, and we here discussed the accumulation and transformation processes of nutrients at the SWI. The nutrients generally decreased from the Pearl River outlets downstream, indicating anthropogenic influences on the nutrient inputs in the estuary. NO3-N concentration was the highest of the three forms of DIN (dissolved inorganic nitrogen, the sum of NH4-N, NO3-N and NO2-N) in the overlying water, and NH4-N was the main component of DIN in pore water. The gradual increase of NH4-N and the rapid decrease of NO3-N with sediment depth provided the evidence for anaerobic conditions below the SWI. Negative fluxes of NO3-N and positive fluxes of NH4-N were commonly observed, suggesting the denitrification of NO3-N at the SWI. The DIN flux direction suggested that the sediment was the sink of DIN in spring, however, the sediment was generally the source of DIN in summer and winter. PO4-P distribution patterns were distinct while SiO4-Si inconspicuously varied in sediment profiles in different seasons. The flux results indicated that PO4-P mainly diffused from the water column to the sediment while SiO4-Si mainly diffused from the sediment to the water column. Generally, the incubated fluxes were the coupling of diffusion, bioturbation and biochemical reactions, and were relatively accurate in this study.