The Bioavailability Of Nitrogen And Phosphorus In Surface Sediment In Daya Bay

Measurements of different species of nitrogen and phosphorus concentrations were made in Daya Bay in Apr. 2001. The results show that the ratio of nitrogen to phosphorus is about 12:1 in seawater of Daya Bay. This suggests that the primary productivity is still controlled by nitrogen. Dissolved nitrogen is the main species of nitrogen and DON composed 62.2% of TN. It is likely to be one of keys to high productivity in water column which is supported by low inorganic nutrients.Incubation experiment of sediments indicated that short-term interconversions between forms of nitrogen are relate to the content of oxygen in water column and that the nitrogen in sediments can transform into dissolved inorganic and organic nitrogen.Exchangeable nitrogen (NEX) and three kinds of phosphorus species in sediments, which were sampled from Daya Bay, South China Sea and Xiamen Bay, are measured and compared. Sediment from Daya Bay has higher content of exchangeable nitrogen than those from South China Sea and Xiamen Bay, average contents are 40.5mg/kg, 35.1mg/kg and 37.4mg/kg respectively. Content of total phosphorus (TP) in Daya Bay sediment is 449.3mg/kg, however, TP in South China Sea sediment and Xiamen Bay sediment is 650.1mg/kg and 643.9mg/kg, respectively. Dissolved inorganic phosphorus in sediments varies from 295.6mg/kg to 498.1mg/kg, which accounts for 52%~77% of total phosphorus. The content of HCl-exchangeable phosphorus ("apatite" fraction) is higher than that of NaOH-exchangeable phosphorus ("non-apatite" inorganic P), which is higher than Mgd2-exchangeable phosphorus ("non-apatite" inorganic P). "Non-apatite" inorganic P value (MgCl2-P+NaOH-P) in Daya Bay sediment (147.6mg/kg) is lower than that in South China Sea sediment (159.6mg/kg) and higher than that in Xiamen Bay sediment (117.8mg/kg), which accounts for 49.9%, 32.0% and 35.0% of TP respectively.It was reported that "non-apatite" inorganic P is potential bioavailable. In this respect, the potential bioavailable P in the sediments from Daya Bay and South China Sea are higher than that from Xiamen Bay.Bioavailability of Daya Bay sediment is estimated and compared with South China Sea sediment and Xiamen Bay sediment by culturing N-deficient or P-deficient algae Chloralla and Isochrysis galbana in dissolved inorganic nitrogen free or dissolved inorganic phosphorus free medium with sediment as the sole nitrogen or phosphorus source. The results show that nitrogen or phosphorus in sediment accelerates the growth of algae. The utilization rate of exchangeable nitrogen in Daya Bay sediment, South China Sea sediment and Xiamen Bay sediment by Chloralla and Isochrysis galbana varies from 9.23% to 27.0%. Similarly, the utilization rate of inorganic phosphorus varies from 21.0% to 27.1%. The data indicated that nitrogen and phosphorus in sediments could be released to water phase and be absorbed by algae, which resulted principally in decrease of the MgCb-exchangeable and NaOH-exchangeable "non-apatite" inorganic phosphorus rather than HCl-exchangeable "apatite" phosphorus. It implied that there are some potential nitrogen and phosphorus in Daya Bay sediment which provide N and P nutrients to phytoplankton and maintain its growth by diffusion, de-adsorption and so on releasing N and P to the water column. So the fact that N and P nutrients providing by sediments is likely to be another key to high productivity in water column, which is supported by low inorganic nutrients.The bioavailability of N and P in Daya Bay sediment by Chloralla and Isochrysis galbana is studied for the first time. Our results may help the study of N and P cycling between sediment-water interface, controlling of primary productivity, aquiculture and managing environment.