Effect of nutrient enrichment on phytoplankton growth in the adjacent waters of the Xisha Yongxing Islands in late fall

To investigate the effect of nitrate and phosphate on phytoplankton growth, we carried out field experiments and enrichment incubation experiments in the adjacentwaters of the Xisha Yongxing Islands ( S0 station: 1730. 364N, 11159. 843E) in late fall of 2014. The added nitrogen concentrations were 4 molL-1 , 8 molL-1 , 16 molL- 1 and 32 molL-1; the added phosphorus concentrations were 0. 25 molL- 1 , 0.5 molL-1, 1 molL-1 and 2 molL-1; the added mixed nitrogen and phosphorus concentrations were 4 molL-1+0.5 molL-1, 8 molL-1+2 molL-1, 16 molL-1+0.25 molL-1 and 32 molL-1 +1 molL-1. The results show that the chlorophyll a increased significantly after adding mixed nitrate and phosphate (P0.05) . The chlorophyll a concentration increased from0.06 mgm-3 to 1.71 mgm-3 and 1. 10 mgm-3 at S0 station after adding 32 molL-1 NO-3-N+1 molL-1PO3-4-P and 4 molL-1NO-3-N+0.5 molL-1 PO3-4-P mixed nitrate and phosphate, respectively. It is suggested that enrichment of mixed nitrate and phosphate (N/P: 8～32) could promote the growth of phytoplankton. However, enrichment of single nitrate and single phosphate did not promote the phytoplankton growth because the chlorophyll a concentration showed no significant increase by adding single nitrate and single phosphate at S0 station. Consequently, the restriction factors of phytoplankton growth at S0 station were both nitrate and phosphate concentrations.