Effects of nitrogen and phosphorus enrichment on growth and photosynthetic assimilation of carbon in a green tide-forming species (Ulva prolifera) in the Yellow Sea

The hypothesis that nitrogen and phosphorus can have a positive effect on the bloom formation of Ulva prolifera along the southern coast of the Yellow Sea was examined. The nutrient enrichment on the growth and photosynthetic carbon assimilation of U. prolifera were investigated in laboratory. Four nitrogen and phosphorus treatments were established: high nitrogen and high phosphorus (HNHP), high nitrogen and low phosphorus, low nitrogen and high phosphorus, and low nitrogen and low phosphorus (LNLP). Fresh weights, relative growth rate (RGR), chlorophyll fluorescence parameters, photosynthetic rate, and the uptake of dissolved inorganic carbon (DIC) in U. prolifera were measured. The results showed that nitrogen and phosphorus enrichment significantly increased RGR of U. prolifera. The chlorophyll fluorescence parameters all reached maximum values under the HNHP treatment. The photosynthetic rate under the HNHP treatment also was the highest, which was 1.52 times that of the LNLP treatment. The DIC uptake under the HNHP treatment was 1.63 times greater than under the LNLP treatment. The photosynthesis and carbon fixation were significantly promoted by N and P enrichment. This work may further clarify the mechanisms of U. prolifera bloom formation and decline in the Yellow Sea.