Effects of macronutrient (NO3(-) -N and NH4(+) -N) on inorganic carbon system of water with Ulva pertusa existed were studied in laboratory simulation experiments. The results demonstrated that nutrient enrichment induced DIC, HCO3- and p(CO2) decreased while pH and CO3(2-) increased. The seawater changed from carbon source to carbon sink. During the experiments, the concentration of DIC, HCO3- and p(CO2) decreased with increasing concentration of nutrient when the NO3(-) -N and N4(+) -N were less than critical concentration. The concentration of DIC changed most at the NO3(-3) and NH4(-)3 groups, which decreased 151 micromol x L(-1) and 232 micromol x L(-1) compared with the control groups in the end of experiment. The increased dry weight of Ulva pertusa (deltam) of nutrient addition groups showed a significant negative correlation with deltaDIC (r = - 0.91, p < 0.0001, n = 11). The main controlling factor to inorganic carbon variation is the adaptation of Ulva pertusa to different DIN. When the concentration of DIN facilitates the growth of Ulva pertusa, the concentration of DIC decreased and dry weight of Ulva pertusa increased. NH(4) -N has more influence on inorganic carbon system than NO(3-) -N.