Nutrient enrichment experiments with nitrogen(N) and phosphorus(P) were conducted with samples from two stations in the coastal waters of Qingdao, China, during summer to identify limiting nutrients. In late July of 2009, low P concentrations and the maximum photochemical efficiency of photosystem II(Fv /Fm) in the initial samples together with Fv /Fm and chlorophyll a(Chl a) responses to P addition indicated P limitation at the two stations. In early August, low P levels still limited phytoplankton growth at station A. Fv /Fm and Chl a were the highest in the NP treatments at station B, suggesting an N/P co-limitation. In mid-September, nutrient concentrations and Fv /Fm were elevated and phytoplankton communities were healthy. Greater Fv /Fm and Chl a in the treatments with added P than those without the addition suggested potential P limitation at station A. Lack of Fv /Fm and Chl a responses following nutrient additions indicated N and P repletion at station B. At the end of July 2010, neither N nor P was limited at station B. Additionally, Fv /Fm coupled with 24-h-long nutrient enrichment experiments can be used to detect P limitation and N/P co-limitation to natural populations. This method can be more accurate for assessing co-limitation than the use of criteria of nutrient concentrations and ratios as indicators, and can provide more rapid results than nutrient addition bioassays using chlorophyll response as an indicator, when a population is potentially limited. Compared with the two conventional methods, the results based on Fv /Fm can also provide more detailed information about physiological states of the populations. Nutrient enrichment experiments with nitrogen (N) and phosphorus (P) were conducted with two stations in the coastal waters of Qingdao, China, during summer to identify limiting nutrients. In late July of 2009, low P concentrations and the maximum photochemical efficiency of photosystem II (Fv / Fm) in the initial samples together with Fv / Fm and chlorophyll a (Chl a) responses to P addition at P limitation at the two stations. In early August, low P levels still limited phytoplankton growth at station A Fv / Fm and Chl a were the highest in the NP treatments at station B, suggesting an N / P co-limitation. In mid-September, nutrient concentrations and Fv / Fm were elevated and phytoplankton communities were healthy. Greater Fv / Fm and Chl a in the treatments with added P than those without the addition the potential P P limitation at station A. Lack of Fv / Fm and Chl a response following nutrient additions indicated N and P repletion at station B. At the end of July 2 010, neither N nor P was limited at station B. Additionally, Fv / Fm coupled with 24-h-long nutrient enrichment experiments can be used to detect P limitation and N / P co-limitation to natural populations. This method can be more accurate for assessing co-limitation than the use of criteria of nutrient concentrations and ratios as indicators, and can provide more rapid results than nutrient addition bioassays using chlorophyll response as an indicator, when a population is potentially limited. Compared with the two conventional methods, the results based on Fv / Fm can also provide more detailed information about physiological states of the populations.