摘要
Nutrient enrichment experiments with nitrogen （N） and phosphorus （P） were conducted with samples from two stationsin the coastal waters of Qingdao, China, during summer to identify limiting nutrients. In late July of 2009, low P concentrations andthe 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 atstation 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 treat-ments with added P than those without the addition suggested potential P limitation at station A. Lack of Fv/Fm and Chl a responsesfollowing 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-limitationto natural populations. This method can be more accurate for assessing co-limitation than the use of criteria of nutrient concentrationsand ratios as indicators, and can provide more rapid results than nutrient addition bioassays using chlorophyll response as an indica-tor, when a population is potentially limited. Compared with the two conventional methods, the results based on F,/F~ can also pro-vide more detailed information about physiological states of the populations.
Nutrient enrichment experiments with nitrogen （N） and phosphorus （P） were conducted with samples from two stationsin the coastal waters of Qingdao, China, during summer to identify limiting nutrients. In late July of 2009, low P concentrations andthe 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 atstation 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 treat-ments with added P than those without the addition suggested potential P limitation at station A. Lack of Fv/Fm and Chl a responsesfollowing 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-limitationto natural populations. This method can be more accurate for assessing co-limitation than the use of criteria of nutrient concentrationsand ratios as indicators, and can provide more rapid results than nutrient addition bioassays using chlorophyll response as an indica-tor, when a population is potentially limited. Compared with the two conventional methods, the results based on F,/F~ can also pro-vide more detailed information about physiological states of the populations.
