Lagged response of phytoplanktonic photosynthetic competence to variation of nitrogen concentration in Jiaozhou Bay, China

Fluorescence-based maximal photochemical efficiency, F _v/F _m, is widely used as an indicator to photosynthetic competence in marine systems. It has been considered a useful parameter diagnosing the nutrient stress on phytoplankton photosynthesis, but many studies argue its usefulness. In the present study, we try to find a temporal relationship between F _v/F _m and nitrogen concentration, and provide a possible explanation on the controversy. We continuously measured F _v/F _m and nitrogen concentration once every 10 days from September 2003 to March 2004 at two stations in Jiaozhou Bay, northern China. It was found that F _v/F _m did not significantly correlate to synchronous nitrogen concentration, but the variation (i.e. the change between two adjacent cruises) of nitrogen concentration of the previous cruise and the variation of F _v/F _m of the current cruise were strongly correlated. This result indicates that a time lag exists between the variation of nutrient status and the subsequent F _v/F _m response. Length of the time lag seems just matched the interval of our measurements (10 days). In the field, direct dependence of F _v/F _m on nitrogen concentration may not be found because of the lagged response of F _v/F _m to nitrogen concentration variations or physiological acclimation. Our results provide a possible way to explain the previously reported conflicting results on the relationship between F _v/F _m and nutrient status. To give a more-accurate estimate about the length of the time lag, an investigation that includes more frequent measurements is needed.