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

Fluorescence-based maximal photochemical efficiency,Fv/Fm,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 Fv/Fm and nitrogen concentration,and provide a possible explanation on the controversy.We continuously measured Fv/Fm 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 Fv/Fm 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 Fv/Fm of the current cruise were strongly correlated.This result indicates that a time lag exists between the variation of nutrient status and the subsequent Fv/Fm response.Length of the time lag seems just matched the interval of our measurements(10 days).In the field,direct dependence of Fv/Fm on nitrogen concentration may not be found because of the lagged response of Fv/Fm to nitrogen concentration variations or physiological acclimation.Our results provide a possible way to explain the previously reported conflicting results on the relationship between Fv/Fm 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.

Fluorescence-based maximal photochemical efficiency, Fv/Fm, 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 Fv/Fm and nitrogen concentration, and provide a possible explanation on the controversy. We continuously measured Fv/Fm 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 Fv/ffm 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 ofFv/Fm of the current cruise were strongly correlated. This result indicates that a time lag exists between the variation of nutrient status and the subsequent Fv/Fm response. Length of the time lag seems just matched the interval of our measurements （10 days）. In the field, direct dependence of Fv/Fm on nitrogen concentration may not be found because of the lagged response of Fv/Fm to nitrogen concentration variations or physiological acclimation. Our results provide a possible way to explain the previously reported conflicting results on the relationship between Fv/Fm 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.