Relationships between electron transport rates determined by pulse amplitude modulated (PAM) chlorophyll fluorescence and photosynthetic rates by traditional and common methods in natural freshwater phytoplankton

The relationships between the electron transport rate (ETR) Measured by pulse amplitude modulated (PAM) fluorometer and the rate of O-2 production and C-14 fixation by traditional and most common methods were estimated in natural phytoplankton communities to determine the molar ratio of oxygen produced and carbon fixation per electron transported. The observations were carried out in Lake Biwa, Japan approximately once a month for a Year. A remarkable linear relationship vas observed between the rate of O-2 production and C fixation to ETR at low-to-medium irradiances during the sampling period, but not at higher irradiances. The annual average O-2 production/ETR and C fixation/ETR ratios at low-to-medium irradiances were 0.117 and 0.095. respectively, and the photosynthetic quotient (O-2 production/C fixation) calculated from the two ratios was 1.23, which is the most common value as the photosynthetic quotient. The same was true for Cultured algal species. These results indicate that the O-2 production/ETR and C fixation/ETR ratios are reliable values at low-to-medium irradiances. and that PAM measurements call be used to quantify primary production rates based oil traditional methods using the ratios. However. it Must be noted that the determination of the average O-2 production/ETR and C fixation/ETR ratios in a natural phytoplankton community requires long-term observation because those ratios Change seasonally over a certain range.