Estimation of primary productivity by chlorophyll a in vivo fluorescence in freshwater phytoplankton

Primary productivity in marine waters is widely estimated by the measuremen ts of C-14 incorporation, the underwater light climate, and the absorption spectra of phytoplankton. In bio-optical models the quantum efficiency of c arbon fixation derived from C-14 incorporation rates, the photosyntheticall y absorbed radiation derived from the underwater light climate, and the phy toplankton absorption spectra are used to calculate time- and depth-integra ted primary productivity. Due to the increased sensitivity of commercially available fluorometers, chlorophyll a in vivo fluorescence became a new too l to assess the photosynthetic activity of phytoplankton. Since fluorescenc e data yield only relative photosynthetic electron transport rates, a direc t conversion into absolute carbon fixation rates is not possible. Here, we report a procedure how this problem can be adressed in freshwater phytoplan kton. We adapted a marine bio-optical model to the freshwater situation and tested if this model yields realistic results when applied to a hypertroph ic freshwater reservoir. Comparison of primary productivity derived from C- 14 incorporation to primary productivity derived from Chi a fluorescence sh owed that the conversion of fluorescence data into carbon fixation rates is still an unsolved problem. Absolute electron transport rates calculated fr om fluorescence data tend to overestimate primary production. We propose th at the observed differences are caused mainly by neglecting the package eff ect of pigments in phytoplankton cells and by non-carbon related electron f low (e.g., nitrogen fixation). On the other hand, the C-14 incorporation ra tes can be artificially influenced by ''bottle effects", especially near th e water surface, where photoinhibition, photorespiration, and Mehler reacti on can play a major role.