|Variable fluorescence normalized to maximal fluorescence, Fv/Fm, determined by Fast Repetition Rate
Fluorometry (FRRF) is being increasingly used to compare photosynthetic electron transport capacity in natural
phytoplankton communities. Interpreting results of such studies is, however, complicated by the fact that
both nutrient status and light history (photoinhibition under in situ conditions) are known to influence Fv/Fm.
Thus, the value of Fv/Fm measurements in the field would be greatly enhanced if the light history signal could
be separated from other influences. Here, both field and laboratory studies demonstrate that dark treatment (30
min–4 h) before FRRF measurement is not sufficient to remove a light history signal in Fv/Fm. The signal could,
however, be essentially eliminated by incubation of samples in low light prior to Fv/Fm determination. For the
study conditions tested, the most effective treatment for removal of the signal was 4 h at 50 µmol m–2 s–1.
However, the effectiveness of the light treatment in removing the light history was influenced by temperature.
Therefore, no universal protocol for eliminating the light history signal can be developed, but recommendations
are given for developing site-specific approaches for separating the light history signal from other factors
influencing Fv/Fm. Carrying out light incubations before determining Fv/Fm not only provides the possibility for
eliminating a light history signal in the measurements but the difference between Fv/Fm measured after light and
dark incubations appears also to be a potentially useful indicator of the degree of photoinhibition experienced
by phytoplankton under natural conditions.|