Interpretation of fast repetition rate (FRR) fluorescence: signatures of phytoplankton community structure versus physiological state

Introduction of active chlorophyll a fluorescence protocols, in particular fast repetition rate (FRR) fluorometry, to oceanography and limnology 15 yr ago has enabled rapid assessment of photosynthetic physiology in situ. The FRR protocol generates simultaneous measurements of Photosystem II (PSII) effective absorption cross sections (termed ?PSII) and photochemical efficiency (termed Fv/Fm). Both Fv/Fm and ?PSII measurements have been utilised to examine the effects of physiological stress on the photosynthetic apparatus of phytoplankton in an ever growing number of fluorescence-based studies. However, it is now becoming clearer that in situ values of Fv/Fm and ?PSII also contain taxonomic information. Here, we present a synthesis of previously unpublished and published data, which show that Fv/Fm and ?PSII vary principally with broad-scale changes in community structure. These patterns observed in situ conform to trends observed in laboratory-grown cultures of a range of phytoplankton taxa. The magnitudes of variability in Fv/Fm and ?PSII driven by changes in phytoplankton community structure often exceed that induced by nutrient limitation (as determined from controlled nutrient addition experiments). An exception to this general trend occurs in high-nutrient, low-chlorophyll a (HNLC) regions, where strong phenotypic changes in Fv/Fm and ?PSII have been repeatedly demonstrated on relief of iron limitation. Overall, FRR fluorescence measurements of both Fv/Fm and ?PSII in natural populations represent a combination of the taxonomic ‘signature’ (values of Fv/Fm and ?PSII determined by the taxa present) within the phytoplankton community that is further modified according to the (photo-) physiological status. As such, fluorescence-based investigations of mixed populations must account for potential variations in phytoplankton community structure before interpretations of physiological status are made.