Satellite-detected fluorescence: Decoupling nonphotochemical quenching from iron stress signals in the South Atlantic and Southern Ocean

Satellite-detected sunlight-induced chlorophyll fluorescence could offer valuable information about the physiological status of phytoplankton on a global scale. Realization of this potential is confounded by the considerable uncertainty that exists in deconvolving the multiple ecophysiological processes that can influence the satellite signal. A dominant source of current uncertainty arises from the extent of reductions in chlorophyll fluorescence caused by the high light intensities phytoplankton are typically exposed to when satellite images are captured. In this study, results from over 200 nonphotochemical quenching (NPQ) experiments conducted on cruises spanning from subtropical gyre to Southern Ocean waters have confirmed that satellite fluorescence quantum yields have the potential to reveal broad regions of iron (Fe) stress. However, our results suggest significant variability in phytoplankton NPQ behavior between oceanic regimes. Dynamic NPQ must therefore be considered to achieve a reliable interpretation of satellite fluorescence in terms of Fe stress. Specifically, significantly lower NPQ was found in stratified subtropical gyre-type waters than in well-mixed Southern Ocean waters. Such variability is suggested to result from differences in incident irradiance fluctuation experienced by phytoplankton, with highly variable irradiance conditions likely driving phytoplankton to acclimate or adapt toward a higher dynamic NPQ capacity. Sea surface temperature empirically demonstrated the strongest correlation with NPQ parameters and is presented as a means of correcting the chlorophyll fluorescence signature for the region studied. With these corrections, a decadal composite of satellite austral summer observations is presented for the Southern Ocean, potentially reflecting spatial variability in the distribution and extent of Fe stress.