To achieve the goals set out above, three oceanographic research cruises were conducted to collect samples and perform experiments to evaluate down regulation of phytoplankton fluorescence under variable incident irradiance and elucidate potential controlling factors. These were located in (i) the tropical North Atlantic, (ii) the equatorial Pacific and the Peruvian upwelling zone, and (iii) the South Atlantic (see blue dots in attached image). Collectively these regions cover multiple oceanographic regimes, with (i) amounts of phytoplankton in seawater varying by three orders of magnitude, (ii) containing different types phytoplankton, and (iii) host to phytoplankton under growth limitation by different nutrients. Using a state-of-the-art fluorometer, experiments were conducted to evaluate the magnitude of fluorescence reductions phytoplankton exhibit upon exposure to progressively increasing irradiance (or ‘non-photochemical quenching’; NPQ). Alongside these experiments, samples were collected to determine how much phytoplankton biomass was present, what types of phytoplankton were present, the concentrations of nutrients (e.g. nitrate and phosphate) and trace elements (e.g. iron), alongside measurements of ocean physics (e.g. how deep phytoplankton in surface waters were being mixed down to). A suite of longer duration experiments directly tested which nutrients were limiting phytoplankton growth. Collectively, thousands of samples were collected on research cruises and were subsequently analysed in GEOMAR Centre for Ocean Research, Kiel (Germany) and Dalhousie University (Canada). Although the experimental work in this project has suggested that an important initial aim of the proposal, to develop a robust NPQ correction to apply to satellite fluorescence data, has considerably greater complexity than previously envisaged, the new data have led to significant advances in understanding processes of nutrient limitation and the complexities of using phytoplankton fluorescence to infer nutrient limitation. The advances have been presented at international conferences and published in international peer reviewed journals (or are under review/in preparation for publication).
