Nutrient limitation regimes control sunlight-stimulated chlorophyll fluorescence in the South Atlantic Ocean

Abstract Sunlight-stimulated chlorophyll fluorescence offers potential for assessing phytoplankton physiological status at broad scales detected by satellites, but controls on these signals are poorly constrained. In this study, we present a comprehensive, high-resolution dataset of passive chlorophyll fluorescence alongside potential physical and biogeochemical drivers collected across the Benguela upwelling system and South Atlantic Subtropical Gyre. The nutrient limitation status of phytoplankton was assessed through 27 onboard bioassay experiments. A consistent and significant difference in light-saturated, passive chlorophyll fluorescence normalized to phytoplankton absorption ( $$F/{{a}_{{{\mathrm{ph}}}}}_{\max }^{{{\rm{passive}}}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>F</mml:mi> <mml:mo>/</mml:mo> <mml:msubsup> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>a</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>ph</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> <mml:mrow> <mml:mi>max</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>passive</mml:mi> </mml:mrow> </mml:msubsup> </mml:math> ) was observed between waters where phytoplankton were either iron or nitrogen limited, with iron limited regions showing threefold higher $$F/{{a}_{{{\mathrm{ph}}}}}_{\max }^{{{\rm{passive}}}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>F</mml:mi> <mml:mo>/</mml:mo> <mml:msubsup> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>a</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>ph</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> <mml:mrow> <mml:mi>max</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>passive</mml:mi> </mml:mrow> </mml:msubsup> </mml:math> compared to nitrogen limited regions. When interpreted alongside the results of the bioassay experiments, we found that neither variability in physical forcing (temperature, mixing, light climate) or phytoplankton community structure could explain the major variability in $$F/{{a}_{{{\mathrm{ph}}}}}_{\max }^{{{\rm{passive}}}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>F</mml:mi> <mml:mo>/</mml:mo> <mml:msubsup> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>a</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>ph</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> <mml:mrow> <mml:mi>max</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>passive</mml:mi> </mml:mrow> </mml:msubsup> </mml:math> . These results provide direct field-based evidence that passive chlorophyll fluorescence is sensitive to phytoplankton nutrient limitation and demonstrates potential for observation at high spatial and temporal resolution using satellite observations.