<p>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 (<InlineEquation ID="IEq1"> <EquationSource Format="TEX">$F/{{a}_{{{\mathrm{ph}}}}}_{\max }^{{{\rm{passive}}}}$</EquationSource> <EquationSource Format="MATHML"><math> <mi>F</mi> <mo>/</mo> <msubsup> <mrow> <msub> <mrow> <mi>a</mi> </mrow> <mrow> <mi mathvariant="normal">ph</mi> </mrow> </msub> </mrow> <mrow> <mi>max</mi> </mrow> <mrow> <mi mathvariant="normal">passive</mi> </mrow> </msubsup> </math></EquationSource> </InlineEquation>) was observed between waters where phytoplankton were either iron or nitrogen limited, with iron limited regions showing threefold higher <InlineEquation ID="IEq2"> <EquationSource Format="TEX">$F/{{a}_{{{\mathrm{ph}}}}}_{\max }^{{{\rm{passive}}}}$</EquationSource> <EquationSource Format="MATHML"><math> <mi>F</mi> <mo>/</mo> <msubsup> <mrow> <msub> <mrow> <mi>a</mi> </mrow> <mrow> <mi mathvariant="normal">ph</mi> </mrow> </msub> </mrow> <mrow> <mi>max</mi> </mrow> <mrow> <mi mathvariant="normal">passive</mi> </mrow> </msubsup> </math></EquationSource> </InlineEquation> 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 <InlineEquation ID="IEq3"> <EquationSource Format="TEX">$F/{{a}_{{{\mathrm{ph}}}}}_{\max }^{{{\rm{passive}}}}$</EquationSource> <EquationSource Format="MATHML"><math> <mi>F</mi> <mo>/</mo> <msubsup> <mrow> <msub> <mrow> <mi>a</mi> </mrow> <mrow> <mi mathvariant="normal">ph</mi> </mrow> </msub> </mrow> <mrow> <mi>max</mi> </mrow> <mrow> <mi mathvariant="normal">passive</mi> </mrow> </msubsup> </math></EquationSource> </InlineEquation>. 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.</p>

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

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2025年12月7日

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Tiera-Brandy Robinson,
Haoran Liu,
Shungudzemwoyo P. Garaba,
Daniela Voss,
Nina Schuback,
C. Mark Moore,
Kevin Oxborough,
Eric P. Achterberg,
Thomas J. Browning

摘要

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 ( $F/{{a}_{{{\mathrm{ph}}}}}_{\max }^{{{\rm{passive}}}}$ $F / {a_{ph}}_{\max}^{passive}$ ) was observed between waters where phytoplankton were either iron or nitrogen limited, with iron limited regions showing threefold higher $F/{{a}_{{{\mathrm{ph}}}}}_{\max }^{{{\rm{passive}}}}$ $F / {a_{ph}}_{\max}^{passive}$ 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 $F/{{a}_{{{\mathrm{ph}}}}}_{\max }^{{{\rm{passive}}}}$ $F / {a_{ph}}_{\max}^{passive}$ . 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.