부산대학교 도서관

The ratio between fluorescence (F) and chlorophyll‐a (Chl)—where fluorescence is measured with a saturating fluorometer—is variable in the world's oceans, with the highest ratios and highest variability observed in the Southern Ocean. While species composition and Chl packaging per cell are strong drivers for the observed variability, additional factors, including iron limitation, have to this date not specifically been evaluated. Radiometers on biogeochemical (BGC)‐Argo floats allow for an independent estimate of Chl concentration that is based on the light attenuation coefficient, Kd. Making use of 4,000 radiometry profiles from BGC‐Argo floats in the Southern Ocean, we estimate Chl based on Kd and investigate the variability in F/Chl. Our analysis reveals a positive correlation between F/Chl and a proxy for iron limitation based on non‐photochemical quenching dynamics. The strong influence of iron limitation on F/Chl is further corroborated by data from Southern Ocean phytoplankton cultures. Plain Language Summary: Phytoplankton fluorescence is a relatively easy and consistent measurement that can be made in the ocean. The measured fluorescence stems directly from the chlorophyll contained in phytoplankton cells and is therefore very specific to the tiny plants. Great oceanic coverage of fluorescence measurements is achieved because fluorometers can be mounted on autonomous platforms such as gliders and floats, yielding reliable data for years without human intervention. But there is a problem with this measurement: fluorescence does not equal chlorophyll, and the factor for the conversion of fluorescence to chlorophyll varies almost 10‐fold across the world's oceans. The largest conversion factors, with the highest variability, are observed in the Southern Ocean. We know that things such as phytoplankton species composition, nutrient status, and acclimation to light all affect the conversion factor. Using data from biogeochemical Argo floats and from laboratory studies, we show in this study that iron limitation also significantly increases the fluorescence‐chlorophyll conversion factor, and that iron limitation may indeed be a key driver for the high conversion factors observed in the Southern Ocean. Key Points: Fluorescence/chlorophyll ratios in the Southern Ocean are consistently higher and more variable than the global averageBiogeochemical‐Argo data and phytoplankton culture studies suggest increased fluorescence/chlorophyll ratios are linked to iron limitationThe mean fluorescence/chlorophyll ratio observed was 3.79, and can be used to scale fluorescence data from the Southern Ocean [ABSTRACT FROM AUTHOR]