TY - JOUR
T1 - The Photophysiological Response of Nitrogen-Limited Phytoplankton to Episodic Nitrogen Supply Associated With Tropical Instability Waves in the Equatorial Atlantic
AU - Sherman, Jonathan
AU - Subramaniam, Ajit
AU - Gorbunov, Maxim Y.
AU - Fernández-Carrera, Ana
AU - Kiko, Rainer
AU - Brandt, Peter
AU - Falkowski, Paul G.
N1 - Funding Information: This study was supported by the NASA Ocean Biology and Biogeochemistry Program (grants NNX16AT54G and 80NSSC18K1416 to MG and PF and 80NSSC21K0439 to AS) and EU H2020 under grant agreement 817578 TRIATLAS project. JS was supported by a graduate fellowship from the Department of Marine and Coastal Sciences at Rutgers University. AS was also funded by the Climate Center, Climate and Life Fellowship of Lamont-Doherty Earth Observatory and NSF Grant OCE-1737128. RK also acknowledges support via a “Make Our Planet Great Again” grant of the French National Research Agency within the “Programme d’Investissements d’Avenir,” reference “ANR-19-MPGA-0012.” Funding Information: Sarat Chandra Tripathy, National Centre for Polar and Ocean Research (NCPOR), India Haimanti Biswas, National Institute of Oceanography, Council of Scientific and Industrial Research (CSIR), India Publisher Copyright: Copyright © 2022 Sherman, Subramaniam, Gorbunov, Fernández-Carrera, Kiko, Brandt and Falkowski.
PY - 2022/1/11
Y1 - 2022/1/11
N2 - In the Equatorial Atlantic nitrogen availability is assumed to control phytoplankton dynamics. However, in situ measurements of phytoplankton physiology and productivity are surprisingly sparse in comparison with the North Atlantic. In addition to the formation of the Equatorial cold tongue in the boreal summer, tropical instability waves (TIWs) and related short-term processes may locally cause episodic events of enhanced nutrient supply to the euphotic layer. Here, we assess changes in phytoplankton photophysiology in response to such episodic events as well as short-term nutrient addition experiments using a pair of custom-built fluorometers that measure chlorophyll a (Chl a) variable fluorescence and fluorescence lifetimes. The fluorometers were deployed during a transatlantic cruise along the Equator in the fall of 2019. We hypothesized that the Equatorial Atlantic is nitrogen-limited, with an increasing degree of limitation to the west where the cold tongue is not prominent, and that infrequent nitrate injection by TIW related processes are the primary source alleviating this limitation. We further hypothesized phytoplankton are well acclimated to the low levels of nitrogen, and once nitrogen is supplied, they can rapidly utilize it to stimulate growth and productivity. Across three TIW events encountered, we observed increased productivity and chlorophyll a concentration concurrent with a decreased photochemical conversion efficiency and overall photophysiological competency. Moreover, the observed decrease in photosynthetic turnover rates toward the western section suggested a 70% decrease in growth rates compared to their maximum values under nutrient-replete conditions. This decrease aligned with the increased growth rates observed following 24 h incubation with added nitrate in the western section. These results support our hypotheses that nitrogen is the limiting factor in the region and that phytoplankton are in a state of balanced growth, waiting to “body surf” waves of nutrients which fuel growth and productivity.
AB - In the Equatorial Atlantic nitrogen availability is assumed to control phytoplankton dynamics. However, in situ measurements of phytoplankton physiology and productivity are surprisingly sparse in comparison with the North Atlantic. In addition to the formation of the Equatorial cold tongue in the boreal summer, tropical instability waves (TIWs) and related short-term processes may locally cause episodic events of enhanced nutrient supply to the euphotic layer. Here, we assess changes in phytoplankton photophysiology in response to such episodic events as well as short-term nutrient addition experiments using a pair of custom-built fluorometers that measure chlorophyll a (Chl a) variable fluorescence and fluorescence lifetimes. The fluorometers were deployed during a transatlantic cruise along the Equator in the fall of 2019. We hypothesized that the Equatorial Atlantic is nitrogen-limited, with an increasing degree of limitation to the west where the cold tongue is not prominent, and that infrequent nitrate injection by TIW related processes are the primary source alleviating this limitation. We further hypothesized phytoplankton are well acclimated to the low levels of nitrogen, and once nitrogen is supplied, they can rapidly utilize it to stimulate growth and productivity. Across three TIW events encountered, we observed increased productivity and chlorophyll a concentration concurrent with a decreased photochemical conversion efficiency and overall photophysiological competency. Moreover, the observed decrease in photosynthetic turnover rates toward the western section suggested a 70% decrease in growth rates compared to their maximum values under nutrient-replete conditions. This decrease aligned with the increased growth rates observed following 24 h incubation with added nitrate in the western section. These results support our hypotheses that nitrogen is the limiting factor in the region and that phytoplankton are in a state of balanced growth, waiting to “body surf” waves of nutrients which fuel growth and productivity.
KW - Equatorial Atlantic
KW - fluorescence lifetimes
KW - photophysiology
KW - phytoplankton
KW - tropical instability waves (TIWs)
KW - variable fluorescence
UR - http://www.scopus.com/inward/record.url?scp=85123436241&partnerID=8YFLogxK
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U2 - https://doi.org/10.3389/fmars.2021.814663
DO - https://doi.org/10.3389/fmars.2021.814663
M3 - Article
SN - 2296-7745
VL - 8
JO - Frontiers in Marine Science
JF - Frontiers in Marine Science
M1 - 814663
ER -