Southern Ocean Phytoplankton Community Structure as a Gatekeeper for Global Nutrient Biogeochemistry

Nissen, Cara; Gruber, Nicolas; Münnich, Matthias; Vogt, Meike

doi:10.1029/2021gb006991

Cited by 15 publications

(7 citation statements)

References 99 publications

(346 reference statements)

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“…The degree of biological drawdown depends on several factors, including limiting micronutrients like iron, and the transit time of upwelled waters before subduction, which is related to the strength of meridional overturning. In this framework, decadal‐ and centennial‐scale variations in SAMW nutrient content and low‐latitude productivity have been interpreted in terms of such factors (Ayers & Strutton, 2013; Marinov et al., 2006; Nissen et al., 2021; Sarmiento et al., 2004). Our analysis calls for a re‐examination of this view by showing that the influx of subtropical waters can be as relevant as biological drawdown in lowering SAMW nutrient concentrations with respect to incoming Antarctic waters.…”

Section: Discussionmentioning

confidence: 99%

Subtropical Contribution to Sub‐Antarctic Mode Waters

Fernández-Castro

Mazloff

Williams

et al. 2022

Geophysical Research Letters

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Sub‐Antarctic Mode Waters (SAMW) form to the north of the Antarctic Circumpolar Current (ACC) through deep winter mixing. SAMW connect the atmosphere with the oceanic pycnocline, transferring heat and carbon into the ocean interior and supplying nutrients to the northern ocean basins. The processes controlling SAMW ventilation and properties remain poorly understood. Here, we investigate the significance and origin of a ubiquitous feature of SAMW formation regions: The seasonal build‐up of a subsurface salinity maximum. With biogeochemical Argo floats, we show that this feature influences SAMW mixed‐layer dynamics, and that its formation is associated with a decline in preformed nutrients comparable to biological drawdown in surface waters (∼0.15 mol m−2 y−1). Our analysis reveals that these features are driven by advection of warm, salty, nutrient‐poor waters of subtropical origin along the ACC. This influx represents a leading‐order term in the SAMW physical and biogeochemical budgets, and can impact large‐scale nutrient distributions.

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Section: Discussionmentioning

confidence: 99%

Subtropical Contribution to Sub‐Antarctic Mode Waters

Fernández-Castro

Mazloff

Williams

et al. 2022

Geophysical Research Letters

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“…The degree of biological drawdown depends on several factors, including limiting micronutrients like iron, and the transit time of upwelled waters before subduction, which is related to the strength of meridional overturning. In this framework, decadal-and centennial-scale variations in SAMW nutrient content and low-latitude productivity have been interpreted in terms of such factors (Ayers & Strutton, 2013;Marinov et al, 2006;Nissen et al, 2021;Sarmiento et al, 2004). Our analysis calls for a re-examination of this view by showing that the influx of subtropical waters can be as relevant as biological drawdown in lowering SAMW nutrient concentrations with respect to incoming Antarctic waters.…”

Section: Discussionmentioning

confidence: 92%

Subtropical contribution to Subantarctic Mode Waters

Fernández-Castro

Mazloff

Williams

et al. 2022

Preprint

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<p>Subantarctic Mode Waters (SAMW), forming in the deep winter mixed layers in the Subantarctic Zone (SAZ) to the north of the Antarctic Circumpolar Current (ACC), connect the ocean thermocline with the atmosphere, contributing to ocean carbon and heat uptake and transporting high-latitude nutrients northward, to fuel primary production at low latitudes. Many aspects of SAMW formation are poorly understood due to the data scarcity during Austral winter. Here, we use biogeochemical Argo float observations to investigate the seasonal development, origin and significance of a subsurface salinity maximum in the SAMW formation regions. This conspicuous feature develops every summer in the seasonal thermocline of the SAMW formation regions as a consequence of the advection along the ACC of warmer and saltier waters from the western boundaries of the subtropical gyres, in particular the Agulhas Return current. The salinity maximum acts as a gatekeeper for SAMW ventilation, since it controls the seasonal evolution of stratification at the base of the mixed layer, modulating its rate of deepening during autumn and winter and re-stratifying the SAMW pool when winter mixing ceases. We also show that the subtropical influx, often overlooked, is key to understand the variability of SAMW properties, since it represents a leading order term in the heat and salt budgets at the formation regions. Finally, the analysis of the nitrate seasonal cycle at the SAMW formation regions as recorded by the Argo floats, revealed that the seasonal salinity increase goes along with a decrease in the concentration of this nutrient, as a consequence of the advection of subtropical waters containing low preformed nitrate. These results suggest that nutrient concentration in SAMW is controlled not only by the rate of upwelling of high-nutrient waters south of the ACC and the degree of biological drawdown during their northward transit, as frequently assumed, but also by the influx of subtropical waters, pointing to previously overlooked feedbacks in the redistribution of nutrients between high and low latitudes.</p>

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“…Consequently, more silicic acid could be available for diatoms outside the Southern Ocean (Hauck et al, 2018). Finally, other modeling studies show that not only the transfer of carbon (Gehlen et al, 2006; Lima et al, 2014), but also the transfer of nutrients (Oka et al, 2008) is largely determined by the mineral load of sinking particles globally, which is directly linked to the plankton community structure (Lima et al, 2014; Nissen et al, 2021).…”

Section: Discussionmentioning

confidence: 99%

“…A reduction in the dissolution of sinking opal by ocean acidification (Taucher et al, 2022) could furthermore strengthen silicic acid deficiencies in surface layers and lead to an ever stronger decrease in diatom biomass and silicification, especially in regions where silicic acid is limiting. The reduced usage of silicic acid in the southern high latitudes due to a lower diatom biomass, however, could reduce the role of the Southern Ocean as a “silicon trap” (Hauck et al, 2018; Holzer et al, 2014; Nissen et al, 2021) as less silicic acid is exported and trapped in the Southern Ocean's interior. Consequently, more silicic acid could be available for diatoms outside the Southern Ocean (Hauck et al, 2018).…”

Section: Discussionmentioning

confidence: 99%

Interaction matters: Bottom‐up driver interdependencies alter the projected response of phytoplankton communities to climate change

Seifert

Nissen

Rost

et al. 2023

Global Change Biology

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Phytoplankton growth is controlled by multiple environmental drivers, which are all modified by climate change. While numerous experimental studies identify interactive effects between drivers, large‐scale ocean biogeochemistry models mostly account for growth responses to each driver separately and leave the results of these experimental multiple‐driver studies largely unused. Here, we amend phytoplankton growth functions in a biogeochemical model by dual‐driver interactions (CO2 and temperature, CO2 and light), based on data of a published meta‐analysis on multiple‐driver laboratory experiments. The effect of this parametrization on phytoplankton biomass and community composition is tested using present‐day and future high‐emission (SSP5‐8.5) climate forcing. While the projected decrease in future total global phytoplankton biomass in simulations with driver interactions is similar to that in control simulations without driver interactions (5%–6%), interactive driver effects are group‐specific. Globally, diatom biomass decreases more with interactive effects compared with the control simulation (−8.1% with interactions vs. no change without interactions). Small‐phytoplankton biomass, by contrast, decreases less with on‐going climate change when the model accounts for driver interactions (−5.0% vs. −9.0%). The response of global coccolithophore biomass to future climate conditions is even reversed when interactions are considered (+33.2% instead of −10.8%). Regionally, the largest difference in the future phytoplankton community composition between the simulations with and without driver interactions is detected in the Southern Ocean, where diatom biomass decreases (−7.5%) instead of increases (+14.5%), raising the share of small phytoplankton and coccolithophores of total phytoplankton biomass. Hence, interactive effects impact the phytoplankton community structure and related biogeochemical fluxes in a future ocean. Our approach is a first step to integrate the mechanistic understanding of interacting driver effects on phytoplankton growth gained by numerous laboratory experiments into a global ocean biogeochemistry model, aiming toward more realistic future projections of phytoplankton biomass and community composition.

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Southern Ocean Phytoplankton Community Structure as a Gatekeeper for Global Nutrient Biogeochemistry

Cited by 15 publications

References 99 publications

Subtropical Contribution to Sub‐Antarctic Mode Waters

Subtropical Contribution to Sub‐Antarctic Mode Waters

Subtropical contribution to Subantarctic Mode Waters

Interaction matters: Bottom‐up driver interdependencies alter the projected response of phytoplankton communities to climate change

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