Southern Ocean phytoplankton under climate change: a shifting balance of bottom-up and top-down control
Phytoplankton form the base of the marine food web by transforming CO 2 into organic carbon via photosynthesis. Despite the importance of phytoplankton for marine ecosystems and global carbon cycling, projections of phytoplankton biomass in response to climate change differ strongly across Earth sys...
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ftcopernicus:oai:publications.copernicus.org:bg114935 2024-09-15T18:37:02+00:00 Southern Ocean phytoplankton under climate change: a shifting balance of bottom-up and top-down control Xue, Tianfei Terhaar, Jens Prowe, A. E. Friederike Frölicher, Thomas L. Oschlies, Andreas Frenger, Ivy 2024-05-24 application/pdf https://doi.org/10.5194/bg-21-2473-2024 https://bg.copernicus.org/articles/21/2473/2024/ eng eng doi:10.5194/bg-21-2473-2024 https://bg.copernicus.org/articles/21/2473/2024/ eISSN: 1726-4189 Text 2024 ftcopernicus https://doi.org/10.5194/bg-21-2473-2024 2024-08-28T05:24:15Z Phytoplankton form the base of the marine food web by transforming CO 2 into organic carbon via photosynthesis. Despite the importance of phytoplankton for marine ecosystems and global carbon cycling, projections of phytoplankton biomass in response to climate change differ strongly across Earth system models, illustrating uncertainty in our understanding of the underlying processes. Differences are especially large in the Southern Ocean, a region that is notoriously difficult to represent in models. Here, we argue that total (depth-integrated) phytoplankton biomass in the Southern Ocean is projected to largely remain unchanged under climate change by the Coupled Model Intercomparison Project Phase 6 (CMIP6) multi-model ensemble because of a shifting balance of bottom-up and top-down processes driven by a shoaling mixed-layer depth. A shallower mixed layer is projected on average to improve growth conditions, consequently weaken bottom-up control, and confine phytoplankton closer to the surface. An increase in the phytoplankton concentration promotes zooplankton grazing efficiency, thus intensifying top-down control. However, large differences across the model ensemble exist, with some models simulating a decrease in surface phytoplankton concentrations. To reduce uncertainties in projections of surface phytoplankton concentrations, we employ an emergent constraint approach using the observed sensitivity of surface chlorophyll concentration, taken as an observable proxy for phytoplankton, to seasonal changes in the mixed-layer depth as an indicator for future changes in surface phytoplankton concentrations. The emergent constraint reduces uncertainties in surface phytoplankton concentration projections by around one-third and increases confidence that surface phytoplankton concentrations will indeed rise due to shoaling mixed layers under global warming, thus favouring intensified top-down control. Overall, our results suggest that while changes in bottom-up conditions stimulate enhanced growth, intensified ... Text Southern Ocean Copernicus Publications: E-Journals Biogeosciences 21 10 2473 2491 |
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English |
description |
Phytoplankton form the base of the marine food web by transforming CO 2 into organic carbon via photosynthesis. Despite the importance of phytoplankton for marine ecosystems and global carbon cycling, projections of phytoplankton biomass in response to climate change differ strongly across Earth system models, illustrating uncertainty in our understanding of the underlying processes. Differences are especially large in the Southern Ocean, a region that is notoriously difficult to represent in models. Here, we argue that total (depth-integrated) phytoplankton biomass in the Southern Ocean is projected to largely remain unchanged under climate change by the Coupled Model Intercomparison Project Phase 6 (CMIP6) multi-model ensemble because of a shifting balance of bottom-up and top-down processes driven by a shoaling mixed-layer depth. A shallower mixed layer is projected on average to improve growth conditions, consequently weaken bottom-up control, and confine phytoplankton closer to the surface. An increase in the phytoplankton concentration promotes zooplankton grazing efficiency, thus intensifying top-down control. However, large differences across the model ensemble exist, with some models simulating a decrease in surface phytoplankton concentrations. To reduce uncertainties in projections of surface phytoplankton concentrations, we employ an emergent constraint approach using the observed sensitivity of surface chlorophyll concentration, taken as an observable proxy for phytoplankton, to seasonal changes in the mixed-layer depth as an indicator for future changes in surface phytoplankton concentrations. The emergent constraint reduces uncertainties in surface phytoplankton concentration projections by around one-third and increases confidence that surface phytoplankton concentrations will indeed rise due to shoaling mixed layers under global warming, thus favouring intensified top-down control. Overall, our results suggest that while changes in bottom-up conditions stimulate enhanced growth, intensified ... |
format |
Text |
author |
Xue, Tianfei Terhaar, Jens Prowe, A. E. Friederike Frölicher, Thomas L. Oschlies, Andreas Frenger, Ivy |
spellingShingle |
Xue, Tianfei Terhaar, Jens Prowe, A. E. Friederike Frölicher, Thomas L. Oschlies, Andreas Frenger, Ivy Southern Ocean phytoplankton under climate change: a shifting balance of bottom-up and top-down control |
author_facet |
Xue, Tianfei Terhaar, Jens Prowe, A. E. Friederike Frölicher, Thomas L. Oschlies, Andreas Frenger, Ivy |
author_sort |
Xue, Tianfei |
title |
Southern Ocean phytoplankton under climate change: a shifting balance of bottom-up and top-down control |
title_short |
Southern Ocean phytoplankton under climate change: a shifting balance of bottom-up and top-down control |
title_full |
Southern Ocean phytoplankton under climate change: a shifting balance of bottom-up and top-down control |
title_fullStr |
Southern Ocean phytoplankton under climate change: a shifting balance of bottom-up and top-down control |
title_full_unstemmed |
Southern Ocean phytoplankton under climate change: a shifting balance of bottom-up and top-down control |
title_sort |
southern ocean phytoplankton under climate change: a shifting balance of bottom-up and top-down control |
publishDate |
2024 |
url |
https://doi.org/10.5194/bg-21-2473-2024 https://bg.copernicus.org/articles/21/2473/2024/ |
genre |
Southern Ocean |
genre_facet |
Southern Ocean |
op_source |
eISSN: 1726-4189 |
op_relation |
doi:10.5194/bg-21-2473-2024 https://bg.copernicus.org/articles/21/2473/2024/ |
op_doi |
https://doi.org/10.5194/bg-21-2473-2024 |
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Biogeosciences |
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21 |
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10 |
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2473 |
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2491 |
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1810481297073635328 |