Preprints
https://doi.org/10.5194/bg-2021-113
https://doi.org/10.5194/bg-2021-113

  10 May 2021

10 May 2021

Review status: a revised version of this preprint is currently under review for the journal BG.

Mixed layer depth dominates over upwelling in regulating the seasonality of ecosystem functioning in the Peruvian Upwelling System

Tianfei Xue1, Ivy Frenger1, A. E. Friederike Prowe1, Yonss Saranga José1, and Andreas Oschlies1,2 Tianfei Xue et al.
  • 1GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany
  • 2Christian-Albrechts-University Kiel, Germany

Abstract. The Peruvian Upwelling System hosts an extremely high productive marine ecosystem. Observations show that the Peruvian Upwelling System is the only Eastern Boundary Upwelling Systems (EBUS) with an out-of-phase relationship of seasonal surface chlorophyll concentrations and upwelling intensity. This "seasonal paradox" triggers the questions: (1) what is the uniqueness of the Peruvian Upwelling System compared with other EBUS that leads to the out of phase relationship; (2) how does this uniqueness lead to low phytoplankton biomass in austral winter despite strong upwelling and ample nutrients? Using observational climatologies for four EBUS we diagnose that the Peruvian Upwelling System is unique in that intense upwelling coincides with deep mixed layers. We then apply a coupled regional ocean circulation-biogeochemical model (CROCO-BioEBUS) to assess how the interplay between mixed layer and upwelling is regulating the seasonality of surface chlorophyll in the Peruvian Upwelling System. The model recreates the "seasonal paradox" within 200 km off the Peruvian coast. We confirm previous findings that deep mixed layers, which cause vertical dilution and stronger light limitation, mostly drive the diametrical seasonality of chlorophyll relative to upwelling. In contrast to previous studies, reduced phytoplankton growth due to enhanced upwelling of cold waters and lateral advection are second-order drivers of low surface chlorophyll concentrations. This impact of deep mixed layers and upwelling propagates up the ecosystem, from primary production to export efficiency. Our findings emphasize the crucial role of the interplay of the mixed layer and upwelling and suggest that surface chlorophyll may increase along with a weakened seasonal paradox in response to shoaling mixed layers under climate change.

Tianfei Xue et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2021-113', Anonymous Referee #1, 07 Jun 2021
    • AC1: 'Reply on RC1', Tianfei Xue, 15 Jun 2021
    • AC3: 'Reply on RC1-2', Tianfei Xue, 16 Aug 2021
  • RC2: 'Comment on bg-2021-113', Anonymous Referee #2, 12 Jul 2021
    • AC2: 'Reply on RC2', Tianfei Xue, 16 Aug 2021

Tianfei Xue et al.

Tianfei Xue et al.

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Short summary
The Peruvian system supports 10 % of the world's fishing yield. In the Peruvian system, wind and earth’s rotation bring cold, nutrient-rich water to the surface and allow phytoplankton to grow. But observations show that it grows worse at high upwelling. Using a model, we find that high upwelling happens when air mixes the water the most. Then phytoplankton is diluted and grows slowly due to low light and cool upwelled water. This study helps to estimate how it might change in a warming climate.
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