Reply on RC2

Tianfei Xue

doi:10.5194/bg-2021-113-ac2

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> The Peruvian upwelling system hosts a marine ecosystem with extremely high productivity. Observations show that the Peruvian upwelling system is the only eastern boundary upwelling system (EBUS) with an out-of-phase relationship between seasonal surface chlorophyll concentrations and upwelling intensity. This âseasonal paradoxâ triggers the following questions: (1)Â what are the unique characteristics of the Peruvian upwelling system, compared with other EBUSs, that lead to the out-of-phase relationship, and (2)Â how does the seasonal paradox influence ecosystem functioning? Using observational climatologies for four EBUSs, we diagnose that the Peruvian upwelling system is the only one to reveal that intense upwelling coincides with deep mixed layers. We then apply a coupled regional ocean circulation biogeochemical model (CROCOâBioEBUS) to assess how the interplay between mixed layers and upwelling regulates the seasonality of surface chlorophyll in the Peruvian upwelling system. Our model reproduces the âseasonal paradoxâ within 200âkm off the Peruvian coast. We confirm previous findings regarding the main contribution of mixed layer depth to the seasonality of chlorophyll, relative to upwelling. Deep mixed layers in austral winter cause vertical dilution of phytoplankton and strong light limitation, impacting growth. The effect of advection, though second-order, is consistent with previous findings for the Peruvian system and other EBUSs, with enhanced offshore export opposing the coastal build-up of biomass. In addition, we find that the relatively colder temperatures of upwelled waters slightly dampen phytoplankton productivity and further slow the build-up of phytoplankton biomass. This impact from the combination of deep mixed layers and upwelling propagates through the ecosystem, from primary production to export and export efficiency. Our findings emphasize the crucial role of the interplay between mixed layer depth and upwelling and suggest that surface chlorophyll may increase, along with a weakened seasonal paradox, in response to shoaling mixed layers under climate change.

Highlights

The Peruvian Upwelling System (PUS) hosts a disproportionally productive ecosystem and supports 10% of the world’s fishing 20 yield while covering only 0.1% of the ocean area (Chavez et al, 2008)
This "seasonal paradox" triggers the questions: (1) what is the uniqueness of the Peruvian Upwelling System compared with other Eastern Boundary Upwelling Systems (EBUS) that leads to the out of phase relationship; 5 (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
For the remainder of the year, in the Canary system enhanced surface nitrate concentrations are positively correlated with chlorophyll, suggesting that phytoplankton is stimulated by enhanced nitrate availability

Summary

Introduction

The Peruvian Upwelling System (PUS) hosts a disproportionally productive ecosystem and supports 10% of the world’s fishing 20 yield while covering only 0.1% of the ocean area (Chavez et al, 2008). The high primary production, together with subsequent export and remineralization in part causes a sub-surface oxygen deficient zone which is shallow and intense in the PUS (Fuenzalida et al, 2009; Stramma et al, 2010; Getzlaff et al, 2016). Via its high productivity, the response of the PUS to climate change is of great 25 social and economic interest (Pauly et al, 1998; Bakun, 1990; Bakun et al, 2010), and a variety of studies have investigated how physical and biogeochemical processes influence the production of phytoplankton and its potential links to ecosystem functioning in the PUS

Results

Discussion

Conclusion