Abstract
Abstract. 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 “seasonal paradox” triggers the following questions: (1) what are the unique characteristics of the Peruvian upwelling system, compared with other EBUSs, that lead to the out-of-phase relationship, and (2) 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 (CROCO–BioEBUS) 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 “seasonal paradox” within 200 km 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, supporting 10 % of the world’s fishing yield while covering only 0.1 % of the ocean area (Chavez et al, 2008)
This study addresses the following key questions: (1) what are the unique characteristics of the PUS, compared to other eastern boundary upwelling system (EBUS), that lead to this seasonal paradox; (2) what are the mechanisms that cause low surface phytoplankton in winter; and (3) how do these mechanisms affect ecosystem functioning?
3.1 Anticorrelation of chlorophyll and upwelling: the seasonal paradox only appears in the Peruvian upwelling system
Summary
The Peruvian upwelling system (PUS) hosts a disproportionally productive ecosystem, supporting 10 % of the world’s fishing yield while covering only 0.1 % of the ocean area (Chavez et al, 2008). As one of the eastern boundary upwelling systems (EBUSs), winds favouring upwelling raise cool, nutrient-rich waters to the surface, supporting high primary production and fish yield. High primary production, together with subsequent export and remineralization, contributes to the formation of 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).
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