Light‐Driven and Nutrient‐Driven Displacements of Subsurface Chlorophyll Maximum Depth in Subtropical Gyres

Abstract

Abstract The mechanism that determines the dynamics of subsurface chlorophyll maximum depth (zSCM) has long been debated. Although a coupling between zSCM and the top of nitracline (znit) has been widely observed in the open ocean, a co‐location of zSCM and an isolume depth (ziso) has often been reported in oligotrophic waters. In this study, based on continuous observations of ten BGC‐Argo floats, we found that the seasonal displacement of zSCM in all subtropical gyres was driven mainly by light, but zSCM displayed a nutrient‐driven pattern occasionally when znit became shallower than ziso. We therefore proposed a “two‐group competition framework”: zSCM in subtropical gyres is determined by the competition of two phytoplankton groups, nutrient‐sensitive picoeukaryotes and light‐sensitive Prochlorococcus. When znit (ziso) is shallower than ziso (znit), picoeukaryotes (Prochlorococcus) dominate the chlorophyll biomass at the SCM, and thus zSCM follows znit (ziso). This paradigm reconciles the inconsistent conclusions drawn from earlier studies.

Keywords

• subsurface chlorophyll maximum depth
• seasonal cycle
• subtropical gyres
• isolume
• nutricline
• picoplankton