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

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. Phytoplankton growth requires both light and nutrients. In most open oceans, the vertical distribution of the chlorophyll-a concentration generally is a maximum at a subsurface depth because of nutrient depletion in surface waters and inadequate light at depth. We took advantage of the continuous observations from autonomous profiling floats of the chlorophyll maximum depth in oligotrophic subtropical oceans. Our analysis of those data showed that the depth of the chlorophyll maximum depth in these areas usually followed either the depth where there was enough light for phytoplankton to grow or the depth where there were enough nutrients for them to grow. This discovery suggests that two different groups of phytoplankton determine where the chlorophyll maximum appears. One group is sensitive to nutrient supply and responds more quickly to variations of nutrient supply than of light, and the other group is sensitive to light availability and responds more quickly to variations of light than of nutrient supply. The depth of the chlorophyll maximum is determined by the interaction between light and nutrient availability, which determines which group dominates. The displacement of subsurface chlorophyll maximum (SCM) depth in subtropical gyres is mainly driven by the isolume depth, which is generally above the top of nitraclineThe top of nitracline impacts the SCM depth when it occasionally becomes above the isolume depth, displaying a nutrient-driven SCM patternThe dynamics of SCM depth are likely due to the competition of light-sensitive Prochlorococcus and nutrient-sensitive picoeukaryotes

Automated summary

The dynamics of subsurface chlorophyll maximum depth (zSCM) are mainly driven by light in subtropical gyres, but occasionally display a nutrient-driven pattern when the top of nitracline (znit) becomes shallower than an isolume depth (ziso). This study proposes a two-group competition framework where zSCM is determined by the dominance of nutrient-sensitive picoeukaryotes or light-sensitive Prochlorococcus. This reconciles previous inconsistent conclusions and highlights the importance of the interaction between light and nutrient availability.