Oceanic Fronts Structure Phytoplankton Distributions in the Central South Indian Ocean

Abstract

In the South Indian Ocean (SIO), high surface chlorophyll concentrations are often observed in mesoscale anticyclonic eddy cores. Yet the role of submesoscale dynamics in modulating phytoplankton distributions remains largely unknown in the region. By using a biogeochemical‐Argo float and a high‐resolution model, we show that elevated chlorophyll concentrations were evident in a frontal region, and the depth‐integrated chlorophyll in this frontal region can be higher than that in the anticyclonic eddy core. Submesoscale frontogenesis was shown to inject nutrients vertically and enhance phytoplankton growth in the frontal region. Frontal dynamics tend to facilitate large phytoplankton such as diatom growth, which consequently increases diatom fraction. Over the entire central SIO, we found a significant correlation between the phytoplankton composition index and frontogenesis. However, the correlation between total phytoplankton biomass and frontogenesis was not significant. These results demonstrate the complex role of frontal dynamics in structuring phytoplankton distributions in the central SIO.

The Impact of Fine-Scale Currents on Biogeochemical Cycles in a Changing Ocean

“…First, eddy transport may directly affect mean reactive processes ⟨BC ⟩. For instance, the strength of the eddy nutrient supply affects the structuring of the planktonic ecosystem, with certain plankton species favored and others not (Mangolte et al 2022, Guo et al 2022; the modified structuring may affect, in turn, the export efficiency (Treguer et al 2018, Serra-Pompei et al 2022. Another example is the feedback between the phytoplankton growth rate and the eddy vertical nutrient flux (Freilich et al 2022), as the former sets the vertical gradient on which the latter acts.…”

Efficient biological carbon export to the mesopelagic ocean induced by submesoscale fronts

“…The upward isopycnal displacement during a CE intensifying stage can increase nutrient supply and primary production 25 – 28 , enhancing POC sinking in subtropical gyres (referred to as the biological gravitational pump) 29 – 31 . Due to the increase in the lateral buoyancy gradient 32 , intensifying submesoscale fronts are often accompanied by vertical velocities as high as 10–100 m d −1 , which is an order of magnitude greater than the vertical velocities induced by mesoscale eddies 14 , 15 . These vertical motions can inject high quantities of POC produced in the euphotic zone into the mesopelagic layer (referred to as subduction pump) 17 , 19 , 22 , 24 , resulting in carbon sequestration 17 , 19 , 20 , 23 .…”

“…These vertical motions can inject high quantities of POC produced in the euphotic zone into the mesopelagic layer (referred to as subduction pump) 17 , 19 , 22 , 24 , resulting in carbon sequestration 17 , 19 , 20 , 23 . The vertical motions induced by submesoscale fronts may also drive the vertical nutrient supply and stimulate the growth of large phytoplankton communities 32 – 36 ; these large phytoplankton may sink fast and affect the carbon export efficiency by creating particles with fast sinking velocities through food web dynamics 16 , 36 , 37 . At the global scale, submesoscale fronts have the potential to contribute to POC export 38 , 39 , which would help to reconcile the imbalance between the upper layer carbon supply and deep ocean carbon consumption 9 , 17 , 40 .…”

Modelling the influence of submesoscale processes on phytoplankton dynamics in the northern South China Sea

“…Nutrients upwelled from subsurface can be taken up by different phytoplankton species that are further modulated by ecosystem dynamics and other biogeochemical processes, which are all subject to different temporal scales (Levy et al, 2018). The short time scales associated with strong submesoscale vertical nutrient injections and the biological response time favour dominance by the large phytoplankton size class (diatoms) of the model (Figure 12; Levy et al, 2012;Guo et al, 2022). Different physical dynamics between mesoscale and submesoscale pr oc e s se s c a n th us le a d t o sp at ial an d t em p ora l heterogeneities in community structure and ecosystem dynamics (d 'Ovidio et al, 2010;Levy et al, 2012;Clayton et al, 2014;Mousing et al, 2016;Clayton et al, 2017).…”

“…Moreover, the intensity of nutrient injection induced by submesoscale processes is also determined by the background nutricline depths that are set by mesoscale or large-scale processes. Consequently, regions with high physical straining and stretching are not always associated with enhanced total chlorophyll concentration (e.g., Guo et al, 2022).…”