Seasonal mixed layer depth shapes phytoplankton physiology, viral production, and accumulation in the North Atlantic

Abstract

Seasonal shifts in phytoplankton accumulation and loss largely follow changes in mixed layer depth, but the impact of mixed layer depth on cell physiology remains unexplored. Here, we investigate the physiological state of phytoplankton populations associated with distinct bloom phases and mixing regimes in the North Atlantic. Stratification and deep mixing alter community physiology and viral production, effectively shaping accumulation rates. Communities in relatively deep, early-spring mixed layers are characterized by low levels of stress and high accumulation rates, while those in the recently shallowed mixed layers in late-spring have high levels of oxidative stress. Prolonged stratification into early autumn manifests in negative accumulation rates, along with pronounced signatures of compromised membranes, death-related protease activity, virus production, nutrient drawdown, and lipid markers indicative of nutrient stress. Positive accumulation renews during mixed layer deepening with transition into winter, concomitant with enhanced nutrient supply and lessened viral pressure.

Both Dissolved Oxygen and Chlorophyll Explain the Large-Scale Longitudinal Variation of Deep Scattering Layers in the Tropical Pacific Ocean

“…In addition, considering the continuous variation in the marine environment, this study also identifies a broad transition zone. The primary cause of this longitudinal environmental gradient in Pacific Ocean is biological oceanographic and biogeochemical mechanisms controlled by physical oceanographic differences, such as seasonal variation of mixed layer depth and transport of water masses (Amos et al, 2019;Diaz et al, 2021), but this environmental gradient is further reinforced by subtle biological processes, such as basin-scale distribution of nitrogen-fixing organisms (Cheung et al, 2020).…”

Seasonal Variability of Phytoplankton Biomass Revealed by Satellite and BGC‐Argo Data in the Central Tropical Indian Ocean

“…At the base of the mixed layer, water column stability also affects the flux of nutrients to the surface layer (Carvalho et al, 2017). Previous studies have shown vertical mixing and phytoplankton biomass are consistent with the critical depth at which photosynthesis occurs (Behrenfeld, 2014;Diaz et al, 2021). Vertical mixing also affects grazing by diluting micro-grazers along with phytoplankton (Marra & Barber, 2005).…”

“…Highly phytoplankton biomass in the subsurface layer in the CIO is the result of a continuous nutrient input into the euphotic zone due to the thermocline shoaling generated by wind‐driven upwelling (Jyothibabu et al., 2015; Thushara et al., 2019). In addition, wind‐driven turbulent mixing process forms an upper‐density layer known as the surface mixed layer, which plays a vital role in explaining the physical processes driving phytoplankton blooms in the upper ocean (Behrenfeld, 2014; Diaz et al., 2021). Mixed‐layer depth (MLD) is thus a central metric commonly studied for understanding phytoplankton variability (Anju et al., 2020; Carvalho et al., 2017).…”

“…Mixed‐layer depth (MLD) is thus a central metric commonly studied for understanding phytoplankton variability (Anju et al., 2020; Carvalho et al., 2017). Variations in the MLD, in general, are due to the input of turbulent kinetic energy by air‐sea fluxes related to surface cooling and strengthened surface winds (Diaz et al., 2021; Liao et al., 2017; Madhupratap et al., 1996). At the base of the mixed layer, water column stability also affects the flux of nutrients to the surface layer (Carvalho et al., 2017).…”

“…Vertical mixing also affects grazing by diluting micro‐grazers along with phytoplankton (Marra & Barber, 2005). Seasonal variability of MLD could contribute to the shifts in phytoplankton accumulation and loss (Behrenfeld, 2014; Diaz et al., 2021). Particularly, the deepening of the mixed layer can enhance upward fluxes of nutrients within the euphotic layer, resulting in a positive accumulation in the water column (Anju et al., 2020; Behrenfeld, 2014).…”