Biogeochemical‐Argo floats show that chlorophyll increases before carbon in the high‐latitude Southern Ocean spring bloom

Abstract

In the Southern Ocean, phytoplankton blooms are an annually recurring prominent feature that play a significant role in ocean CO2 uptake. Understanding the timing of the phytoplankton bloom is necessary to provide insights into the underlying physiological drivers, for the study of ecosystem dynamics and consequent patterns in downward carbon export. Previous studies have used chlorophyll (chl) and particulate organic carbon, from either satellites or biogeochemical‐Argo (BGC‐Argo) floats, to investigate bloom phenology, but provide inconsistent findings regarding bloom timing. Here, we compare bloom dynamics based on three diagnostics from 7114 BGC‐Argo float profiles, south of 60°S. Bloom onset consistently occurs earlier when calculated using chl than when based on phytoplankton carbon or nitrate uptake, and the decoupling increases with latitude. This suggests that phytoplankton synthesize increased chl to acclimate to low‐light conditions, before increasing their biomass. These results highlight the importance of considering phytoplankton physiology when choosing proxies for phytoplankton growth.

Plankton blooms over the annual cycle shape trophic interactions under climate change

“…Similarly, zooplankton exhibit multiple phenological patterns with peaks occurring over the seasons (Calbet et al 2001;Winder and Varpe 2020). Yet, most of the plankton phenology studies focus on spring dynamics (Thackeray 2012;Yamaguchi et al 2022;Vives et al 2024) because of their substantial contribution to annual production and well-defined bloom periods (Behrenfeld and Boss 2014;Hjerne et al 2019). However, to predict temporal mismatches across trophic levels, understanding plankton phenology over the annual cycle and their response to the changing environment is critical but often understudied in marine systems.…”