Light limitation of phytoplankton biomass and macronutrient utilization in the Southern Ocean

TLDR

The Antarctic Circumpolar Current has abundant nutrients yet low phytoplankton biomass, a paradox attributed to low temperatures, weak stratification, limited sunlight, and strong winds that constrain growth. The study evaluates whether the ACC could support massive, nutrient‑exhausting phytoplankton blooms using a photophysiological model driven by mixing depth and surface irradiance data. The model incorporates persistent >50 m mixed layers and a minimal phytoplankton loss rate of ~0.2 d⁻¹, simulating growth under observed mixing and irradiance. The model shows phytoplankton would use less than 10 % of available macronutrients, and even large Fe additions would not substantially reduce atmospheric CO₂ without stronger stratification.

Abstract

The Antarctic Circumpolar Current (ACC) is unique in that it has continually high concentrations of major plant nutrients but low phytoplankton biomass. This enigmatic phenomenon is the focus of significant speculation that trace nutrients, including Fe, may limit phytoplankton crop size. Global climatologies indicate that the ACC is a region with low surface temperatures, weak density stratification, little summertime surface solar irradiance, and strong wind stress. These physical phenomena act to limit growth rates of the phytoplankton community. Using a photophysiological description of phytoplankton growth in a simple one-dimensional ecosystem model forced by observations or climatologies of mixing depth and surface irradiance, we make an evaluation of the potential for massive, nutrient-exhausting, phytoplankton blooms forming in the ACC. The ACC has persistent mixed layers in excess of 50 m. Literature values and model optimization indicate that the minimal aggregate specific loss rate for phytoplankton, including respiration, sinking, and grazing, is ∼0.2 d‒1. For a minimal loss rate and typical physical conditions of stratification and surface irradiance, the model predicts that phytoplankton in the ACC would not utilize > 10% of the available macronutrients. Without a mechanism for increasing the strength of stratification, we predict that massive Fe additions to the Southern Ocean would fail to significantly mitigate the atmospheric CO2 derived from fossil fuel.