Global Patterns of Marine Phytoplankton Pigment Package and Composition Influencing Light Absorption

Abstract

Phytoplankton are fundamental to marine ecosystems, influencing global biogeochemical cycles and climate. A key to understanding their abundance and function lies in their light absorption properties (a_ph), which are primarily governed by two intrinsic factors: the package effect (Q_a*, the efficiency loss in specific absorption caused by cellular self-shading) and pigment composition (a_pig*). However, our understanding of the non-linear bio-optical relationships between a_ph, a_pig*, and Q_a* has been largely constrained by localized studies, leaving a significant gap in our knowledge of their dynamics and interactions across the global ocean. This study aims to quantify the global-scale (60°S–60°N) causal relationships between a_ph and its drivers, Q_a* and a_pig*, based on 22 years of satellite data by refining the bio-optical models. Our results reveal a latitudinal structuring of influence: the package effect was the dominant driver of a_ph variability in mid-latitudes, contributing 40%–50% of the changes in the Atlantic, while its influence was minimal in tropical regions. Pigment composition acted to reduce uncertainty in a_ph, particularly in subtropical zones. Overall, the package effect demonstrated a more active and significant interaction with phytoplankton absorption than pigment composition on a global scale. This work provides a global causal perspective on the mechanisms controlling phytoplankton optical properties. It moves beyond local correlations to reveal a coherent latitudinal pattern in bio-optical control. By elucidating these global latitudinal causal patterns, this study provides a mechanistic basis for refining regionally parameterized bio-optical schemes in Earth System Models, and enhances the interpretation of satellite ocean color data.