Phytoplankton package effect in oceanic waters: Influence of chlorophyll-a and cell size.

In this study, the interaction between the packaging effect (Q) and total chlorophyll-a concentration (C) or total size index (SI) was investigated to explore the potential bio-optical mechanism in phytoplankton cells in the global oceans. In addition, the long-term spatiotemporal characteristics of these interactions were necessary for grasping their variation. Numerous in situ surface measurements (phytoplankton pigment and absorption coefficients) from the global oceans were analyzed first, and then correlation and causality analyses were performed on the satellite-deduced Q, C, and SI in the global oceans during 2002-2020. The results show a negative correlation between Q and C or SI in the low latitudes (30°S-30°N) and a positive correlation in the middle latitudes (30°S-55°S and 30°N-55°N). The causality analysis reveals a mutual and asymmetric cause-effect relationship between Q and C or SI in the low latitudes. The stabilization effect of Q contributes to a 10%-50% variation in C and SI, with 40%-60% uncertainty of Q caused by C and SI in the low latitudes, which is inverse in the middle latitudes. The remaining contribution to each variable mainly originates from long-term trends and noise. Combining the analysis between Q and the irradiance, the balancing processes in phytoplankton cells are different in the low (phytoplankton-driving mode) and middle latitudes (irradiance-driving mode), which is related to photoacclimation and photoinhibition. The analyses provide insights into the quantitative interpretation of the relationship between Q and C or SI, which contribute knowledge that has not been previously reported.