Phytoplankton Package Effect in Oceanic Waters : Influence of Chlorophyll a and Cell Size

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