Assessing Phytoplankton Primary Productivity Variability in the Changjiang Estuary, East China Sea From Coupled Fast Repetition Rate (FRR) Fluorometry and Chlorophyll-a Measurements

Phytoplankton primary productivity (PP) varies significantly over environmental gradients, particularly in physically-dynamic systems such as estuaries and coastal seas. During summer, runoff peaks in the Changjiang River driving large environmental gradients in both the Changjiang estuary and adjacent East China Sea (ECS), likely driving significant variability in PP. As satellite models of PP often underperform in coastal waters, we aimed to develop a novel approach for assessing net PP variability in such a dynamic environment. Parallel in situ measurements of Fast Repetition Rate (FRR) fluorometry and carbon (C) uptake rates were conducted for the first time in this region during two summer cruises in 2019 and 2021. A series of C-13-incubations (n = 31) were performed, with measured PP ranging from similar to 6 to 1,700 mgC m(-3) d(-1). Net PP values were significantly correlated with salinity (r = 0.45), phytoplankton chlorophyll a (Chl-a, r = 0.88), Photosystem II (PSII) functional absorption cross-section (sigma(PSII), r = -0.76) and maximum PSII quantum yield (F-v/F-m, r = 0.59). Stepwise regression analysis showed that Chl-a and sigma(PSII) were the strongest predictors of net PP. A generalized additive model (GAM) was also used to estimate net PP considering nonlinear effects of Chl-a and sigma(PSII). We demonstrate that GAM outperforms linear modeling approaches in estimating net PP in this study, as evidenced by a lower root mean square error (similar to 140 vs. 250 mgC m(-3) d(-1)). Our novel approach provides a valuable tool to examine carbon cycling dynamics in this important region.