Effects of phosphorus limitation on sinking velocities of phytoplankton during summer in the Changjiang River Estuary

The sinking of phytoplankton is critical to organic matter transportation in the ocean and it is an essential process for the formation of coastal hypoxic zones. This study was based on a field investigation conducted during the summer of 2022 in the Changjiang River (Yangtze River) Estuary (CJE) and its adjacent waters. The settling column method was employed to measure the sinking velocity (SV) of different size fractions of phytoplankton at the surface of the sea and to analyze their environmental control mechanisms. The findings reveal significant spatial variation in phytoplankton SV (−0.55–2.41 m/d) within the CJE. High-speed sinking was predominantly observed in phosphate-depleted regions beyond the CJE front. At the same time, an upward trend was more commonly observed in the phosphate-rich regions near the CJE mouth. The SV ranges for different size-fractionated phytoplankton, including micro- (>20 µm), nano- (2–20 µm), and picophytoplankton (0.7–2 µm), were −0.50–4.74 m/d, −1.04–1.59 m/d, and −1.24–1.65 m/d, respectively. Correlation analysis revealed a significant negative correlation between SV and dissolved inorganic phosphorus (DIP), implying that the influence of DIP contributes to SV. The variations in phytoplankton alkaline phosphatase activity suggested a significant increase in SV across all size fractions in the event of phosphorus limitation. Phytoplankton communities with limited photosynthetic capacity (maximum photochemical efficience, Fv/Fm < 0.3) were found to have higher SV than that of communities with strong capacity, suggesting a link between sinking and alterations in physiological conditions due to phosphate depletion. The findings from the in situ phosphate enrichment experiments confirmed a marked decrease in SV following phosphate supplementation. These findings suggest that phosphorus limitation is the primary driver of elevated SV in the CJE. This study enhances the comprehension of the potential mechanisms underlying hypoxic zone formation in the CJE, providing novel insights into how nearshore eutrophication influences organic carbon migration.