Macroalgal influence on phytoplankton sinking and decomposition: Unraveling the carbon flux dynamics in the Sargassum zone.

Tian, Xiaofei; Ye, Yufeng; Li, Yucheng; Wang, Yihang; Yang, Xiaolong; Tang, Feng; Zhang, Xiumei

Recent Sargassum horneri blooms, altering marine ecosystems, highlight an urgent need to understand their impact on phytoplankton and the regulation of organic matter export from the euphotic zone. To this end, we examined the effects of floating Sargassum on the phytoplankton size composition and their rates of sinking and lysis in the area of floating Sargassum horneri near the Zhoushan archipelago. We found that the presence of floating Sargassum horneri significantly influences the marine ecosystem by increasing the proportion of pico-sized phytoplankton, indicating community miniaturization. This shift suggests a novel mechanism by which floating macroalgae may alter the structure and function of phytoplankton communities. Our findings also indicate that these macroalgae enhance the sinking and lysis rates of phytoplankton, which could have profound effects on the vertical transport of organic matter and biogeochemical cycling in the ocean. The potential aggregation of pico-sized phytoplankton may facilitate their sinking in the surface layer, possibly leading to a reduction of lysis rates because of competition with floating seaweed. Conversely, in deeper layers, the decomposition of phytoplankton could result in the formation of aggregates of pico-sized phytoplankton, accelerating their sedimentation. These results highlight the complex interactions between phytoplankton size, lysis and sinking rates, and underscore the importance of considering these dynamics in understanding carbon export and sequestration in marine environments.

EUPHOTIC zone; CARBON sequestration; MARINE ecology; BIOGEOCHEMICAL cycles; SARGASSUM

Marine Biology, 2025, Vol 172, Issue 1, p1

0025-3162

Academic Journal

10.1007/s00227-024-04574-4