Growth phase-linked Sargassum-associated DOM structures microbial succession and carbon-nutrient coupling in a...

Benthic Sargassum canopies influence overlying waters by releasing growth phase-linked dissolved organic matter (DOM) that interacts with microbes and phytoplankton. We investigated nutrient stoichiometry, fluorescent DOM (FDOM) and chromophoric DOM (CDOM), extracellular enzyme activities (EEAs), and microbial assemblages across initial growth (March), peak growth (May), and senescence (June) in a Weizhou Island (Sargassum bed) system with limited terrestrial input. At peak growth, dissolved inorganic nitrogen (DIN) and phosphorus (DIP) reached seasonal minima, with DIP especially scarce, while dissolved silicate (DSi) was relatively elevated; protein-like DOM increased, BIX rose and HIX and SUVA₂₅₄ declined, and EEAs were higher, aligning with increases in Bacteroidia and Alphaproteobacteria and stronger nano-phytoplankton and Coscinodiscophyceae signals. In senescence, DIN and DIP rebounded and humic-like signatures increased, consistent with higher micro-sized chlorophyll a and a resurgence of Mamiellophyceae. These results suggest that growth phase-linked DOM helps structure bacterial succession, phytoplankton composition, and carbon-nutrient coupling in nearshore waters, informing ecosystem models and blue-carbon management.