For nearly two decades, the Yellow Sea has experienced recurrent green tides, which are now considered the largest of their kind globally; the mechanism behind these outbreaks remains highly complex and not fully understood. This study investigates the pre-outbreak distribution, abundance, and species composition of suspended macroalgae in the Southern Yellow Sea (SYS) during 2023–2024, along with environmental parameters. The results indicate that suspended macroalgae were predominantly distributed in the nearshore waters, particularly along the shallow beaches of northern Jiangsu. Furthermore, their abundance in the surface water layer significantly exceeded that in the bottom water. A total of 1353 and 493 algal filament samples were collected in 2023 and 2024, respectively. Dominant species included Ulva prolifera, Ulva linza, Ulva flexuosa, and Blidingia sp. Nutrient levels positively correlated with filament abundance. As a primary means of rapid proliferation for U. prolifera, suspended macroalgae contribute significantly to the initial expansion of green tides. Furthermore, their abundance holds promise as a biological indicator for forecasting the scale and extent of impending blooms, thereby providing a critical foundation for elucidating the underlying outbreak mechanisms.

Large-scale green tides dominated by Ulva species have recurred annually in the Southern Yellow Sea for nearly two decades, yet early detection remains challenging due to the patchy distribution of incipient floating macroalgae. This study investigated the spatiotemporal dynamics of Ulva micropropagules during the 2020 outbreak using a systematic cultivation assay. Seawater samples were collected from 23 stations across the Subei Shoal and adjacent waters in April, May, and July, and incubated under controlled laboratory conditions to enumerate Ulva germling densities. Results revealed that Ulva micropropagule abundance peaked in April, with high-density foci concentrated in the Subei Shoal region—particularly in aquaculture areas of Neopyropia J. Brodie & L.-E. Yang, 2020—confirming this zone as one of the important sources. Abundance declined progressively through May and July as macroalgae drifted northward under wind and current forcing. This method effectively identified putative source regions and reconstructed initial dispersal patterns prior to satellite-detectable macroalgal aggregation. These findings demonstrate that Ulva micropropagule monitoring provides a cost-effective, sensitive tool for early warning and Ulva source tracking, offering finer-scale propagule distribution data to inform precision management strategies for mitigating green tide impacts on coastal marine ecosystems. Future research should expand investigations into Ulva micropropagule dynamics to elucidate their mechanistic processes and ecological significance in green tide initiation and development.

Green tide disasters have erupted consecutively in the Southern Yellow Sea, yet their driving mechanisms remain debated. The “raft-origin-hypothesis” identifies the source as epiphytic Ulva macroalgae attached to laver aquaculture rafts in the Subei Shoal, whereas the “micropropagule-hypothesis” speculates that Ulva micropropagules settling into open sea sediments constitute a cross-year seed reserve. The latter has hitherto relied primarily on laboratory culture simulations, with field-scale assessment lacking. This study established 46 surface sediment survey stations from 2020 to 2022, covering periods before, during, and after green tide outbreaks. Surface sediments were obtained using a box corer, and Ulva micropropagules were quantitatively analyzed through laboratory cultivation and morphological identification. Results showed that micropropagule abundance was generally low and exhibited spatial heterogeneity; high-value areas persistently existed in the Subei Shoal aquaculture zone, extended into the open sea during outbreaks, and generally disappeared in the open sea during and after the decline period. Monthly variations indicated that micropropagules released and settling into sediments (e.g., in the open sea) during large-scale outbreaks could not survive until the following year’s floating period; the initial biomass of subsequent eruptions did not originate from the revival of the seafloor “seed source” in the open sea, which remains speculative.