Spatiotemporal Distribution of Ulva Micropropagules in Surface Sediments: Insights from Field Sampling Data in the Southern Yellow Sea (2020–2022)

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.