Ship-based eDNA tracking unveils early dispersal patterns and microecological dynamics of Ulva prolifera micropropagules in yellow sea green tide outbreaks

Green tides caused by Ulva prolifera in the South Yellow Sea (SYS) pose significant ecological and economic threats. Neopyropia aquaculture rafts in the Subei Shoal are considered to be the main source of green tides in the SYS, where micropropagules are the ‘Seed Bank’ for green tides. But the early dynamics of the U. prolifera remain unclear due to the limitations of conventional monitoring methods. This study systematically investigated the spatiotemporal distribution characteristics and key environmental driving mechanisms of environmental DNA (eDNA) from U. prolifera in development areas of Yellow Sea green tide, using eDNA technology combined with environmental parameters. Surveys in April and June 2024 across Neopyropia raft-farming (RD transect) and non-raft (SY transect) areas revealed ecological differences during early green tide development. Key findings include:(1) In April, U. prolifera eDNA abundance in RD exhibited a "high near rafts, low farther away" distribution, regulated by temperature and total nitrogen, while SY followed similar trends driven primarily by temperature. No environmental factors were found to be significantly correlated with the distribution of U. prolifera eDNA in June. (2) High U. prolifera eDNA abundance at SY4 in April was driven by hydrodynamics (tidal asymmetry), temperature, and biological interactions (e.g., Cercozoa), identifying a hotspot for monitoring. (3) Raft-farming areas exhibited stronger species interactions, with Chlorophyta and Cercozoa closely linked to U. prolifera. The identification of Pirsonia sp. and other species provides a new perspective for elucidating the competitive mechanisms underlying early-stage U. prolifera outbreaks and for developing early biological control strategies against green tides. This study provides molecular ecological evidence for analyzing the aggregation driving mechanism of the development areas of early green tide in the SYS, highlights the utility of eDNA in monitoring, and offers theoretical support for marine ecosystem management under climate change.