Dissipation of Ulva prolifera green tides across various spatial and temporal scales and the short-term effects on marine environments

The floating Ulva prolifera green tides have been observed annually in the South Yellow Sea (SYS) since 2007, and most researches indicate that Ulva prolifera typically appears near the Jiangsu Shoal in May, then floats and spreads northward, and eventually millions of tons of Ulva prolifera (4539600 t in 2021) accumulates and disappeared offshore of the Shandong Peninsula. The decomposition of Ulva prolifera in this region releases substantial amounts of nutrients and organic matter, which significantly affect the coastal ecosystem. To study the extinction of green tides on the environment, this study examines environmental parameters across three characteristic areas and analyzed the impact of Ulva prolifera death on the coastal seawater of Qingdao city from 2017 to 2022. The findings indicated that while green tides outbreaks do not significantly affect the annual nitrogen and phosphorus budgets of the entire SYS, while have notable impacts in regions where Ulva prolifera decays. The nutrients released during Ulva prolifera decay promote phytoplankton growth, increased the mean chlorophyll-a concentration from 1.45 μg/L to 2.85 μg/L during May to August in the Qingdao coastal area. Nutrient concentrations in the Qingdao coastal waters increased after the decay of Ulva prolifera and its’ associated phytoplankton. Dissolved inorganic nitrogen and phosphate concentration increased from 3.89 to 6.44 μmol/L and 0.17–0.20 μmol/L during August to October in the Qingdao coastal waters, respectively. This finding indicates that Ulva prolifera decay could cause short-term effects on the marine environment especially in nutrient pools.