Density-dominated aggregation and spatiotemporal differentiation of yellow sea green tides revealed by long term observations

Green tides dominated by Ulva prolifera have bloomed in the Yellow Sea since 2008, posing persistent threats to coastal ecosystems. While multiple indicators (including bloom extent, pixel area, coverage area, and biomass) have been used to characterize bloom dynamics, their inconsistent use may obscure assessments of bloom intensity and spatiotemporal variability, leaving their variations across spatial perspectives poorly understood and limiting their practical utility for green tide management. Here, we investigate how the interannual dynamics of Yellow Sea green tides during 2008–2024 differ when evaluated in terms of biomass rather than bloom extent, and the spatial patterns underlying these differences. The annual biomass variability peaked at 3.6 times the long-term average, exceeding the 2.0 times observed for bloom extent. This indicates increasingly dense and aggregated U. prolifera distributions and an underestimation of interannual bloom intensity by bloom extent. In the Subei Bank Front (SBF) perspective, MODIS observations show that U. prolifera first appear near the SBF in early May. U. prolifera biomass in the northern region closely tracked annual biomass across the Yellow Sea, making it the dominant contributor to interannual bloom variability, while the southern region accounted for only 17 %. From the nearshore perspective (within 20 km of the coastline), nearshore green tide biomass has largely remained below the 2008 level (49.90 kt) over the past 16 years, except in 2015 (53.54 kt) and 2019 (70.33 kt), which exceeded it by 7 % and 41 %. The cross-validation identified the annual biomass series excluding the 20 km coastal zone and the region north of the SBF as robust proxies for annual mean biomass under data gaps, with the former yielding a substitution error of 8 %. These findings provide critical quantitative evidence for Yellow Sea green tide monitoring and sustainable coastal ecosystem management in support of SDG 14.2.