Interpreting the progressive eutrophication behind the world's largest macroalgal blooms with water quality and ocean color data

World's largest macroalgal blooms (MAB) caused by Ulva prolifera outbreak every summer in the Yellow Sea since 2007, which bring potential damage to regional marine environment and economics. Assessing the evolution of historic eutrophication status is of importance for exploring the reasons for the blooms. In this study, we used an area-weighted nutrient pollution index (AWCPI-NP) and ocean color data to assess temporal changes in eutrophication status in the Yellow Sea basin. Results show that significant shifts occurred from the pre-MAB phase (2001-2006) to MAB phase (2007-2012): the average AWCPI-NP was higher than that in the pre-MAB phase (2001-2006) by about 45 %; chlorophyll a concentration, an indicator of eutrophication, increased by about 15 % in the region of MAB. Meanwhile, in the Jiangsu Shoal, the origin place of the floating macroalgae, the annual in situ nutrient concentrations increased rapidly from 2000 to 2011. The result obtained through our approach highlights that the MAB in the Yellow Sea might be primarily linked to the progressive eutrophication. In order to reduce this hazard, policy makers should include this new knowledge for an effective countermeasure planning.