Progressive eutrophication behind the world-largest super floating macroalgal blooms in the Yellow Sea

Abstract. World-largest super floating macroalgal blooms of Ulva prolifera have lasted 7 years by now in every summer in the Yellow Sea, the outer part of a semi-enclosed coastal sea. Evaluation of the inter-annual variability in the trophic status is one of fundamental tasks for prediction and management of the blooms. We show the new findings of a progressive eutrophication in the large Yellow Sea basin behind the super floating macroalgal blooms. The inter-annual variability in human-induced nutrient pollution from 2001 to 2012 was assessed by a nutrient pollution index weighted by area (AWCPI-NP), and a significant increase in the trophic status was found in the macroalgal bloom (MAB) phase (2007–2012): the average AWCPI-NP was higher than that in the pre-MAB phase (2001–2006) by about 45%; meanwhile, in the Jiangsu Shoal, origin place of the drifting macroaglae, the annual in-situ nutrient concentrations increased rapidly from 2000 to 2011. Chlorophyll a concentration (Chl a), an indicator of eutrophication, increased by about 15% in the bloom region from the pre-MAB to MAB phase. This progressive eutrophication might lead to the non-linear outburst in the growth of macroalgae, i.e., green tides, in the Yellow Sea since 2007.