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

Abstract

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( ) to MAB phase (2007: the average AWCPI-NP was higher than that in the pre-MAB phase (2001)(2002)(2003)(2004)(2005)(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.

Long-term trend of Ulva prolifera blooms in the western Yellow Sea

“…Overall, maximum daily CoPA (Table 1) is significantly lower than reported in Hu et al (2010) because of the much higher upper bound used in pixel unmixing in this study, and also significantly lower than all previously reported coverage when all algaecontaining pixels were treated the same (e.g., Xing et al, 2015b; Table 1). Interestingly, the ratio of CoPA to coverage of algaecontaining pixels appears to be separated for low and higher coverage: 5-6% when CoPA is <160 km 2 but 10-11% when CoPA is >500 km 2 .…”

“…After 2008, some moderate blooms (i.e., >10% but <50% in the maximum daily CoPA referenced against maximum daily CoPA in 2008, Table 1) were found between 2009 and 2012, after which the bloom area increased annually to reach a maximum in 2015. Indeed, in 2015, the annual mean CoPA reached 116 km 2 , representing the largest Ulva bloom in this region (Xing et al, 2015b;SOA, 2016) and almost 4 times of the annual bloom size in 2008. However, in coastal waters near Qingdao (red circles in Fig.…”

“…Water samples along China coasts have been collected regularly by SOA, and the marine environmental parameters such as TIN and PO 4 -P are determined from water samples according to China's standard marine sampling and measuring protocols (GB17378.4-1998) (as presented by Xing et al, 2015b). These water sample data were then used to determine water quality levels and to estimate the area of polluted waters.…”

“…These water sample data were then used to determine water quality levels and to estimate the area of polluted waters. On the basis of these data, an Area-Weighted Composite Pollution Index of Nitrogen and Phosphorus (AWCPI-NP) was proposed and used to quantify the water quality; subsequently the inter-annual variations in nutrient pollution level of a given region can be assessed with AWCPI-NP (Xing et al, 2015b).…”

“…Among all environmental factors that could possibly influence Ulva amount, the following have been well documented by a number of studies: seaweed aquaculture size and production (which may determine the amount of Ulva initially released to the ocean), temperature, light, wind, and nutrient availability (Liu et al, 2009Hu et al, 2010;Keesing et al, 2011;Xing et al, 2015b).…”