On the recurrent Ulva prolifera blooms in the Yellow Sea and East China Sea

Abstract

A massive bloom of the green macroalgae Ulva prolifera (previously known as Enteromorpha prolifera) occurred in June 2008 in the Yellow Sea (YS), resulting in perhaps the largest “green tide” event in history. Using a novel index (Floating Algae Index) and multiresolution remote sensing data from MODIS and Landsat, we show that U. prolifera patches appeared nearly every year between April and July 2000–2009 in the YS and/or East China Sea (ECS), which all originated from the nearshore Subei Bank. A finite volume numerical circulation model, driven by realistic forcing and boundary conditions, confirmed this finding. Analysis of meteorological/environmental data and information related to local aquaculture activities strongly supports the hypothesis that the recurrent U. prolifera in the YS and ECS resulted from aquaculture of the seaweed Porphyra yezoensis (or nori) conducted along the 200 km shoreline of the Subei Bank north of the Changjiang (Yangtze) River mouth. Given the continuous growth in aquaculture efforts in the region, similar macroalgae bloom events, such as the summer 2008 event, are likely to occur in the future, particularly between May and July. This was confirmed by the 2009 bloom event in the same regions and the same period. The profit of the local P. yezoensis aquaculture industry (∼16,000 Ha in 2007) is estimated as U.S. $53 million, yet the cost to manage the impact of the summer 2008 U. prolifera bloom exceeded U.S. $100 million. Therefore, better strategies are required to balance the economic benefit of seaweed aquaculture and the costs of environmental impacts.

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

“…In essence, the visually-determined algae slicks were used as the truth. Note that similar to those demonstrated in Hu et al (2010), although the total number of Ulva-containing pixels is sensitive to the choice of FAI L , the CoPA after linear unmixing is not. Once Class 1 and Class 2 pixels were determined, the remaining pixels in an image were Class 3 pixels.…”

“…This region was selected because most of the Ulva macroalgae blooms occurred in this region (Hu et al, 2010;Xing et al, 2015a). All relevant data were selected to cover this region as well.…”

“…1) for all summer months (May-August) between 2007 and 2015 were obtained from the NASA Goddard Space Flight Center (OBPG, http://oceancolor.gsfc.nasa.gov). No data between 2000 and 2006 was obtained because previous studies (Hu et al, 2010;Xing et al, 2015a) showed no blooms in other months or other years for the western YS. Following Hu (2009), the data were processed using the SeaWiFS Data Analysis System software (SeaDAS, Version 7.0) to derive the Rayleighcorrected reflectance, R rc (l), in the spectral bands of 469, 555, 645, 859 and 1240 nm for each image pixel.…”

“…Numerous studies have documented their growth cycles, seasonality, transport patterns, and their linkage with local aquaculture, eutrophication, and ocean circulations (Hu et al, 2010;Liu et al, 2009Liu et al, , 2013Wang et al, 2015). However, to date, although a number of studies used remote sensing to estimate their spatial distributions and temporal changes, nearly all focused on how to effectively detect the presence of the macroalgae (Cui et al, 2012;Hu, 2009;Hu et al, 2010;Keesing et al, 2011;Lee et al, 2011;Garcia et al, 2013;Bao et al, 2015;Xing and Hu, 2016). Several studies used individual images to show the maximum coverage during the bloom season or the maximum impacted water area to show the severity (Keesing et al, 2011;Hu et al, 2010;Garcia et al, 2013;Xu et al, 2014;Xing et al, 2015a,b).…”

“…However, to date, although a number of studies used remote sensing to estimate their spatial distributions and temporal changes, nearly all focused on how to effectively detect the presence of the macroalgae (Cui et al, 2012;Hu, 2009;Hu et al, 2010;Keesing et al, 2011;Lee et al, 2011;Garcia et al, 2013;Bao et al, 2015;Xing and Hu, 2016). Several studies used individual images to show the maximum coverage during the bloom season or the maximum impacted water area to show the severity (Keesing et al, 2011;Hu et al, 2010;Garcia et al, 2013;Xu et al, 2014;Xing et al, 2015a,b). To our best knowledge there has been little attempt to quantify the Coverage of Pure Algae (CoPA, i.e., waters fully covered by algae without gaps) from coarse-resolution satellite measurements, not to mention documenting the shortterm changes and long-term trend of CoPA in a statistically meaningful way.…”