Abstract
The world’s largest macroalgal blooms, Ulva prolifera, have appeared in the Yellow Sea every summer on different scales since 2007, causing great harm to the regional marine economy. In this study, the Normalized Difference of Vegetation Index (NDVI) index was used to extract the green tide of Ulva prolifera from MODIS images in the Yellow Sea in 2016–2018, to investigate its spatiotemporal patterns and to calculate its occurrence probability. Using the standard deviational ellipse (SDE), the morphological characteristics of the green tide, including directionality and regularity, were analyzed. The results showed that the largest distribution and coverage areas occurred in 2016, with 57,384 km2 and 2906 km2, respectively and that the total affected region during three years was 163,162 km2. The green tide drifted northward and died out near Qingdao, Shandong Province, which was found to be a high-risk region. The coast of Jiangsu Province was believed to be the source of Ulva prolifera, but it was probably not the only one. The regularity of the boundary shape of the distribution showed a change that was opposite to the variation of scale. Several sharp increases were found in the parameters of the SDE in all three years. In conclusion, the overall situation of Ulva prolifera was still severe in recent years, and the sea area near Qingdao became the worst hit area of the green tide event. It was also shown that the sea surface wind played an important part in its migration and morphological changes.
Highlights
In 2007, a small-scale green tide formed by Ulva prolifera was observed for the first time in the coastal area of Qingdao, China [1]
In 2018, due to continuous thick cloud cover, the Moderate Resolution Imaging Spectroradiometer (MODIS) images of the Yellow Sea region in May were deleted during the screening stage
The earliest time and place of U. prolifera could not be accurately determined by interpretation of the MODIS images
Summary
In 2007, a small-scale green tide formed by Ulva prolifera was observed for the first time in the coastal area of Qingdao, China [1]. Ulva prolifera has been confirmed as the only dominant species in the Yellow Sea green tides, by both morphology and molecular analyses [6,7,8]. Remote sensing (RS) technology has the advantage of fast data acquisition, large coverage, and dynamic monitoring. It can monitor the origin, spatial distribution, dynamic process, scale, and migration of green tides in real time. It has become one of the irreplaceable means for green tide monitoring and early warning. A mass of past studies have documented the appearance, statistics, and causes of U. prolifera through RS
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