Tracing floating green algae blooms in the Yellow Sea and the East China Sea using GOCI satellite data and Lagrangian transport simulations

Abstract

In an image from the Geostationary Ocean Color Imager (GOCI) taken on 13 June 2011, a recurrent floating green algae bloom was detected around Qingdao. To detect the bloom an index based on multi-spectral band ratios using three wavelengths (555, 660, 745nm) was utilized. One month later, the GOCI images detected widespread floating green algae patches (east–west elongated) across the Yellow Sea and East China Sea (YS, ECS). The presence of the patches was confirmed from various oceanic observation cruises.Lagrangian particle tracking experiments were conducted to understand the pathway of the floating green algae patches and interpret the physical forcing factors that affect the distribution and advection of the floating green algae. The numerical simulation results indicated that dominant southerly winds during June and July 2011 were related to offshore movement of the floating green algae, especially their eastward extension in the YS/ECS. An infrequent and unusual event occurred in June 2011: a typhoon MEARI, caused the green algae to detach from the coast and initiated movement to the east. After the typhoon event, sea surface temperature recovered rapidly enough to grow the floating green algae, and wind and local current controlled the movement of the massive floating algae patches (coastal accumulation or offshore advection in the YS/ECS). Analysis of the floating green algae pixels' movement during passage of Typhoon MAON in July 2011 revealed that the floating green algae patches were significantly controlled by both ocean currents and enhanced winds. These findings suggest that the floating green algae bloom off Qingdao and in the middle of the YS and ECS in the summer of 2011 occurred due to the combined effects of recent rapid expansion of seaweed aquaculture, strong winds, and the wind patterns in blooming regions. Our combined approach, using satellite data and numerical simulations, provides a robust estimate for tracing and monitoring changes in green algae blooms on a regional scale.