Drifting trajectories of green algae in the western Yellow Sea during the spring and summer of 2012

Abstract

The northward drift of green algae (Ulva prolifera) from Subei Shoal in the western Yellow Sea, China, during the spring and summer of 2012, was investigated using satellite data and numerical modeling. Past studies have suggested that the green algae, documented offshore of Shandong province since 2007, originate in Subei Shoal region of the Yellow Sea. To test this hypothesis, drift bottles and satellite-tracked surface drifters were released from Subei Shoal and used to investigate the trajectories of green algae. Subei Shoal is characterized by complex bathymetry such as broad tidal flats and radial sand ridges. To identify processes that drive drift of the green algae around the shoal, a coastal ocean model based on the Finite Volume Coastal Ocean Model (FVCOM) was used. This model is forced by tides and surface winds, and has sufficient resolution to include tidal flats and sand ridges during both wetting and drying. The results of numerical experiments indicated that sand ridges limit the trajectory of particles. Without wind, particles scattered from their initial positions displayed a tendency to move northward, but were unable to move out of Subei Shoal. When a southerly wind was introduced to the model, particles traveled further north, out of the shallow waters. After leaving Subei Shoal, drifters remained limited by tide and topography until reaching 34°30.0′N. North of 34°30.0′N, 33% of the trajectory vectors can be explained by Ekman theory, and the remainder are probably controlled by the strong baroclinic processes in this area. For the six surface-following drifters deployed, the mean drift speed was 11.1 cm s−1 (288.8 km month−1), close to the speed observed for patches of U. prolifera. Numerical models and the results from drifter bottles demonstrated that green algae could leave Subei Shoal, but only when aided by a southerly wind. Satellite-tracked drifters provided strong evidence that if floating particles do leave Subei Shoal, they could be transported to the Qingdao coast or even further north.