The mechanism of the banded structure of drifting macroalgae in the Yellow Sea

Abstract

At the end of May 2008, a massive bloom of macroalgae occurred in the western Yellow Sea off China and lasted for nearly two months, and annual blooms have occurred since then on. During bloom period, the surface-drifting macroalgae have showed an interesting pattern dominated by a banded structure, and the distance between neighboring bands ranged from hundreds of meters to about 6 km with a peak at 1–1.5 km, which is an order of higher than the scale of Langmuir circulation of 50–100 m. In order to explain this new phenomenon, ocean current data obtained from a Doppler current profiler off Qingdao was used to implement stability analysis. By numerically solving the resulting differential Orr-Sommerfeld equation, the secondary circulation induced from the instability of the Emkan current was found to fit well with the observed spatial scale of the surface-drifting macroalgae’s banded structure. As the wind driven Emkan current exist universally in the global ocean, it is reasonable to conclude that the banded structure with kilometers distance between adjoining bands is ubiquitous. We found a new circulation in the upper ocean which is important for exchange of energy, materials and gas between the upper ocean and subsurface layer.