Green macroalgae blooms in the Yellow Sea during the spring and summer of 2008

Abstract

During the spring and summer of 2008, a large‐scale green macroalgae (Enteromorpha prolifera) bloom broke out in the Yellow Sea. The bloom eventually aggregated in the coastal region off Qingdao in China, significantly affecting the city's preparations for the 2008 Summer Olympic sailing games. An algorithm using the Normalized Difference Algae Index, in combination with the shortwave infrared (SWIR) based water optical signature retrievals over macroalgae coverage, was developed with measurements from the Moderate Resolution Imaging Spectroradiometer (MODIS) on Aqua to reveal the start, evolvement, coverage, and aggregation density of this bloom. The bloom started in the southern Yellow Sea during mid May of 2008, moved to the north with the ocean currents, reached its peak in late June with bloom areal coverage of ∼4000 km2 on 25 June, then gradually died down in late July. The SWIR atmospheric correction algorithm has been used to study the optical features of the green macroalgae patches during this event and quantify the macroalgae aggregation in the coastal region. The optical spectra of the bloom are characterized with significantly enhanced ocean surface‐leaving radiance at the near‐infrared wavelength with a maximum value exceeding ∼5 mW cm−2μm−1 sr−1 at the wavelength of 859 nm off the Qingdao coastal region in late June. In contrast, the optical spectra of macroalgae show little difference from the clear ocean waters in the visible band. The causes of the macroalgae bloom are also discussed and attributed to the eutrophication and bloom‐favorable physical conditions (e.g., sea surface temperature, winds, ocean current, etc.) in the Yellow Sea.