Investigation of Land-Based Pollution and Algae Blooms in Chinese Coastal Waters
Alpers, Werner1; Hansen, Morten W.2; Zhang, Yuanzhi3; Tang, DanLing4; Lin, Jingrou4
1University of Hamburg, Institute of Oceanography, Hamburg, GERMANY; 2Nansen Environmental and Remote Sensing Center, Bergen, NORWAY; 3The Chinese University of Hong Kong, HONG KONG; 4South China Sea Institute of Oceanology, Guangzhou, CHINA
Synthetic aperture radar (SAR) images acquired by ERS SAR and Envisat ASAR over Chinese coastal waters have been analyzed with the aim of locating man-made sea pollution that has its origin on land. Pollutants of anthropogenic (man-made) origin are transported into the sea usually by rivers located in heavily populated or industrialized areas. Pollutants can only be captured by SAR when they are associated with surface-active material floating on the sea surface, which consists of organic material as encountered almost always in waste water of industrial and sewage plants. These surface films, which are usually only one-molecular layer thick, damp the short sea surface ripples almost as strongly (of the order of 8 dB) as mineral oil films originating from ship spills. The polluted areas become visible on SAR images as recurrent dark patches attached to the coastline. We have identified several areas at the east coast of the Pearl River delta (e.g. the mouths of the Dongboahe and Dashabe rivers and the Shenzhen Bay) and at the west cost of Taiwan (Gaoping river), where such dark patches are almost always present provided that the wind speed is not too strong (below 6-8 m/s). However, dark patches adjacent to the coast line are not always caused by anthropogenic pollution, but also by wind shadowing by coastal mountains, by cold upwelled water, or by biogenic surface films originating from phytoplankton blooms. In order to discriminate between dark patches originating from land-based anthropogenic pollution and from other phenomena, the so-called look-alikes, we have used also other sources of information, like data from weather services and other satellite sensors. The satellite data include near-surface wind fields derived Quikscat and ASCAT data, chlorophyll-a concentrations derived from SeaWiFS, MODIS, and MERIS data, and sea surface temperatures derived from AVHRR, MODIS, and ATSSR data. Areas covered with biogenic surface have been identified in upwelling areas north of Taiwan and in the Strait of Taiwan.