Using MODIS medium-resolution bands to monitor harmful algal blooms

MODIS medium-resolution (250- and 500-m) bands were successfully used to detect and map the distribution of a harmful phytoplankton bloom (HAB) in the Paracas Bay, Peru, that caused economic losses estimated at about $28.5 million. A Red-Green-Blue combination of bands 1, 4 and 3 was used to visually distinguish the HAB while the turbidity index, a semi-quantitative measure of the amount of particulate material in the near-surface water, was used to estimate the intensity of the HAB. The turbidity index was inversely correlated with oxygen concentration in the bay. Temporary anoxia caused by the HAB was probably the main mechanism causing fish kills. The 250-m resolution provided by MODIS bands 1 and 2 is essential to detect localized HABs in coastal areas. While turbidity is not specific to algal blooms, it is a quantitative estimate of the intensity of the bloom once the existence of the bloom is detected by the RGB images.