Some of the HABs are known as ‘red tides’ because of the water being coloured red by numerous concentrated red algae, but HABs can have different colours. However, not all HABs have optical signatures detectable by remote sensors and many HABs occur deeper down in the water column. In addition cloud cover may interrupt remote sensing. But despite these limitations remote sensing is very valuable because it provides rapid detection of increases of chlorophyll-a over large areas in a small period of time. One of the criteria used for assessing water quality, is the concentration of phytoplankton distributed in the water. While naturally occurring phytoplankton are common in coastal waters, increasing eutrophication and sea surface temperatures (SSTs) cause these microscopic algae to bloom. Apart from oxygen depletion caused by the massive death and decomposition of algae, a small percentage of algae produce biotoxins, posing a great threat to nature and public health by contaminating the water, sediments and aquatic organisms. Long-term regional, national and international monitoring plans and programmes provide information about trends and frequencies of these so called ‘harmful algal blooms’ (HABs), their resulting toxicity for marine life, and suspected causes (e.g., climatological, hydrographical, or nutrient changes). To monitor HAB species occurrence and HAB toxins it is recommended to combine in-situ measurements with remote sensing. The dramatic increase of phytoplankton biomass causes the HABs to have distinct spectral characteristics. In particular, chlorophyll-a, a coloured pigment found in phytoplankton, is detectable by satellites, at least for HABs occurring in surface waters. |