Case I waters are defined as waters for which phytoplankton and their associated materials (such as debris, heterotrophic organisms and bacteria, excreted organic matter) control the optical properties. Chlorophyll retrieval in Case I water is relatively simple because only plankton influence optical properties, and scattering and absorption by debris is correlated with chlorophyll concentrations.
Case II waters are found in coastal zones influenced by land drainage or suspended sediment In addition to phytoplankton. Optical properties are typically controlled by three independent components:
Concentrations of these three must be retrieved simultaneously; and this is a complex problem that is not yet fully solved, although progress is being made, taking advantage of extra MERIS channels in the red part of the spectrum. The standard MERIS product, algal 2 is derived using a neural net algorithm, which simultaneously retrieves yellow substance and suspended sediment.
Case I waters are typical of the open ocean, away from coasts and river mouths. Chlorophyll-a concentration ranges from very low values (~ 0.02 mg m-3) in the oligotrophic waters of the subtropical gyres, up to high values, 5-20 mg m-3, in coastal upwelling areas such as the Benguela. The chl-a concentration to be detected from space spans over 3 orders of magnitude, and the probability of a pixel having a given [Chl] concentration is inversely proportional to the [Chl] value itself. For this reason chlorophyll concentrations are usually presented on a logarithmic scale.
Phytoplankton blooms such as those frequently occurring in upwelling areas are a particular problem for ocean colour processing. Formally these are Case I waters, for their optical properties are still controlled by phytoplankton and their associated material, but enormous number of phytoplankton cells in the water means that these water-leaving radiance may be high enough to trigger the Case II classification flag, and some blooms may have sufficient reflectances above 700nm to cause problems for aerosol retrieval, and thus for the atmospheric correction.
Coccolithophore blooms are commonly in this category, and flagging for coccolithophores before processing from L1B to L2, has been suggested as part of the solution to the problem of aerosol correction in the presence of these blooms.