Bio-optical properties of high chlorophyll Case 1 waters and of yellow-substance-dominated Case 2 waters

Abstract

Case 1 waters with high chlorophyll content can be encountered as soon as the nutrient availability is high enough and the terrestrial influence by land drainage is negligible. Offshore oceanic blooms and upwelling zones along arid coasts are instances of such waters. Their bio-optical properties are less documented compared to those of mesotrophic or oligotrophic waters. A coherent set of measurements of bio-geochemical properties (algal pigments, suspended particulate matter), inherent optical properties (absorption and scattering by water bodies and by particulate material), and apparent optical properties (hyperspectral reflectance and diffuse attenuation coefficients) was obtained within the Benguela Current, i.e. in an upwelling area with arid climate and no runoff. These data allow the bio-optical relationships in eutrophic Case 1 waters to be analyzed, and their natural variability to be compared with that previously observed in less productive waters. In addition, a comparison between eutrophic Case 1 waters and yellow substance dominated Case 2 waters can be made, since such waters are also present in the area under investigation. The coherence between the inherent and apparent optical properties is also analyzed via inversion. Despite some deficiencies in their parameterization, the existing bio-optical models for Case 1 waters were proven to be valid such that they can be extended without significant discontinuities toward the domain of high concentration (up to 30 mg m −3). In particular, those models in use for the interpretation of remotely sensed ocean color continue to apply, even if the sensitivity of current algorithms for the chlorophyll retrieval weakens owing to inescapable physical limitations in the case of high concentrations.