Bio‐optical and biogeochemical properties of different trophic regimes in oceanic waters

To examine the source and magnitude of the variability of bio-optical properties in open ocean, we simultaneously measured inherent optical properties (IOPs) and biogeochemical quantities during late summer from the eutrophic waters of the Moroccan upwelling to the oligotrophic waters of the northwestern Mediterranean and the ultraoligotrophic waters of the eastern Mediterranean. Vertical distributions of spectral absorption and attenuation coefficients were measured with a high-resolution in situ spectrophotometer (WETLabs ac9) together with biogeochemical measurements that included phytoplanktonic pigments and particulate organic carbon concentrations, particle size distributions, and picoplankton abundance. The variability in specific IOPs (i.e., per unit of biogeochemical constituent concentration) was examined, and an optical index of particle size was derived. The fine-scale vertical distributions of various biogeochemical properties were thus described from ac9 profiles. Particle attenuation and carbon budgets, estimated from a combination of optical and biogeochemical measurements, underlie a major contribution of nonalgal stocks in oceanic waters. We show that first-order variations in IOPs in oceanic waters are explained by the trophic state (i.e., chlorophyll a concentration) and that second-order variations are the result of changes in the composition of phytoplankton assemblage, the balance between algal and nonalgal stocks, and lightrelated processes (colored dissolved organic material photo-oxidation and algal photo-adaptation). At the interface between marine optics and biogeochemistry, bio-optical studies (Smith and Baker 1978) aim to characterize the biological and biogeochemical state of natural waters through their optical properties, and to quantify the role of the ocean in global biogeochemical (particularly carbon) budgets. These studies rely on the direct dependence of the water’s inherent optical properties (IOPs) and apparent optical properties on the concentration and nature of optically significant biogeochemical constituents. In open ocean case I waters (Morel and Maritorena 2001 and references therein), these constituents are, by definition, phytoplankton and their accompanying and covarying retinue of material with biological origin, namely nonalgal particles (including biogenous detritus and heterotrophic organisms) and yellow substances (so-called colored dissolved organic material [CDOM]). Thus, optical properties generally are modeled as a function of chlorophyll a concentration (Chl a; used as a proxy for phytoplankton) in generic remote sensing algorithms for case I waters (Morel and Maritorena 2001 and 1

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