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

Abstract

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).

Partitioning total spectral absorption in phytoplankton and colored detrital material contributions

“…Eleven stations were sampled: the MIO site in the Ionian Sea, the DYF site in the North-Western Mediterranean and stations 1-9 in various regions of the Mediterranean (for station locations see fig. 1 in Oubelkheir et al 2005). Trophic conditions were contrasted, from the eutrophic waters of the Malaga Upwelling (Sta.…”

“…Vertical profiles of total and dissolved absorption coefficients were determined in situ at nine wavelengths with a WETLabs ac-9 (without or with a 0.2μm filter at the intake of the measuring tubes). Ac-9 data acquisition and analyses are detailed in Oubelkheir et al (2005). Water samples also were collected for particle absorption coefficient measurements, a p (λ), following a protocol explained in more detail below, as it is not described elsewhere.…”

“…Therefore, the partitioning method proposed here allows us to estimate the slope of the particulate spectral attenuation coefficient from simultaneous measurements of total spectral absorption and attenuation coefficients made using an ac-9 (or ac-s). Past and recent studies (Boss et al 2001a(Boss et al , 2001bOubelkheir et al 2005; and references therein) have shown that the slope of the particulate attenuation spectrum can be related to changes in the size of the particle assemblage. Inde- Fig.…”

“…Vertical profiles: PROSOPE Campaign: The model presented here is applied to in situ vertical profiles of total minus water spectral absorption coefficient acquired with an ac-9 over different trophic regimes in the Mediterranean Sea (PROSOPE campaign, table 1; Oubelkheir et al 2005). The computed CDM absorption coefficient at 412 nm is compared with measurements derived from the sum of CDOM absorption coefficients measured with an ac-9 with a filter, and non-algal particle absorption coefficients determined on discrete samples (see Methods).…”

Modelling the plankton groups of the deep, peri-alpine Lake Bourget

“…Di erent phytoplankton species and detritus have been observed to have di erent speci c-light extinction coe cients. Oubelkheir et al (2005) suggests 0.02 m 2 mmolC −1 for non-algal particles, i.e., P OC here (which was estimated from P OP assuming the Red eld ratio). The same study provides a range from 0.012 to 0.045 m 2 mmolC −1 for various phytoplankton species, but for the sake of simplicity, we assume here a value of 0.03 m 2 mmolC −1 for all phytoplankton species.…”